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https://ntrs.nasa.gov/search.jsp?R=19730013425 2020-07-29T01:14:31+00:00Z
DEVELOPMENT OF A DC MOTOR WITH
VIRTUALLY ZERO POWERED
MAGNETIC BEARING
Prepared under Contract NAS5-21587
CAMBRIDGE THERMIONIC CORPORATION
Cambridge, Massachusetts
For Goddard Space Flight Center
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
(
TABLE OF CONTENTS
SECTION I --
VZP DEMONSTRATION
MODEL
SECTION II --
VZP CONTROL
TECHNIQUES
SECTION III --
TORQUE MOTOR
SECTION IV--
WORKING MODEL
SECTION V --
SUMMARY
PREFACE
1.1 Magnets
1.0 Magnet Tests
1.2 Virtually Zero Power Principle
1.3 General Difference Between Electromagnetic& Electromagnetic/Permanent Magnet Systems
1.4.0 Development of Breadboard Model of the VZPMagnetic Bearing System
1.4.1 New Technology
1.4.2 Mechanical Configuration
1.4.3 Displacement Sensing System
1.4.4 Rate Sensing System
1.4.5 Description of Behavior of Assembled System
1.4.6 3rd Servo of Centering System
1.4.7 Conclusions
2.0 Magnetic Bearing Motor and Motor DriveCircuitry
2.1 Magnetic Bearing Scanner Mount
2.2 Close Gap Bearing
2.3 "Bang-Bang" Magnetic Bearing AxialCentering Servo
2.4 Clamshells
2.5 Velocity Only Axial Control
2.6 Conclusion of Control Experimentation
3.0 Instability
3.1 Torque Motor
4.0 Electronic Controller
4.1 Automatic Starting
4.2 NASA Magnetic Bearing and MotorSpecifications
4.2.1 Supplementary Data
5.0 Recommendations and Conclusions
Page
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1
3
4
9
10
11
11
12
12
12
14
14
16
16
18
20
21
24
33
35
36
37
37
38
40
41
I
PREFACE
Previous magnetic bearing - DC motor system delivered to NASA by
Cambridge Thermionic Corporation required 20-30 watts of continuous support
power. Although this system performed will the continuous support power was
a disadvantage for space applications.
The object of this contract was to develop a magnetic bearing - DC motor
with a minimum of power required for the support system. It was felt that
permanent magnets could be made to carrythe steady state bearing loads
while the electromagnets could be required only to counteract the transient
base disturbance inputs.
The period of this contract was to be nine months in length. However,
within three months the basic principal was reduced to practice. This principal
was called "Virtually Zero Power" (VZP) and was demonstrated in a working
device at NASA, Goddard Space Flight Center on 25 February 1971.
Following this demonstration, work proceeded to increase the radial
support capability, design and fabricate a drive motor and design and fabricate
the electronic drive circuitry.
Subsequent investigation showed that the radial stiffness could be increased
by armoring the permanent magnet with steel. This also required the use of
relatively narrow magnetic air gaps which put a greater burden on the
mechanical alignment and electronic servo system. A magnetic bearing using
steel armoring on the permanent magnets was designed and built. This device
was to be a Magnetic Bearing Scanner Mount (MBSM).
NASA provided a motor to be integrated into the magnetic bearing system.
This newly developed motor was considered to have qualities suitable for
scanner pointing applications.
The motor was integrated into the magnetic bearing system and the complete
device was delivered and demonstrated at NASA, Goddard Space Flight Center
on 19 September 1972.
ill
SECTION I - VZP DEMONSTRATION MODEL
1.0 MAGNETS
It became evident early in the development that there was need for a
feasibility study to determine if it would be technically possible to carry
all steady state bearing loads (axial, radial, and torsional) solely through
the use of permanent magnets (requiring zero power).
The NASA Engineer stressed the importance of utilizing a high magnetic
flux density (B) (through the use of permanent magnets), in the formula2
F - kB . It was felt that the required magnetic modulation, for stability
in the presence of disturbances could be obtained more efficiently in this
manner.
The selection of the most efficient geometrical configurations among
the large number of possibilities became a basic problem. It was early
concluded that there was insufficient information available to fix on a
particular design. It was therefore decided to: (1) Examine a number of
possible designs, (2) Make measurements, (3) Further investigate the use
of the new Samarium-Cobalt magnets. (See Figures 1 and 2)
In Figures 3 through 9 in the Appendix are graphs taken from data on
several configurations, including some with very small (1" x 1/4" discs 25
gram) Samarium-Cobalt magnets and more with very large (50 lb.) Alnico
magnets. In connection with the use of the large magnets, two extremes
in L/D ratios were examined. The large diameter tube (L/D = 1.337) is on
the edge of torsional instability while the smaller diameter tube (L/D = 2.675)
exhibits excellent torsional stability.
There is very obviously a discrete point on an L/D curve ratio where one
passes from stability to instability - for any given ring geometry and gap.
Although not a specified part of the project - since it would be sufficient to
employ an arbitrarily high L/D ratio.
Engineering data on Samarium-Cobalt magnets, as well as samples, were
provided by the Raytheon Company of Waltham, Massachusetts. Because of
the newness of these magnets and the consquent lack of understanding of their
behavior, some of the data concerning Force/Displacement attached to this
report must necessarily be regarded as preliminary. It has become evident, how-
ever, that one is dealing with a very unusual material which must be handled
by quite different methods than those normally employed with older "alnico"
type magnets. Specifically, it is evident that if is desired to obtain the very
high force levels possible with this light weight material, one must reduce the
"armoring" (by ordinary steels) to the shortest possible path lengths. The
Samarium-Cobalt magnet must be thought of as a large "air-gap" magnet.
A possible geometrical configuration began to evolve. It developed partly
from the absolute necessity to keep the pole piece lengths - in the use of
Samarium-Cobalt magnets - extremely short. Additionally, it is believed
essential to have very high speed (low inductance) servo operation, an attempt
has been made to utilize a linear ("ironless") motor in the high field provided
by the supporting permanent magnets. Such an approach does appear to have
merit, expecially due to its simplicity and apparent high order of theoretical
efficiency.
A test of such a system was conducted on the "magnetic gate" fixture. It
showed that a + 3 lbs. of force change (electric modulation) is provided through
the step-function application of 2.4 watts. (This is in a 24 lb. permanent
magnet total available force level (at zero gap)). Results were obtained at a
.050" air-gap. Due to flexural limitations of the test fixture, smaller air-gaps
could not be examined. The mechanical assembly was not rigid enough to
handle to higher force levels. It was apparent however, that percentage
modulation was tending to increase with gap reduction. Even so, such a 10%
modulation might prove sufficient for servo control of a complete system.
Because of the clearly demonstrated superior properties of the Samarium-Cobalt
materials, it was then proposed to abandon any further activity with Alnico
-2-
type magnets so as to be able to concentrate on the efficient use of the new
Samarium-Cobalt material. Subsequent emphasis was directed toward (1)
linear generators, (2) linear motors suitable for use with Samarium Cobalt and
"Virtual Zero Powered".
1.1 MAGNET TESTS
After selection of Samarium-Cobalt magnets, emphasis was next placed on
construction of apparatus for evaluation and test. Two new cup cores were
manufactured to contain the Samarium-Cobalt 1/4" x 1" magnets. Each magnet
of the mating pair was fitted with a 1/2" diameter (.025" square cross-section)
flux concentrating ring. On one of them there was built on, as an integral
part, a shaft extension (1.5" x .150") so that additional devices might be
installed on the end of the system. These new cup cores were not as thick as
the set previously made because there is no armoring other than the rings. A
miniature rate generator was constructed. A small linear motor coil was also
built to fit into the leakage flux path of the "rotor" cup core, while being
fixed to the "stator" core.
In addition to the above, a rigid stand was constructed to hold the stator
coils approximately six inches apart and to clamp them securely in this position.
(See Photographs 1 through 6.)
Quite clearly the elimination of the steel armoring to the Samarium-Cobalt
magnets resulted in a higher effective force level both axially and, to some
exent, radially as well. Axial measurements were reasonably straightforward.
(See Figures 10-13)
Shims were employed (.003") to fix the air-gap and the breakaway force
measured for each increment in displacement as shims were added from zero air-
gap to the maximum of interest. The dial of the Chatillion force gauge recorded
maximum force encountered and, since the breakaway point was precipitous,
the method yielded high accuracy. All elements had to be made very rigid to
preserve pole face alignment and prevent "cocking", in the face of high force
levels.
-3-
Measurement of radial forces was not quite so easily accomplished. In
fact, considerable effort was actually put into the design and construction
of the apparatus before a satisfactory method was developed. The difficulty
was in preserving accurate pole face alignment at small air gaps, in the
presence of the high axial pulling force. This method finally worked out is
illustrated in Figure II. The data from this test is shown in Figures 12 and 13.
It was found very important to have the "stabilizing" rod long and stiff
in order to preserve pole face alignment. However, when all had been put
in order, it was readily possible to measure radial forces down to .003" air-
gaps with excellent repeatability.
Some testing was also done with the idea of using one end of the magnetic
bearing as the velocity (linear) motor. The constituted a negative feedback
system and was attractive because of its simplicity. However, results were
inconclusive, largely because sufficient force could not be obtained from the
motor.' In the space available within the cup core only 56 turns of #28 wire
could be inserted and still allow for the necessary .025" radial "play". If
the scheme was to receive further attention, larger cup cores would have to
be employed. Tests did show that response time of the whole system was
sufficiently short, and this was the point of most interest at the time.
Also the miniature rate generator was attached to the shaft and was
tested. An unsatisfactory low output level was encountered, but again the
time constant appeared reasonably short. (This small unit was built with the
idea that it might later be placed within the shaft of the rotor).
1.2 VIRTUALLY ZERO POWER PRINCIPLE
Relatively early in the investigation, a rather far reaching decision was
made: namely, "to fabricate a complete system as quickly as possible to
prove out the overall theory of the Virtually Zero Power System". This was
done and the principle was proved and there was no longer any doubt whatso-
ever concerning the validity of the theory of "VZP".
-4-
To construct a complete operable system in a short period of time
required making numerous compromises. Previously built "cup cores" were
known from tests to be quite efficient. Their effective use, however,
entailed working at close air gaps (of the order of .005" - .010") where
dimensional tolerances are critical. However, to achieve a time table of
one month for completion of the apparatus made it impossible to consider
their use.
It was found that a triplet configuration of I" diameter "raw" Samarium-
Cobalt magnets yielded 2 lbs. of radial force at air gaps of .100" to .200".
To be sure, such a system could not be expected to give a steep radial force/
displacement curve, but if fairly large radial clearances were employed, it
appeared sufficiently high to allow construction of a workable demonstration -
if rotor weight was kept low. Unfortunately, it was not possible to secure
more than two Samarium-Cobalt magnets with large enough clearance holes
from the Raytheon Company in time. This meant that only a pair of doublets
were available.
The system was designed to separate the permanent magnet radial support
magnets by approximately 5". Two axial force coils were buil It to act
oppositely against a very light central disc of BFM steel. These force coils
had to develop sufficient force to enable starting the system from zero air
gap. It could be accomplished by application of 16 watts from the amplifier.
Next, a very simple displacement system was built. This system consists of
two photo-cells looking at two micro-miniature lamps. The rotor carries on
one end a ring, mounted on transparent plastic, which divides the light
equally to the two sensor cells when the central disc is halfway between the
two force coil cup cores. Such a simple system proved adequate. See
Figure 14. -
A light shield was made to avoid effects of stray light and surrounded
the whole sensor system. The rate generator employed makes use of one of
the rotor carried support magnets - a temporary but effective measure.
-5-
When the rotor is precisely in the central position (unstable) the rotor
"falls" to one side or the other to the stopped position (+ .025") in 8-17
seconds, depending on the amplifier gain. This free fall situation is
naturally dependent upon amplifier gain, P.M. air gap, etc., but in
general is a measure of rate generator effectiveness.
Upon assembly of the apparatus, it was immediately found that the
force coils vibrated as a tuning fork. This required that the free ends be
rigidly clamped, which was done - the clamp being additionally used to
support the force coil terminals.
By 2 February 1971, the system was operational. Some time was spent
in observing the behavior of the unit. It was soon found that:
I. The bearing could be operated with virtually zero power to the force
coils (Bearing Horizontal). Actually .05 watts.
2. That this situation also applied to Vertical Operation IF the amplifier
was re-balanced in this position.
3. That the system would self-start in any position. Starting required
approximately 16 watts peak and was completed in a measured 150
milliseconds.
It was found that initial balancing adjustments were critical, in that it
was easily possible unwittingly to balance one unbalanced part of the system
against another oppositely unbalanced part. When this was done, a totally
unstable system resulted! A procedure, outlined below, was then worked
out to avoid the problem:
I. Balance the amplifier with power stages shut off. This is done by
adjusting the amplifiers recommended bias potentiometer while viewing
the zero-center output meter.
2. Adjust the relative positions of the P.M.'s along the rotor shaft so that
the equa! force (unstable) position occurs with the central disc is pre-
cisely midway between the force coil cup cores.
3. Then physically move the fixed portion of the displacement sensor
system (with the force coils ON) to make the central disc arrive in
-6-
the previously determined central position. (Note that the disc's
axial motion follows the sensor motions, i.e. has the same sense).
4. Observe the zero-center meter. It should now read zero. If it does
not precisely conform to zero, then minor amplifier balance adjust-
ment may be made.
5. Note that the central axial position is that position at which the
rotor is equally likely to move left (or right!). With the displacement
sensor shut-off and this position is achieved, then the rotor will hold
this position for a considerable time - staying away from the destina-
tion pole (whichever) for as much as a minute. (On rare occasions
when all balance adjustments are nearly perfect.)
It was found that there was a small double zero position, but it
indicated further detailed examination as its elimination would permit a
closer approach to absolute zero power.
A very poor coast time was immediately noted. This was attributed to
the open field resulting from the use of "raw" P.M.'s reacting on the
axial shaft which penetrated two of them.
As experimental work continued, it was learned that with the displace-
ment sensor shut-off ("rate alone") that center position could be manually
maintained for extended periods. This was done by watching the zero-
center meter while manually adjusting the amplifier balance control.
Practice was required, after which it was possible to hold close to center
positions for a long as was desired. Such controlling required no appreci-
able skill since ample time was available for introduction of correctional
signals. This was due to the long time constant of the rate system as a
whole. The next logical step was to introduce the 3rd servo or centering
system. This was done on 3 February 1971 and proved highly effective.
It should be remembered that this system senses the unbalanced conditions
across the force coils (essentially the zero-center meter signal) and serves
to remove this unbalance by biasing the amplifier in the proper direction
by the proper amount.
-7-
Due to the low voltage level of the force coils (< 4V) it was not possible,
at that time, to introduce the desired dv/dt signal. In spite of this, it was
found possible to secure reasonably stiff servo response.
With the "3rd" servo in operation, it was immediately possible to secure
reasonably stiff servo response. With the "3rd" servo in operation, it was
immediately possible to see automatic compensation for applied axial loads.
To be sure, nothing like optimum operation was achieved, but, in principle,
the system was certainly a success. The rotor moved in opposition to axially
applied loads with total motion of + .020" before control was lost. Within
these limits, and with the axial loads not exceeding several pounds, the
system was stable and obviously kept the required power to low levels -
usually less than 1/2 that required to counter the loads without the system
in operation.
The concept of the Virtually Zero Power magnetic bearing system was thus
proved out. Clearly, much remained to be done before a sophisticated motor-
bearing could be fabricated. But it then became quite apparent that VZP
magnetic bearing systems were practical.
At this time, an additional interesting experiment was conducted. The
2nd (displacement) servo was disconnected (after starting) and reliance was
placed only on the 1st (rate) and 3rd (force balancing system).
In spite of the previously mentioned non-optimum characteristics of the
3rd servo system, operation was quite stable. The possibility of such a system
had been predicted before testing so it was particularly gratifying to observe
the actual validity of the idea.
In order to conserve overall power, it would be possible to disconnect the
#2 servo system whenever the rotor is axially centered. This could be done
(simply) by deriving power for the sensor lamps from un-differentiated output
of the amplifier. This power is essentially zero when the rotor is in the center
positions when starting is required, (thus the lamps are "On").
-8-
As a result of the work accomplished, during this early period it was
concluded that the concept of a Virtually Zero Powered magnetically
supported rotor was feasible. Further work was then pointed toward adding
two additional magnets, and increasing the overall performance of the #3
servo system, and to add a motor. See schematic in Figure 15.
1.3 GENERAL DIFFERENCES BETWEEN ELECTROMAGNETIC &
ELECTROMAGNETIC/PERMANENT MAGNET SYSTEMS
The following is a listing of the differences between electromagnetic
and electromagnetic/permanent magnet systems:
I. No thermal problems. Control coils operate cold (very close to
ambient.)
2. There is no mechanical strain accross the rotor and, therefore, a
lighter supporting structure is possible. (In all fairness, this could
also be done with the EM System).
3. Almost exactly 1/2 the number of electronic components are re-
quired. (The circuit is differential rather than push-pull).
4. There are no large steady state currents in electronic system -
they operate in standby condition, except for transients. (About
2 watts + unresolved #3 servo differential power of approximately
< 0.1 watts.)
5. It is readily possible to secure a satisfactory displacement signal
with I (one) micro-miniature lamp only. Multiple redundancy is
also possible.
6. Due to the "empty weight" being lower in the permanent magnet
system, the payload to empty weight ratio can be higher.
7. The configuration is more attractive from a dynamic standpoint since
outboard weights are very much smaller.
8. Cocking problems are reduced.
-9-
1.4.0 DEVELOPMENT OF BREADBOARD MODEL OF THE VZP MAGNETIC
BEARING SYSTEM
There then followed the following series of events:
I. Preparing for demonstration of the breadboard model unit.
2. Demonstration of the unit at Goddard Space Flight Center on 25 February
1971.
3. Collection of data on the unit.
In Figure 16 is a plot of amplifier performance which shows that there is
little, if any, "cross-over" distortion. It will be recalled that earlier mention
has been made of a "double-zero" position. It became evident that in view of
the linearity noted in the amplifier characteristics, it clearly could not be
attributed to the amplifier. Therefore, the "double-zero" condition needed to
be explained in some other way. It then was believed to be a fundamental
characteristic of the system as a whole.
It was also noted that the rotor must accelerate (axially) either to the
right or to the left from the neutral position as determined by the magnetic
fields of the permanent magnets. The function of the force coils is to induce
this acceleration to approach zero, as a limit.
Assume, for example, that the rotor is accelerated to the left but is very
close to the zero acceleration position. We now move the bias control so as
to start the acceleration through zero and beyond - to the right. One can
see that to counteract the right hand acceleration (if we wish to approach
zero again) the bias must now be moved through zero - to the left again.
There is, therefore, an unstable "flip/flop" condition existing about the
zero acceleration-zero power position. This is'fundamental to the system as
conceived at the time.
It became important to examine the magnitude of the unstable region.
Measurements showed that the actual axial motion, in the unit, was of the
order of plus or minus 0.0003" and that the power required to work against
this instability was about .05 watts.
-10-
It was then believed that the unstable region could be reduced in
magnitude by a number of means such as (I) increased displacement system
gain, (2) increased rate system gain, (3) reduced permanent magnet force
level - this being the least desirable. But it was thought that the "back-
lash", as in mechanical devices, could not easily be eliminated entirely -
but must be allowed for in bearing use. In some applications, for instance,
the system may be deliberately biased to one side so that the rotor cannot
accelerate through zero. This, however, would cost power. The Principal
Investigator, Mr. Joseph Lyman, completed his portion of the task assign-
ment at this point with the verification of the Virtually Zero Power principle
by practical demonstrations and test.
1.4.1 NEW TECHNOLOGY
A fundamental technological development confirming previous work of
the principal investigator was then reported. A stable Magnetic Suspension
system was fabricated and a fundamental basic electromagnetic principle
reduced to a practice, wherein a Magnetically Supported rotor utilizing
permanent magnet primary support had been made stably operational utilizing
Virtually Zero Power. The system is described as follows:
1.4.2 MECHANICAL CONFIGURATION
1. A triplet of 1" diameter "raw" Samarium-Cobalt magnets yielded >2 lbs.
of radial force at air gaps of .100" to .200". Such a system could not
be expected to give a steep radial force/displacement curve, but if
fairly large radial clearances are employed, it appeared sufficiently
high to allow construction of a workable demonstration - if rotor weight
was kept low. Unfortunately, it was not possible to secure more than
two Samarium-Cobalt magnets with large enough clearance holes from
the Raytheon Company in time. This meant that only a pair of doublets
were available. This proved to be sufficient, but provisions were made
to complete the triplet pairs as soon as the additional magnets were
received.
-II-
2. The system was designed to separate the P.M. radial support magnets by
approximately 5". Two axial force coils were built to act oppositely
against a very light central disc of BFM steel. These force coils had to
develop sufficient force to enable starting the system from zero air gap.
This could be accomplished by application of 16 watts from the amplifier.
1.4.3 DISPLACEMENT SENSING SYSTEM
1. A very simple displacement system was built. The system consisted of two
photo-cells looking at two micro-miniature lamps. The rotor carried on
one end a ring, mounted on transparent plastic, which divided the light
equally to the two sensor cells when the central disc was halfway between
the two force coil cup cores. This simple system proved adequate and is
diagrammed in Figure 14. A light shield was made to avoid effects of
stray light and surrounded the whole sensor system.
1.4.4 RATE SENSING SYSTEM
1. The rate generator employed made use of one of the rotor carried support
magnets - a temporary but effective measure. When the rotor is precisely
in the central position (unstable) the rotor "falls" to one side or the other
to the stopped position (+ .025") in from 8-17 seconds. This free fall
situation is naturally dependent upon amplifier gain, P.M. air gap, etc.,
but in general is a measure of rate generator effectiveness.
1.4.5 DESCRIPTION OF BEHAVIOR OF ASSEMBLED SYSTEM
1. When the system was operational, some time was spent in observing the
behavior of the unit. It was soon found that:
a. The bearing could be operated with zero detectable power to the
force coils (Bearing Horizontal). Actually .05 watts.
b. That this situation also applied to Vertical Operation IF the amplifier
was re-balanced in this position.
c. That the system would self-start in any position. Starting required
approximately 16 watts peak and was completed in a measured 150
mi ll iseconds.
-12-
2. It was found that initial balancing adjustments were critical, in that it
was easily possible unwittingly to balance on unbalanced part of the system
against another oppositely unbalanced part. When this was done, a totally
unstable system resulted. A procedure, outlined below, was then worked
out to avoid the problem:
a. Balance the amplifier with power stages shut off. This is done by
moving the amplifiers recommended bias potentiometer while viewing
the zero-center output meter.
b. Adjust the relative positions of the P.M.'s along the rotor shaft so that
the equal force (unstable) position occurs with the central disc is
precisely midway between the force coil cup cores.
c. Then physically move the fixed portion of the displacement sensor
system (with the force coils ON) to make the central disc arrive in the
previously determined central position. (Note that the disc's axial
motion follows the sensor motions, i.e. has the same sense.)
d. Observe the zero-center meter. It should now read zero. If it does
not precisely conform to zero, then minor amplifier balance adjustment
may be made.
e. Note that the central axial position is that position at which the rotor
is equally likely to move left or right. With the displacement sensor
shut-off and this position is achieved, then the rotor will hold this
position for a considerable time - staying away from the destination
pole (whichever) for as much as a minute. (On rare occasions when all
balance adjustments are nearly perfect.)
3. Also, it was found that there was a small double zero position. No serious
attempt had yet been made to uncover the cause of this situtation, but it
deserved further detailed examination as its elimination would permit a
closer approach to absolute zero power.
4. A very poor coast time was immediately noted. This was attribute to the
open field resulting from the use of "raw" P.M.'s reacting on the axial
shaft which penetrates two of them.
-13-
5. As experimental work continued, it was learned that with the displacement
sensor shut-off ("rate alone") that center position could be manually main-
tained for extended periods. This was done by watching the zero-center
meter while manually adjusting the amplifier balance control. A little
practice was required after which it was possible to hold close to center
positions for as long as was desired. Such controlling required no appreci- -
able skill since ample time was available for introduction of correctional
signals. This is due to the long time constant of the rate system as a whole.
1.4.6 3RD SERVO OR CENTERING SYSTEM
1. This system senses the unbalanced conditions across the force coils (essentially
the zero-center meter signal) and serves to remove this unbalance by biasing
the amplifier in the proper direction by the proper amount.
2. Due to the low voltage level of the force coils (<4V) it was not possible,
within the developmental period, to introduce the desired dv/dt signal. In
spite of this, it was found possible to secure reasonably stiff servo response.
3. With the "3rd" servo in operation, it was immediately possible to see automa-
tic compensation for applied axial loads.. To be sure, nothing like optimum
operation was achieved, but, in princip!e, the system was certainly a success.
The rotor moved in opposition to axially applied loads with a total motion of
+ .020" before control was lost. Within these limits, and with the axial loads
not exceeding several pounds, the system was stable and obviously kept the
required power to low levels - usually less than 1/2 that required to counter
the loads without the system in operation.
1.4.7 CONCLUSIONS
In effect, the concept of the Virtually Zero Power magnetic bearing system
had been proved out. Clearly, much remained to be done before a sophisticated
motor-bearing could be fabricated. But at this point it appeared that it could
certainly be done.
It has now been shown that permanent magnets can be made to carry the
steady state bearing loads while the electromagnets are only required to counter-
act the transient base disturbance inputs.
-14-
The servo system for this type of magnetic suspension is not unique in itself.
However, the combined system provides some unusual characteristics. The
first being the reduction of power consumption to a minimum. The second how-
ever, is that the rotor moves in opposition to axially applied loads so that the
permanent magnets continue to carry the load and maintain the power consump-
tion at a minimum.
-15-
SECTION II - VZP CONTROL TECHNIQUES
2.0 MAGNETIC BEARING MOTOR AND MOTOR DRIVE CIRCUITRY
Attention now turned to the drive motor for the magnetic bearing. The
rotor consisted of a 1/2" diameter Samarium-Cobalt magnet magnetized
across the diameter. The stationary motor coils were arranged around the
diameter of the rotor magnet at 90° intervals. This was done to provide a
two phase system. Hall Effect devices were installed (I per phase) in the
motor coils to provide non-contacting commutation.
In the motor drive circuitry current feedback was utilized to provide
maximum torque and efficiency. (See Figure 17 in Appendix.) Speed
control was obtained by use of a negative voltage feedback loop around
the amplifier. Top speed was ultimately controlled by the back EMF
generated inthe coils. See Parts List- Figure 18.
Pertinent motor data can be found in Figure 19 in the Appendix.
Subsequent improvements in the motor drive circuit components may
be seen in Figure 20 in the Appendix.
2.1 MAGNETIC BEARING SCANNER MOUNT
At this point in the performance of the contract, Cambion was requested
to submit a proposal for a modification to the contract to include a magnetic
bearing scanner mount, MBSM, using the techniques developed during the
first part of the contract.
It was recognized that the use of the magnetic bearing device as a
scanner made accurate speed regulation necessary. Such regulation is obtain-
able if the device is rotated by a synchronous motor. The average rotational
speed is then as constant as the frequency of the supply, and if the waveform
is sinusoidal the instantaneous torque is constant producing constant instantane-
ous speed. Figure 21 shows the schematic of a 2-phase, .8 Hz power supply.
It is made of a classical phase shift oscillator built arount Z1, an operational
amplifier. The frequency is set by C1 and resistance R1 and P1 in series and
C2 and the resistance in parallel R2 and P2. P14, R14 and T14 are used to
-16-
adjust the gain to give a good sine wave at a reasonable amplitude. T14 is
a thermistor which is traditionally used to stabilize the amplitude. R3, C3,
R20 and C20 provide the 90° phase shift with Z2 making up for the resultant
attenuation. P15 permits amplitude equalization of the two phases and P3 sets
the phase difference of 90° .
The motor is driven by the transistors Q1 through Q4 which are all 3 ampere
Darlington power transistors.
Use of larger (1" diameter) diametrically magnetized rare earth cobalt
magnets permitted a more potent motor and the larger diameter made it easier
to provide windings with less space unoccupied by copper.
All the motor drive circuitry, like the support, was powered from a + 12
volt source.
The prime purpose of the Magnetic Bearing Scanner Mount, MBSM, device
was to rotate a scanning mirror in a satellite.
It includes an integral motor which, like all the rotating elements, would
be supported by magnetic rather than mechanical bearings. This avoids the
problems that often affect mechanical bearings in the high vacuum of deep
space. The mirror was not included, although the mounting provisions were
provided.Specifications:
Mechanical
(I) Length (C.G. mirror to opposite end MBSM) 9 + 1/2"
(2) Diameter 5 + 1/2"
(3) Weight 7 + I lb.
(4) Potential speed 48 rpm
(5) Support Capability I (with load applied symetrically 5 lbs.to shaft not mirror case)
(6) Stiffness (as above under support capability) 350 lb/inch
(7) Support Capability 11 (with plan or load at 4 5°angle 5 oz.with respect to shaft mirror case)
(8) Mirror mountingprovision 1/4-28 NF thread
Electrica I
Input voltage: + 12 volts
Input power: Less than 8 watts
-17-
2.2 CLOSE GAP BEARING
An attempt was made to provide a bearing with magnet gaps of less than
.010" therefore increasing the radial stiffness. This close gap bearing was
connected to electronics used to run VZP magnetic bearings previously but
it was not possible to levitate the rotor. See Figure 22. (
A large number of experiments to determine the cause of this ensued.
The electromagnets were insulated from their aluminum plates, the plates
were slit, and the same experiment performed with one of the electromagnets
in the same coaxial position without any of the bearingstructure. This
latter test showed that the single pole at 500 Hz. was due to the structure
and it was found that it could be caused by simply inserting the force ring
aluminum partition in front of the rate coil. Slitting this partition radially
improved matters slightly - zero phase shift moved up to 600 Hz.
Finally, the partition was shielded with a piece of .014" transformer
silicon steel on each side. The two silicon steel discs being joined by
ferromagnetic screws through the partition, which also held the shields on.
This eliminated the problem and pushed the zero phase shift up to 10 KHz.
It was decided to widen the .008" air gaps to .018" to reduce the stiffness
and thus the band width requirement.
Even then, levitation was not achieved. The rate coil did not fit the
one inch diameter rate magnet too closely - there is an eighth inch
clearance on the diameter - but the assembly and disassembly of the bearing
had made it somewhat eccentric, and it was thought that we were troubled
by acoustic feedback via rate coil I bobbin/magnet contact. To prevent
this, the magnet was moved so that it came close to but did not enter the
bobbin. This sacrificed half the rate coil output. We also shifted to the
pulse mode of operation and achieved levitation. In this mode the output
of the rate coil is fed to a Schmitt trigger circuit whose trip points are
-18-
symetrical about zero voltage. When the preset velocity was reached in
one direction the electromagnet that was pulling the rotor toward it was
shut off and the other turned on. A circuit that accomplishes this is shown
in Figure 23.
There are various advantages and disadvantages to this arrangement.
One of the plus features is that the electromagnets are driven from the
highest voltage available thus insuring the fastest response time. With the
bearing levitated, it was possible to make some observations impossible
without levitation. The pulse mode of operation meant that the rotor was
constantly subject to reversals of the electromagnets and the frequency at
which this took place was of interest. One would expect that as the
Schmitt trigger trip points approach each other and also zero voltage the
frequency would increase. This did happen, and it was possible to main-
tain levitation from 250 Hz. to 1,500 Hz. with less power being drawn at
the higher frequency. A trip point of 100 or 200 millivolts was sufficient.
The actual output of the rate coil did not look just as one would have
expected from the theory of operation. Although it should not have matter-
ed which way one sent the current through the nonpolorized electromagnets,
operation was possible only with proper poling.
It appeared that there was a considerable direct transformer coupling
from electromagnet to rate coil . Whether this inhibited linear mode
operation more than pulse was not known.
With so many changes made to secure levitation, it would have been
necessary to analyze each variable at a time to see which was critical.
However, success with wider gap bearing led the development to return to
previously proven approaches.
During this period of investigation two other areas were explored. A
"Bang-Bang" type servo control was considered and "Clamshell" type
magnetic circuits were also considered. The results of these two con-
current investigations are detailed in the following two sections.
-19-
2.3 "BANG-BANG" MAGNETIC BEARING AXIAL CENTERING SERVO
In Figure 23 is shown a circuit used to axially center the rotor without
displacement measurement. The following explains the physics behind this
circuit.
In a passive radial bearing (although the use of this method is not restrict-
ed to this case) there is a null force position somewhere. But this no force
position is unstable because if the rotor approaches either magnetic pole it
is accelerated toward same.
Now assume that in some unspecified manner one applies a servoed force
field to the rotor such that the rotor motion is vectorially opposite to any
other force applied. To make this statement more concrete let us assume that
the radial support magnets providing the passive radial, and the unstable
axial force are temporarily removed. If one now pushes the still servoed rotor
with one's finger it pushes one's finger back. Imagine that the radial support
magnets are again active. The servoed force field will cause the rotor to flee
from the magnet poles rather than be drawn to same and come to rest where
the nonservoed force field iszero i.e. at the formerly unstable equilibrium
point.
One method of producing such a servoed force field is as follows:
Let the servoed force field be of the simple "bang-bang" type i.e. two
valued either +Fs or -Fs. When the rotor achieves a +V0 axial velocity let
the -F force be applied and maintained until it causes the rotor to reach
-V0, when the +Fs force is applied. This simple servo will produce the
desired result as demonstrated below.
Triggering the servo force from +Fs to -Fs when +V0 is achieved and vice
versa is equivalent to triggering whenever the kinetic energy of the rotor
reaches 1/2 mV02 . The successive triggers will occur when the various
forces acting on the rotor have made a change in its kinetic energy of mV0 2 .
-20-
/-
If
Sometimes the net force on the rotor is Fe+Fs and sometimes Fe-Fs, where
Fe is the external (perhaps finger) force. But the work done is (Fe+Fs) x i
in the first case and (Fe-Fs)(-x2) in the second. Where x i and x2 are the
displacements. But both the increments of work are equal being equal to
mV0 2 . Thus:
mVO0 2 = (Fe+Fs)xl = (Fe-Fs)(-x2 ) = (Fs-Fe)x2
If Fs>Fe, which is essential for proper operation, it is seen that x2 >xI.
But xl is the direction in which Fe would move the rotor in the absence of
Fs, thus the net actual movement per cycle, xl-x2 , is against Fe.Q.E.D.
The circuit shown in Figure 23 carries out this motif quite simply. It
happens to be the exact one in use when the NASA representative
visited us and the values etc. are by no means sacred but are put in to be
concrete. The blocking capacitor C is used to prevent strain on power
supplies and Ql, Q2 during experiments. It may be replaced by a direct
connection. The voltage follower ZI
is not essential, we have used
similar circuits with the rate coil going directly to Z2 . Z2 , a Schmitt
trigger, is the essential element, doing the triggering discussed above.
The diodes DI
and D2 are worthy of mention. They determine which
force coil gets the current, but they also permit the energy from one force
coil to enter the other when the switch is made and prevent inductive kicks
from sending currents into QI and Q2 then.
2.4 CLAMSHELLS
During the Magnetic Suspension work it was found to be of interest to
increase the magnetic forces by armoring Samarium-Cobalt magnets. When
it was found that NASA was interested in having such done, Cambion
utilized some of the work previously done to produce what was called the
"Clamshell" structure.
The largest Samarium-Cobalt magnets available at the time of this work
were hollow cylinders 1" O.D., 1/2" i.D. and 1/4" long available from
three suppliers. Subsequently, larger magnets were made available.
-21-
In addition, extensive work was done on a previous NASA Contract
Number NAS5-11585 showing that when utilizing fringing to make a radially
passive device, the ring width to air gap length could not profitably be de-
creased below 5, which restricts one practically to high gap permeances,
excluding leakage.
The clamshell structure did not maximize the air gap energy. It employed
the Samarium-Cobalt magnet to provide a fairly high remnant, Br, and was
also advantageous because it would stay magnetized with an adversely low
L/D ratio.
Ideally, the polarized material would be as close to the air gap as possible
with a small surface area (to prevent leakage) flux concentrator used to bring
the pole tips close to saturation. There are two things to mention about the
output area of the concentrator. One mentioned previously is that the ring
width should not be less that 5 times the air gap, and conversely it should not
be too large, say above 10 times. If the magnet had sufficient volume to
saturate an unduly large ring, say 20 times the air gap distance, one should
make two concentric rings etc.
The clamshell structure was selected as bringing the polarized material as
close to the air gap with as little leakage as possible considering that the
magnet was axially magnetized.
Figure 24 shows the first clamshell configuration tried. The high permeance
of the load meant the BD would approach Br, or about 8,000 gauss for Samarium-
Cobalt. Neglecting leakage, the area of the ring structure would be 1/3 that2 2
of the magnet to aim for 24,000 gauss. The area of the magnet is T'1/4(1 -1/2)
= 3/16 or 0.6 sq. inch. The first trial was made without thinning the ring from
.050 inches radially to .035 as shown - it being easier to remachine after trial
than making a new piece. Thus the ring area was 1.3 iti'x .05 = 0.2 sq. inch.
The single force ring partition shown mechanically grounded in Figure 24 was
clamped in a 3 jaw lathe check. A half inch diameter aluminum rod, 18" long,
had all the other parts slid on and affixed to one end of same. The other end
-22-
had a short piece of flexible wire joining it and the lathe tailstock. It was
then possible to axially pull the clamshell via the tailstock to nearly center
the ring, thus simulating the magnetic bearing situation.
The parts of Figure 24 were all designed so that the air gap should come
out zero. Then a piece of .014 inch transformer lamination was used to make
a washer and increase the air gap. This produced an air gap of about .008"
each side.
Force measurements were made by pushing on the clamshell with a
Chatillon force gauge in 4 directions and averaging the results.
As described, the clamshell took 2 pounds to push it beyond its maximum
travel. The latter occurred at about .050" in radial displacement.
Theclamshell was then cut down to conform to Figure 24 as drawn i.e. with
a .035 inch radial ring. This produced 3.5 pounds.
A single magnet in "opposition" as shown in Figure 25 was permitted to
stick itself to the outside of the clamshell. This did more good than had been
expected. The force rose to 5 pounds.
A second outboard magnet brought it up to. 7.5 pounds. The configuration
conformed exactly to Figure 25.
Two 2 inch diameter washers on the outside of the outboard magnets
brought the support capability up to 10 pounds. This is shown in Figure 26.
This seemed like a useful force and we turned to making a complete bearing
using this structure. We did, however, try three other experiments which may
be of interest.
First, since it was easy, we put in an additional washer making the air gaps
.014 instead of .007 inches. This always cut the force in half. This statement
applies to Figures 24 through 26.
Second, clamshells were made using Vanadium-Cobalt steel available at
the time. Although it is doubtful that saturation was reached in the cold rolled
steel used in the preceeding, it was felt that any increase in force using the
high B material would indicate that we had saturated the cold rolled. Actually,S
-23-
there was no improvement. However, while it is certain that the high field
alloy can go higher than cold rolled when both are correctly annealed we
annealed neither, and there doesn't seem to be any data available on the
performance of unannealed and fairly heavily machined high field material
versus similarly treated (or untreated) cold rolled.
The ring was then cut down from. 035 to .025 inches on the radius. This
reduced the radial support to 8 pounds. (See Figure 26) It should be noted
that this brought the ring/width/air gap ratio below 5.
2.5 VELOCITY ONLY AXIAL CONTROL
An important investigation was the attempt to correlate the theories re-
garding velocity only axial control with emperical observations. In particular,
operation of the suspension in the linear mode was repeatedly attempted. This
was done because it was felt that this mode may require less bandwidth and thus
permit one to operate stiffer bearings without increasing the frequency response
of the circuit elements - particularly the force electromagnets. Increasing the
bandwidth of the electromagnets means subdividing the solid silicon steel
(laminating) currently composing same, to reduce eddy currents. However,
such an approach is difficult and makes the structure less rugged. The results
are summarized in deltail in Section 8.1.
The non-linear or bang-bang mode was discussed previously. As mentioned,
suspension can definitely work while the whole system is oscillating by pure
electromagnetic feedback. In the simple non-self-oscillating theory one can
easily visualize the waveform that will be at the rate coil terminals. With
some simplifying assumptions (immediate appearance of the force in one electro-
magnet and disappearance in the other and assuming that the electromagnet
force is constant over the range of displacement permitted) the velocity will be
a symetrical triangular function of time as shown in Figure 27.
Figure 28 shows the observed wave shape. It is simply the triangular
function riding on the electromagnetically induced square wave as shown.
-24-
It is apparent that the device cannot function according to the simple theory
if the Schmitt trigger point is set below the electromagnetically induced
voltage - in this case about 100 mi ll ivolts.
But the suspension actually worked well below this voltage. The whole
system goes into oscillation at a high frequency e.g. 5KHz. The current
switches from one coil to another at this rate. Whether the mechanically
generated voltage phase modulates the 5 KHz carrier mentioned previously
to maintain axial control or whether the device works a la the simpler theory
is now known. This latter is possible because it may be that under these
conditions the mechanically generated voltage is large enough to swamp out
the electromagnetic induced EMF. It may also be a combination of these two
effects.
While the circuitry is similar to that earlier shown in a National Semi-
conductor, LM311H comparator, used in the Schmitt trigger in lieu of the
1/2 747 operational amplifier previously shown.
This was done because the 747 had a very low corner frequency (approximate-
ly 10 Hz) so that it would be stable as a voltage follower, without any external
capacitance for compensation. The schmitt trigger circuit used did need an
amplifier - a comparator is even more appropriate. The comparator needs no
internal roll off and the LM311 as used had a rise time of only 1.5 microseconds.
With the 747 the highest self-oscillating frequency was about two KHz.
Apparently its response and the natural parallel resonant frequency of the rate
coils (5 KHz) worked together to limit the self-oscillating frequency thus.
With the LM311, the oscillations take place at 5 KHz- the band-width of
the compartor being infinite in comparison.
The above discussion of the use of the 747 versus the LM311 is merely
reporting. No significant improvements in practical levitation were noted
using one versus the other. The use of the high speed comparator merely made
the experimental apparatus correspond more closely to the idealized theory.
-25-
Using the simple theory, not contemplating electromagnetically coupled
oscillation, one would expect that if the field direction of rate magnet, poling
of rate coil, choice of force coil connections were once made properly that
any of four different combinations of the above would be equivalent (the origi-
nal configuration plus two reversals of three polings). In addition, since the
force electromagnets were used in the soft iron attractive mode the direction
of current flow in same were immaterial.
Actually one particular combination of the variables gave optimum results.
In fact, it was not certain that we had a device which would work at all with
two combinations. We had not made a special effort to ascertain whether this
was possible. There was a minor exception to the above. One could reverse
the rate magnet and rate coil connections with impunity.
From the above it appeared certain that the direct mutual rate coil - force
coil inductance played an important role in practical suspension operation.
While, as mentioned heretofore, it was not known that the coupling and
sometimes oscillation of the circuit degrading the suspension performance, it
was suspected that this inhibited the linear mode of operation (described later
on) and it certainly did no good. This last statement was based on the following
reasoning:
If oscillation was helpful the necessary feedback could always be
provided by circuitry rather than by stray coupling. Furthermore,
with large electromagnetic feedback the percentage of electro-
mechanical voltage went way down.
Having decided that it would be beneficial to reduce the electromagnetic
feedback, some thought and simple experimentation was devoted to this end.
Actually what one wants to reduce is the ratio of stray electromagnetic
coupling to electromechanical. The actual values of voltage was immaterial
until one reduced the electromechanical voltage below the point where
amplifier noise (including long term drift) became important.
-26-
The resulting rate coil structure was not very efficient. The magnetic
field intensity, H, vector had components parallel to the rotational axis
and perpendicular (radial). If the magnet had a permeability more than
three we are assured by the textbooks' thumb rule that we may consider H
to be perpendicular to the magnet surface. (Apparently three is a good
approximation to infinity). The material has a permeability of about 1.05
so this simplification was not good. By using a test similar to the familiar
iron filing set up it has been concluded that the lines hug the cylindrical
surface of the magnet very closely. Thus a coil that would change its flux
linkages considerably with a small movement of the magnet would want to
hug the corner of the Samarium-Cobalt cylinder closely. Increasing the
O.D. of the coil would always further increase the voltage output of the
coil, but the increase would be small because of the weak field a little
way from the edge of the cylinder. The stray field pickup would however
rise more than linearly with the coil radius thus degrading the signal to
noise ratio.
Assuming that one continues to use a structure similar to the present,
ther rules are: (I) hug the corner as closely as possible - considering the
fact that the magnet moves (2) make the O.D. as small as possible and
(3) use as fine wire as possible. (2) and (3) are interdependent. Fine
wire permits more turns with lower O.D.
It should be noted that the circuitry need not be d-c coupled. This
is one of the is one of the advantages of the VZP principal. Previously
this fact was used to prevent the Schmitt trigger from causing an annoying
large steady current from flowing in the force electromagnet when during
experiments the Schmitt stayed locked one way.
One may capacitively couple the rate coil to the input circuitry, thus
eliminating the effect of offset drift. Similarly one may transformer
couple the rate coil to the input. These variations would enable one to get
-27-
along with less rate coil output voltage. A typical rate coil is 1.2"
I.D., 1.50" O.D. and axial length about 0.25". This does not hug
the magnet corner very well. As a positive by-product of this construc-
tion it is not useful to bring the magnet extremely close to the coil.
Little difference in output was noted between having the rate coil near
plane coincide with the magnet edge or 0.1" away. The 0.1" spacing
was convenient to prevent bumping.
It was best if the inevitable radial motion of the rate coil caused
no change in flux linkage and thus no spurious signal. Actually
lateral motion did generate a signal and the ratio of the axially genera-
ted signal to radially generated for equivalent sinusoidal excursions of
the same frequency provided a rejection figure of merit. Minimizing
this might invalidate the three rules set down previously. At the rota-
tional speeds we had employed the spurious signal never caused problems.
The axial component of the rate magnet flux was not useful in genera-
ting a rate output. Thus the magnet was used inefficiently. The rate
coil is to a dynamic loudspeaker as a generator is to a motor. Thus one
might be led to use a loudspeaker like structure. The dimensions of the
magnets practically available did not encourage one to attempt this.
The reasoning being that the addition of soft iron to the 1" O.D., 1/2"
I.D. and 1/4" long Samarium-Cobalt magnet would cause it to work into
too high a permeance to fully utilize the energy product. However
working into a permeance of 2 instead of I, only costs about 10% of the
energy product. Merely putting a large (2" diameter) soft iron washer
on the magnet surface away from the rate coil nearly eliminates the
reluctance of half of free space and approximates this. The increase of
flux density in the wire was thus roughly 2 x .9 = 1.8. Experiments
showed that the simple addition of such a washer almost doubled the rate
coil output with no complications.
-28-
Axial stability will be achieved if the levitated object is subject to
a servo system such that the object moves in the algebraically opposite
direction to any non-servo force. This omits the desirability of damping,
but with the velocity signal available damping is produced in the usual
way although, as will be seen, there is a switch of algebraic sign.
Take Newton's Law with the mass constant.
F = ma (I)
Let the servo force be, (Ka), proportional to the acceleration, (a),
and (K) be a positive quantity. Equation (I) becomes:
F + Ka = ma (2)
or---
F = (m - K)a (21)
If (K>m) the coefficient of (a) is negative and the object will acceler-
ate in a direction opposite to (F). It will achieve a velocity and displace-
ment that moves it against (F). The real mass, (m), has been given the
attributes of the negative mass, (K-m).
In the case at hand, if the levitated object approaches pole I, and
leaves pole 2, it is subject to a net force that without the servo would
cause it to soon collide with pole I. According to Equation 2 this would
be prevented by adding a force proportional to the acceleration to (F)
with (K) being positive. If the actual acceleration, (a), is directed away
from pole I, the (Ka) would appose (F) and the situation seems sensible.
This argument is circular in that it assumes what it is to prove, but it
does show that no inconsistency existed in this case. Conversely, the
equation (F + Ka = ma) with (K>m) cannot be satisfied for a positive (F)
and positive (a) since (Ka) alone exceed (ma). Thus (F) and (a) cannot
both have the same sign.
In a sense we have changed the real mass, (m), into the artificial
mass, (m-K), a negative quantity. As will be seen later, this connects
up with the non-linear or bang-bang case. It is easy to show that either
-29-
the bang-bang or linear all electronic system essentially turns positive
circuit elements into negative e.g. positive inductances into negative.
The practical servo system used must provide damping as well as the
function discussed above. This may be done by adding to (Ka) the
quantity (Dv) where (D) is a positive constant and (v) is the velocity.
F + Ka + Dv = ma (3)
In order to apply linear theory to this equation one must assume,
traditionally but somewhat inaccurately, that (F) is a linear function of
(X). Taking (x = 0) to be halfway between the magnetic poles, assume
that (F = +AX), with (A) a positive quantity. The positive coefficient
(A) corresponds to reality in that this will cause the pole collision we
are seeking to avoid. Writing (3) over, with this modification, using
Newtonian notation, and putting all the terms on the left side yields
(K -m)x+ Dx + AX = 0 (4)
or in transform notation
(K -m)s2 x+ Dsx + A = 0 (4)
To avoid positive exponentials in the solution, and thus bound (x),
this must be a Hurwitz polynomial. This requires that (K-m) and (D) be
of the same sign. (K-m) is positive and thus the choice of (D) as posi-
tive is correct. This is convenient as it allows the acceleration and
velocity signals to come from the same operational amplifier.
Figure 29 shows one of the circuits tried. Z3 and Z4
were used to
reduce the crossover distortion which would be produced if the output of
Z2 went directly to the bases of QI and Q2 . The negative feedback
divided the three diode drops that would trouble one without Z3 by a
factor equal to the open loop gain of Z3 .
ZI
is a voltage follower buffer that preventsthe rate coil inductance
from affecting the wave shaping elements connected to Z2 . Z2 and its
-30-
associated network did the real work. Its transfer function is:
e R0 =-I0
~~e. R C - I ~(R s + R5)e. oR s' o0 0
RoThe (RoCIs) term is the acceleration and the term the velocity.
The signs are proper. Selecting (Ro), (Ci) and (R2) permits one to adjust
the magnitudes of acceleration component and velocity component appear-
ing at the output of Z2 .
It is often desirable to test using straight velocity feedback only. Feel-
ing a strong viscous drag as the rotor is moved from pole to pole is reassuring
and it also permits one to phase things properly. With wrong phasing, the
rotor clicks from pole to pole more snappily with the electronics than with-
out because of the positive feedback. However when (K>m) the poling
should be reversed. This was rather annoying since when the poling was
reversed with (K<m) one really clicks rapidly from pole to pole. The
existence of a small (K) has reduced the effective mass but not made it
negative and the permanent magnet force knocks the rotor around like a
feather.
One way of looking at this whole matter is to note that the addition of
an acceleration term turns the free fall differential equation under an
attractive force proportional to (x), into the equation of damped simple
harmonic motion.
Presumably one can then take over all the mathematical paraphenalia
used with this classical stable second order system. However, the whole
notion of negative effective mass etc. is not very satisfying intuitively.
In trying to fully understand the operationof the VZP a few all electron-
ic models of the electromechanical system have been drawn. Newton's Law
with the mass constant is modeled by the primitive circuit shown in Figure 30.
-31-
diE = Lq = Ld i (6)
dt
with E = force
L = mass
q displacement
q = i - velocitydi
q = dt - acceleration
Figure 31 shows an all electronic analog that makes the real positive
inductance look like a negative inductance, (L-AM), (AM L). Here and
in what follows (A) is not the large number tending to infinity associated
with operational amplifiers. It is a finite number like 5 etc.
Figure 31 brings one into the currently popular area of active circuits,
where negative impedance convertors, gyrators etc. abound. Tying the
VZP to the powerful theorems of active circuit theory seemed to be the
best way to proceed, but this had just been started.
Figure 32 shows the analog of an electromechanical bang-bang. It
is a bang-bang negative impedance convertor. Note that, as with the
bang-bang VZP, no obvious differention of the current (velocity) was done.
The drop across RI
was analogous to velocity or current not its derivative,
unlike Figure 31, where the mutual inductance performed the differentia-
tion.
Figure 33 shows a circuit which was an imperfect, but interesting
analog of a suspension without any servo control. It was imperfect since
it lacked an inductive element, the mass analog, and only lead to a first
order equation not a second. But like the un-servoed bearing it had an
equilibrium point, which was unstable viz: (ei = eo
= 0). The slightest
departure from these conditions caused run away in one or the other direct-
ion.
Figure 34 shows a bang-bang mode of preventing this, and keeps
the charge (analog of displacement) on C in the vicinity of zero. A2
and its associated resistors form a Schmitt trigger symetrical around zero
volts input.
-32-
4
2.6 CONCLUSION OF CONTROL EXPERIMENTATION
The following conclusions were drawn from the work done on Velocity
only Axial Control:
I. It was still not clear whether operation in the linear mode was possible.
The equations show that it was, but the analogous all electronic equiva-
lent systems "looked" stange as will be seen.
2. While the axial control system can be made to operate just as described
previously, one more often obtains operation in a mode that is a modifi-
cation of same.
3. As far as simple user tests of the quality of suspension are concerned,
the modified mode was superior. By simple user tests one means ease of
hand starting of the bearing by pushing it off a pole and resistance to
the stopping of suspension by introducing outside forces axially and/or
radia I ly.
4. The modified mode was simply a mode in which the whole circuit was
normally oscillating by reason of the direct electromagnetic coupling
(transformer) between the force electromagnets and the rate coil. The
additional mechanical/electrical coupling via force electromagnets
acting to change the velocity of the axial motion was superimposed on
the above.
5. While it was possible to operate the suspension in accordance with the
theory set forth in previous sections this could only be done over a
small range of critical velocities (critical velocity is where the force
magnets are switched). Attempting to reduce this velocity by lowering
the Schmitt trigger voltage caused the Schmitt to be triggered
"immediately" by the square wave voltage induced in the rate coil
when the current started its approximately linear rise in the newly
"on" electromagnet. This did not prevent levitation. Setting this
velocity too high did prevent levitation, presumably by allowing the
armature too close to a pole.
-33-
6. The above was, as mentioned, caused by the fact that there was
considerable magnetic coupling between force coils and rate coil.
It was felt that this coupling may prevent operation in the linear
mode. This was because amplitude stable oscillations were only
possible in non-linear systems.
-34-
SECTION III - TORQUE MOTOR
3.0 INSTABILITY
Two basic problems remained in the magnetic bearing system. One was the
lack of mechanical rigidity and second was the inadequate frequency response
in the system.
The lack of mechanical rigidity caused vibrations to occur during high axial
force levels which destabilized the servo system.
The frequency response of the system was inadequate for the fast response
required by the narrow( + .007") magnetic gaps. This was because of the high
non-linear force levels in the axial direction.
At axial gaps of + .017" it was possible to operate the magnetic suspension
system with an additional one pound weight attached to the rotor. However,
stability was poor and power consumption was relatively high. The system was
very sensitive to outside disturbances which would disrupt its operation and it
was found that a soft mounting surface was necessary.
With the magnetic suspension system operating the following measurements
were made:
Radial Stiffness 125 lb./in.(each bearing)
Axial Support Capability 10 lbs.
Power Requirement + 13 VDC at 50 ma+ 11 VDC at 50 ma(2.4 watts total)
Time Constant 0.16 seconds(time for rotor to fall from centerto one pole - .017")
Rotor Weight 1.5 lbs.
A two phase DC motor as described earlier in this report was connected to the
magnetic bearing. The bearing was rotated up to 2600 RPM where rotor vibration
was encountered. Below 2600 RPM operation was very smooth.
-35-
To reduce the instability a change was made to the force coil configuration.
One of the two force coils was removed and a Samarium-Cobalt permanent
magnet armature was added to the rotor. What resulted was a force coil that
could push and pull. The flux path of the permanent magnet armature passed
through the force coil thus providing a bias level for the force coil to work on.
This method reduced the inherent time lag in the force coil thus increasing the
response of the magnetic suspension system. The system then operated in a more
stable manner. However, one drawback was that the additional permanent
magnets added a radial and axial destabilizing force thus adversly effecting the
axial and radial stiffness.
3.1 TORQUE MOTOR
The NASA representative recommended at this time that Cambion consider
incorporating into the magnetic bearing system a motor built for NASA by Sperry
Marine Systems Division. The motor was a three-phase ironless armature type
and was developed with specific characteristics in mind appropriate for magnetic-
ally supported systems.
A study was made of the possible integration and it was found that the motor
could be added to one end of the bearing with a minimum of changes.
Cambion designed and made the mounting fixtures for the motor and installed
the motor on the magnetic bearing. Bearing operation did not change except
for some increased sag (003") at the motor end when in a horizontal position.
A complete wiring diagram may be seen in Figure 35.
-36-
SECTION IV - WORKING MODEL
4.0 ELECTRONIC CONTROLLER
The electronic controller had evolved somewhat from the circuitry that had
been used on the first VZP model.
A schematic diagram of the final controller may be seen in Figure 35. The
position signal is generated by the six lamps and the six photocells. The velocity
signal is generated by a permanent magnet mounted on the rotor which induces a
voltage in the rate coil proportional to the axial velocity. The rate coil consists
of 50,000 turns of #50 AWG wire wound on a 1.2" diameter bobbin.
The velocity and/or position signals are amplified by an operation amplifier
(Burr-Brown 1506/15). The operational amplifier is operated open-loop and has
a gain of 104 dB. A balance control is included so that the output of the opera-
tional amplifier may be set to zero with zero input. The amplified signal then
goes to a complementary-symetry power amplifier. The power amplifier consists
of a bias network and two Motorola darlington silicon power transistors MJ4032
(PNP) and MJ 4035 (NPN). The signal from the power amplifier then goes to
the force coil which controls the position of the rotor. The force coil is made up
of 250 turns of #23 AWG wire with an inside diameter of 1.15", and outside
diamter of 1.5" and is .725" long.
A portion of the signal from the operational amplifier is fed to a RC integrator
and then back to the non-inverting input of the operation amplifier. This signal
is required for virtually zero power operation or, in other words, is necessary for
the force coil to position the rotor at the neutral force position.
4.1 AUTOMATIC STARTING
Attention now turned to automatic starting of the bearing upon application of
power. Because of the high axial forces present in the "clamshell" type magnetic
bearing, automatic starting would not take place. At an axial magnetic gap of
.017" the radial stiffness was 250 Ibs./in. (both bearings). However, for this
-37-
value of radial stiffness 4000 Ibs./in. of axial stiffness would be required to lift
the rotor off the magnetic pole faces. The servo system could only provide 500
Ibs./in. which was enough to control the rotor when near its center position but
not enough to lift if off the pole face.
Temporary mechanical stops were installed in the air gaps and it was found
that with a mechanical air gap of + .005" the bearing would start automatically.
However, this method was not completely dependable and was not pursued further
because of work needed to be done on the motor.
4.2 MAGNETIC BEARING STIFFNESS
The three-phase D.C. motor, supplied by NASA, was integrated into the
magnetic bearing assembly. The added weight of the motor did not permit
operation in the horizontal position because the bearing sag exceeded the radial
clearance of the motor.
Efforts were made to increase the stiffness of the magnetic bearing and, there-
fore, decrease the sag. These efforts were ineffective because of a bent shaft. The
1/2" diameter aluminum shaft was replaced and much better results were obtained.
However, rotor-stator contact still occurred in the horizontal position.
The magnetic gapswere decreased to increase the radial stiffness. On the
motor end the gap was reduced to + .007" and the opposite end was reduced to
+ .014". However, because of the destabilizing force of the permanent magnet
force coil the radial stiffness was not as high as expected. The + .014" gap
should have been 300 Ibs./in. radial stiffness but because of the P.M. force coil
being reduced to 75 Ibs./in., the opposite gap (+ .014") was 150 Ibs./in. as it
sould have been. This resulted in a total radial stiffness of 225 Ibs./in.
After conferring with the NASA representative, it was decided that the motor
air gap would have to be widened. The maximum radial clearance for the motor
as designed was + .0075". This occurred between the outside diameter of the
stator and the inside diameter of the rotor. Since this was the limiting clearance,
we decided to take .010" off the inside diameter of the rotor outer ring. Removing
this amount of material would increase the radial gap to + .0125".
-38-
Because of the construction of the rotor and the close proximity of the magnets,
the outer steel ring had to be removed by machining.
The new steel ring was constructed from carpenter BFM Iron to the new dimen-
sions. The new ring was installed on the rotor and the motor was assembled on the
magnetic bearing.
Although the motor gap was now + .0125", adjustment for the motor centering
was still difficult. Adjustment was completed and the magnetic bearing-motor was
hand carried to NASA-Goddard and demonstrated in operation on 19 September
1972.
On the following page is a list of the specifications for the magnetic bearing-
motor as delivered. (Performance specifications for the motor itself are not
included.) For a layout diagram of magnetic bearing and motor see Figure 36.
-39-
P
4.2 NASA MAGNETIC BEARING AND
Description
Length
Diameter
Weight (with motor)
Weight (less motor)
Potential Speed
Radial Support Capability I(symetrical load)
Stiffness (radial)
Maximum Force (radial)
Horizontal Support Capability II(with mirror)
Input Voltage
Input Power
Contract
9 + 1/2 inches
5 + 1/2 inches
7+1 lbs.
48 RPM
5 lbs.
350 Ibs./in.
5 oz.
+ 12 VDC
Less than 8 watts
Actual I
10 inches
4.5 inches
6.25 lbs.
4.15 lbs.
2200 RPM
6.5 lbs.
225 Ibs./in.
6.75 lbs.
9 oz.
+ 12 VDC
0.5 watts(2.4 watts inposition mode)
4.2.1 SUPPLEMENTARY DATA
Description
Magnetic Gap (motor end)
Magnetic Gap (opposite end)
Axial Stiffness
Axial Support Capability
Servo Time Constant
Sag (rotor horizontal in IG environment)
System Resonant Frequency(Operating)
Electromagnet Gap
Radial Stiffness (motor end)
Radial Stiffness (opposite end)
Actua I
+ .007 inches
+ .014 inches
3570 Ibs./in.
10 lbs./amp
4.8 seconds
.002 inches at motor
4384 Hz
.032 inches nominal
150 Ibs./in.
75 Ibs./in.
-40-
r end
MOTOR SPECIFICATIONS
5.0 RECOMMENDATIONS AND CONCLUSIONS
Previous to this contract magnetic bearings, in general, had three main
faults: 1. They required relatively large amount of power to operate, 2.
They contained considerable dead weight and 3. They were large in size as
compared to ball bearings.
The device developed under this contract plainly shows that continuous
power consumption can be reduced to extremely low levels - milliwatts.
Further investigation showed that the size and weight of a magnetic bearing
can be reduced by a ratio of better than 2 to 1.
By the addition of steel armoring to rare earth - cobalt permanent magnets,
high levels of radial and axial stiffness are achieved with very little expended
in weight and volumn. This then produces a magnetic bearing that can perform
jobs that were previously done by mechanical bearings but without the problems
of lubrication and friction.
The use of rare earth - cobalt permanent magent material in the force
driver also reduces the size and weight because of a reduction in steel and
copper required to do the same job. An added benefit is the increase in the
response time of the force driver because of the biasing effect of the permanent
magnet material.
Although the device developed under this contract shows a great improve-
ment in magnetic bearings it is only the beginning. Further research and
development can produce a new generation of magnetic bearing that will be
useful in both space and commercial applications.
-41-
APPENDIX
4,/-4
Figure I -
B-H Curve Showing Characteristics of Rare Earth-Cobalt, Platinum Cobalt, Indox VI
and Alnico V
.~~~~~~~~~~~~~~~~~~~~~. ~12000
1. RARE EARTH COBALT
2. PLATINUM COBALT 11000
3. INDOX Vl BB
4. ALNICO V
10000
kg B
45-O
36. 9000
27
' ~~~~~~~18 '~70009~~~~~~~~~ ~~~~8000
27 36 45 54, koe
7000
6000
5000
3000
2000
1000
000 8000 7000 6000 5000 4000 3000 2000 1000
BHc (oersteos) 661959-31-16-70
PR I NT ED
IN U.S.A.
IF:35
Figure 2
RAYTHEON COMPANYMicrowave and Power Tube Division
Waltham, Massachusetts
SAMARIUM COBALT MAGNETS
MAGNETIC PROPERTIES
Coercive force -
Intrinsic coercive force
Residual induction
Energy product, max
Curie temperature
Temperature coefficient
7, 500 - 9, 000
25, 000
7, 500 - 9, 000
20
850
0. 02
Oersted
Oersted
Gauss
MG Oe0°C
%per °C
PHYSICAL PROPERTIES
Specific gravity
Electrical resistivity
MECHANICAL PROPERTIES
Tensile strength
Compressive strength
Flexural strength
8 g/cc
5x10-4 ohm-cm
8,000 psi \
>> 10, 000 psi
12,000 psi.
___ Figure 312-280
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Rli 470Q 2 WATT 10% CARBON
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R13__ _ 10KQ2 1/2 WATT 10% CARBON.
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Figure 19
MOTOR DATA
1. O.D. Sam. Co. Magnet
2. I.D. Iron Collar (Return Path)
3. Mean Dia. Air Gap
4. Mean Field at Above
5. Magnet Length
6. Wire Size
7. Turns/Coil (There are 2 coils/phase, 4 per motor)
8. Active Wire Length/Phase
9. Back EMF @1200 RPM
10. Inactive Wire Length/Phase (Est.)
11. Resistance/Phase
12. Peak Torque/Ampere (1 Phase)
13. Average Torque for 1 Ampere
in Each Phase
14. Average Torque @0.05 Amp/Phase
1.5 Cm.
3.0 Cm.
2.25 Cm.
2,000 Gauss
1.5 Cm.
#36(25 Circular Mils)
600 Turns
3600 Cm.
10 Volts
5000 Cm.
160 Ohms
26 Oz. In.
26 Oz. Inc.
1.3 Oz. In.
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$g4iz717L(uXzuFigure 23
RATE COIL
= 100 KS2,1/2W,10%
= 10 Kf,1/2W,10%
= 100Q ,1/2W,10%
;= 1000L-,1W,10%
= 2,200f ,1/2W,10%
= 0.1fL,5W,10%
= 1/2 747 OP. AMP.
= 6A, 50V DIODE
= 16A, NPN DARLINGTON
= 16A, PNP DARLINGTON
= 25 MFD, 20V, NON POL ELECTROLYTIC
CHECKED CHECKED
VERIFIED APPROVED CAMBRIDGE THERM IONIC CORPORATI ON445 Concord Avenue Cambridge 38, Massachusetts, U.S.A.
DRAWN SCALE
D9DATE
CAMBRIDGE THERMIONIC CORPORATION Printed ,n USA~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ii
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FRAME
Figure 25
.035
Figure 26
075
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RADIAL FORCE
_m FERROMAGNETIC
MATERIAL
CHECKED CHECKED
CAMIBRIDGE THERM IONIC CORPORATIONVERIFIED APPROVED
445 Concord Avenue Cambridge 38, Massachusetts, U.S.A. I................. _DRAWN SCALE
70DATE
DIS E. .. .............. i..___ .. , .......-..--------...... .CAMBRIDGE THERMIONIC CORPORATION
Printed in USA
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Figure 27
TIMEMILLISECONDS
-- -4 MILLISECONDS
Figure 28
CAMBRIDGE THERMION-IC CORPORATIONCambridgo 38, Massachusetts, U.S.A.
CAMBRIDGE THERMIONIC CORPORATION
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VERIFIED APPROVED CAMBRIDGE THERMIONIC CORPORATION445 Concord Avenue Cambridge 38, Massachusetts, U.S.A. l
__________ ~~~~~~72_~~ . . ._ EON DRAWTN SCA LE
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CHECKED CHECKED
CAMBRIDGE THERMIONIC CORPORATIONVERIFIED APPROVED
445 Concord Avenue Cambridge 38, Massachusetts, U.S.A. ALE.......................................... )RAWN | SCALE
.73DATE
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CAMBRIDOGE THEORIONIC CORPORATION445 Concord Avenue Cambridgo 38, Massachusetts, U.S.A.
CAMBRIOGE THERMIONIC CORPORATION
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Figure 33
Figure 34
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