me 407: mechanical engineering design assistant prof. melik dölen department of mechanical...
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ME 407: Mechanical Engineering Design
Assistant Prof. Melik Dölen
Department of Mechanical Engineering
Middle East Technical University
Electric Motors/Drivers& Their Selection
ME 407 2
Outline – Electric Motors* • Classification of Electric
Motors– Stepper Motors
– DC Motors
– Brushless DC Motors
– Induction Motors
• Fundamentals of Motor Drives– DC Motors
• Motor/Driver Selection Procedure– Load Analysis
– Performance Curves• Applications
• Summary
[*] W. Voss, A Comprehensible Guide to Servo Motor Sizing, Copperhill Tech.Corp. 2007.
ME 407 3
Electrical Motors
• In most industrial applications, electrical motors are extensively used as actuators.
• Four motor systems are common alternatives in machine tool designs:– Stepper motors: Simple applications (e.g. desktop manufacturing
tools)– DC motors: Earlier CNC machine tools and specialized machine
tools– Brushless DC motors: Principle axis drives for contemporary CNC
machine tools– AC (Induction) motors: High-power spindle drives.
ME 407 4
Stepper Motors
• Stepper Motors– Permanent Magnet
• Relies on rotor magnets– Variable Reluctance
• Relies on rotor saliency– Hybrid Motors
• Relies on both rotor saliency and magnets
• Each pulse moves rotor by a discrete angle (i.e. “step angle”).
• Counting pulses tells how far motor has turned without actually measuring (no feedback!).
ME 407 5
Advantages / Disadvantages
Low costSimple and ruggedVery reliableMaintenance freeNo sensors neededWidely accepted in
industry
Resonance effects are dominant
Rough performance at low speed
Open-loop operation Consume power even
at no load
ME 407 6
(Simplified) Full-Step Operation
• Rotor of a PM stepper motor consists of a permanent magnet: – Stator has a number of
windings.
• Just as the rotor aligns with one of the stator poles, the second phase is energized.
• The two phases alternate on and off to create motion.
• There are four steps.
N
S
Current
S
N
Coil A
Coil B
Coil C
Coil D
S N
Coil A
Coil B
Coil C
Coil D
Current
NS
S
N
Coil A
Coil B
Coil C
Coil D
Current
N
S
SN
Coil A
Coil B
Coil C
Coil D
Current
N S
1 2
34
ME 407 7
(Simplified) Half-Step Operation
N
S
Current
S
N
Coil A
Coil B
Coil C
Coil D
S N
Coil A
Coil B
Coil C
Coil D
Current
NS
Coil A
Coil B
Coil C
Coil D
N SSN
Coil A
Coil B
Coil C
Coil D
Current
NS
1 2
78
Current
S
N
Coil A
Coil B
Coil C
Coil D
S N
Coil A
Coil B
Coil C
Coil D
Current
NS
S
N
Coil A
Coil B
Coil C
Coil D
Current
N
S
SN
Coil A
Coil B
Coil C
Coil D
Current
NS
3 4
56
S
N
S N
NS
Current
S
N
Current
S
N
Current
SN
S
N
Current
Current
ME 407 8
Half-Step Operation (Cont’d)
• Commutation sequence has eight steps instead of four.
• The main difference is that the second phase is turned on before the first one is turned off.
• Sometimes, both phases are energized at the same time.
• During the half-steps, the rotor is held in between the two full-step positions.
• A half-step motor has twice the resolution of a full-step motor. – Very popular due to this reason.
ME 407 9
Actual Stepper Motor*
• The stator of a real motor constitutes more coils (typically 8).
• These individual coils are interconnected to form only two windings:– one connects coils A, C, E,
and G:• A and C have S-polarity• E and G have N-polarity
– one connects coils B, D, F, and H:
• B and D have S-polarity• F and H have N-polarity
A
E
B
C
DF
G
H
N
SNN
SS
[*] Courtesy of Microchip.
ME 407 10
PM Stepper-Motor Animations*Full-step: Half-step:
[*] Courtesy of Motorola, Inc.
ME 407 11
Conventional DC Motor• The stator of a DC motor is composed of
two or more permanent magnet pole pieces.
• The rotor is composed of windings which are connected to a mechanical commutator. In this case the rotor has three pole pairs.
• The opposite polarities of the energized winding and the stator magnet attract and the rotor will rotate until it is aligned with the stator.
• Just as the rotor reaches alignment, the brushes move across the commutator contacts and energize the next winding.
• A spark shows when the brushes switch to the next winding.
Courtesy of Motorola, Inc.
ME 407 12
Brushless DC Motor
• A brushless DC motor (BLDC) has a rotor with permanent magnets and a stator with windings.
• It is essentially a DC motor turned inside out. The brushes and commutator have been eliminated and the windings are connected to the control electronics.
• The control electronics replace the function of the commutator and energize the proper winding.
• he energized stator winding leads the rotor magnet, and switches just as the rotor aligns with the stator.
• BLDC motors are potentially cleaner, faster, more efficient, less noisy and more reliable.
ME 407 13
AC (Induction) Motor
• Motor is essentially driven like an AC synchronous motor by applying sinusoidal current to motor windings.
• The drive needs to generate 3 currents that are in the correct spatial relationship to each other at every rotor position.
• High-resolution optical encoder is needed to control the commutation accurately.
• Very smooth low speed rotation.• Negligible torque ripple.
Servo-Motor Drivers• Most servo-motor drivers incorporate
motion controllers that allow the user to control– Torque (phase currents)– Speed– Position
• Once the user selects the control mode, the motor drivers must be connected to multi-axis controller unit (industrial PC, motion control card etc.):– Wiring configuration – Setting motor parameters via
• Manually (Control Panel / Memory Stick)
• Software assistance
ME 407 14
Torque Control Mode
• In this mode, the motor driver accurately regulates the motor phase currents in respect to the rotor’s position (namely, rotor magnetic flux linkage vector).
• Servo-motor acts like an ideal torque modulator to yield the electro-magnetic torque being demanded by the (position) control system.
• Torque command is issued through an analog input (usually a bipolar voltage).
• In precision motion control applications, this mode is frequently preffered.
ME 407 15
Velocity Control Mode
• Motor driver regulates the rotor’s angular velocity. – Relies on built-in incremental position encoder to measure
velocity.– Generally, a digital PI controller is employed to control the
velocity.• Velocity controller feeds torque commands to the current/torque regulator. • User must upload the relevant gains and parameters of the “hardwired”
controller to the driver.
• Velocity command is usually issued through an analog input.– Use of control data buses (such as CAN, SERCOS, Profibus,
RS-485, etc.) to send digital commands out to the driver is also common in industry.
ME 407 16
Position Control Mode• Motor driver regulates the rotor’s angular position.
– Motor driver again employs built-in incremental position encoder to measure position.
– Generally, a digital PID controller is utilized to control the position.
• Position controller feeds torque commands to the current/torque regulator. • User must upload the relevant gains and parameters of the “hardwired”
controller to the driver.
• For convenience, command is usually issued through two digital inputs (i.e. direction and pulse).– The servo-motor behaves like a position controlled stepper
motor.
• Advanced drivers support data buses (such as CAN, SERCOS, Profibus, RS-485, etc.) to send/receive digital information.
ME 407 17
ME 407 18
Operating Modes of DC Motor
M
+
_
ia
Va
Forward Motor
Tm
m
M
+
_
ia
Va
Forward Generator
M
+
_
ia
Va
Reverse Motor
M
+
_
ia
Va
Reverse Generator • In motor mode, the machine drives the “load” and needs energy from the supply.
• In generator mode, the “load-side” drives the machine and it generates power.
ME 407 19
“Forward Motor” Control
M+
VDC
Electronically controlledpower switch
+
_
Va
Va
VDC
t
Va
Tp
Td
• Electronically-controlled (unidirectional) switch is turned on/off rapidly.– Pulse width modulation
• Desired (average) voltage at the terminals of DC motor is obtained via controlling switching times:
La
+
S1
D1
Ra
ea
+
_
VDCBack
E.M.F.
DC Motor
ia dVTT
VV DCp
dDCa
where Tp is PWM period(constant) and Td/Tp = d is called duty cycle.
ME 407 20
Forward Motor Control (Cont’d)
• When S1 is turned off, ia flowing through the motor cannot be cut off immediately.– It must flow somewhere!
• The “clamp” diode allows current flow in Mode 2: – La drives a decaying
current.
• If D1 isn’t in place, a very large voltage will build up across S1 and blow it up.
La
D1 :off
Ra
ea
+
_
VDC
ia
S1 :on
Mode 1:
La
D1 :on
Ra
ea
+
_
VDC
ia
S1 :off
Mode 2:
ME 407 21
Four-Quadrant Motor Control
S1
S2
D1
D2
D3
D4
S3
S4
VDC
+
M
Half-Bridge Half-Bridge
• “H” bridge is used to operate the motor in four quadrants.
• Driver is composed of two half-bridges.
• Switches in a half-bridge cannot turned at the same time.– causes short-circuit.– If one of the switches is
turned, the other must be off.
ME 407 22
Forward Motor
• To go forward,– S3 is fully turned on;– PWM and ~PWM (inverted PWM) signals are applied to S2 and S1
respectively.
• Unidirectional switch S1 can carry current only in the indicated direction.
S1
S2
D1
D2
D3
D4
S3
S4
M
ia
VDC
Mode 1:
S1
S2
D1
D2
D3
D4
S3
S4
M
ia
VDC
Mode 2:
ME 407 23
Reverse Motor
• To go backward,– S1 is fully turned on;
– PWM and ~PWM signals are applied to S4 and S3 respectively.
S1
S2
D1
D2
D3
D4
S3
S4
Mia
VDC
Mode 1:
S1
S2
D1
D2
D3
D4
S3
S4
M
ia
VDC
Mode 2:
ME 407 24
Indirect Control System
CommandGenerator
Tor
que
com
man
d
Axis Control System
MotionController
Servomotor
Angular position feedback
Pos
itio
nco
mm
and
MotorDrive
Position sensor
Part
Ball Screw Shaft Nut
Table
Power
Courtesy of Heidenhain Corp.
ME 407 25
Direct Control System
CommandGenerator
Tor
que
com
man
d
Axis Control System
MotionController
Servomotor
Angular position feedback
Pos
itio
nco
mm
and
MotorDrive
Position sensor
Part
Ball Screw Shaft Nut
Table
Power
Linear Scale
Direct position feedback
Courtesy of Heidenhain Corp.
Generic Servo-Control System
CommandGenerator
Torquecommand(voltage)
Programmable Controller
Pos
itio
nco
mm
and
Drive + Motor
Mech.SystemO
utpu
tIn
terf
ace
Sen
sor
Inte
rfac
e
PositionSensor
Disturbance
ControlAlgorithm
e(k) m(k) x(t)
x(k)
x*(k) +
_
m(t) m(t)
Motortorque
Load's position
ME 407 26
Servo-Control (Cont’d)
ME 407 27
Factors to Consider
• The drive requirements must be defined before proceeding to motor selection:– How fast and at which torques does the load
move?– How long do the individual load phases last?– What accelerations take place?– How great is the mass-moment of inertia?
ME 407 28
Factors (Cont’d)
• Often the motor is indirectly coupled to the load shaft, this means that there is a mechanical transformation of the motor output power using belts, gears, screws and the like.
• The drive parameters, therefore, are to be reflected onto the motor shaft.
ME 407 29
Motor Selection
• Decide the motor technology to use (DC brush, DC brushless, stepper, etc.)
• Select a motor/drive combination
• Does motor support the required maximum velocity? If no, select next motor/drive.
• Use rotor inertia to calculate system (motor plus mechanical components) acceleration (peak) and RMS torque
ME 407 30
Motor Selection (Cont’d)• Does motor’s rated torque support the
system’s RMS torque? If no, select next motor/drive.
• Does motor’s intermittent torque support the system’s peak torque? If no, select next motor/drive.
• Does the motor’s performance curve (torque over speed) support the torque and speed requirements? If no, select next motor/drive.
ME 407 31
Selection Procedure*
ME 407 32[*] Courtesy of Omron, Corp.
Load Analysis
• Calculate inertia of all moving components– Determine inertia reflected to motor
• Determine velocity, acceleration at motor shaft– Calculate acceleration torque at motor shaft
• Determine non-inertial forces such as gravity, friction, pre-load forces, etc.
• Calculate constant torque at motor shaft• Calculate total acceleration and RMS
(continuous over duty cycle) torque at motor shaft
ME 407 33
Load Calculation
ME 407 34
sgn( )LL L L CL L D L
dJ b T T T
dt
sgn( )M DM M M CM M M
d TJ b T T
dt N
ML N
sgn( )M Le e M Ce M M
d TJ b T T
dt N
2ˆ Le M
JJ J
N
2ˆ Le M
bb b
N ˆ CL
Ce CM
TT T
N
Gearbox(N:1)
Servomotor Load
TMTL
TD
M L
Load Side:
Motor Side:
Gearing Ratio:
When combined:
where
Other Load Types*
ME 407 35[*] Courtesy of Omron, Corp.
Common Motion Profiles*
ME 407 36[*] Courtesy of Omron, Corp.
Performance Curves* (Brushless DC)
• There are two operating regions:1. Continuous Duty Region: Motor can deliver the torque continuously without
overheating. • Steady-state regime
2. Limited (Intermittent) Duty Region: Large torque can be developed with decreased overall efficiency.
• Transient regime (during acceleration/deceleration)
ME 407 37
[*] Courtesy of Pasific Scientific, Inc.
Performance Curve* (Brushed DC)
ME 407 38[*] Courtesy of Baldor, Inc.
Selection Criteria
• The motor’s rated speed must be equal to or exceed the application’s maximum speed.
• The motor’s intermittent (max) torque must be equal or exceed the load’s maximum (intermittent) torque.
• The motor’s (continuous) rated torque must be equal to or exceed the load’s RMS torque.
• The ratio of load inertia to motor inertia should be equal to or less than 6:1.
ME 407 39
Selection Criteria (Cont’d)
ME 407 40
,, ,( )L ss
M e M ss Ce C M ss
TT b T T
N
At Steady-State: (0 < M,ss < R)
,max,max ,max ,max( )L
M e M e M Ce P M
TT J b T T
N
Worst Case: (0 < M,max < R)
2, ,max
0
1( ) ( )
dutyt
M rms M C Mduty
T T t dt Tt
Note that duration to stay in the intermittent duty zone varies from oneservo-motor to another (0.05 s to 30 s)
In Duty Cycle: (0 < M,max < R)
RMS Torque
• The RMS torque (“Root Mean Squared”) represents the average torque over the entire duty cycle.
ME 407 41
Inertia Matching
• For optimum power transfer, the rule of thumb is that the motor’s (mass) moment of inertia should match to that of the load. – Ratio of 1:1 between load and motor inertia
would be the ideal scenario.– Rather, impractical – some mismatch is
allowed.
ME 407 42
Inertia Mismatch
• If the ratio of load over rotor inertia exceeds a certain range (for servo motors 6:1) consider the use of a gearbox or increase the transmission ratio of the existing gearbox. Servo motors should not be operated over a ratio of 10:1.
• Bosch Rexroth, for instance, recommends the following for inertia mismatch:
– 2:1 for quick positioning– 5:1 for moderate positioning– 10:1 for quick velocity changes
ME 407 43
Some Applications
ME 407 44
Proper Application:TM,rms < TC()TM,max < TP()
Failure:TM,rms > TC()TM,max > TP()
Applications (Cont’d)
ME 407 45
Low Speed Operation:< R
TM,rms < TC()
High Speed Operation: R
TM,rms < TC()
Typical Data Sheet*
ME 407 46[*] Courtesy of Pasific Scientific, Inc.
Data Sheet (Cont’d)
ME 407 47
Data Sheet (Cont’d)
ME 407 48
Summary
ME 407 49