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The development of sensors for CMMs

Sir David McMurtry Chairman and Chief Executive, Renishaw plc

Abstract

This paper reviews the technical development of 3D tactile sensors and co- ordinate measuring machines (CMMs) over the years and examines the changes in their ability to perform the inspection task in terms of accuracy, uncertainty and speed.

Introduction

Drawing standards such as ANSI Y14.5 and BS8888 define exactly the allowable geometric size, position and shape of part features. However, none are inspection standards and give no guidance whatsoever to an inspector on how the part should be inspected or how the measured value is defined. In fact, there is no internationally agreed standard for dimensional inspection. It is usually up to the inspector to interpret the drawing and determine what procedure is needed to verify the part, given the inspection tools at hdher disposal.

The same measuring instrument can give vastly different results depending on how it is used. In the case of the micrometer (Figure l), it is not possible to determine the length of the part shown as the micrometer cannot handle the form of the part on its ends (as illustrated).

In the case of the CMM (Figure 2), the difference in the apparent diameter of the part with a form error can vary greatly depending on the number of points taken and where they are taken. This can lead to bad parts being accepted and good parts being rejected.

The CMM started to become a tool of choice for measuring prismatic parts in the late 60s and early 70s, but there has been a heightened concern about its capability after a GIDEP alert was issued by an engineer from Westinghouse in the USA. A GIDEP alert is a US Government industry data exchange

Transactions on Engineering Sciences vol 44, © 2003 WIT Press, www.witpress.com, ISSN 1743-3533

206 Laser Metrology und Muchiize Performance VI

programme designed to publish data of national importance. In this case the data showed clearly that the only available equipment to measure a safety-critical aerospace part, namely a CMM, gave inconsistent results that could pass a bad part.

(Figure 1)

(Figure 2)

The military could not ignore this and insisted that all parts must conform to the drawing, even if there was no standard on how h s could be acheved or equipment that could acheve it. This led to an impasse. A high level meeting of recognised experts in the field was convened in Washmgton to rectify the situation. Working parties were set up to establish a mathematical definition of ANSI Y14.5 drawings and create a new standard. To date, no standard that identifies how many data points and where they should be taken has emerged. In the light of all this uncertainty why was it that 3D touch trigger sensors on CMMs still became so popular?

The majority of prismatic parts were produced on lathes, grinding machines and machining centres which can be equipped with boring bars. These machines proved that parts with consistent form could be produced over long periods of time, necessitating only infrequent circular form checks to verify quality. Sample parts could be checked on form measuring equipment such as a Talyrond roundness-measuring machine, which was an agreed international method on


Laser Metrology and Machine Performance VI 207

how to measure circular form. The low frequency of checks on circular form meant that this was a commercially viable solution.

However, once there was a h g h confidence level, for example, of the form of a hole, then a minimum of three data points was needed to confirm the diameter as defined by the drawing standard. Size has to be controlled far more frequently than fonn, as size depends on tool wear; whereas form usually depends on the deterioration of the spindle bearing. CMMs became very popular for checlung the feature size and position on precision parts using only a limited number of measured points, and so became a very cost-effective tool.

The early 70s saw the introduction of ornni-directional touch trigger probes from Renishaw (Figures 3a - 3h), followed shortly by 3-axis active measuring probes, for example, from Zeiss (Figure 7), capable of talung single data points a. Both reduced the measurement uncertainty of a CMM compared to the original machmes equipped with hard probes. In particular, the omni-directional touch-trigger probe significantly reduced the time to measure a part on manual as well as a DCC (direct computer controlled) machine as a result of its ability to take data from any direction. Despite its ability only to measure size and position in a cost-effective manner, ths did not prove to be a barrier to success over a number of decades.

(Figure 3 a) (Figure 3b)

Over the past three decades a number of different types of probes were developed based on established tough-trigger and measuring probe technology with varying success in both cases. These developments were to improve accuracy, and also speed in the case of the measuring probe.

The more successful improvements over the touch trigger principle was the introduction of piezo sensing, using acoustic emissions generated by the probe's tip htting the work-piece and transmitting the acoustic wave up through the stylus to the piezo.


208 Laser Metrology und Muchiize Performance VI

Elect r~cal switchmg sectmnthrough ~ ~ n c m r l i c . eledncal clrcultthrough k'"'moc'

contacts

rerlstance measured

a contact patches reduce In '"'OUgh

m e as stylus forces b u ~ ~ d kinemat'cs

Klnematlcr bonded to (and ,n$ulated born) prob. body

stylus bendmg under contact F- loads before tngger threshold 1s reached ' pre-travel depends on F,and L

trigger Is generated a short dstance after the stylus flnt touches the component

F,xL=F,xR L and F. are constant

:.F, is proportional to R F,

(Figure 3e)

Pre-travel variation - ' lobing9

trigger force in Z diredon is higher than in XY plane

-no mechaniai advantage over spnog

-F,;.F,

kinematic resiswe prober exhibit 30 (XYZ) pretavel variation

- Oornblnsbon of Z and XI tnggei efficb

-low XYZ P N useM for mntoursd pit nrpection

E l ~ t r l c a l switching

resistance breaches ~es la tmra threshold and probe trlggers

kinematics are st11i in Force on kl".rndIC,

when rty,vr is in contact when pmbe toe space triggers

-stylus in defind rrige.r POSltlO"

current cut before

(Figure 3 d)

p$g+y~;;w~6 aw$p7;*:

Pre-travel variation - ' lobing3 Top view

trigger force depends an probing direction, since pivot point vaiies

-F, IS pmplbonal to R

therefore, pre-bavei varies around the XY plane

(Figure 3f)

Typical pre-travel variation

(Figure 3g) (Figure 3h)



The frst commercially available probe using this material was from Zeiss, (Figure 4a), followed by Renishaw's TP12 (Figures 4b, 4c), and TP800 (Figures 5a, 5b, 5c, 5d). Although the piezo based probe gave extremely good results, virtually eliminating pre-travel variation or lobbing which caused small errors in the first touch-trigger probes, they tended to suffer drawbacks (side-effects) that the original probe did not have. They were more sensitive to acoustic noise in the environment. Factory noise, or a door slamming, could cause the probe to false trigger. There were developments to recognise false triggering and prevent bad data from being processed, but too many occurrences could prove unacceptable in some cases.

( Figure 4a courtesy of Zeiss) (Figure 4b) (Figure 4c)

(Figure 5a) (Figure 5b)

Piezo sensors - 6 off

U

(Figure 5c)

Another drawback with this technology is in the measurement of soft materials, or parts with oil on the surface. The amount of acoustic energy generated could be insufficient to be recognised by the piezo. A further development to overcome this effect was to calibrate the signal given off by the acoustic emission, together with the signal given off when a force is applied to the stylus tip occurring later


2 10 Laser Metrology und Muchiize Performance VI

in time. If the more accurate "first" acoustic signal was not recognised, the second one was used, with only a small loss of accuracy, to establish the data point - a principle of Renishaw's TP800. Although this class of probes was more accurate, they did not replace the original technology due to their complexity. In fact, traditional touch trigger probes vastly outsell them.

shock travels at speed of sound through the stylus and probe

- 800 m per second (2,600 Wsec)

- response time is 1 .X gsec / mm

pre-travel depends on stylus length and probing speed

- pre-travel is the same in all directions since mechanical signal path is constant

- lobing effect limited ta ball sphericity!

(Figure 5d)

A more successful probe technology to reduce the effect of pre-travel variation or lobbing, was to sense the force on the tip using strain gauges, for example Renishaw's TP7 (Figure 6a) and TP200 (Figures 6b - 6g). This proved to be a far better compromise between accuracy and environmental interference. The use of three micro strain gauges meant the output could be combined to produce non- lobbing in the transverse direction to the axis and a very small error (0.2 microns) along the axis.


Laser Metrology and Machine Performance V1 21 1


(Figure 6c)

(Figure 6d) (Figure 6e)

Car1 Zeiss in1973 introduced the first commercially successful 3-D parallel acting active measuring probe head. The approach to taking a data point was quite different to that of the touch trigger probe. Ths was achieved by mounting a probe head on the end of the quill of a CMM which was, in effect, a miniature DCC CMM in its own right (Figure 7) in that it had three independent axes



stacked on top of one another in series, each axis containing a powered actuator, a linear measuring system (LDVT) and axis locks.

(Figure 6f)

Unidirectional repeatability IS0 f 0360-2 2D form

Pm 3.5

3.0

2.5

2.0

1.5

1.0

0.5

0 0 50 100 150 200 250 0 50 100 150 200 250

Stylus length Stylus length

(Figure 6g)

The system's original operating sequence was to select a direction of travel along one of the axes of the probe head, advancing the probe tip using the axis actuators. With the other axes locked, the probe head is advanced in the direction of the live axis until the work-piece is contacted. The probe head and machine are then driven until the tip is back to zero position in the middle of the measuring range of the LDVT and a defined force is applied to the part by the


Laser Metrology and Machine Performance VI 2 13

actuator. Combining the LDVT readings with the CMM's scale readings and by averaging a number of readings, the system was able to achieve a data point.

In 1974, Leitz introduced a passive measuring probe head that was similar to the Zeiss head in that it had three stacked axis, but with no axis actuators or locks. This meant that the force to hold the probe tip against the work-piece was achieved by a spring force since each axis was biased back to the centre position by the springs. The mode of operation of ths head was to advance the probe onto the part until a pre-determined deflection was achieved. A series of readings of the position of the probe transducers were taken, and the machine's axis position; combining them to calculate a single data point. Later t h s method of was changed to that shown in Figure 8.

(Figure 7) (courtesy of Zeiss) (Figure 8) (courtesy of Leitz)

Thls method comprised driving the probe onto the surface of the part to a pre- determined deflection, then removing the probe tip from the part slowly, recording data from the probe and the machne's position on the way out. T h s enabled the readings to be combined to calculate a point in space just as the probe leaves the surface i.e. when there is no load of the probe tip on the part. Zero force data can be taken and dispenses with compensation for machme quill deflection. Both of these probes could carry out scanning functions by using feedback from the probe outputs to servo the probe, enabling it to stay on the surface while being driven in the pre-determined direction. As the moving mass was quite high in this type of probe, the scanning speeds in this mode were extremely slow but ensured accurate results. Other examples of passive scanning probes were developed as shown in Figures 9a - 9e.

As far as 3-axis measuring probe heads were concerned, the technology did not change much until the early 90s, with the introduction of lvgher speed scanning probes. Examples of these are the Zeiss Vast, Leitz and the Renishaw SP600. Zeiss and Leitz pioneered high-speed scanning, predominantly for prismatic parts, whereas Renishaw pursued high-speed scanning predominantly for the measurement of complex forms (digitising) for the use of the mould and die industry. However, both solutions can be used for either application.



(Figure 9a courtesy of SIP) ( Figure 9b courtesy of Movomatic)

(Figure 9c courtesy of API) (Figure 9d courtesy of Brown & Sharp)

(Figure 9e)

High speed scanning of prismatic parts was developed to satisfy the demand to measure size, position and form on a CMM in a cost-effective manner. This was



achieved by driving the CMM around a known path that would ensure probe contact while observing the deflection of the probe head together with the machine position, in order to acquire data. In the case of Zeiss, the probe tip was held in contact with the work-piece using the probe's axis motors. This maintained a relatively constant force on the part, apart from dynamic effects. In the case of passive probes (Leitz), the force to keep the stylus on the part was applied by springs in each axis and gave a force on the part proportional to deflection, again apart from the dynamic effects. Ths type of scanning is known as open loop or known part scanning. In each case, the probe had to be calibrated to cater for these effects.

Both systems produced similar results which were much improved over unknown part scanning or closed loop for the same speed, as both of these probes had relatively high moving masses caused by the weight of the parallel acting mechanism and the stylus change mechanism. Their ability to follow an unknown part, such as a mould or die, at speed was still limited although the known part method did increase the speed significantly.

Renishaw introduced the SP600 probe in the early 90s that had been designed to give good unknown part or closed loop performance. This was achieved by using the passive probe approach with a parallel acting mechanism, but the transducers were moved from each axis and replaced with sensors mounted to the probes static body (Figures 10a, lob). The need for flexible leads was removed as well as reducing the moving mass. A frictionless damper was also added to control the amplitude of the natural oscillations when the probe was not in contact with the part.

(Figure 10a) (Figure lob)



The result of reducing the mass for a given stylus stiffness resulted in a much improved dynamic performance when used in closed loop scanning mode. However, this characteristic also improved open loop scanning due the hgher natural frequency. The size of the SP600M (40mm diameter) meant it could be mounted on articulated heads, giving much more flexibility for a given stylus compared with fixed head probes.

The introduction of Renishaw's SP25M introduced a new technical approach to contact scanning, greatly reducing the moving mass and improving flexibility of use. This was achieved by replacing the parallel acting spring mechanism with a pivoting mechanism described in Figure 11. Also, the transducer system located in a separate module (Figure 12a, 12b) meant that not only the stylus could be changed but also the spring module, while the sensing electronic body remained fitted to the head. The spring module may be changed to accommodate larger styli if necessary, providing better accuracy with longer styli. The speed and accuracy improvements acheved by this arrangement makes it ideally suited for the volume end of the CMM market.

Positioning sensing device - 2 off

Return spring z,y,z only

(Figure 11)

However, where the highest accuracy is required at the cost of speed, Renishaw's SP80 represents the latest generation of passive scanning probes. By removing the stylus ejection mechanism to the probe changer and applying attention to detail on the weight of all moving parts together with the introduction of a scale and readhead transduction system (Figure 13a), the moving mass has been reduced. All flying leads to the transducers have also been eliminated. The use of hgh-resolution scales allows a long very accurate measuring travel on all axes and has eliminated the need for a tare system to accommodate different weights of styli.



Sensing

- modules

(Figure 12a)

(Figure 12b)



The improvement in accuracy was obtained not only by the accuracy and h g h resolution of the scale, but the fact that the X and Y axis straightnesses are generated off the lines of the scale and not from the mechanical parallel acting spring system (Figure 13b).

3 moving scales 3 static read heads


So what of the future of scanning probes? Back in LAMDAMAF' 1997 I reported progress on a new approach to obtaining vastly increased scanning speeds by combining the virtues of passive scanning and active scanning. The passive element senses the surface and the active element ensures the probe stays on the surface by using feedback from the passive sensor. By using a rotary active head instead of parallel active mechanism, the probe tip can be orientated in any direction, eliminating the need for an indexing or conventional two axis- positioning head.

A very high band width passive sensor is achieved by measuring the position of the stylus very near the ball using the laser beam reflected back from the tip (Figures 14a, 14b, 14c). To improve the servo response and eliminate torque reaction back into the quill of the machine, Renishaw has incorporated an inertia balancing system.



( Figure 14a)

A CMM is at its most accurate when it is standing still or travelling at a constant velocity in a straight line, as in these cases there is no bending of the machme structure caused by dynamic effects. It is in one of these conditions that the rotary active head is used to collect data points by moving the tip where necessary to take data.

As it is inertia balanced and very Low Mass, hgh-speed data collection can be acheved without adversely influencing the CMM's structure. The mode of operation is shown in Figure 14a. This technology uses precision air bearings and encoders in order that roundness may be measured on a CMM, independent of the accuracy of the CMM. This head should be on Beta test at CMM manufacturers later this year. I have no doubt that this approach is the way forward, both for contact and non-contact scanning.

Note: The ultimate accuracy of all these classes of scanning probes is highly dependent on error mapping software, both static and dynamic as well as the machine drive algorithms. This subject has not been addressed in this paper.


220 Laser Metrology ulzd Muchine Performance VI

Probing technology H~gh frequency response Ultra low movlng mass. jiast the stylus hall. Law scanning force.

(Figure 14b)

Enclosed laser directed onto a reflector at the stylus tip.

The stylus touches the part and bends. The reflector is d~splaced The altered return path of the laser is sensed by a PSD

Light beam is not affected by gravity, inertia or thermai effects

The exact tip position 1s known because the reflector and the stylus ball are unified.

Return path to PSD when stylus deflected.

(Figure 14c)


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