TOUCH SENSITIVE PROBE FOR PRECISION MEASURING
MACHINES
FIELD OF THE INVENTION
This invention relates to Precision Measuring Machines used in the Engineering Industry. In particular, this invention relates to a very high sensitive Probe to be used for position determination in measuring machines.
There are various kinds of probes known in the prior art. The following specification gives a presentation of different kind of probes being used by the industry. Inspite of their efficient design and construction, these probes are not able to fulfil the current day demands of accuracy and precision due to the constraints in their constructional design. The object of the invention is to precisely study the various probes known in the art and determine the inherent defects in the conventional probes and find out a solution to overcome the problem associated with prior art probes. Accordingly, a literature survey has been conducted by the company to ascertain the leading manufacturers in this particular industry and study the various probes manufactured by the aforesaid company. A description in detail explaining various aspects of the Touch Sensitive Probe known in the art is given. The following description refers to US patent application / specification and various other related documents pertaining to precision measuring machines.
References cited
US Patents
US patent Ser No: 4,138,823 dated Feb 13, 1979 Probe for use in Measuring Apparatus
US patent Ser No: 4.473,955 dated Oct 2, 1984 Probe for use in Measuring Apparatus
US patent Ser No: 5,669,152 dated Sep 23, 1997 Touch Probe
US patent Ser No: 4,701,704 dated Oct 20, 1987 Non-Directional Touch Signal Probe
Other Publications
Renishaw TP12 Dynamic Trigger Probe User's Guide - Pages 10,11 and 12 about pretravel variation
Renishaw Touch Trigger Probe System Users Guide - Page 7 with Specification
Practical aspects of Touch Trigger Probe error compensation - a paper by W. Tyler Ester, S.D. Philips, B. Borchardt, T. Hopp, M. Levenson. K. Eberhardt, M. McClain and Y. Shen and X. Zhang, published in Precision Engineering 21: p1-17, 1997
Error compensation for CMM Touch Trigger Probes - a paper by W. Tyler Ester, S.D. Philips, B. Borchardt, T. Hopp, C. Witzgali, M. Levenson, K. Eberhardt, M. McClain and Y. Shen and X. Zhang, , Precision Engineering, 19: p85-97, 1996
Pretravel Compensation for Vertically Oriented Touch Trigger Probes with Straight Styli - a paper by Yin-Lin Shen and Xianping Zhang

On perusal of the cited patents and other publications, we have noted that the lobing error ascends with the increase in stylus dimensions and adversely affect the accuracy of measurement in measuring machines. The industry is aware of the problem and a software compensating method has been adopted to compensate for the error. This is only a compensating technique and not a correcting technique that completely eliminates the error. The object of this invention is to eliminate the error in the device and improve the accuracy.
DESCRIPTION OF PRIOR ART
The description regarding prior art is with respect to Figures 1 and 2 of the drawings which accompany the specification.
Figure 1 shows the principle of operation of a conventional probe used in measuring machines for high precision measurement. The probe body (1) carries the kinematic arrangement (2), which in tum carries the stylus (3) with a ruby ball tip (4). The kinematic arrangement has 3 electrically conducting spheres (5) each seated on a pair of two electrically conducting cylinders (6) all connected together in series when the probe is at rest and fed from an electric source (7). The kinematic arrangement is kept in place by a force provided by a compression spring (8). The kinematic arrangement serves two purposes. It is used for detection of contact on workpiece clamped in the machine and for repeatable reference positioning of the stylus after probing.
When the ruby ball tip touches a workpiece (52) clamped in the machine, the kinematic arrangement deflects, the contact between the balls is broken and the probe output (9) changes from logic high state to logic low state. When the stylus tips releases from the workpiece, the kinematic arrangement goes back to its normal position. The balls come in contact again and the probe output changes to logic high state.
The accuracy of dimensional measurement in a measuring machine depends largely on the accuracy of the probe being used along with other factors such as environment and structural accuracy of the machine. The conventional probe has many inherent errors:
• Lobing error or Pretravel variation
• Directional dependence of probe response
• Probing force dependence of probe response
• false triggering
• increase in error magnitude with the increase in length and weight of stylus
• increase in error magnitude while probing at high speed
• error due to dragging of the cable used to connect the probe to subsequent electronics

The lobing error or Pretravel variation accounts for the majority of the probe errors. When the ruby ball tip touches the workpiece, the force developed at contact point is not high enough to overcome the compression spring force to deflect the kinematic arrangement. As a result, after touching the workpiece, the probe travels in the probe approach direction a certain distance called pretravel (12) causing the stylus to bend and the contact force to increase until the force is high enough to deflect kinematic arrangement. This pretravel distance is not constant and depends upon the probing direction, probing force, length and weight of the stylus, compression spring force etc., It increases with increase in length of stylus. The error in measurement caused by this defect is called as lobing error.
Figure 2 shows the lobing error pattern of a conventional probe. When a circular workpiece placed in the XY plane of the machine is measured, the ideal plot of the circle is shown by dotted circle (10). Due the lobing error inherent in the conventional probe, the actual plot will be a lobe as shown by the solid pattern (11). The difference between the dotted circle (10) and solid pattern (11) is the pretravel (12).
False triggering is another problem common in conventional probes. The compression spring force can be set at a minimum value, so that the deflection of probe on contact is very fast. But, on the negative side, this causes the kinematic arrangement to deflect when there is vibration in the machine or in the external environment, which is high enough to overcome the spring force, the force being set at a lesser value. This is termed as false triggering as it gives a false response as if it contacted a workpiece.
Probes using dynamic sensors are also used in measuring machines. These probes sense the shock generated at the instant of impact to detect contact on workpiece. The response of this sensor depends upon the speed of probing, direction of probing and the nature of the workpiece.
A Probe using quartz tuning fork oscillator is used in electronics, plastic and precision industry. This probe operates at a fixed resonant frequency of oscillation and can be used only with a particular stylus. This is because, the resonant frequency of oscillation of the probe depends on the dimensions of stylus. If a different stylus has to be used, the probe must be tuned to that resonant frequency. There is no mechanism in this probe to adjust the frequency of oscillation.

OBJECT OF THE INVENTION
It is the primary object of the invention to invent a novel Touch Sensitive Probe which is unique in construction.
It is another object of the invention to invent a novel Touch Sensitive Probe which has extensive industrial application.
It is yet another object of the invention to invent a novel Touch Sensitive Probe which is sturdy and economical in construction and efficient in operation.
Further objects of the invention will be clear from the following description.
SUMMARY OF THE INVENTION
The invention addresses the issue of errors in measurement. It further addresses the components causing the error. The invention incorporates a novel probe mechanism, which surmounts the problems associated with conventional probes.
A novel Touch Sensitive Probe comprising of cylindrical / circular / rectangular body having a probe head adaptable to fit in a machine, the said probe having three parts comprising an interface part at the upper portion, a kinematic arrangement part positioned below the interface part and a sensor part placed at the bottom portion, a stylus adapter to be fitted to the sensor part, and an electronic control logic circuitry fitted to the interface part, the constnjction of the probe is such that the kinematic arrangement part and the sensor part are separated so as to eliminate the influence of inaccuracies generated by the kinematic arrangement part in the sensor part
Now, the invention will be described in detail with reference to drawings accompanying this complete specification. The following description describes in detail various aspects of the invention, including the constructional features of the invention and the manner in which the device according to the invention is operated.
BRIEF DESCRIPTION OF DRAWINGS
Figure 1 shows the constructional details of a conventional probe
Figure 2 shows the error pattern in a conventional probe
Figure 3 shows cross section elevation of the components of the probe according to
the invention
Figure 4 shows the cross section elevation of the sensor part

Figure 5 shows the electrode configuration on the piezoelectric elements
Figure 6 shows the block diagram of the electronic circuitry of control logic
Figure 7 shows the schematic elevation and section drawing of Coordinate Measuring
Machine wherein our novel Touch Sensitive Probe is fitted
DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION
The design and construction of the probe according to the invention is illustrated in Figure 3 of the drawings.
In conventional probes, mechanism for detection of contact and the seating are built into the same mechanism. These are two different design needs addressed by the same mechanism. The errors inherent in one affect the other. This is the cause for the lobing or pretravel variation and false triggering in such probes.
The invention has a Kinematic arrangement part (17-21,5,6) and a sensor part (21-24) that are separated from each other so that the inaccuracies in the kinematic arrangement part does not influence the operation of the sensor part. The sensor part is used only to detect contact on workpiece. The kinematic arrangement part is used only for repeatable reference positioning of the stylus.
The probe according to the invention has a body (1) wherein three parts are housed: the interface part (13 - 16) in upper portion within the body, the kinematic arrangement part (5,6,17-21) positioned below the interface part and the sensor part (21-24) positioned just below the kinematic arrangement part.
The sensor part consists of a special aluminium cylinder (21) connected to the kinematic arrangement part at one end and carries a stylus holder (24) on the other end to which a stylus with ruby ball tip is screwed. A piezoelectric exciter element (22) and a piezoelectric pickup element (23) are stuck on to the inner and outer surfaces of the aluminium cylinder (20) respectively. The function of the sensor part only is to detect contact on the workpiece.
An electronic circuitry (16) in the interface part excites the piezoelectric exciter element and sets the stylus into oscillation at resonant frequency The piezoelectric pickup element senses the oscillation. When the oscillating stylus contacts the workpiece, there is a shift in the frequency of oscillation and phase and this is sensed by the piezoelectric pickup

element. This signal is further processed in the electronic circuitry to generate a trigger signal on contact.
The kinematic arrangement part consists of 2 layers, one is the sphere support plate (18) carrying six electrically conducting spheres (5), the other is the layer formed by six cylinders (6). Out of the six cylinders, three are fitted 120° apart in a circular fashion in a fixed cylinder holder plate (20) and the other three cylinders are fitted 120° apart in a circular fashion in the movable cylinder holder plate (21). A tension spring (19) keeps these layers in tact. A compression spring (8) at the top of the sphere holder (17) is used to adjust the force set on the kinematic arrangement part. This arrangement is commonly used in probes for repeatable reference positioning of the stylus and for probing in six directions.
The interface part has M8 threaded clamping bush (13) adaptable to be screwed to the machine quill (50) or the probe head (51). A conducting bush (15) in the center of the clamping bush (13), is used to get the power supplied from the probe head. This power is used to power up the electronic control logic circuitry (16) in the interface part. An insulator (14) insulates the conducting bush (15) and the clamping bush (13).
Figure 4 shows the elevation and cross sectional views of the sensor part of the probe.
Figure 5 shows the top and elevation view of the electrode arrangement of the piezoelectric exciter element and pickup element. Both the piezoelectric elements are cylindrical in shape and are made up of PZT ceramic (27). The inner surface (25) and outer surface (26) of the both the piezoelectric cylinders are coated with silver and form the two electrodes for making electrical connection.
The electrode in the inner surface (25) of the piezoelectric element is full faced as shown in Figure 5a (top view of electrode configuration) by continuous hatched lines and contains no segmentation. The electrode in the outer surface (26) is segmented both in the horizontal and vertical direction as shown in Figure 5b (elevation view of the electrode configuration). With this arrangement, signal propagation is uniform throughout the piezoelectric exciter element and the piezoelectric pickup element gives a maximum sensitivity and quick response on contact.
Figure 6 shows the detailed block diagram of the electronic circuitry. This circuitry performs two functions. Firstly, it gives the required signals to the sensor part and interprets the signals from the sensor part. Secondly, it converts the signals from the sensor part into a conventional probe requirement for compatibility with their 2-wired technology.

The Pnase LOCKea Loop (zo), caneu
voltage controlled oscillator in the PLL supplies the exciting signal (29) to the piezoelectric exciter element to set the stylus into oscillation at resonant frequency. The PLL maintains a finite frequency relation between the exciting signal and the piezoelectric pickup signal (30) through a control voltage (31) and is said to be in lock, when the probe is at rest. When the ruby ball tip (6) touches the workpiece, there is a shift in frequency of oscillation that throws the PLL out of lock. This is reflected as a change in the control voltage output from the PLL along with another reference voltage output (32).
The control voltage is filtered in a multistage filter (33) and the filtered signal (34) is subtracted from the reference voltage output (32) in the Differential Amplifier (35). The difference voltage (36) is compared with a threshold voltage (38) in the Level comparator (39), the output of which is called the trigger signal (40). The threshold voltage is the minimum change in control voltage required to detect the very first instant of contact on workpiece. It sets a window for the control voltage and is generated in the threshold voltage generator (37). When the probe is at rest, the control voltage is within the window of the comparator and the trigger signal (40) is in logic low state. When the workpiece is touched, the control voltage (31) changes, bringing the comparator out of the window at the immediate instant of contact and the trigger signal (40) goes to logic high state.
The response of the piezoelectric elements in the sensor part on probing is very high and fast when the stylus oscillates at resonant frequency. The resonant frequency of oscillation depends upon the length and weight of the stylus. Each time, a stylus of different dimension is used, it has to be tuned to oscillate at its resonant frequency to get maximum response. This is done automatically by the digital self tuning circuit (41) which is connected to the voltage controlled oscillator in the PLL. The control logic (42) generates the required signals to control the tuning circuit and threshold voltage generator.
The trigger signal output (40) from the Level comparator is modulated in the RF transmitter (43) and radiated into space via an antenna (44) to be picked up by the measuring machine computer
Figure 7 of the drawings shows the elevation and the side view of a Co-ordinate Measuring Machine wherein the Touch Sensitive Probe (43) is fitted. The coordinate measuring machine is a quality control system for high precision 2D and 3D measurements of components in industries. The machine has a reference base (44) on which two support columns (45) are erected. The support columns slide on the reference base without any friction. To inspect any component in the machine, their X, Y and Z coordinate points are to

be determined with respect to a referral point called the origin. One end of the reference base is identified as the origin (46).
The right hand side of the reference base forms the Y axis guideway (47). The to and fro movement of support columns on the reference base forms the Y axis. The support columns carry a horizontal X beam (48) which carry the Z carriage (49). The horizontal movement of the Z carriage on the X beam forms the X axis. The Z carriage carries a vertical quill (50). The up and down movement of the quill forms the Z axis.
The quill (50) carries a probe head (51), which in turn carries the Touch Sensitive Probe (43), which in turn carries a stylus (3) with ruby ball tip (4). All the three axes are fitted with length measuring transducers (not shown in the figure) that keep track of the position of probe as it moves.
The Probe according to the invention is used to detect the contact of the of the ruby ball tip (4) on a workpiece (52) clamped in the reference base (44) and issue a trigger signal output (40) to the computer (not shown in the figure) connected to the Coordinate Measuring Machine without any pretravel variation or lobing error. On receiving this signal, the computer reads the coordinate of the contact point on workpiece from the length measuring transducers and with the aid of measurement software, computes the geometry of workpiece.
ADVANTAGES OF THE INVENTION
The invention has the following advantages:
• Instantaneous contact detection
• Lobing error free measurement as there is no pretravel or stylus bending
• Uniform Omnidirectional response on probing as the piezoelectric elements used for contact detection exhibit constant, uniform response in all the probing directions
• Force free probing as the piezoelectric elements have high three dimensional sensitivity and there is no need of force build up for contact detection
• No false triggering
• High repeatability
• Probing in all the six directions
• Built in digital self tuning capability to operate with a wide range of stylus and extensions without degradation in accuracy
• High speed probing without loss of accuracy

We claim
1 - A novel Touch Sensitive Probe comprising of cylindrical / circular / rectangular body having a probe head adaptable to fit in a machine, the said probe having three parts comprising an interface part at the upper portion, a kinematic arrangement part positioned below the interface part and a sensor part placed at the bottom portion, a stylus adapter to be fitted to the sensor part, and an electronic control logic circuitry fitted to the interface part, the construction of the probe is such that the kinematic arrangement part and the sensor part being separated so as to eliminate the influence of inaccuracies generated by the kinematic arrangement part in the sensor part
2 - A novel Touch Sensitive Probe as claimed in claim 1 wherein the kinematic arrangement part does not act as a switch to detect the contact, the said kinematic arrangement part being used for repeatable reference positioning of the stylus, and said the sensor part being used to detect contact
3 - A novel Touch Sensitive Probe as claimed in claim 1 wherein the sensor part is made up of two sub assemblies / parts, one is the piezoelectric exciter element and the other is the piezoelectric sensor element, the said piezoelectric exciter element is made of PZT BM400 and the said piezoelectric sensor element is made of PZT BM500
4 - A novel Touch Sensitive Probe as claimed in claim 1 wherein the piezoelectric exciter element has a cylindrical tube shape and having either a plate, spherical, hemispherical, ring, or a solid cylindrical shape and the piezoelectric sensor element has a cylindrical tube shape, and having either a plate, spherical, hemispherical, ring, or a solid cylindrical shape.
5 - A novel Touch Sensitive Probe as claimed in claim 1 wherein the size of the piezoelectnc exciter element is very critical as it determines the resonant frequency of oscillation of the stylus which should be beyond the audible range, the said size includes the length, interna! diameter and external diameter for the cylindrical shape.
6 - A novel Touch Sensitive Probe as claimed in claim 1 wherein the piezoelectric exciter element and the piezoelectric sensor element are fixed in a concentric manner with the piezoelectric exciter element forming the inner member of the concentric cylinder and the piezoelectric sensor element forming the outer member of the concentric cylinder.

7 - A novel Touch Sensitive Probe as claimed in claim 1 wherein the piezoelectric exciter element has a specific electrode configuration illustrated by the way of drawings and the piezoelectric sensor element has a specific electrode configuration illustrated by the way of drawings.
8 - A novel Touch Sensitive Probe as claimed in claim 1 wherein the sensor part has a single stylus and not a twin stylus as in conventional probe to obtain better orientation and physical configuration of stylus.
9 - A novel Touch Sensitive Probe as claimed in claim 1 wherein the detection of contact is
direct and involves a single operation and does not involve multiple steps like force build up
at contact point, transmission of force to sensor part and then the detection
10 - A novel Touch Sensitive Probe as claimed in claim 1 wherein the interface part has an electronic control logic circuitry with built in digital self-tuning capability to accommodate a wide range of stylus and extensions.
11 - A novel Touch Sensitive Probe as claimed in claim 1 wherein the transmission between the probe and the CMM controller is RF wireless and the interface has an electronic control logic circuitry for the RF wireless transmission, the transmission can either be infrared, ultrasonic, sound wave or through wires
12 -A novel Touch Sensitive Probe is described in the specification and illustrated by the
virtue of drawings