GB2260410A - Thermal monitoring devices for dimension gauge - Google Patents

Abstract

A thermal monitoring device is provided in a dimension gauge either as a part of or as an attachment to a gauging probe, allowing temperature measurements to be made at the point of measurement of the probe for use in compensating for errors in dimension measurements due to changes in temperature of the work-piece. As shown a probe tip 10 is mounted on a threaded stud 12 to be screwed into a gauging probe. The tip comprises a thermally conducting shell 11 which is attached to the stud 12 via a thermal insulation layer 14. A temperature sensor 15 within the shell is coupled to it by embedding in epoxy resin. For more rapid read-out, the true work-piece temperature may be calculated from one or more timed measurements after applying the probe. <IMAGE>

Description

THERMAL MONITORING DEVICE The present invention relates to gauging apparatus and particularly to an improved measuring transducer for use in such apparatus.

It is known to check the accuracy of workpieces by using gauging apparatus including a transducer having a workpiece engaging portion which is moved over a surface of the workpiece and supplies signals to a monitoring circuit where the exact dimensions can be computed and, if desired, compared with standard dimensions. It is also known that such gauging is only truly accurate if carried out at a predetermined temperature e.g. 20"C and that if the workpiece has just completed a process step the workpiece may be at a temperature which is different to the predetermined temperature. In recent years, industrial trends have followed a demand for faster machining of parts, as the technology becomes available.

This often results in the workpieces being monitored while at a temperature well above the predetermined temperature.

In the past, a separate sensor has been provided to supply to the monitoring equipment a signal indicative of the temperature of the workpiece. As well as requiring a separate holding mechanism in addition to that for the transducer measuring displacement, this results in the temperature being measured at a point other than that at which the displacement transducer is positioned. Thus, if the temperature sensor and the displacement transducer are in contact with different materials of different heat capacities or other thermal characteristics, this may result in inaccuracy even after attempts to compensate for the temperature of the workpiece.

It is an object of the present invention to provide a transducer having provided in its workpiece engaging portion a temperature sensing device, allowing accurate compensation to be made for the effects of temperature on the workpiece.

According to this invention, there is provided a thermal monitoring means for gauging apparatus wherein a temperature sensing device is mounted on a workengaging portion of a displacement transducer, said temperature sensing device providing a signal corresponding to the temperature at the point of measurement of a gauging head.

Preferably the gauging apparatus provides means by which the signal from the temperature sensing device can be processed and combined with a signal from the displacement transducer to reproduce a resultant temperature compensated gauging reading.

In another aspect of this invention, the actual temperature at the point of measurement is estimated by monitoring the change of temperature of the temperature sensing device as this approaches the actual temperature. The time taken for the process of monitoring the temperature in order to provide for compensation of the measurements of displacement can then be reduced.

The transducer may be a specially constructed transducer with integral temperature sensing device and associated wiring or may be simply a tip member which can be used to replace the existing tip member in an existing transducer, in which case separate provision has to be made for the wiring.

In order that the present invention be more readily understood, an embodiment thereof will now be described by way of example with reference to the accompanying drawings in which: Fig. 1 shows a sectional side view of a thermal measuring element according to the present invention; and Fig. 2a, b and c show diagrammatically three ways of connecting the element shown in Fig. 1 to monitoring apparatus.

Fig. 3 shows a typical graph of change of temperature with respect to time for a temperature sensing device put in contact with a workpiece of higher temperature.

Fig. 4 shows a circuit for processing signals from a temperature sensing device so as to produce a result for temperature compensation.

This invention concerns the inclusion of a thermal measuring element into a LVDT or half bridge differential measuring transducer of a gauging apparatus. The thermal measuring element is provided in the tip of the transducer and is used for the specific purpose of measuring the temperature or rate of change of temperature of the component to be gauged.

As shown in fig. 1 the transducer tip 10 is made up of a rigid hollow member 11 made of a thermally conducting metal/ceramic material which is mounted on a stud 12 via a thermal isolation layer 14. A thermal measuring element 15 is located in the hollow of the member 11 and lead wires 16 from the element 12 extend through the thermal isolation layer 14 and the periphery of the head 12a of the stud 12. The space between the member 11 and the element 15 is filled with a thermally conductive material such as an epoxy resin which intimately bonds the element 15 and the member 11 without damaging either. A resilient gater 18 surrounds the stud 12 and the lead wire 16. The stud 12 has a screw threaded portion so as to enable the tip 10 to be mounted on a suitable transducer.

In order to be effective, the member 11 has to be made of a material which will transmit mechanical forces away from the thermal measuring element to the main body of the transducer but have a minimum thermal mass and a low level of linear coefficient of expansion when subjected to heat.

Similar considerations apply to the thermal barrier layer 14.

Additionally it is important that a rigid mechanical contact is made between the threaded portion of the stud 12 and any transducer arm to which it is connected in order that any displacement of the transducer tip is accurately passed on to the displacement transducer via the transducer arm. In the case where the transducer tip of Fig. 1 is used to replace the existing tip member in an existing displacement transducer, this requirement may be satisfied by way of the threaded portion of the stud 12 having a length sufficient for the end face 13 of the stud to abut with an end-face of the arm of the existing transducer, thus preventing the lead wires 16 from becoming pinched between the arm of the existing transducer and the periphery of the head 12a of the stud 12.Alternatively, the lead wires 16 may be wound on a wide diameter and the periphery of the head 12a of the stud 12 may have a wider diameter, sufficient to allow the threaded portion of the arm of the existing transducer to pass between the lead wires 14 and the threaded portion of the stud 12, so as to allow the end face of the arm of the existing transducer to abut with the underside of the periphery of the head 12a of the stud 12.

In the case where the transducer tip of Fig. 1 is an integral part of a specially constructed transducer, similar possibilities to those above are available in order to satisfy the requirement that the transducer tip is fixed accurately on to the transducer arm.

The connecting wires either pass through the body of the transducer or are transferred by a sleeve over a standard transducer. As shown in Fig. 2, the method of transmitting the measured value of temperature may be direct electrical coupling (Fig. 2a), electromagnetic coupling (Fig. 2b) or optical coupling (Fig. 2c).

Prior to explaining the circuit of Fig. 4 for processing signals from the thermal measuring element, the form of the temperature characteristic to be measured will be explained, with reference to Fig. 3.

In use of gauging apparatus, it is generally considered advantageous if readings can be made quickly.

Most temperature sensing devices take time to respond, since the probe of the device takes time to reach the temperature of the object whose temperature is being measured. Assuming that the probe of such a device is of a relatively small thermal mass in comparison with that of the object being measured, the temperature of the probe will rise on an asymptotic exponential curve approaching the temperature of the object. In the example of Fig. 3 the temperature of the probe is shown to have almost reached the temperature of the sample T.AIM after 5 seconds. It has been found experimentally that the time constant of the exponential curve is a constant for each particular probe tip.Thus by taking measurement of the output signal from the temperature sensing device at two times a short interval apart, the temperature to which the probe tip is tending can be calculated by suitable mathemtical analysis.

In the example of Fig. 3 there is an interval of one second between successive temperature measurements the temperature initially being at T.START and rising to T after one second. From these two readings and knowledge of the time constant for the probe tip, the temperature to which the probe tip is tending T.AIM can be calculated.

Fig. 4 shows an example of a circuit for processing the signals from the temperature sensing device electronically to produce a temperature compensated gauging reading, using two signals from the temperature sensing device correspording to temperature measurements made at two times a short interval apart.

In this example, the temperature sensing device produces a voltage signal corresponding to the temperature of the probe tip. The temperature may be rising or falling; in either case the circuit has the same function and operating procedure. Assuming that the temperature is rising, a clock pulse A from the logic block 45 transfers the signal stored in buffer B2 to analogue/digital converter A/D(L) at time X=0. This signal corresponds to the temperature T.START. At time X=l second, a clock pulse B from the logic block 45 transfers the signal stored in buffer B1 to the analogue/digital converter A/D (H). This signal corresponds to the temperature T .The digital signals corresponding to the lower and higher temperature readings are then available in digital bus L and digital bus H, for use by any digital device, for example a computer or microprocessor.

The analogue/digital converters A/D (H) inverted by the two signals are fed into a summing amplifier 40 whose output represents the difference between the temperatures T.START and T . The gain of the summing amplifier output can be controlled by means of R and is then available for temperature GAIW compensation of the readings from the probe.

Depending on the possible range of temperatures of the probe tip and the accuracy of measurement required, the temperature compensation can be done by a full projection of the exponential temperature against time curve using the two measured values of temperature and the time constant for the probe tip as found by calibration tests. Alternatively, it may be estimated that in one second the temperature of the probe rises by a known fraction of the temperature difference between the initial temperature T.START and the projected temperature T.AIM. If this estimate is considered sufficiently accurate then the output of the summing amplifier, multiplied by a known factor, will be the temperature compensation amount that may be added to the initial temperature T.START to give the projected temperature T.AIM.This allows the output of the summing amplifier to be used directly in the processing to calculate the temperature compensated gauging reading of the position of the displacement transducer.

sing the projected estimate as to the temperature to which the probe tip is tending, either by full projection of the exponential curve or by an estimated correction factor, the final signal thus produced by the signal processing circuit can then be processed by the gauging apparatus to compensate the gauging reading for errors due to the temperature of the workpiece being different to the predetermined temperature.

The arrangement of the thermal measuring element described above has the combined functions of dimensional measurement and temperature compensation.

The part of the element for dimensional measurement and the part for temperature measurement preferably have as small an effect on each other as possible while allowing both types of measurement to be made in relation to the same point. The temperature measurements can therefore justifiably be used to compensate for temperature effects on the dimensional measurements. The element shown in Fig. 1 could however be used simply to detect the temperature or rate of change of temperature of a workpiece.

Claims (9)

1. A thermal monitoring means for gauging apparatus wherein a temperature sensing device is mounted on a work-engaging portion of a displacement transducer, said temperature sensing device providing a signal corresponding to the temperature at the point of measurement of a gauging head.

2. A thermal monitoring means according to claim 1, wherein the temperature sensing device is an integral part of the gauging apparatus.

3. A thermal monitoring means according to claim 1, wherein the temperature sensing device is attached to existing gauging apparatus.

4. A thermal monitoring means according to claim 1, 2 or 3, wherein the signals from the temperature sensing device are transmitted from the temperature sensing device by means of direct electrical coupling.

5. A thermal monitoring means according to claim 1, 2 or 3, wherein the signals from the temperature sensing device are transmitted from the temperature sensing device by means of electromagnetic coupling.

6. A thermal monitoring means according to claim 1, 2 or 3, wherein the signals from the temperature sensing device are transmitted from the temperature sensing device by means of optical coupling.

7. A thermal monitoring means according to any of the preceding claims, wherein the signals from the temperature sensing device are processed so as to provide compensation data for effects of temperature on measurements of the gauging head.

8. A thermal monitoring means according to claim 7, wherein the compensation data is calculated from the measurements of temperature of the temperature sensing device.

9. A thermal monitoring means according to claim 7, wherein compensation data is calculated from measurements of the rate of change of temperature of the temperature sensing device.