Opto-electronic devices have been used for position sensing in process control and inspection, either as simple limit switches giving a signal when a position has been reached (e. g. slotted opto-switches) or as complex systems for absolute and relative measurement (e. g. digital readout systems).

In many manufacturing situations it is necessary to monitor the dimensions of sub- assemblies or finished parts or the position of components in a manufacturing process. The data gathered on the dimensions or positions is used to ensure that a process is kept in control and that the finished product is to specification.

Opto-electronic devices have limited use either because they only give limited information (on or off), or they are expensive and delicate.

In the type of manufacturing situation described above, measuring equipment is needed that operates over only short distances relative to the specification tolerances. Typically the distances that are measured are less than one millimetre relative to the normal design specification.

Often the cost of measuring equipment is prohibitive and simple gap gauges are used, alternatively standard measuring equipment is used such as digital callipers, micrometers etc. The use of gap gauges provides only pass or fail data. while standard measuring equipment may not always be ideally suited to the application and requires training and understanding of basic metrology principles. In either case there are situations such as the measurement of delicate or flexible components (e. g. thin wall plastic mouldings) that will be affected by subjective judgements of feel and pressure.

It is therefore desirable to have a simple, robust device that can be incorporated into gauge fixtures or assembly equipment to provide consistent repeatable measurement data over short distances relating to a specification tolerance.

It is an object of the present invention to provide a position sensor that mitigates at least some of the aforesaid problems.

A further object of the invention is to provide a sensor that gives an output signal in proportion to the distance sensed. The sensor can be used as both a"GO"and"NO GO" gauge or to supply data for statistical process control.

According to the present invention there is provided a position sensor including a body member, a probe means. a light source and a light detector, said body member including a first bore defining an optical path between the source and the detector and a second bore that intersects the first bore, said probe means being mounted for movement in said second bore and including an aperture that extends through the probe means. the arrangement being such that. depending on the position of the probe means relative to the body member, the aperture may be aligned with the first bore or displaced from the aligned position, whereby when the aperture and the first bore are aligned the intensity of light reaching the detector is a maximum. and when the aperture is displaced from the aligned position the intensity of light reaching the detector is reduced.

The sensor can be very small and enables position or size to be measured with accuracy.

It is very simple and reliable in operation. and does not rely on making or breaking electrical contacts.

Advantageously. the light detector provides an output signal that is proportional to the amount of light reaching the detector. The output signal may be either directly or indirectly proportional to the amount of light reaching the detector, depending on the size and shape of the apertures. Using a proportion output detector makes it possible to adjust the tolerance of the sensor when used as a GO/NO GO gauge. and allows distances and/or dimensions to be measured.

Advantageously. the aperture is displaceable to both sides of the aligned position.

Alternatively. the aperture may be displaceable to just one side of the aligned position.

The aperture and the first bore are of substantially the same size. or alternatively they may be of different sizes.

Advantageously, the aperture ard the first bore are substantially circular. This enables simple manufacturing processes to be used. Other shapes, for example rectangular, are also possible.

Advantageously, the first and second bores are substantially perpendicular.

Advantageously, the probe means includes resilient biassing means.

Advantageously, the sensor includes stop means for limiting movement of the probe means.

Advantageously, the sensor includes means for locating the probe means with the aperture and the first bore in an aligned position. This is helpful during setting-up.

According to a further embodiment of the invention there is provided a position sensor that uses a system of masks and slide (s) to restrict the path of light from a light source to a photo detector. the output of the photo detector being in relation to the position of a slide within the systems of masks and slides.

Advantageously, the slide is arranged to have a linear motion. Alternatively, it may be arranged to have a rotational motion Advantageously, the sensor includes a return mechanism such as a spring or weight.

Advantageously, the sensor includes a combination of one or more slides and one or more masks to restrict the path of light from one or more light sources to one or more photo detectors.

According to a preferred embodiment, the invention comprises of a light source and a photo sensor mounted relative to each other such that the light source projects a beam of light onto the photo sensor. Between the light source and photo sensor there is a system of masks and a slide such that when the slide is moved it restricts the amount of light incident on the photo sensor. The output from the photo sensor can be used to create an electrical current, which is in relation to the position of the slide. The masks and slide are arranged so the output is a maximum when all elements are fully co-incident and reduces in relation to the misalignment of the slides and masks.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows the general arrangement of a linear position sensor; Figure 2 shows the general arrangement of a rotary position sensor; Figures 3 and 3a show the slide and masks in an aligned position giving maximum output; Figures 4 and 4a show the slide and masks in a partially mis-aligned position giving a reduced output ; Figure 5 shows the slide in a fully mis-aligned position giving a minimum output; Figure 6 is a graph of output vs. distance for circular apertures ; Figure 7 is a graph of output vs. distance for rectangular apertures; Figure 8 is a graph of output vs. distance for different size apertures ; Figure 9 shows a simple block diagram of a typical control circuit; Figures 10 and I Oa are detailed sectional side views of a first linear position sensor ; Figure 11 is a detailed sectional top plan view of the first linear position sensor; Figure 12 is a detailed sectional end view of the first linear position sensor; Figure 13 is a partial sectional end view of the first linear position sensor. on an enlarged scale; Figure 14 is a detailed sectional side view of a second linear position sensor; Figures 15a and 15b are side and end views of a plunger assembly from the second linear position sensor: Figures 16a. 16b and 16c are end sectional views of the second linear position sensor, showing the plunger assembly in three positions; Figure 17 is a graph of output vs. distance for the second linear position sensor;

Figures 18a. 18b and 18c are end sectional views of a modified form of the second linear position sensor. showing the plunger assembly in three positions; Figure 19 is a graph of output vs. distance for the modified form of the second linear position sensor ; Figure 20 is a detailed sectional side view of a third linear position sensor; Figure 21 is a detailed sectional top plan view of the third linear position sensor; Figure 22 is a detailed sectional end view of the first linear position sensor on line XXII- XXII ; and Figure 23 is a detailed sectional end view of the first linear position sensor on line XXIII- XXIII.

As shown schematically in Figure 1, the sensor includes a slider 1 and two masks 2 and 3.

The first mask 2, the slider I and the second mask 3 are provided with apertures 4,5 and 6, which are aligned when the slider 1 is in the aligned position shown in Fig. 1. A light source 7 and a photo detector 8 are positioned adjacent the mask apertures 4,6. The masks 2 and 3 are arranged so as to restrict the path of the light travelling from the light source 7 to the photo detector 8. and reduce the angle of the light beam to approximately a parallel beam. The slider 1 is free to move relative to masks 2 and 3 and the amount of light passing from the light source 7 to the photo detector 8 depends on position of the slider aperture 5 relative to the mask apertures 4,6. The amount of light reaching the sensor 8 is a maximum when the slider aperture 5 is aligned with the mask apertures 4 and 6, and falls to a minimum when they are not aligned.

A return device 9 such as a spring or weight may be used to ensure that the slider is returned to a home position. A probe 10 may be attached to the slider to ensure appropriate contact with the object to be sensed. Stops 11 and 12 may be used to restrict the slider travel.

Figure 2 shows the same elements as Figure 1 but arranged as a rotary position sensor. The same reference numbers have been used throughout. with the addition of the marking'. In this embodiment, the slider 1'and the masks 2', 3'are curved and the slider is arranged for rotary movement relative to the masks.

Figure 3 shows the condition where the light output at the photo detector is at a maximum.

All three apertures 4, 5 and 6 are aligned. as shown in the view from the photo detector 8 shown in Figure 3a.

Figure 4 shows the condition where the slider 1 has been moved a small distance (less than the diameter of the slider aperture) relative to the masks 2 and 3. The displacement of the slider aperture 5 relative to the mask apertures 4. 6 results in the aperture of the light path from the light source 7 to the photo detector 8 being restricted, as shown in the view from the photo detector 8 shown in Figure 4a. The output of the photo detector 8 is therefore reduced relative to the output when the slider is in the position shown in Figure 1. The output of the photodetector thus provides an indication of the position of the slider 1 relative to the masks 2 and 3.

Adjustment of the output can be made by several means such as: a) varying the output of the light source, b) varying the distance from the light source 7 to the photo detector 8, c) moving one mask 2 or 3 relative to the other mask 2 or 3 to induce a misalignment, d) adding extra slides 1 or masks 2 and 3, which can be adjusted relative to the masks 2 and 3 to restrict the effective size of any of the apertures 4,5 or 6, e) varying the relative size or shape of any of the three apertures 4. 5 or 6, or f) including plain or graduated light filters in the light path.

Any combination of the above can be used to adjust the range of operation of the invention.

Figure 5 shows the condition where the slider I is moved downwards a larger distance (more than the diameter of the slider aperture) relative to the masks 2 and 3. The apertures 4,5 and 6 are now completely misaligned and the light path from the light source 7 to the photo detector 8 is blocked. The output from the photo detector 8 is therefore at a minimum. Similarly. if the slider is moved in the opposite direction (upwards) to a

misaligned position, the light path will be blocked and the output of the photo detector will be a minimum. The output from the photo detector 8 therefore has two minimums, one with the slider moved downwards as shown in Fig. 5 and the other with the slider 1 moved vertically in the opposite direction. This gives the sensor the ability to act as a"GO"and "NO GO"sensor. The two minimum conditions can be seen as a minimum and maximum tolerance condition and the condition shown in Figure 3 as the nominal tolerance condition.

Figure 6 shows a typical graph of displacement versus photo detector 8 output for the sensor when using circular apertures. Point 13 represents the aligned condition shown in Figure 3, point 22 represents the partially mis-aligned condition shown in Figure 4 and positions 23a and 23b represent the two misaligned conditions. one of which is shown in Figure 5. From this graph it can be seen that trigger points can be set from the photo detector 8 output to coincide with set positions such as warning limits or tolerance limits: thus the sensor can be used as a'GO''and"NO GO"gauge. Alternatively, the output of the photo detector 8 can be linked to a positional readout system or it can provide an output to a Statistical Process Control (SPC) system.

Using circular apertures gives a non-linear output as shown in Figure 6. Alternatively, using a rectangular or square aperture results in a linear output as shown in Figure 7.

Using different sizes of apertures results in a flattened output curve as shown in Figure 8.

Other shapes or combinations of shapes for the apertures can be used to give different effects on the output of the photo detector 8.

Figure 9 shows a simple block diagram of a typical control circuit for the sensor. The circuit includes a constant current source 14, a signal-conditioning unit 15 that generates an output signal 16 and an indication system that may include, for example, a comparator circuit 17. an analogue to digital converter (ADC) 18, or a digital readout 19. A computer 20 with a statistical process control program may be connected to the ADC 18 and/or an indicator 21 (e. g. a light emitting diode) may be connected to the comparator circuit 17.

The constant current source 14 is used to power the light source 7, and the output from the photodetector 8 is fed into the signal-conditioning unit 15. The signal-conditioning unit 15 converts the output from the photodetector 8 into an appropriate signal (e. g. voltage,

current, frequency etc.) and this signal 16 can then be used by an appropriate indication system such as: a. a comparator circuit 17 set to give an output to an indicator 21 when the object being sensed is within a positional tolerance, b. a digital readout 19 giving an indication of the position of the slide 1, or c. an analogue to digital converter 18, outputting to a computer 20 with a statistical process control program.

As shown in Fig. 6, the output of the photodetector 8 is a maximum when apertures are fully aligned and falls gradually with increasing mis-alignment to a minimum when the apertures are fully mis-aligned and no light reaches the photodetector. The photodetector 8 thus provides an analogue output voltage that depends on the relative positions of the apertures, rather than a simple on/off output. This analogue output can be used by the control circuit for various purposes, as described below.

When the device is to be used as a tolerance gauge to indicate, for example, whether the size of an object falls within a predetermined range, the tolerance range can be adjusted by selecting the output voltage at which cut-off takes place. If cut-off takes place at a high intermediate voltage 22 (just below the maximum voltage 13) the tolerance range will be narrow, whereas if cut-off takes place at a low intermediate voltage 22 (just above the minimum voltage 23a. b) the tolerance range will be wide.

Alternatively, instead of providing a simple GO/NO GO signal, the difference between the measured size and the desired size can be calculated from the output voltage and displayed, for example using the digital readout 19.

A further possibility is to use the analogue output in a statistical process control programme to monitor and correct consistent or progressive discrepancies in the size of manufactured articles. Alternatively, a logic output may be used in a control system such as a programmable logic controller (PLC).

A further embodiment of the position sensor will now be described in detail with reference to Figures 10-13. In this embodiment, the sensor has three operating states (off/on/off).

The sensor includes a body member 50, a shaft 52 with a biassing spring 54, a light emitting diode 56 and optical sensor 58, both of which are connected to a printed circuit board 60, a top plate 62 with a drive rivet 64, a plunger 66 with a biassing spring 68, and an aligning pin 70 (shown in Fig. 10a) The body member 50 is preferably made as an investment casting and is approximately 26mm long, 16mm wide and 14mm high. It includes a base 72 having four fixing holes 74 (two of which may be tapped for fixing screws and the other two being plain for bolts or dowels), and a central housing 76 that extends upwards from the base.

Two orthogonal bores extend through the base 72 parallel to the plane thereof, these being a longitudinal bore 78 having a main portion 78a with a diameter of approximately 4mm and a rear portion 78b with a diameter of approximately 2.5mm. and a transverse bore 80 having a diameter of approximately I mm, which intersects the longitudinal bore 78. The ends of the transverse bore 80 are drilled to a larger diameter, for reasons that will be explained later.

Two rectangular recesses 82 extend vertically downwards through the central housing 76 on either side of the longitudinal bore 78 and intersect the transverse bore 80. These recesses, which are designed to receive the LED 56 and the sensor 58, are linked together at their upper ends by a further recess 84 for receiving the printed circuit board 60.

A cylindrical bore 86 extends vertically through the front part of the housing 76 and intersects the longitudinal bore 78. This bore houses the plunger 66 and the spring 68.

The top plate 62 covers and seals the upper ends of the rectangular recesses 82 and the cylindrical bore 86, and is held in place by the drive rivet 64. A cable entry port 90 is provided in the plate 62 for making an electrical connection to the printed circuit board 60.

A guide sleeve 92 is provided on the front portion of the base 72. and includes a vertical bore 94 that intersects the longitudinal bore 78. The guide sleeve houses the aligning pin 70. and may include a rubber bung 96 or a screw for sealing the bore 94 when the pin 70 is removed.

The shaft 52 extends through, and has a sliding fit with, the longitudinal bore 78. The front portion of the shaft 52a has a diameter of approximately 4mm and the rear portion 52b has a diameter of approximately 2.5mm. Both ends of the shaft 52 extend beyond the base 72, the front end including a contact surface 98 for contacting an object to be measured, and the rear portion 52a being attached to a knob 100. The front end 98 may alternatively be provided with a male or female thread, for attaching a separate contact device.

A flat 102 is provided on the upper surface of the shaft 52, the ends of that flat forming stop surfaces 104 that are engaged by the plunger 66.

The compression spring 54 surrounds the rear portion 52a of the shaft and is compressed between the rear wall of the base and the annular face at the junction between the main and rear portions 52alb of the shaft. The spring biases the shaft 52 forwards and the shaft may be pulled backwards against this bias by grasping the knob 100.

A transverse bore 106 having a diameter of approximately I mm extends through the front portion 52a of the shaft. When this bore 106 is aligned with the transverse bore 80 in the body member 50, it defines an optical pathway between the LED 56 and the sensor 58.

A vertical bore 108 is provided in the front portion 52a of the shaft, which may be engaged by the aligning pin 70 to set the shaft with the bores 106 and 80 aligned. The aligning pin is also used during the manufacturing process to hold the shaft in position while the bores 108 and 80 are drilled in a single operation through the body member 50 and the shaft 52.

The plunger 66 comprises a cylindrical rod that is mounted for sliding movement in the vertical bore 86. The plunger 66 is biassed resiliently downwards by the spring 68, which is captured in the space between the upper end of the rod and top plate 62. The plunger engages the flat 102 to ensure that the bores 106,80 are always parallel, and acts with the stop surfaces 104 to limit movement of the shaft.

The LED 56 and the optical sensor 58 are both sidewavs looking devices. each of which includes a hemispherical lens 110 that is engaged in the enlarged end portions of the transverse bore 80, to maintain them in alignment with the bore. The LED 56 and the sensor 58 also each include a rectangular backing plate 112 that fits into one of the rectangular recesses 82. The backing plates are connected to the printed circuit board 60

and are encapsulated in a potting compound 114. Two holding plungers 116 are inserted into the enlarged ends of transverse bore 80 during manufacture, to hold the LED 56 and the sensor 58 in position while the potting compound 114 cures.

Operation of the sensor is substantially as described above. When the bores 80,106 are aligned, the output of the optical sensor 58 is a maximum, and when those bores are misaligned. the output falls as shown in Fig. 6. For calibration purposes, the shaft 52 may be retained in the aligned position using the aligning pin 70. While in this position, the light output of the LED 56 may be adjusted to ensure that the photodetector 58 is not swamped.

To permit insertion of an object whose position or size is to be sensed. the shaft 52 may first be retracted using the knob 100. When the knob is released, the spring 54 urges the shaft 52 against the object with a consistent force. ensuring uniform measurement. The output voltage of the photodetector 58 is connected to an indication system, for example as described above. This system provides an indication, based on the position of the shaft 52, of the size or position of the sensed object. For example, the system may indicate whether the size of the object meets a predetermined tolerance condition.

A further embodiment of the invention, for use as a simple two-state (on/off) sensor, for example as a replacement for a conventional microswitch, will now be described with reference to Figs. 14 and 15.

The sensor includes a housing 120, a guide plate assembly 122, a plunger assembly 124, a biassing spring 125, an LED 126, and an optical sensor 128.

The housing 120 may be manufactured as a investment casting and is matched to the size and shape of a conventional microswitch. It has the shape of a flat, rectangular box that is open on one side 130, and is provided with two fixing holes 132 and a seat 134 for the spring 125.

The guide plate assembly 122 fits into the open side 130 of the housing 120 and includes two parallel bores 136,138 for guiding movement of the plunger assembly 124. A transverse bore 140 extends through the guide plate assembly perpendicular to the first 136

of the two parallel bores, which it intersects. The ends of the transverse bore 140 are countersunk to receive the lenses of the LED 126 and the optical sensor 128.

The plunger assembly 124 includes a cylindrical plunger 142 having a transverse bore 144, and a guide pin 146 that extends parallel to the plunger 142. The plunger 142 and the guide pin 146 are attached at their lower ends to a rivet plate 148.

The plunger 142 and the guide pin 146 have a sliding fit in the two parallel bores 136,138 in the guide plate assembly 122. and the spring 125 engages the underside of the rivet plate 148 to bias the plunger assembly upwards to a position in which the plunger42 extends beyond the housing 120, by a distance of about 2.8mm.

The guide pin 146 prevents rotation of the plunger 142. so that the transverse bore 144 in the plunger 142 is maintained in alignment with the transverse bore 140 in the guide plate assembly 122.

The LED 126 and the optical sensor 128 are both sideways-looking devices and are mounted on either side of the guide plate assembly 122 with their lenses aligned with the bore 140 and engaged in the countersunk ends thereof. The LED 126 and the optical sensorl28 are connected to a printed circuit board (not shown).

There are two versions of this device. both of which are mechanically as described above.

In the first version, shown in Figs. 16a-16c, the bore 144 is positioned such that when the plunger is in its rest position as shown in Fig. 16a, the plunger bore 144 is fully aligned with the bore 140 in the guide plate assembly. In this version of the sensor. the device operates as an adjustable trigger microswitch.

In the second version. which is show in Figs. 18a-18c, the plunger bore 144 is positioned slightly closer to the free end of the plunger 142, such that when the plunger is in its rest position, the bore 144 is not fully aligned with the bore 140 in the guide plate assembly 122. The plunger must then be depressed slightly to the position shown in Fig. 18b before the bore 144 becomes fully aligned. In this version of the sensor, the device operates as a tolerance gauge microswitch.

Operation of the first version is as follows: when the plunger 142 is at rest as shown in Fig.

16a the bores 140,144 are fully aligned and the light reaching the optical sensor 128 is a maximum. The voltage output of the sensor is also therefore a maximum, as shown graphically in Fig. 17. As the plunger is depressed, the light reaching the optical sensor 128 and the voltage output of the sensor both fall, reaching a minimum when the plunger has been depressed approximately 0.8mm to the fully mis-aligned position shown in Fig. 16c.

The cut-off point can be selected electronically by adjusting the sensor output voltage at which cut-off is deemed to have taken place.

Operation of the second version differs from that of the first version in that when the plunger 142 is at rest as shown in Fig. 18a, the bores 140, 144 are not aligned and the light reaching the optical sensor 128 is a minimum. The voltage output of the sensor is also therefore a minimum. As the plunger is depressed, the bores 140,144 first move into alignment as shown in Fig. 18b and the output of the optical sensor 128 reaches a maximum. Then, as the plunger is depressed further, the bores move out of alignment and the output of the sensor falls again. This is shown graphically in Fig. 19. Maximum output is reached when the plunger has been depressed approximately 0.8mm, and the output reaches the second minimum at a depression of about 1. 3mm. Again, the cut-off point can be selected electronically by adjusting the sensor output voltage at which cut-off is deemed to have taken place.

Various modifications of the device are possible, so examples of which are described below.

Instead of the knob 100, an actuator, for example a solenoid or a pneumatic actuator, can be connected to the rear end 52b of the shaft, to effect automatic retraction of the shaft.

The return spring 54 may be omitted so that the shaft 52 is free-floating for direct connection to other equipment.

A bracket, pivot and lever may be provided for adjustable trigger applications (similar to variations of standard micro-switches).

A second aperture and an additional pair of optical devices may be added, to sense whether the travel has exceeded the nominal position when used as a measuring device. The second aperture may be offset from the first aperture by about 50% of the aperture diameter.

Various applications of the sensor are envisaged, some of which are discussed below.

The main application is as an inspection device with three types of output (or a combination of these): a) a simple LED output arrangement to indicate either pass (a GO gauge) or pass/fail (a GO/NO-GO gauge); b) a logic output suitable for connection to a control system such as a PLC (programmable logic controller) or an industrial computer etc., or c) an analogue voltage output that is interpreted by a signal conditioner/control device. the output could be linked to a statistical process control system (SPC).

The device can also be used as a short-range measuring device. This may require a second set of opto devices. Applications for this mode could include: a) adaptive feedback control in machining centres; b) positional sensing in automatic assembly equipment; and c) control feedback.

The device can also be used as a fine adjustment trigger switch. replacing the use of micrO- switches (or opto-switches) and mechanical adjustment devices with a single device. The trigger position would be set electronically rather than mechanically. Such applications include: a) adjustable tool safety switches in mould and press tools, b) automatic assembly equipment, and c) consumer equipment. such as VCRs.

A further embodiment of the position sensor will now be described in detail with reference to Figures 20-23. This embodiment is similar in certain respects to the embodiment shown in Figures 10-13, and is designed to be the same size and have the same mounting arrangement as a dial test indicator.

The sensor includes a body member 250, a bush 251. a shaft 252 with a biassing spring 254, a light emitting diode 256 and an optical sensor 258, both of which are connected to a printed circuit board 260, a top plate 262 secured by a drive rivet 264, a cross block 266 seated on a high density sponge 268 that serves as a biassing element, and an aligning pin 270. The sponge 268 may alternatively be replaced by some other resilient member, for example a leaf spring.

The body member 250 is preferably made as an aluminium casting and is approximately 35mm long, 35mm wide and 20mm high. It includes a base 272 having four fixing holes 274 and two sets of internal support walls 276,277 that extend upwards from the base. The housing may accommodate a battery (not shown) for electrical power and/or may include electrical connections for an external power source.

The bush 251, which is made as a brass casting, is mounted within the body member 250 between the first pair of support walls 276. It includes a tubular sleeve 278 having an outside diameter of 8mm that extends beyond the housing 250. Two orthogonal bores extend through the bush 251, these being a longitudinal bore 279 with a diameter of approximately 4mm that extends along the axis of the sleeve 278, and a transverse bore 280 having a diameter of approximately I mm, which intersects the longitudinal bore 279. The ends of the transverse bore 280 are countersunk, to accommodate the LED 256 and the optical sensor 258.

Two rectangular recesses 282 are provided in the support walls 276, to receive the LED 256 and the sensor 258.

A cylindrical bore 283 extends vertically through the front part of the bush 251 and intersects the longitudinal bore 279. This bore accommodates the aligning pin 270. A plug (not shown) may be provided for sealing the bore against dust when the pin is removed.

The top plate 262 covers and seals the upper end of the housing 250 and is held in place by the drive rivet 264. A cable entry port (not shown) is provided in the plate 262 for making electrical connections to the printed circuit board 260.

The shaft 252 extends through, and has a sliding fit with, the longitudinal bore 279. The front portion of the shaft 252a is provided with a female thread 284, for attaching a contact device (not shown). The rear portion of the shaft 252b is attached to a knob 286.

A flat 288 is provided on the lower surface of the shaft 252, the ends of the flat forming stop surfaces 290. The flat 288 is engaged by the cross block 266, which is biassed against the flat by the high density sponge 268. This prevents rotation of the shaft 252 about its longitudinal axis and limits travel of the shaft in the direction of that axis.

The compression spring 254 surrounds the rear portion 252b of the shaft and is compressed between the rear wall of the housing and a circlip 292 mounted on the shaft. The spring biases the shaft 252 forwards and the shaft may be pulled backwards against this bias by grasping the knob 286.

A transverse bore 294 having a diameter of approximately 1 mm extends through the shaft 252. When this bore 294 is aligned with the transverse bore 280 in the bush 251 it defines an optical pathway between the LED 256 and the sensor 258.

A vertical bore 296 is provided in the front portion 252a of the shaft, which may be engaged by the aligning pin 270 to set the shaft with the bores 294 and 280 aligned. The aligning pin is also used during the manufacturing process to hold the shaft in position while the bores 294 and 280 are drilled in a single operation through the bush 251 and the shaft 252.

The LED 256 and the optical sensor 258 are both sideways looking devices, each of which includes a hemispherical lens that is engaged in the enlarged end portion of the transverse bore 280, to maintain the device in alignment with the bore. The LED 256 and the sensor 258 also each include a rectangular backing plate that fits into one of the rectangular recesses 282. The backing plates are connected to the printed circuit board 260.

Operation of the sensor is substantially the same as that of the sensor shown in Figs. 10 to 13, and will not be described in detail. When the bores 280,294 are aligned, the output of the optical sensor 258 is a maximum, and when those bores are misaligned, the output falls as shown in Fig. 6. For calibration purposes, the shaft 252 may be retained in the aligned position using the aligning pin 270.

An output signal may be sent to a external indicator device or, alternatively, a signal conditioning circuit may be provided within the housing and indicators such as LEDs (not shown) may be provided on the housing 250 or the top plate 262. Batteries for powering the device may also be mounted within the housing, allowing the sensor to be entirely self- contained.

The device may be mounted either by clamping on the sleeve 278 or by using screws to engage the threaded holes 274 in the housing 250.