GB2046518A - Electric switch for use in time measurement - Google Patents

Abstract

An electric switch for use with a device for measuring the stopping time or the stopping distance of an industrial machine initiates the measurement when the separate switch contacts of a stop button are operated. The switch comprises a housing 55, 66 having at least one adjustable abutment 57 enabling the housing 55, 66 to be held firmly at an adjustable distance from the stop button (not shown). A plunger 63, 65 slides in the housing 55, 66 and can be struck by a person testing the machine to depress the stop button whereby a permanent magnet 71 operates a magnetic read contact unit 72 arranged to produce a timing signal which is substantially simultaneous with the operation of the stop button. <IMAGE>

Description

SPECIFICATION The measurement of the efficiency of the brakes, particularly of industrial machines This invention relates to the measurement of the efficiency of brakes of industrial machines, e.g. by measuring the time that the machine takes to come to a halt or to an approximate halt; or by measuring the distance through which a moving part of the machine moves after actuation of the brakes before it comes to a halt (or an approximate halt).

It will be appreciated that, in orderto measure the braking efficiency, it is necessary to provide some means for producing a signal atthe instant when the braking sequence (the efficiency of which is to be tested) is initiated.

The braking sequence can be considered to begin when an operator first touches the emergency stop button or other actuator of the machine. Alternatively it can be considered to begin at the moment when the contacts of the emergency stop button are operated. The present invention relates particularly to the latter case and aims to provide a suitable timing signal which defines accurately the instant when the contacts of an emergency switch are operated.

Considerable difficulty has been experienced in designing a suitable device for producing this timing signal, which device can be used in conjunction with any particular type of emergency stop button which might be encountered; it being appreciated that there are a multitude of different shapes and sizes of such buttons which may be mounted in various different parts of machines.

It should also be explained that it would be impracticable to use the contacts of the emergency button to provide the timing signal used for testing purposes (a) because this would involve partial dismantling of the emergency stop button and (b) because taking the timing signal from the contacts of the stop button/switch may interfere with the functioning thereof.

This invention provides a device for producing a timing signal indicative of the instant of actuation of the contacts of a switch having a depressable actuating member, the device comprising a housing, at least one adjustable abutment member whereby the housing can be held firmly at an adjustable distance from the actuating member a plunger mounted moveably by the housing so that it can be struck by an operatorto depress the actuating member, and means for producing an electric signal as a result of movement of the plunger.

In operation the device is held against the emergency stop button of any machine and the adjustable abutment member or members is/are adjusted so that the contacts of the emergency stop button/switch are just not operated. The operator then strikes the plunger which simultaneously operates the contacts of the stop button and produces the aforementioned electric signal which can be used for testing the efficiency with which the brakes of the machines work.

In a preferred form of the invention three or more of said adjustable abutment members are provided and these are mounted on the housing in such a way that they can be adjusted both in the direction of movement of the plunger and in a sense perpendicu larto that direction. This enables the abutment members to be adjusted so that they can be placed against any suitable surface adjacent the stop button of the machine whilst also enabling them to be adjusted so that the plunger just does not depress the button sufficiently to operate the contacts before the plunger is struck by the operator.

The "means for producing an electric signal as a result of movement of the plunger" is preferably a simple switch which can, for example, be a reed switch operated by a magnet on the plunger or can be an opto-switch which is operated when the plunger moves.

Further features of the invention will appear from the following description and the accompanying drawings of a system embodying the present invention. In the drawings: Figure 1 shows a guillotine whose braking efficiency is under test; Figure 2 illustrates the braking mechanism of the machine illustrated in Figure 1; Figure 3 illustrates the optical system incorporated in the optical apparatus indicated at 13 in Figure 1; Figure 4 is a block diagram of the electrical circuitry employed in the system illustrated in Figure 1 and also shows the mechanical arrangement of a device for interrupting a protective light curtain in front of the moving blade of the machine; Figure 5 is a cross-section through a mechanism for initiating operation of the brakes by pressing an emergency stop button on the machine of Figure 1; and Figure 6 illustrates a touch-switch also for cooperation with the emergency stop button of Figure 1.

Referring firstly to Figure 1, there is shown a guillotine 1 having a moving blade 2 with a leading edge 3. It will be understood that the invention is not only applicable to guillotines. It is also applicable to pressing tools and any other machines where there are dangerous moving parts.

In front of the blade 2 of the guillotine is a beam of light or other radiation (e.g. infra-red) projected by a projector 4 to a photo-sensitive receiver 5. When the light beam is interrupted during movement of the blade 2 a stop signal is generated by the receiver 5 which applies the brakes of the machine. The brakes can also be applied by pressing a stop button 6. This generates a similar stop signal.

In alternative machines, stop signals may be generated by other means; for example by the release of a so called dead man's handle or by the application of pressure to a pressure sensitive mat on the floor.

Another possibility is to provide two plates in place of the units 4 and 5 and to detect changes in the capacitance between these plates, such changes in the capacitance indicating that the operator's hand may be dangerously positioned close to the moving part.

The way in which the brakes are applied is illustrated very schematically in Figure 2. Air is supplied along a line 7 to an accumulator 8 and thence through a venting valve 9 to pneumatic cylinders 10.

The latter act on brake caliper arms 11 to hold the arms open against the action of brake application springs 12. On receipt of a stop signal, e.g. from button 6 or receiver 5, the valve 9 vents the air in the accumulator 8 to the atmosphere resulting in the application of the brakes. An object of the apparatus to be described is to measure the efficiency of the braking system and of the systems (e.g. 4 and 5) which lead to the actuation of the braking system.

In order to measure the braking efficiency it is necessary to produce two timing signals; the first of which initiates operation of the braking sequence; and the second of which defines the time when the member 2 comes to a halt or reaches a velocity sufficiently low to preclude risk of accidents.

Reverting now to Figure 1,there is attached to the blade 2 of the machine a sheet 10 of rectroreflective material carrying a series of curved bars 1 OA as described in our co-pending Patent Applications 16322/77 and 3961/78. The bars are of rectroreflective material on a relatively non-reflective, e.g. matt black, background. Alternatively, the bars could be matt black on a rectroreflective background.

The sheet 1 is viewed by an optical device 13 which projects a beam of light onto the sheet 10 and receives light reflected from the sheet 10. The device 13 is mounted on a tripod 14 which is adjustable in height above the factory floor; and has a handle 15 which enables the device 13 to be set at any desired angle. This enables the lens system to be aligned with the sheet 10.

The device 13 is shown in more detail in Figure 3.

The light source 16 generates light which passes through a lens system 17 and then through an aperture stop 18. The latter has a rectangular aperture with castellated edges for a purpose which will become apparent later. After passing through the stop 18 the light is reflected by a partially reflective and partially transmissive mirror 19 and passes through a zoom lens system 20 which focusses an image of the aperture stop 18 on the sheet 10. The sheet 10 has two vertical lines 19 on it and, after adjusting the focus by means of the zoom lens 20, the attitude of the device 13 is adjusted, using the handle 15, so that the castellations are aligned with the lines 19. The apparatus is then correctly set for observing movement of the bars of the sheet 10 and thus the movement of the blade 2.

Light reflected from the bars on the sheet 10 is focused by the zoom lens 20, onto photo-sensors 21 and 22 after being split by a partially reflective, par tiallytransmissive mirror 23. The photo-sensors give identical electric signals which vary cyclically in amplitude as the blade 2 moves and as the images of the bars 10A (not shown in Figure 3) traverse the photo-sensors 21 and 22.

Referring now to Figure 4, the two photo-sensors 21 and 22 are used to produce a signal which indicates when the blade 2 has almost halted. The outputs of sensors 21 and 22 are,amplified at 24 and 25 respectively, the amplifier 25 being an inverting amplifier. The outputs of circuits 24 and 25 are then added at 26 to give a signal which is similarto that at the output of 21 but less subject to variations caused by differences in the reflectivity of different parts of the sheet 10 The output of the adding circuit 26 is passed to a high pass filter 27 designed to remove frequencies of less than about ten hertz. The purpose of this is to remove signals derived from very low velocities of the blade 2, e.g. due to vibrations as it comes to a halt.

The output of the adder 26 is also passed to an indicator 28 which mayfor example be a moving coil voltmeter. It serves to indicate to the operator that a signal is being received and helps him to ensure that the apparatus 13 is correctly aligned.

The output of the filter 27 is fed to a threshold detector 29 which produces an output pulse every time its input traverses positive and negative threshold values of about 200 millivolts.

The pulses from the detector 29 are received by a timing circuit 30 and this produces a logic signal on line 31 which indicates whether or not a pulse from circuit 29 has been received during the preceding 50 milliseconds. This logic signal thus provides an effective indication of when the speed of the member 2 has been reduced to a value very close to zero.

The system for producing a signal which initiates the actuation of the braking sequence will now be described. The initiation of the sequence is, for testing purposes, preferably at a time when the blade 2 or the equivalent moving member reaches its maximum velocity. This typically occurs during midstroke but differs from machine to machine.

In the system shown in Figures 1 and 4 the brake actuation signal is derived by placing a special lug 32 on the blade 2 where it is held in position by a permanent magnet 33 to which the lug as attached.

The lug 32 operates with a signal generating device 34 (Fig. 1) which includes an opto-switch 35 (Fig. 4) and a solenoid driving circuit 36 (also Fig. 4).

The solenoid driving circuit 36 produces a 150 millisecond pulse commencing at the time when the lug 32 interrupts the beam of light or other radiation produced in the opto-switch 35. It should be explained here that an opto-switch is a miniature device which has, in one encapsulated unit, a source of light or other radiation such as infra-red. This radiation is passed across a gap to a receiver. Entry of a projection intothe gap breaks the beam and the receiver consequently gives out a signal which can be used for any desired purpose.

The 150 millisecond pulse from the solenoid drive circuit 36 is passed to a curtain interrupting device 37 which is also shown in Figure 1. Like the device 34 it is mounted adjustably on a stand which is held magnetically to the bed 38 of the machine. The curtain interrupting device 37 will now be described in detail with reference to Figure 4.

A solenoid 38 has a bore 39 containing an iron slug 40 which is drawn into the solenoid when the latter is actuated by the pulse from the solenoid drive circuit 36. The slug 40 is connected to a bracket 41 which in turn is connected to the horizontal arm of an L shaped sheet 42. The horizontal arm has on its upper surface a notch 43 and extends through the gap of an opto-switch 44. The light beam of the opto-switch 44 is arranged so that, when the solenoid is not actuated, it, i.e. the light beam, passes through the notch 43. However, as soon as actuation of the solenoid occurs, the light beam of the switch 44 is interrupted and a signal is passed along the line 45. The optoswitch 44 is mounted adjustably on a bracket 46 fixed to the solenoid.By suitably adjusting the mounting of the opto-switch on this bracket it is possible to ensure that the signal appears on the line 45 at virtually the same instant as the solenoid is actuated. The solenoid is mounted in a casing 46 from which the vertical part 47 of the arm 42 projects.

This part 47 constitutes a flag which is used to interrupt the light beam projected between the devices 4 and 5 shown on Figure 1. In order to set up the apparatus the curtain interrupter 37 is placed approximately in the correct position on the machine bed 38 and the knurled knob 48 is then turned. This adjusts the position of the solenoid relative to the housing 46. The flag 47 is held by a spring 48 and an abutment (not shown) relative to the housing 46 and so the adjustment of the solenoid also means that the position of the flag 47 is adjusted. This adjustment is continued until the flag 47 is in a position where it just does not interrupt the light beam between devices 4 and 5 sufficiently to cause stopping of the machine.

From the above it will be appreciated that the moment that a signal is generated by the solenoid drive circuit 36, the flag 47 interrupts the light beam between devices 4 and 5 thereby initiating the stopping sequence of the machine. At the same instant the opto-switch 44 generates a signal which is fed along line 45 to a gate 50. This opens the gate thereby allowing pulses, occurring at 1 kilohertz, to be fed from an oscillator 51, through a 50 millisecond delay circuit 52 (to compensate for the delay introduced by circuit 30) to a counter 53. The counter thus starts accumulating a count which is terminated when the signal appears on the line 31 indicating that the blade 2 has come to a halt. The count accumulated at 53 thus indicates the time taken for the blade 2 to stop.This count is displayed on a digital display device 54 which is also shown on Figure 1.

If it is desired to measure the distance through which the blade 2 travels before coming to a halt (instead of the time it takes to come to a halt) it would be possible to replace the oscillator 51 with a connection as shown in broken lines. This will mean that the counter 53 counts the number of signals produced by the photo-sensors 21 and 22 and this, of course, represents the distance, as measured by the bars 19, through which the blade 2 has moved.

Another possible modification would be to produce the stop signal, which is fed to the counter 53, by a system as described in our Application 3961/78 where the sheet 10 is marked with a series of triangular or similar shapes. The images of these are projected onto a further photo-sensor through two slits and the output from this further photo-sensor characterises the direction of movement of the blade 2 and the instant when it comes to a halt. A suitable halt signal can thus be derived for application to the counter 53.

In the system described so far, the initiation of the braking sequence has been carried out by interrupting the light beam between the devices 4 and 5. It may, however, on some occasions be desired to test the braking efficiency of the machine when braking is effected by some other means, for example by pressing the button 6. When it is desired to do this the apparatus shown in Figure 5 or Figure 6 can be used.

Referring firstly to Figure 5, the apparatus comprises a plate 55 having, spaced around its periphery, three bores 56 of which only two are shown in the drawing. These bores 56 receive legs 57 which can be locked in the bores 56 by locking screws connected to knurled knobs 58. The legs 57 cannot only slide within the bores 56 but can also rotate so that the operator can adjust the spacing between the ends of the legs 57. This is because of the bends 59 in the latter.

The plate 55 is connected to a cylindrical housing 60 having an end panel 61 into which is set a threaded bush 62. The bush 62 supports a shaft 63 which also passes through an aperture 64 in the plate 55. Fixed to one end of the shaft 63 is a pad 64 and fixed to the other end is a push member 65. The shaft 63 also carries a disc 66 which is fixed at a given position along the shaft 63 by two spring washers 67 and 68. The disc 66 is guided by two rods 69 and 70 which pass through holes in the disc 66.

The purpose of the disc 66 is to carry a permanent magnet 71 which co-operates with a reed switch 72 mounted on the outside of the casing 60. The components 63,64,65,66,67 and 71 are held in the positions illustrated by a spring 73. In this position the magnet 71 is just not sufficiently close to the switch 72 to close the contacts of the latter.

To use the device shown in Figure 5 the legs 57 are adjusted by means of the knobs 58 so that when the ends of the legs 57 are pressed against the surrounds of the emergency stop button 6 (Figure 1) the pad 64 engages the button 6 but does not quite depress it sufficiently to actuate the brakes. In order to commence the braking sequence the push member 65 is struck manually and this instantaneously depresses the stop button 6 and closes the contacts of the switch 72. The resulting signal from the switch 72 is passed along line 45 to the gate 50 in exactly the same way as was the signal from the curtain interrupter37 as described with reference to Figure 4.

A feature of the device shown in Figure 5 is that it is readily adaptable for co-operation with virtually any type or size of stop button in current use.

The apparatus shown in Figure 5 is designed to commence the timing sequenceatthe instantthat the contacts of the emergency stop button 6 are closed. The possible time period between the instant when the operator first touches the button 6 and the instant when the contacts close is not taken into con sideration. The apparatus shown in Figure 6 is for use in circumstances where it is desired to take this into consideration. Referring to part A of Figure 6, the illustrated device is a touch switch. It has two contacts 74 and 75 which take the form of interdigitated electrodes as illustrated. These electrodes are formed by a thin layer of metal on an insulating sheet 76. They can be formed by etching or by vapour deposition or by any other suitable method.The electrode 75 is connected to a conductive sheet 77 on the reverse side of the insulator 76. This sheet or layer is earthed and has been found to improve the reliability of the touch switch in that it is, because of the conductor 77, less subject to false actuations.

When an operator touches the surface constituted by electrodes 74 and 75, the potential of the electrode 74 is immediately reduced to earth potential, or a potential close to earth. This reduction in potential is used as a signal which, like the signal derived from the switch 72, Figure 5, and like the signal derived from the switch 44 of Figure 4, is passed along the line 45 to the start gate 50.

In order to give even further improved reliability the device used in Figure 6B is used. This device includes a metal cup 78 which fits like a thimble over the index finger of the operator. The closed end of the cup 78 is attached to a pad 79 of foam conductive rubber. This is a composition containing rubber and conductive particles, usually of carbon. By using the device shown in Figure 6B, when the operator touches the touch switch of Figure 6 with the pad 79 a more reliable signal is generated on the line 45.

It will be appreciated that the start signal applied to the gate 50, in the case of the apparatus shown in Figure 6, will be atthe instant that the operator first makes contact with the emergency button 6. This is at an earlier time than the time of the corresponding signal generated by the apparatus as shown in Figure 5.

Another possible measurement which one may wish to make when testing the braking of industrial machines is the time taken for a machine to stop after the brakes are actually applied. Alternatively it may be wished to measure the distance moved during this time. In the case of pneumatically operated brakes such as have been described with reference to Figure 2, this measurement can be made by using a microphone as shown at 80 on Figure 2. When the brakes are applied by venting the air to atmosphere through the vent valve 9 a very loud noise is generated. This noise contains a very wide spectrum of frequencies and is detected by microphone 80. The resulting signal is filtered by a circuit 81 which is designed to remove frequencies associated with noises commonly occurring in industrial premises.

The output of the circuit 81 can thus be assumed to be derived from the venting of the pneumatic system of the machine. This output is fed to a pulse shaping circuit 82 to produce a signal on line 45 which signal is passed to the start gate 50 as has been described before.

Claims (6)

1. A portable device for producing a timing signal identifying an instant of operation of a depressable actuating member of a machine, the device comprising: a support structure; a plunger carried on the sup port structure and movable relative thereto between withdrawn and depressed positions; adjustable abutment means allowing the support structure to be held firmly relative to the machine at a spacing therefrom which can be adjusted so that the plunger partly depresses the actuating member when in its withdrawn position, the plunger being arranged to further depress the actuating member when moved towards its depressed position; and means co- - operating with the plunger to produce the said timing signal in response thereto.

2. A device according to claim 1 in which the abutment means comprises three or more abutment members, each mounted on the support structure in such a way that it can be adjusted relative thereto both in the direction of movement of the plunger and in a sense perpendicular to that direction.

3. A device according to claim 1 or 2 in which the said means co-operating with the plunger is a switch arranged to be operated when the plunger moves.

4. A device according to any preceding claim in which the plunger is urged by a spring towards its withdrawn position.

5. A device according to any preceding claim in which the support structure comprises a housing in which the plunger is adapted to move.

6. A device according to claim 1 and substantially as described with reference to Fig. 5 of the accompanying drawings.