GB2046381A - Testing pneumatic brakes - Google Patents

Abstract

When testing the efficiency of a pneumatic braking system (e.g. for a power guillotine) it is sometimes desired to measure the time which elapses between the venting of the pneumatic system and the instant when the machine comes to a halt. In order to make this measurement it is necessary to detect the exact instant when the pneumatic system is vented; this instant not normally being the same as the instant when the brake actuating sequence is commenced. This is because there may be a delay between the time when, for example, an emergency stop button is pressed and the time when this results in a pneumatic venting valve being opened. In accordance with the invention the instant when the pneumatic system is operated is detected by a microphone 80 connected to a sensing circuit 81, 82 so as to detect the noise produced by the venting of the pneumatic system 7, 8, 9, 10. <IMAGE>

Description

SPECIFICATION Improvements in or relating to the measurement of the efficiency of pneumatic brakes This invention relates to the measurement or testing of the efficiency of pneumatic brakes. The invention arose in the design of a system for testing the pneumatic brakes of an industrial machine but it would also be applicable to any pneumatic braking system.

Various ways of testing the time taken for a machine to come to a halt (or to reach a low velocity which is considered safe) have been proposed in the past. However, difficulty has arisen in establishing, if the test proves that the braking efficiency is satisfactory, where the fault lies. This can be a very difficult problem since the fault would not normally result in the complete failure of the brakes to operate and the fault may, therefore, be difficult to isolate. The problem is made even more difficult by the fact that the fault may not be a fault of operation of the braking system. It may be a design fault somewhere in the system.

With the aforementioned problem in mind the invention provides a system which enables one to measure the time which elapses between actual operation of the pneumatic system and the time when the machine comes to a halt (or reaches an acceptably slow speed). It can thus be established whether a fault lies in the brakes themselves or in the system which initiates operation of the brakes.

In order to establish the instant when the pneumatic system is operated in the brakes of a machine the invention provides a microphone connected to a sensing circuit so as to detect the noise produced by the venting of the pneumatic system.

It will be appreciated that at the time when the brakes are applied the venting of the pneumatic system produces a very loud noise. The invention arose from a realisation that noise provided an accurate indication of the instant when the brakes are operated.

The venting of the brake system normally produces what is known as "white noise", i.e. the noise contains a very wide spectrum of frequencies. The sensing system connected to the microphone is, therefore, preferably provided with a frequency discriminator especially design to remove those frequencies which are likely to arise from interferring effects in the precincts where the machine is likely to be used. This is to avoid false actuation of the system.

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; Figure2 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 tQ 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 effeciency 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 focusse 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 moving 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, partially transmissive 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 similar to 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 may for 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 initiaton 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 mid-stroke but differs from machine to machine.

In the system shown in Figures 1 and 4the 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 (Figure 1) which includes an opto-switch 35 (Figure 4) and a solenoid driving circuit 36 (also Figure 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 into the 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 opto-switch 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 effeciency ofthe 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 Figures 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 bore 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 interrupter 37 as described with reference to Figure 4.

Afeature 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 sequence at the instant that 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 consideration. 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 at the 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 the machine to stop after the brakes are actually applied. Alternatively it may be wished to measured 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 to 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 move 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 (3)

1. Apparatus for testing the effectiveness of pneumatic braking apparatus of the type in which brakes are applied in response to the venting of a pneumatic system, the apparatus comprising means for commencing the testing operation in response to sound produced by the venting.

2. Apparatus according to claim 1 in which the means for commencing the testing operation includes a microphone and a sensing circuit which receives the output of the microphone and rejects specific frequency components of that output.

3. Apparatus according to claim 1 and substantially as described with reference to Figure 2 of the accompanying drawings.