GB1602178A

GB1602178A - Apparatus for testing the braking of a moving machine part

Info

Publication number: GB1602178A
Application number: GB1632277A
Authority: GB
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1977-04-20
Filing date: 1977-04-20
Publication date: 1981-11-11

Description

(54) APPARATUS FOR TESTING THE BRAKING OF A MOVING MACHINE PART (71) We, THE MARCONI COMPANY LIMITED, a British Company, of Marconi House, New Street, Chelmsford, Essex, CMI lPL, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to apparatus for testing the braking of a moving machine part.

Such apparatus are known in which light reflected by a moving surface is received by a sensor, which provides an electrical output signal having a frequency component which is related to the velocity of the moving surface.

One aspect of the present invention is concerned with the obtaining of measures relating to the braking effect of a moving member such as, for example, part of an industrial machine fitted with emergency braking.

It is frequently required to obtain a measure of the time which elapses between the shutting off of, or the application of an emergency brake of a machine in order to check the potential hazard of that machine in an emergency brake of a machine in order to check the potential hazard of that machine in an emergency situatinn. The measure may be required to be expressed in time as such or in distance moved by a moving part of the machine, be it linear or rotational, during the braking time. Often physical contact with the moving member is difficult or inconvenient.

One object of the present invention is to provide an improved apparatus enabling such measurements to be taken.

According to the invention there is provided apparatus for testing the braking of a moving machine part including: a patterned member attached or adapted for attachment to the machine part and an optical detector arranged so that there is relative movement between the detector and the patterned member when the machine part is moving with the pattemed member attached thereto, the detector including a light sensitive member and means including a zoom lens for focussing the pattern onto the light sensitive member so that, as the machine part moves, the light sensitive member is traversed by an image of different parts of the pattern, thereby producing a signal which represents the movement of the machine part and from which signal an indication of the braking effect can be obtained.

The pattem on the patterned member is preferably a series of lines or bars. They are advantageously concave on their leading sides having regard to the direction of relative movement between said surface and the detector. The patterned member may take the form of a sheet adapted to be attached to said moving part, for example, magnetically.

Preferably said pattern comprises, side by side, two sets of curved bars, one being the mirror image of the other. The two sets of bars may be in contact along a median line and each set is preferably inclined with respect to the direction of movement of said pattern relative to the detector whereby along the median line between said two sets of curved bars, the end of each bar is further advanced in said direction of travel than the remainder of the said bar.

Normally the widths of said bars and the widths of the spacings between bars are all equal and the radius of curvature of each bar is the same.

Preferably the relationship between the width W of one set of curved bars, the angle 0 of inclination of each bar relative to a line at right angles to said median line and the radius R of curvature of each curved bar has a relationship sin 0 = RW and W is chosen to be approximately half of the width of the field of view of the sensor of said optical detector at said pattern.

Where said patterned member comprises a sheet which is adapted to be attached to said moving part, said sheet may comprise a matt black coated copper foil upon which said lines or bars are formed by reflective adhesive material, the latter preferably comprising small spherical glass lenses on self adhesive plastic material.

The optical detector preferably includes means for receiving the signal from the light sensitive member and determining therefrom the velocity of movement of the machine part.

This determination of velocity can be used to terminate a measurement relating to braking effectiveness when the velocity reaches zero or a predetermined speed approaching zero.

In one embodiment of the invention said means for terminating said measurement comprises a dual polarity threshold circuit to which said signal is applied, such that, at the output of said threshold circuit, a pulse appears when said signal is between the positive and negative thresholds. Means responsive to the period of said pulse is used to generate a stop control signal when said period attains a predetermined length. This stop control signal is used for terminating said measurement.

Preferably a logarithmic amplifier is provided to amplify said signal prior to application to said dual polarity threshold circuit whereby the waveform between the positive and negative thresholds of said dual polarity threshold circuit is substantially linear.

Preferably an electronic counter is provided to effect said measurement, means being provided to initiate counting of said counter at a time corresponding to the commencement of braking and said counter being arranged to be arrested upon the occurrence of said stop control signal.

Said counter may be arranged to count the transitions at the output of said dual polarity threshold circuit and/or the pulses generated by a clock oscillator.

Preferably switch means are provided whereby said counter may be connected to count either said transitions or said clock pulses.

In all cases said member may be part of an industrial machine and its movement may be linear or rotational.

The invention is illustrated in and further described with reference to Figures 1 to 3 of the drawings accompanying provisional specification number 16322/77 and Figures 4 to 6 of the accompanying drawings, in which, Figure 1 is a highly schematic diagram illustrating one arrangement for checking the velocity of a moving surface in accordance with the present invention Figure 2 has an explanatory diagram relating to the configuration and dimensions of the grating pattern on the sheet 3 of Figure 1, Figure 3 illustrates one practical embodiment of the sheet 3 of Figure 1, Figure 4 is a block schematic diagram of one apparatus for checking the emergency braking distance and/or times of a moving member in accordance with the present invention, Figure 5 is an explanatory graphical diagram relating to Figure 6 and Figure 6 is a block schematic diagram of another apparatus for checking the emergency braking time and/or distance of a moving member in accordance with the present invention.

Referring to Figure 1, a moving part of an industrial machine, such as a power press, is represented at 1. The requirement is to determine the emergency braking time of the machine, that is to say the time which elapses between the initiation of emergency shut down and the moment at which the member 1 comes to a stop, and, in this instance, also the distance travelled by the member 1 during this time.

In order to determine this time the velocity of the member 1 is monitored, as known per se, by means of an optical velocity responsive apparatus represented at 2.

In order to enhance the signal-to-noise ratio in the optical signal received by the optical velocity responsive apparatus 2, a sheet 3 having a grating pattern 4 thereon is attached to the surface of member 1 so that the sheet 3 moves within the field of view of the sensor of the apparatus 2 in the direction of the arrow 5 during the 'power' stroke of the machine. The nature of the grating pattern 4 and the sheet 3 will be described in greater detail with reference to Figures 2 and 3.

Referring to Figure 2, the grating pattem consists of two sets 6 and 7 of curved reflective grating bars of which, in Figure 2, only three are shown in each set these being referenced respectively 6A, 6B, 6C and 7A, 7B and 7C.

The curved bars of each set meet along a median line 8 and in fact set 7 is a mirror image of set 6. All of the curved bars and the spacings therebetween, in both sets, are equal. The width of each set is W, whilst the radius of curvature of each curved bar is R. The curvature of each bar in Figure 2 is exaggerated for the sake of illustration.

As will be seen the bars of each set are inclined with respect to a line 9 at right angles to the median line 8 by an angle 0, so that the end of each bar adjacent the median line 8 is further advanced in the direction of travel indicated by arrow 5 than is the remainder of the bar.

The relationship between, R, 0 and W is sin w 0 = R. W is chosen to match the sensitivity width of the receiving optics of the optical velocity responsive apparatus 2, which normally equals 2W. In other words, W is chosen to be approximately half of the width of the field of view of the sensor of the optical velocity responsive apparatus 2, at the grating pattern 4.

The angle 0 is chosen in dependence upon the amount of vertical misalignment which is to be tolerated in setting up the arrangement. In practice this tolerance may amount to 5 , as is appropriate to the case of the practical grating pattern illustrated in Figure 3.

Clearly in applying the sheet 3 to the member 1 in Figure 1 an operator will endeavour to obtain alignment of the median line 8 with the direction of travel through the centre of the field of view of the apparatus 2. However, utilising the pattern shown in Figure 3 a reasonably uniform response will be obtained over alignment variations of up to 5 .

In order to attach the sheet 3 to the member 1, the sheet carries small magnets (not shown) on its underside.

In order to obtain optimum signal-to-noise ratio, the spatial frequency of the grating pattern should be matched to the grating of the optical velocity responsive apparatus at the image plane of the sensor of the apparatus. For this reason the optical velocity responsive apparatus is provided with a zoom lens of sufficient dynamic range such that the grating pattern 4 may be adjusted to have a constant spatial frequency at the image plane. In addition, the enhancement surface of the sheet 3 should be of high contrast in the spectral band of the detector (0.6 to 0.9 ,u for a silicon detector) of the optical velocity responsive apparatus. The sheet illustrated in Figure 3 is formed of ligh absorbent matt black coated copper foil to which are applied strips of self adhesive reflective plastic forming the reflective bars. The self adhesive plastic reflective strip used in the case of the sheet shown in Figure 3 is of the kind making use of small spherical glass lenses on a self adhesive plastic backing. This is available commercially as 'High Gain 7610' or 'High Gain 7611' under the brand name 'Scotchlite' (R.T.M.).

Referring to Figure 4, the block 9A represented the optical system of the apparatus 1 of Figure 1. The optical system 9A focuses the grating pattern 4 of Figure 1 on the photodiode 10, which, whilst the grating pattern 4 with the member 1 of Figure 1 is moving, provides a fluctuating electrical signal having a component the frequency of which is proportional to the speed at which the member 1 is moving.

This fluctuating electrical signal derived from the photodiode 10 is applied via a preamplifier 11 and a high pass filter 12 to a logarithmic amplifier 13. The high pass filter 12 is provided to remove the d.c. off-set in the electrical signal generated by the photodiode 10 and caused by the ambient light level. The high pass filter 12 also attenuates low frequency signals generated by system vibration. It maybe noted that a filter with high pass characteristics may be used since the processing circuits are not required to handle signal frequencies representing machine speeds below the nominal 'stopped' speed (typically 10 Hz), a predetermined positive speed approaching zero.

The output of logarithmic amplifier 13 is connected to a dual polarity threshold circuit 14. The positive and negative thresholds of the dual polarity threshold circuit 14 are set about j 5% of the typical peak to peak signal and are not crossed by system noise or vibration signals. The logarithmic amplifier 13 acts to control signal amplitude and maintain a substantially linear signal waveform between the positive and negative thresholds of the dual polarity threshold circuit 14.

The output of the dual polarity threshold circuit 14 is connected to a detector 15 with time constant characteristics, the arrangement being such that the detector 15 provides a control output pulse when the time taken for the signal waveform to pass between the positive and negative thresholds of the dual polarity threshold circuit 14 exceeds the time constant of the detector 15. This is arranged by consideration of the slope of the signal waveform to occur at a signal frequency of, typically 10 Hz, representing a machine speed of 1All per second.

This last mentioned speed is taken to indicate that the moving member 1 has stopped. In fact, of course, the member 1 will not physically have stopped but will continue moving for a marginally longer period and for a marginally further distance. Utilising a threshold in order to determine when the machine has almost stopped, as opposed to attempting to determine precisely the amount of stopping, is preferred since at such very low speeds erratic movement can occur, for example vibration may occur as the machine actually comes to rest.

The control pulse generated by detector 15 is applied to a counter 16 to stop the same.

The counter 16 is controlled to be started by a pulse applied to terminal 17, which is generated as the braking system of the machine under test is operated. The generator (not shown) of the 'break initiation' pulse applied to terminal 17 may be a hand operated device which can be operated as an operator applies the emergency brake to the machine. Where practicable however, the generator will be coupled to the emergency brake mechanism of the machine so that it automatically generates a 'brake initiation' pulse as an operator applies the emergency brake.

A crystal oscillator 18 is provided as a source of clock pulses for the counter 16. The count in the counter 16 as it is stopped by a control pulse from detector 15 is arranged to be displayed by a display unit 19.

It will be seen that the output of the crystal oscillator 18 is connected to terminal 20 of a two way switch 21. The alternative terminal 22 of the switch 21 is connected directly to the output of the dual polarity threshold circuit 14.

Thus the input of the counter 16 may be connected by the switch 21 either to the crystal oscillator 18 or to the output of the dual polarity threshold circuit 14. In the first case the counter will count the clock pulse provided by the crystal oscillator 18 and thus provide a count representing the time from the generation of the 'brake initiation' pulse applied to terminal 17 to the time at which detector 15 generates a control 'stop' pulse. In the second case the counter 16 will count the number of transitions of the dual polarity threshold circuit 14 occurring between the 'break initiation' pulse applied to terminal 17 and the control 'stop' pulse generated by the detector 15. Each count in this last mentioned case represents the half cycle signal, or, typically 1/40th" of movement. A suitable scaling factor is incorporated in the display unit 19.

Where the requirement is to determine the number of revolutions of a revolving machine which elapse during the emergency braking time, the sheet 3 may be provided as a cylinder attached to revolve with the machine. Typically the arrangement will be one hundred grating lines around the circumference of the cylinder so that for each revolution of the machine two hundred transitions of the dual polarity threshold circuit 14 will occur. In this case, with the input of the counter 16 connected to terminal 22 of switch 21, the count of the counter will indicate the number of revolutions of the machine during the emergency braking time.

A further arrangement for checking the stopping distance and/or time of a moving member in accordance with the present invention, will now be described with reference to Figures 5 and 6.

Referring to Figure 5, the curve V represents the velocity of a moving member of an industrial machine the emergency stopping distance and/or time of which it is required to check. It will be seen that the curve V is linear initially.

At a given point in time the emergency braking system of the machine is initiated as represented by the dashed arrowed line BIP. The initials BIP represent 'brake initiation pulse' since at the same time an electrical pulse may be considered to have been generated indicating the commencement of emergency braking.

It will be seen that for a period of time after BIP the curve V remains linear. This is typical and is known as the 'brake reaction time'. It is due to delays between the operation of the emergency braking control and the commencement of braking. Thereafter deceleration occurs which, in general, will be linear over the major portion of the deceleration part of the curve.

As V approaches zero however the deceleration of the machine often becomes erratic due to either 'grab' causing the curve to steepen to zero, or 'fade' causing the curve to level off.

In addition, a moving member may not stop cleanly but may be subject to vibration. All of these factors tend to make it difficult to obtain a clean reliable measure of the point at which the machine may be considered to be stopped.

In addition, where, as described above the stopping distance and/or time of a moving member is checked utilising an optical velocity measuring apparatus for monitoring purposes, at very low speeds the frequency of the alternating electric wave which is generated by the optical velocity responsive apparatus is low and processing the signal, which may involve for example filters, tends to be difficult at such low velocities approaching zero.

The arrangement about to be described relies upon a prediction of the rate of deceleration from a value which is conveniently high as to avoid the above mentioned difficulties. If, for example, the lowest velocity which can conveniently be handled is V1 the deceleration time from the point at which the velocity equals Vl to zero is predicted as an extension of the linear curve which the deceleration has followed previously. Broadly speaking, in the apparatus hereinafter to be described this prediction is made by increasing the clock rate of a clock, which was started at time BIP, over a period of time from V2 to Vl and stopping the clock at Vl, the increase in clock rate being such that the reading of the clock when stopped at V1 is the same as that which it would have been at V = 0 if the deceleration had remained linear from V1 to 0 and the clock rate had not been increased. As a simple example, if V2 is twice V1 then the clock rate between V2 and V1 would be doubled.

Referring to Figure 6, the practical apparatus thereby schematically illustrated will now be described.

The optical velocity responsive apparatus is arranged as described generally with reference to Figure 1. It is assumed that the optical velocity responsive apparatus used in the present case is one having two light sensors which are in anti-phase as known per se. It will of course be appreciated that in the embodiment described with reference to Figure 4 a similar arrangement of two anti-phase sensors could be used.

The signals from the two sensors are applied respectively to terminals 23 and 24 which are connected to a signal conditioning circuit 25 which contains band limiting filters and acts to produce at its output 26 the difference between the two signals applied to terminals 23 and 24. At this point the signal is an alternating electric wave which has a frequency corresponding to the instantaneous speed of the moving member.

The output 26 of signal conditioning circuit 25 is applied to a frequency-to-voltage converter 27 which comprises a thirty micro-second monostable arranged to be triggered by the zero crossings of the difference signal appearing on lead 26. The output of the monostable is applied via a 15 Hz low pass filter within frequency-tovoltage converter 27 to the output 28 of the latter. The output 28 of the frequency-to-voltage converter 27 is applied to a threshold circuit arrangement 29 which provides an output on lead 31 when it detects that the voltage V (see Figure 5) has reduced to the value V2 and, later, a signal on output lead 30 when it detects that the voltage V (Figure 5) has reduced to the value V1.

A crystal clock 32 provides clock pulses to a dividing counter 33 which in this case is controllable to provide a division factor of sixteen or six. The counting of counter 33 is controlled by a control circuit 34 which is either set so as to permit counter 33 to commence counting by a signal on 'set' input lead 35 or is inhibited to arrest the counting of counter 33 in response to an input signal on its 'inhibit' input lead 36. It will be noted that 'inhibit' input lead 36 is connected to the output lead 30 ofthe threshold circuit arrangement 29. Signals applied to 'set' input lead 35 are derived from a terminal 37 via a delay compensating circuit 38. Terminal 37 corresponds to terminal 17 in Figure 4 and is the terminal to which a pulse is applied as the emergency brake control of the machine is operated (time BIP) in Figure 5. Delay compensating circuit 38 is provided to introduce a small delay interval to compensate for filter propagation delays. The output of the dividing counter 33 is connected to clock a three decade counter 39, the count of which is arranged to be displayed on a display unit 40. The display unit 40 has a 'blank display' latch 41, manually operable to cancel the display when required in order to economise in power consumption.

As has already been mentioned, dividing counter 33 is of selectable division. When enabled by enabling circuit 34 its normal division factor is sixteen. Upon the application of a signal to count control input 42 the division factor is changed from sixteen to six. It will be noted that count control input 42 is connected to output lead 31 of the threshold circuit arrangement 29.

In operation a pulse applied to terminal 37 as the emergency braking control is operated causes enabling circuit 34 to trigger dividing counter 33 which then divides the clock pulse from crystal clock 32 by a factor of sixteen to provide a standard 1 kHz output to count in milliseconds. The output of counter 33 is clocked into three decade counter 39 which registers time elapsed in milliseconds. As the velocity of the member decreases to V2 (Figure 5) threshold arrangement 29 applies an output to the count control input 42 of the dividing counter 33 to cause its division factor to decrease to six. Thus the rate at which signals are clocked into counter 39 increases. As the velocity of the member decreases still further to V1 Figure 5) threshold circuit arrangement 29 applies a signal via its output 30 to the inhibiting input 36 of enabling circuit 34 which arrests the count by counter 33 which in turn causes the count in decade counter 39 to be frozen. The count of decade counter 39 is displayed by display unit 40 which in milliseconds will indicate a measure of the time elapsed from the operating of the emergency braking control of the machine to the point at which the moving member comes to rest, including a prediction for the final period of deceleration from V1 to zero.

With the dividing counter 33 having the alternative division rates of sixteen or six, V2 of Figure 5 is 12.8% of may and Vl is 8% of Vmax.

It will be noted that signal output sockets are provided at 45 in respect of signal conditioning circuit 25 and at 43 in respect of the frequency-to-voltage converter 27. By means of the output sockets 42 and 43 signal output and speed profile output may be derived for separate processing if required.

It will also be noted that the pulse applied to terminal 37 is also applied to the threshold circuit arrangement 29 and, as represented by the arrowed lead 44, to the other logic circuits involved, in order to provide for reset.

Furthermore it will be found advantageous to vary the threshold levels of the threshold circuit arrangement 29 in dependence upon the initial velocity of the moving member. The voltage applied to threshold circuit arrangement 29 provides a measure of this initial velocity immediately before the initiation of emergency braking and this voltage may be utilised within the threshold circuit arrangement 29 to adjust the threshold levels as the brake initiation pulse is applied thereto from terminal 37.

WHAT WE CLAIM IS: 1. Apparatus for testing the braking of a moving machine part including: a patterned member attached or adapted for attachment to the machine part and an optical detector arranged so that there is relative movement between the detector and the patterned member when the machine part is moving with the patterned member attached thereto, the detector including a light sensitive member and means including a zoom lens for focussing the pattern onto the light sensitive member so that, as the machine part moves, the light sensitive member is traversed by an image of different parts of the pattern, thereby producing a signal which represents the movement of the machine part and from which signal an indication of the braking effect can be obtained.

2. Apparatus according to claim 1 in which the pattern includes a pattern of differently reflective parts and in which the optical detector is arranged to receive light reflected from the more reflective of said parts.

3. Apparatus according to claim 2 in which the said different parts are lines.

4. Apparatus according to claim 1, 2 or 3 in which the optical detector includes means for receiving said signal and determining therefrom the velocity of movement of the machine part.

5. Apparatus according to claim 4 in which the optical detector includes means for providing an indication of the deceleration of the machine part.

6. Apparatus according to claim 3 in which the lines are curved.

7. Apparatus according to claim 6 in which

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. the voltage V (Figure 5) has reduced to the value V1. A crystal clock 32 provides clock pulses to a dividing counter 33 which in this case is controllable to provide a division factor of sixteen or six. The counting of counter 33 is controlled by a control circuit 34 which is either set so as to permit counter 33 to commence counting by a signal on 'set' input lead 35 or is inhibited to arrest the counting of counter 33 in response to an input signal on its 'inhibit' input lead 36. It will be noted that 'inhibit' input lead 36 is connected to the output lead 30 ofthe threshold circuit arrangement 29. Signals applied to 'set' input lead 35 are derived from a terminal 37 via a delay compensating circuit 38. Terminal 37 corresponds to terminal 17 in Figure 4 and is the terminal to which a pulse is applied as the emergency brake control of the machine is operated (time BIP) in Figure 5. Delay compensating circuit 38 is provided to introduce a small delay interval to compensate for filter propagation delays. The output of the dividing counter 33 is connected to clock a three decade counter 39, the count of which is arranged to be displayed on a display unit 40. The display unit 40 has a 'blank display' latch 41, manually operable to cancel the display when required in order to economise in power consumption. As has already been mentioned, dividing counter 33 is of selectable division. When enabled by enabling circuit 34 its normal division factor is sixteen. Upon the application of a signal to count control input 42 the division factor is changed from sixteen to six. It will be noted that count control input 42 is connected to output lead 31 of the threshold circuit arrangement 29. In operation a pulse applied to terminal 37 as the emergency braking control is operated causes enabling circuit 34 to trigger dividing counter 33 which then divides the clock pulse from crystal clock 32 by a factor of sixteen to provide a standard 1 kHz output to count in milliseconds. The output of counter 33 is clocked into three decade counter 39 which registers time elapsed in milliseconds. As the velocity of the member decreases to V2 (Figure 5) threshold arrangement 29 applies an output to the count control input 42 of the dividing counter 33 to cause its division factor to decrease to six. Thus the rate at which signals are clocked into counter 39 increases. As the velocity of the member decreases still further to V1 Figure 5) threshold circuit arrangement 29 applies a signal via its output 30 to the inhibiting input 36 of enabling circuit 34 which arrests the count by counter 33 which in turn causes the count in decade counter 39 to be frozen. The count of decade counter 39 is displayed by display unit 40 which in milliseconds will indicate a measure of the time elapsed from the operating of the emergency braking control of the machine to the point at which the moving member comes to rest, including a prediction for the final period of deceleration from V1 to zero. With the dividing counter 33 having the alternative division rates of sixteen or six, V2 of Figure 5 is 12.8% of may and Vl is 8% of Vmax. It will be noted that signal output sockets are provided at 45 in respect of signal conditioning circuit 25 and at 43 in respect of the frequency-to-voltage converter 27. By means of the output sockets 42 and 43 signal output and speed profile output may be derived for separate processing if required. It will also be noted that the pulse applied to terminal 37 is also applied to the threshold circuit arrangement 29 and, as represented by the arrowed lead 44, to the other logic circuits involved, in order to provide for reset. Furthermore it will be found advantageous to vary the threshold levels of the threshold circuit arrangement 29 in dependence upon the initial velocity of the moving member. The voltage applied to threshold circuit arrangement 29 provides a measure of this initial velocity immediately before the initiation of emergency braking and this voltage may be utilised within the threshold circuit arrangement 29 to adjust the threshold levels as the brake initiation pulse is applied thereto from terminal 37. WHAT WE CLAIM IS:

1. Apparatus for testing the braking of a moving machine part including: a patterned member attached or adapted for attachment to the machine part and an optical detector arranged so that there is relative movement between the detector and the patterned member when the machine part is moving with the patterned member attached thereto, the detector including a light sensitive member and means including a zoom lens for focussing the pattern onto the light sensitive member so that, as the machine part moves, the light sensitive member is traversed by an image of different parts of the pattern, thereby producing a signal which represents the movement of the machine part and from which signal an indication of the braking effect can be obtained.

3. Apparatus according to claim 2 in which the said different parts are lines.

6. Apparatus according to claim 3 in which the lines are curved.

7. Apparatus according to claim 6 in which

the bars are concave on their leading sides having regard to the direction of movement of the machine part.

8. Apparatus according to any preceding claims in which the patterned member is a sheet.

9. Apparatus according to any preceding claim in which the patterned member is adapted to be attached magnetically to the machine part.

10. Apparatus according to any preceding claim in which the pattern includes areas of reflective material comprising spherical lenses.

11. Apparatus according to claim 4 including means for performing a measurement relating to the braking effect and for terminating said measurement when the speed of the machine part has been reduced to a predetermined value.

12. Apparatus according to claim 11 in which the predetermined value is zero.

13. Apparatus according to claim 11 in which the predetermined speed is a positive speed.

14. Apparatus according to claim 11, 12 or 13 including a dual polarity threshold circuit to which an output from the light sensitive member is applied, such that at the output of said threshold circuit a pulse appears only when the output from the light sensitive member is between the positive and negative thresholds, means responsive to the period of said pulse from the threshold circuit between the positive and negative thresholds for generating a stop control signal when said period attains a predetermined length and means for utilising said stop control signal for terminating said measurement.

15. Apparatus according to claim 14 including a logarithmic amplifier provided to amplify the output from the light sensitive detector prior to application to said dual polarity threshold circuit.

16. Apparatus according to any of claims 11 to 15 including an electronic counter to effect said measurement and means to initiate counting at a time corresponding to the commencement of an event leading to the application of the braking effect, the counter being arranged to be arrested upon the occurrence of the stop control signal.

17. Apparatus according to claim 16 in which the counter is arranged to count signals from the output of the dual polarity threshold circuit and/or pulses generated by a clock oscillator.

18. Apparatus according to claim 17 in which the counter is associated with a switch so that it can be connected to count either the signals from the threshold circuit or from the oscillator.

19. Apparatus according to claim 1 and substantially as described with reference to Figures 1 to 3 of the drawings accompanying the Provisional Specification filed with Application 16322/77.