GB2395002A

GB2395002A - Apparatus for determining angular position of a rotatable mechanical element

Info

Publication number: GB2395002A
Application number: GB0224484A
Authority: GB
Inventors: Mark Lewis Whatley; John Shepherd; Roger William Brassington
Original assignee: JC Bamford Excavators Ltd
Current assignee: JC Bamford Excavators Ltd
Priority date: 2002-10-22
Filing date: 2002-10-22
Publication date: 2004-05-12
Also published as: GB0224484D0

Abstract

A mechanical element 10 comprises a cylinder, mounted for rotation about an axis A and featuring an optically readable marking 12 thereon. The marking is provided on a part of the cylinder having a first reflectivity and includes a plurality of areas having a second reflectivity 12a. The marking may be located about the circumference of the cylinder or on an end face in a generally circular array. The marking is such that it encodes positional information such that the absolute angular position of the cylinder can be determined to a desired resolution. The coding maybe in the form of pseudorandom binary sequences, and the apparatus may be used to control the position of the mechanical element 10.

Description

Title: An Apparatus for Determining the Angular Position of a Rotatable

Mechanical Element Description of Invention

The invention relates to an apparatus for determining the angular position of a rotatable mechanical element, particularly but not exclusively for determining the position of a mechanical element rotatably mounted on an excavator or other similar earth-moving machine.

It is known to measure the angular position of a rotatable mechanical element using a potentiometer or a rotary Hall effect device. Such devices are not ideal for use in an excavator or other similar earth- moving machine as they have a restricted arc of measurement or non-linear operation over their arc of measurement, and therefore they do not perform well in applications such as these where linear operation over a full 360 arc is required.

Moreover, the cost of the electric components and circuitry necessary for a potentiometer device to provide a desired resolution is high, and therefore is would not be commercially viable to use a high resolution potentiometer device in such applications.

According to one aspect of the invention we provide an apparatus for determining the angular position of a first mechanical element with respect to a second mechanical element wherein the first mechanical element has a marking which is optically readable, the optically readable marking comprising at least part of the mechanical element with a surface having a first reflectivity and a plurality of areas having a second reflectivity.

One of the mechanical elements may be provided on a body part of an earth moving machine, and the other mechanical element may be provided on a part pivotally mounted with respect to the body part of the earth moving machine.

Preferably the first mechanical element is rotatable whilst the second mechanical element is stationary.

Preferably, the marking may comprise a plurality of code elements.

Preferably each code element encodes a numerical value in binary digits.

Preferably each code element comprises a unique part of a pseudorandom binary sequence.

The reflectivity of each portion of a predetermined width of the marking may determine the binary digit represented by that portion of the marking.

Alternatively, the areas having a second reflectivity may have a width which is generally constant in a direction parallel to the direction of movement of the mechanical element, in which case, each binary digit may be indicated by the distance between adjacent areas having the second reflectivity. Further alternatively, the distance between adjacent edges of the areas having a second reflectivity may be generally constant in a direction parallel to the direction of movement of the mechanical element, in which case, each binary digit may be indicated by the width of the areas having a second reflectivity in a direction parallel to the direction of movement of the mechanical element.

Preferably, the apparatus further comprises reading means to provide an output dependent on the angular position of the first mechanical element with respect to the second mechanical element, the reading means comprising illuminating means, detector means, and decoding means, at least the detector means being mounted on the second mechanical element.

The apparatus may comprise a plurality of optical fibres whereby light is transmitted from the illuminating means to the marking.

The apparatus may further comprise focusing means to focus light reflected from the surface of the mechanical element or from the areas of second reflectivity onto the detector means.

The illuminating means may comprise a light emitting diode.

The illuminating means may comprise a plurality of light emitting diodes, the light from each light emitting diode being directed onto a different area of the marking.

The detector means may be disposed to detect light from the illuminating means reflected from the surface having a first reflectivity and the areas having a second reflectivity, the detector means comprising a detector which provides a signal having a value within a first range when light from a part of the surface having a first reflectivity is reflected thereon and having a value within a second range when light from an area having a second reflectivity is reflected thereon. The detector means may comprise an array having a plurality of said detectors, in which case each detector may be adapted to collect generally only light reflected from a portion of the illuminated surface of the marking.

The apparatus may comprise a plurality of optical fibres whereby reflected light from the marking means is transmitted to said array.

The array may provide an intermediate signal to the decoding means, the intermediate signal comprising a signal from each of said detectors.

The decoding means may comprise electronic means to decode the intermediate signal to provide the output.

The decoding means may comprise means to divide the intermediate signal into a plurality of portions each corresponding to a bit of the binary sequence encoded by the marker, and allocating each portion the binary digit 1 or O depending on the reflectivity of each portion. Alternatively, the decoding means may comprise means to detect a detector signal of the intermediate signal corresponding to a transition corresponding to an edge of an area of second reflectivity and means to identify a plurality of binary digits from the separation of a plurality of transitions.

The decoding means may comprise means to identify a code element from the binary digits and provide the output indicating the angular position of the mechanical element dependent on the code element.

The decoding means may further comprise means to identify the detector of the detector array on which a selected one of said transitions falls, to provide a fine position.

The apparatus may further comprise means for controlling the amount of light produced by the illumination means in response to a level of light collected by the detector means so that the level of light collected by the detector means is within a predetermined range.

One of the mechanical elements may be mounted on a body part of an earth moving machine, and the other mechanical element may be pivotally mounted on the body part of the earth moving machine. Thus, for examples, the apparatus may be used to determine the angular position of a ground engaging portion such as tracks, a steering part, or a tool, with respect to the upper body part of the earth moving machine.

The first mechanical element may comprise a cylindrical rod mounted for rotation about a longitudinal axis, and the marking may be provided either around the circumference of the rod, or on an end face of the rod.

According to a second aspect of the invention, we provide a control system for a rotable mechanical element, the control system comprising an apparatus according to the first aspect of the invention and a controller operable to receive angular position information from the apparatus and cause the mechanical element to rotate.

According to a third aspect of the invention we provide a method of determining the angular position of a first mechanical element with respect to a second mechanical element, the first mechanical element being provided with a marking which is optically readable and which comprises at least part of the mechanical element with a surface having a first reflectivity and a plurality of

areas having a second reflectivity, the method comprising the steps of using a reading means to optically read the marking, and providing an output indicating the angular position of the first mechanical element with respect to the second mechanical element.

The reading means may have any of the features of the reading means of the first aspect of the invention.

The marking may have any of the features of the marking of the first aspect of the invention.

Preferably the method further comprises the steps of comparing the output with a previous output, the previous output having been obtained from a reading taken a known time interval before, to determine the change in angular position of the first mechanical element with respect to the second mechanical element, and rejecting the output if the change in angular position exceeds a specified amount.

The invention will now be described, by way of example only, with reference to the accompanying drawings, of which, FIGURE 1 is a schematic view of a first embodiment of a mechanical element and an apparatus according to the first aspect of the invention, FIGURE 2 is a schematic view of a second embodiment of a mechanical element and an apparatus according to the first aspect of the invention, FIGURE 3_ is an enlarged view of a portion of the mechanical element of Figure 1, FIGURE 3_ is a graphical representation of an intermediate signal, FIGURE 3_ is a graphical representation of a waveform derived by the decoding means from the intermediate signal of Figure 3_, and FIGURE 4 is a further enlarged view of a portion of the mechanical element of Figure 1.

FIGURE 5 is a view of a fbre-optic illumination means for an apparatus embodying the present invention, and

( FIGURE 6 is a schematic arrangement of a control system incorporating the invention.

Referring now to figures 1 and 2, there is shown a first mechanical element 10 comprising a cylinder with an optically readable marking 12 thereon. The cylinder 10 is mounted for rotation about a longitudinal axis A. The marking 12 is provided on a part of the cylinder with a surface having a first reflectivity and includes a plurality of areas having a second reflectivity 12_ (shown as the dark areas in the Figures). The marking 12 may be located around the circumference of the cylinder 10, as shown in Figure 1, or it may be located on a end face of the cylinder 10 in a generally circular array as shown in Figure 2.

Whilst in this example, two levels of contrast are provided in the marking 12, i.e. light and dark, more than two levels of contrast may be provided in the marking.

The requirements of the optically readable marking 12 are that it should encode positional information using contrast between areas of the marking 12 such that the absolute angular position of the cylinder 10 can be determined to a desired resolution, be tolerant of errors introduced by loss of dark areas 12_. It is also desirable that the marking be durable and corrosion resistant, and as far as possible not impair the corrosion resistance of the cylinder 10 itself.

Preferably, the optically readable marking is provided on a mechanical element 10 once it has been manufactured, for example, by grinding, to the diameter required. In one embodiment, the cylinder 10 is steel and the appropriate parts of it are first coated with a bronze substrate. The cylinder 10 is preferably ground or wet polished before coating to have a surface finish of 0.8 rim or less. The bronze substrate has a thickness in the range 0.038 to 0.051 man and comprises 88% to 92% copper and 8% to 12% tin. After coating, the bronze substrate should show no sign of porosity, and is polished to achieve a surface Amish of 0.2 to 0.4 1lm. A chromium surface is deposited on the bronze

at a rate of 0.001 mm/hour by a conventional electro-deposition technique to provide a surface having a first reflectivity. The surface may have micro cracks at approximately 400 cracks/linear cm minimum. The final chrome thickness may be varied for different components depending on the final use of the component. Other suitable techniques or parameters may be used as desired.

The chromium is then removed by scanning the desired areas with a laser to produce exposed areas of bronze having a second, lower, reflectivity.

To produce the marking of the Figure 1 embodiment of the invention, the laser is scanned along the cylinder 10 to provide a series of marks parallel to the longitudinal axis A of the cylinder 10. The cylinder 10 is rotated about axis A through a small angle and another region scanned until an area 12_ having the desired width has been generated. This process is repeated for each area 12_ as desired. To produce the marking 12 of the Figure 2 embodiment of the invention, the laser is scanned generally radially over the end face 10a of the cylinder 10 to provide a mark along a radius of the cylinder 10. The cylinder 10 is then rotated about its longitudinal axis A The areas 12_ may alternatively be formed by partially removing or discolouring the chromium without entirely removing the chromium.

Typically, markings of width 0.3mm may be reliably produced using this method. The markings are durable and corrosion resistant, the bronze substrate resisting any impairment of the corrosion resistance of the cylinder arising from the laser marking of the chromium.

Although nickel is conventionally used as a substrate when depositing chromium onto steel since it is generally regarded as providing better corrosion resistance than a bronze substrate, contrary to expectations a nickel substrate was found to be less effective at countering any impairment of the corrosion resistance caused by the laser making of the chromium. In particular, it is

difficult to control the laser power so that the chrome is marked but so that the laser does not significantly affect the substrate and the mechanical element.

Alternatively, where only chromium is provided, the laser opens corrosion paths through micro cracks in the chromium. A bronze substrate provides a corrosion resistant substrate which is highly reflective to the marking laser. The laser power need may not be controlled so accurately, since once the chrome surface has been marked, the laser light is subsequently reflected by the bronze substrate, making it less likely that the laser will burn through the substrate to the mechanical element below or open corrosion paths through to the element. Further, the use of bronze, which has a very different reflectivity to chrome, provides areas 12_ of high contrast.

The marking method provides a readable marking which lasts for a predetermined lifetime without significant degradation. The marking are also sufficiently durable not to be easily eroded by normal use or damaged by accidental impact.

Where the mechanical element 10 is provided with a conventional seal which wipes the chrome surface, any dirt or oil on the element will serve to heighten contrast between the surface and areas 12a since the areas 12_ will be slightly recessed and will accumulate some oil or dirt, thus lowering their reflectivity further.

The method can be used on induction hardened steel, as well as other grades of steel including stainless steel or non-induction hardened steel.

It is possible to use an encoding method where all the marks 12_ are of constant width, as discussed below with reference to Figure 4, and the use of constant width areas of second reflectivity 12a (or marks) has the advantage that it resists impairment of the corrosion resistance caused by laser marking of the surface.

If only chromium is provided, it is desirable that the marks 12_ do not penetrate through the chromium to the mechanical element, requiring control of

the depth and penetration and hence power of the laser. Where a substrate is provided, it may still be desirable that the marks 12_ do not penetrate the chromium, or alternatively that the marks 12_ do not result in significant penetration or heating of the substrate.

When a mark 12_ is formed by a laser on a mechanical element, the amount of heating caused to the element is dependent on the size of the mark 12_, which further affects the depth of penetration of the laser. By making the marks 12_ of constant width, the heating of the mechanical element is normally constant and the depth of penetration will be more easily controlled, since the effect of making the mark 12_ will be known without having to calibrate the laser power for different sizes of mark 12a. The method can be used in addition to or separate from the method of providing a bronze substrate described above to provide corrosion resistance.

The areas 12_ having the second reflectivity may, however, be provided by any other suitable process such as inkjet printing, chemical etching or painting, or any other means of producing precise, durable, and, where necessary, corrosion resistant areas on the surface of the cylinder 10 with a contrast ratio between them.

The optically readable marking 12 comprises a series of code elements each encoding a numerical value based on the position of the code element relative to the cylinder 10. An example of a portion of a coding scheme is shown in Figure 3_.

Figure 3 shows one code element 12b, that is, a distinguishable part of the optically readable marking 12 encoding one numerical value in binary digits (bits). The numerical value indicates the position of the code element 16a on the rod 12. The code element 12_ comprises a part of the surface of the cylinder 10 having a first reflectivity upon which are marked areas 12_ having a second, lower, reflectivity. The areas 12_ have a fixed width T. Disposed between the areas 12a are spaces 20. The contrast level of each portion of the

marking 12 determined the logic value of each bit, i.e. a dark area 12a of width T indicates a bit having the value "0", and a space 20 of width T indicates a bit having a value "1". The code element 12k shown in Figure 3 thus encodes the binary sequence 100111010.

More preferably, the marking uses a pseudorandom binary sequence to encode the positional information. Pseudorandom sequences and methods generating pseudorandom sequences are well known. The advantage is that for any pseudorandom sequence of order N. any N-bit segment of the sequence occurs only once, i.e. each N-bit segment is unique. The marking hence comprises a series of unique, overlapping code elements of length N bits. For example, the portion of the marking 12 shown in Figure 3 shows five nine-bit code elements, namely 01001 110 1, 100 1 11010 (i.e. 12_), 00 1 110 101, 01 1101011, and 111010110.

Moreover, a pseudorandom binary sequence is cyclical, and therefore the sequence may be joined end to end, for example when marked around the circumference of a cylinder 10, and still any N-bit segment of the sequence occurs only once.

As each code element is unique, the pseudorandom binary sequence may be used to determine an absolute angular position of the mechanical element 10 on switching on, without the need for any calibration procedure.

Where a sequence of order N is used as the pseudorandom sequence, the length in bits of the sequence is given by 2N-1. If such a sequence is marked around the circumference of a cylinder 10, there are provided 2N- 1 unique N-bit segments, which occur at intervals of 360/(2N-1) degrees around the cylinder circumference. Thus the angular resolution provided by the marking 12 is 360/(2N-1) degrees or 2/(2N-1) radians. The minimum resolvable arc length at a working radius of R is thus 2R/(2N- 1).

Using this formula it can be demonstrated that, at a working radius R of 1 Om, a psuedo-random binary sequence of order 3, which contains 7 bits, gives a minimum resolvable arc length of 8975mm, a sequence of order 9, which contains 511 bits, gives a minimum arc length of 123mm, and a sequence of order 15, which contains 32767 bits, gives a minimum arc length of 1.9mm.

The use of a pseudorandom sequence has the advantage that a single track of markings can be used to provide absolute position sensing with a resolution of the width of one binary digit. The angular resolution of the system can be selected by varying the order of the pseudorandom binary sequence, and the marking is scaleable for different diameters of mechanical elements by altering the value of T or the order of the sequence.

In this example, each bit is represented by an area of width T. an area 12 of second reflectivity encoding 1, and a space 20 encoding 0, or vice versa. In this case, the length of pseudorandom binary sequence of order N is (2N-1)T. If this is marked in a continuous loop around a cylinder 10, the cylinder circumference must equal the length of the sequence, and so the cylinder diameter is given by (2N- 1)T/.

Alternatively, it is possible to encode the pseudorandom binary sequence by providing marks 12_ having a second reflectivity of a generally constant width in the direction of movement of the mechanical element 10, and varying the width of the spaces 20. For example, to encode the sequence by representing logic value O as a space 20 having width T. and logic value 1 as a space having width 2T, or vice versa. Of course it is also possible to vary the width of the areas 12 of second reflectivity to encode 1 or 0, whilst the width of the spaces 20 is generally constant.

The same portion of binary code sequence as shown in Figure 3, is shown in Figure 4 encoded using this method. It will, of course, be appreciated that for a given value of T. a pseudorandom binary sequence encoded using this method will occupy more space than the same sequence encoded using the

method illustrated in Figure 3, and that the formula given above for calculating the required cylinder 10 diameter is not valid for this method.

Also shown in Figures 1, 2 and 5 is a reading means 16 adapted to read the marking 12 and to provide an output dependent of the angular position of the cylinder 10 with respect to a second, stationary mechanical element, the reading means 16 comprising illuminating 16_ and detector means 16_ and decoding means 18.

The illuminating means 16_ comprises a number of light emitting diodes (LEDs) which are arranged to provide a constant level of illumination across the part of the optically readable marking 12 viewed by the detector array. This is typically achieved by mounting the LEDs remotely from the cylinder 10 surface, and providing diffusing means to even out the light distribution across the viewed area.

Alternatively, light directing means, such as an optical fibre, may be provided for each LED. The light directing means may extend to a point directly adjacent to the cylinder 10 surface, so that the light from each LED falls on a separate area of the marking 12. Thus, light from each LED is directed to a point very close to the cylinder 10 surface, and this ensures that the light level provided on each area of the cylinder 10 surface may be matched with the light level on adjacent areas.

The detector means 16_ comprises a semi-conductor device comprising an array of discrete photo-detectors arranged in an arc which extends around the circumference of the cylinder 10 normal to the longitudinal axis of the rod 12 such that all of the photo-detectors are spaced by the same distance from the cylinder surface, for the embodiment shown in Figure 1. For the embodiment shown in Figure 2, the photo-detectors are arranged such that each is spaced the same distance as each other from the end face of the cylinder lO and from the longitudinal axis A.

The photo-detectors maybe located remotely from the cylinder, and light directing means such as an optical fibre may be provided for each photo-

detector to collect light from a region directly adjacent to the cylinder 10 surface and direct it to the photo-detector. Thus the area of the cylinder lO surface from which light is collected by each photo-detector may be relatively accurately controlled by positioning of the optical fibre relative to the cylinder 1 O. and as the light directing means may be positioned very close to the cylinder surface, the amount of reflected light not collected by the photo-detectors may be reduced.

An example of an illuminating means 16_ and detecting means 16_ using optical fibres is illustrated in Figure 5. In this example, delivery optical fibres 16_' direct light from the LEDs 16_ to the cylinder 10 surface, and the reflected light is collected by collecting optical fibres 16b' and directed to the photodetectors 16k, each collecting optical fibre 16k' directing light to a corresponding single photodetector. An end of each of the collecting optical fibres 16b' is positioned closely adjacent to an area illuminated by one of the delivery optical fibres 16_', thus the light detected by each photodetector corresponds to the light delivered by one delivery optical fibre l 6a'.

It will be appreciated, however, that it is possible to provide delivery optical fibres 16_' only, and collect the light using an array of photodetectors 16b positioned close to the cylinder surface. Similarly, it is also possible, to provide collecting optical fibres 16_' only, and to delivery light using an array of LEDs positioned close to the cylinder surface.

Focusing means, preferably a lens, may be provided for each photo-

detector to focus light reflected from the marking 12, whether via a waveguide such as an optical fibre or not, onto the photo-detector.

Each photo-detector produces a signal whose voltage is proportional to the amount of light which falls upon the photo detector. The intermediate signal from the detector array comprises a series of voltage values, each value

corresponding to the signal of one of the photo-detectors. Each discrete value may be referred to by the position (hereinafter referred to as its 'detector number') of its producing detector in the array. The focusing means and bit l width T is preferably selected such that the light from any given region of the surface having a width T falls upon at least two photo-detectors. For example, detector arrays with separation between detectors, or where provided between the collecting optical fibres 16_', of 0.0635mm are known, and where T is 0.3mm, each bit would be covered by 4.7 detectors.

It is essential to ensure that the maximum size code element 12b, (in the example of Figure 3 having 9 bits, the code element 12b encoding the value 100111010), will fit within the length of the detector array. Thus, in the above example where each marking is covered by 4.7 photo-detectors, the photo-

detector array must include at least 43 detectors in order to decode a 9 bit code element 12_.

Most preferably, the detector can detect more bits than the minimum needed to identify a unique code element. In case of the pseudorandom binary sequence, the additional bits can be used for the purpose of error checking, as described below.

An example of the intermediate signal from the detector array relating to the code element 12b shown in Figure 3a is shown in Figure 3_. For simplicity, in this example, the detector separation is taken as O.lmm, and therefore there are three detectors covering each bit of width 0.3mm. The areas 12a having the second reflectivity, which correspond to logic value 0, appear as minima 22, whilst spaces 20 corresponding to logic value 1 appear as peaks 24.

A possible configuration for the reading means 14 is shown in Figure 6 in schematic form, and the reading means operates as follows.

The marking 12 is illuminated with the illuminating means 16_, and the intermediate signal, representing light level measurements from the photo-

detector array 16b is transmitted to the decoding means 18, in this case a micro

processor, via an analogue input port 28. If necessary, the microprocessor sends a control signal to the illuminating means 16_ via a digital output port 30 to adjust the intensity of light generated by the illuminating means 16_ according to the light level received by the photo- detectors 16b, to ensure that the detectors 16_ or neither saturated with light or under-illuminated.

In this example, once an intermediate signal similar to that shown in Figure 4 is obtained and transmitted to the microprocessor 18 via the analogue input port, an analogue to digital converter is used to produce a numerical representation of the intermediate signal for further processing.

It is, however, possible for the analogue to digital converter to be integral with the detector array 16b, and the numerical representationobtained from the intermediate signal to be transmitted to the microprocessor via a digital input port.

The numerical representation is then supplied to a discriminator which compares the value of the intermediate signal with an upper V+ and lower V threshold voltage. Where the value of the intermediate signal rises above the upper threshold voltage, the discriminator 34 generates a signal voltage of +5 volts, and where the value of the intermediate signal falls below the lower threshold voltage, generates a signal voltage of OV. The discriminator thus converts the numerical representation into a waveform comprising S volt "marks" which correspond to the spaces 20 and 0 volt "gaps" which correspond to the areas 12a, as illustrated in Figure 3c.

The waveform is decoded by dividing the waveform into bits corresponding to the portions of the marking 12 of width T. and allocating SV bits with logic level 1, and OV bits with logic level 0.

Where the logic levels 1 and 0 are coded by varying the width of the spaces 20 or areas 12a, the separation of the transitions between SV and OV is used to obtain a binary sequence. For example, a separation of the O to SV

l transition and 5V to 0V transition of 2T may correspond to a logic level of 1, and a separation of T to a logic level of 0.

The number of bits corresponding to a word of the pseudorandom sequence is extracted from the binary sequence and is validated to ensure it is a valid part of the pseudorandom sequence. Once it has been confirmed as valid, the angular position of the mechanical element can be found from a look-up table, in which each word of the pseudorandom binary sequence is matched against the corresponding angular position of the rod.

Such a method gives the angular position of the mechanical element to within resolution of the width of one binary digit of the pseudorandom sequence marked on the mechanical element.

Where more than one detector is provided for each T width or the marker 12, a fine position can then be calculated by identifying the detector number X of the detector on which the first transition in Figure 4 falls. The position of the detector X thus gives the start of the code element 12b from a given reference point. Since it is known how many detectors correspond to a width T of the pseudorandom sequence, by counting the number of detectors between X and 0, a fine position of the piston rod can be calculated to within the width of one detector of the detector array 16b.

The angular position measurement may be verified by, for example, comparing the latest position measurement with the previous one. The time elapsed between measurements is known, and if the angle-change between a pair of measurements exceeds a specified amount, for example is larger than possible for the particular mechanical element 10 being monitored, the reading can be considered to be suspect and rejected, and a new reading taken.

Moreover, where the detector array 16_ can detect more bits than are required to identify a unique code element 12_, the additional bits may also be used for the purposes of error checking. For example, if the pseudorandom sequence is of order 9, as in the example shown in Figure 3a, the detector 1 6b is

required to read only a 9 bit segment. If, however, the detector l 6b, is capable of reading an 11 bit sequence, three consecutive unique code elements may be identified. The validity of a code element 12b can then be checked for consistency by comparing it and the two additional code elements on either side of the code element 12_ against the known pseudorandom binary sequence. If the three consecutive code elements do not form a pattern that is contained within the total pseudorandom binary sequence, the reading can be considered to be suspect and discarded.

Such an apparatus for determining the position of a mechanical element l O could be used in a conkrol system operable to conkol the mechanical element lO in response to the output signal of the apparatus. In the present example, suitable valve means may be provided whereby supply of fluid pressure to the cylinder is electronically controlled. A possible schematic arrangement is shown in Figure 6, and comprises a mechanical element 10 like that shown in Figure 1, a reading means 16 and decoding means 18 comprising angular position information as hereinbefore described.

The output from the decoding means 18 is passed to an electronic controller 26, typically by means of a communications protocal such as Conkroller Area Network (CAN), I2C, or a Synchronous Serial Interface (SSI).

Signals from other sensors or control means as desired may also be passed to the electronic controller 26. The electronic controller 26 may then operate a fluid pressure control means 28 comprising, for example, a solenoid valve by sending a signal to control the supply of fluid to the mechanical element in response to the positional information from the decoding means 18 and any other sensor information. The rod 12 will move accordingly and its new position will be detected by the reading means 16 and decoding means 18 and passed to the electronic controller 26.

The invention may be used to Tack the angular position of a rotatable part of an excavator or similar earth-moving machine, for examples, to track the

position of an upper body as the upper body rotates with respect to ground engaging means such as tracks or of a tool mounted on an upper body of the excavator as the tool pivots with respect to the upper body, or to measure the steering wheel angle or steering king-pin angle.

The invention has the following advantages over existing transducers used in the above applications. In order to provide a comparable level of resolution using a resistive, or potentiometer device, requires far more expensive instrumentation than is required for the present invention. Moreover, other known transducers, such as potentiometers and rotary Hall Effect devices have a restricted arc of measurement or non-linear operation over their arc of measurement, whereas the present invention is capable of operation over 360 rotation of the mechanical element 10, and operates linearly over the entire range. Whilst in the example hereinbefore described, the marking 12 was provided on the rotatable mechanical element 10, and the reading means 14 is mounted on a relatively stationary mechanical element such as the excavator ground engaging means. Thus on rotation of the mechanical element 10, wires connecting the reading means 14 and the controller 26 should not become tangled. The marking 12 may be provided on a stationary element, and the reading means 14, or at least the detector means 16b, provided on the rotatably mounted mechanical element 10, provided the problems of tangling of wiring between the reading means 14 and the decoding means 18, or between the parts ofthe reading means 14 is solved.

In the present specification "comprises" means "includes or consists of"

and "comprising" means "including or consisting of".

The features disclosed in the foregoing description, or the following

claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any

combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

1. An apparatus for determining the angular position of a first mechanical element with respect to a second mechanical element wherein the first mechanical element has a marking which is optically readable, the optically readable marking comprising at least part of the mechanical element with a surface having a first reflectivity and a plurality of areas having a second reflectivity.

2. An apparatus according to claim 1 wherein one of the mechanical elements is mounted on or integral with a body part of a material handling machine, and the other mechanical element is pivotally mounted on the body part of the material handling machine.

3. An apparatus according to claim 1 or claim 2 wherein the first mechanical element is rotatable and the second mechanical element is stationary.

4. An apparatus according to any one of the preceding claims wherein the marking comprises a plurality of code elements.

5. An apparatus according to claim 4 wherein each code element encodes a numerical value in binary digits.

6. An apparatus according to claim 4 wherein each code element comprises a unique part of a pseudorandom binary sequence.

7. An apparatus according to claim 5 or 6 wherein the reflectivity of each portion of a predetermined width of the marking determines the binary digit represented by that portion of the marking.

8. An apparatus according to claim 5 or 6 wherein the areas having a second reflectivity have a width which is generally constant in a direction parallel to the direction of movement of the mechanical element, and each binary digit is indicated by the distance between adjacent areas having the second reflectivity.

9. An apparatus according to claim 5 or 6 wherein the distance between adjacent edges of the areas having a second reflectivity is generally constant in a direction parallel to the direction of movement of the mechanical element, and each binary digit is indicated by the width of the areas having a second reflectivity in a direction parallel to the direction of movement of the mechanical element.

lO. An apparatus according to any preceding claim further comprising reading means to provide an output dependent on the angular position of the first mechanical element with respect to the second mechanical element, the reading means comprising illuminating means, detector means, and decoding means, at least the detector means being mounted on the second mechanical element.

11. An apparatus according to claim 10 comprising a plurality of optical fibres whereby light is transmitted from the illuminating means to the marking.

12. An apparatus according to claim 10 or claim 11 wherein the apparatus further comprises focusing means to focus light reflected from the surface of

the mechanical element or from the areas of second reflectivity onto the detector means.

13. An apparatus according to any one of claims lO to 12 wherein the illuminating means comprises a light emitting diode.

14. An apparatus according to any one of claims 10 to 12 wherein the illuminating means comprises a plurality of light emitting diodes, the light from each light emitting diode being directed onto a different area of the marking.

15. An apparatus according to any one of claims 10 to 14 wherein the detector means is disposed to detect light from the illuminating means reflected from the surface having a first reflectivity and the areas having a second reflectivity, the detector means comprising a detector which provides a signal having a value within a first range when light from a part of the surface having a first reflectivity is reflected thereon and having a value within a second range when light from an area having a second reflectivity is reflected thereon.

16. An apparatus according to claim 15 where dependent on claim 14 wherein the detector means comprises an array having a plurality of detectors.

17. An apparatus according to claim 16 comprising a plurality of optical fibres whereby reflected light from the marking is transmitted to said array.

18. An apparatus according to claim 16 or claim 17 wherein the array provides an intermediate signal to the decoding means, the intermediate signal comprising a signal from each of said detectors.

19. An apparatus according to claim 18 wherein the decoding means comprises electronic means to decode the intermediate signal to provide the output.

20. An apparatus according to claim 18 or 19 wherein the decoding means comprises means to divide the intermediate signal into a plurality of portions each corresponding to a bit of the binary sequence encoded by the marker, and allocating each portion the binary digit 1 or 0 depending on the reflectivity of each portion.

21. An apparatus according to claim 18 or claim 19 or claim 20 wherein the decoding means comprises means to detect a detector signal of the intermediate signal corresponding to a transition corresponding to an edge of an area of second reflectivity and means to identify a plurality of binary digits from the separation of a plurality of transitions.

22. An apparatus according to claim 20 or 21 wherein the decoding means may comprise means to identify a code element from the binary digits and provide the output indicating the angular position of the mechanical element dependent on the code element.

23. An apparatus according to claim 22 wherein the decoding means further comprises means to identify the detector of the detector array on which a selected one of said transitions falls, to provide a fine position.

24. An apparatus according to claim 10 or any one of claims I 1 to 23 where dependent directly or indirectly on claim 10 wherein the apparatus further comprises means for controlling the amount of light produced by the illumination means in response to a level of light collected by the detector

means so that the level of light collected by the detector means is within a predetermined range.

25. An apparatus according to any preceding claim wherein the first mechanical element comprises a generally cylindrical rod mounted for rotation about a longitudinal axis, and the marking is provided around the circumference of the rod.

26. An apparatus according to any one of claims 1 to 25 wherein the first mechanical element comprises a generally cylindrical rod mounted for rotation about a longitudinal axis and the marking is provided on an end face of the rod.

27. An apparatus substantially as hereinbefore described with reference to and / or as shown in the accompanying drawings.

28. A control system for a ratable mechanical element, the control system comprising an apparatus according to any one of claims 1 to 27, and a controller operable to receive angular position indication information from the apparatus and cause the mechanical element to rotate.

29. A method of determining the angular position of a first mechanical element with respect to a second mechanical element, the first mechanical element being provided with a marking which is optically readable and which comprises at least part of the mechanical element with a surface having a first reflectivity and a plurality of areas having a second reflectivity, the method comprising the steps of using a reading means to optically read the marking, and providing an output indicating the angular position of the first mechanical element with respect to the second mechanical element.

30. A method as claimed in claim 29 wherein the method further comprises the steps of comparing the output with a previous output, the previous output having been obtained from a reading taken a known time interval before, to determine the change in angular position of the first mechanical element with respect to the second mechanical element, and rejecting the output if the change in angular position exceeds a specified amount.

31. A method substantially as hereinbefore described with reference to the . accompanying drawings.

32. Any novel feature or combination of feature substantially as hereinbefore described and / or as shown in the accompanying drawings.