BACKGROUND TO THE INVENTION Precise monitoring of hole depth and slope during digging is required in many situations. The simples method of achieving this is for a person to periodically check the depth with a measuring device while the hole is being dug. This approach is both slow and dangerous for the person making the measurement. It is preferable if the measurements can occur concomitantly with the digging process.

Laser based devices have been used to measure depth and slope for trench digging however this technology suffers considerably from excess vibration and dust. These problems necessitate frequent cleaning and realignment. Laser based systems are also expensive and therefore unattractive in many situations.

Another manner of achieving depth monitoring is described in United States patent number 3997071 assigned to Laserplane Corporation. This patent describes a method and apparatus for indicating effective digging depth of a backhoe in which a trigonometric equation is used to calculate the

lowest point of the backhoe bucket from a knowledge of the length of the booms and a measurement of the relative boom angles. The relative boom angles are determined from transducers that produce electrical signals proportional to the angular displacement of the transducer shaft relative to the body. In order to use such an arrangement the transducers must be suitably mounted on a pivot axis of each boom. Such an arrangement is difficult to realise in practice because of problems associated with mounting the transducer. Furthermore, compounding inaccuracies in the transducers result in unacceptable errors in the system.

An alternate arrangement of depth monitoring has been described by Bachmann et al in United States patent number 4491927 assigned to The Digger Meter Corporation. The Bachmann system utilises a pair of inclinometer sensors on the boom and dipper stick of a digging machine to electrically measure the angle of inclination of these elements relative to the horizontal and calculates the depth in similar manner to Laserplane. The inclinometer sensors are pendulous mechanical devices that fail in the rugged environment of a digging machine.

None of the known prior art systems provide slope information as well as depth information. Furthermore, the known systems are insufficiently robust to be reliable in rugged applications.

OBJECT OF THE INVENTION It is an object of the present invention to provide a system of instrumenting a multi-element boom to determine the depth and slope of an end of the boom.

It is a further object of the invention to provide a robust system for depth and slope monitoring of a multi-element boom.

It is a yet further object to provide the public with an economic alternative to known depth monitoring systems.

Further objects will be evident from the following description.

DISCLOSURE OF THE INVENTION In one form, although it need not be the only or indeed the broadest form, the invention resides in a monitoring arrangement for a multi-element boom comprising: an essentially fixed, reference inclinometer for providing a reference signal; a plurality of further inclinometers arranged one per boom element; a processor means for calculating the position of an end of the multi- element boom using a trigonometric equation having as inputs the length of each boom element and processed signals from the inclinometers; and display means for displaying the position of the end of the multi- element boom.

In preference, the inclinometers are micromachined semiconductor accelerometers providing output signals proportional to the angle of the

accelerometer from the horizontal. The accelerometers suitably provide an output signai proportional to the sine of the angle of the accelerometer from the horizontal.

The reference inclinometer, processor means and display means may suitably be incorporated in a control unit.

In preference the display unit displays the position of the end of the multi-element boom in terms of displacement and slope.

The processor means suitably includes analogue to digital conversion means for converting analogue signals from the accelerometers to digital form suitable for processing in the processor means.

The monitoring arrangement for a multi-element boom may further comprise input means for setting a desired displacement and slope. An alert means suitably alerts an operator when the set displacement and slope are reached.

The control unit may also include memory means associated with the processor means. The memory means may store boom element length data for a number of commercially available booms. The storage means may also store calculated data from the processor means.

In a further form, the invention resides in a method of monitoring the position of an end of a multi-element boom including the steps of: positioning the end of the multi-element boom at a reference location and recording a zero value; setting a desired displacement and slope; moving the multi-element boom;

reading the output signals of inclinometers associated with each element of the multi-element boom; determining in a processor means the displacement and slope of the end of the multi-element boom by reference to the zero position; and displaying the displacement and slope on a display means.

The step of determining the displacement and slope of the end of the multi-element boom is suitably performed by reference to a look-up table.

Alternatively, the step may be performed by calculation of functions defining a trigonometric relation between elements of the multi-element boom.

In preference the method may further include the steps of : setting a desired displacement and slope; and providing an alert if the displacement and slope equal or exceed the set displacement and slope.

BRIEF DETAILS OF THE DRAWINGS To assist in understanding the invention preferred embodiments will now be described with reference to the following figures in which: FIG 1 is a schematic of a typical digging machine fitted with the monitoring arrangement; FIG 2 is a schematic of a sensor unit; FIG 3 is a schematic of a control unit; FIG 4 depicts the front of the control unit of FIG 3; FIG 5 is a vector diagram of the digging machine of FIG 1; and FIG 6 is a flowchart of the operation of the monitoring arrangement.

DETAILED DESCRIPTION OF THE DRAWINGS In the drawings, like reference numerals refer to like parts.

Referring to FIG 1, there is shown a schematic of a digging machine, generally indicated as 1. The digging machine is of the type having a body 2 movable across the ground on wheels 3. A multi-element boom 4 extends from the body 2. In the embodiment shown the multi-element boom 4 consists of a near arm 5, far arm 6 and bucket 7.

The near arm 5 is pivotable with respect to the body 2 at pivot 8.

Similarly, the far arm 6 is pivotable with respect to the near arm 5 at pivot 9.

The bucket 7 is pivotable with respect to the far arm 6 at pivot 10. It will be appreciated that the digging machine described is just one version of a class of machines that employ multi-element booms. In fact, the invention can be applied to any multi-element boom and is not restricted to digging implementations, although it is envisaged by the inventor that this will be the primary application.

Each element 5,6,7 has a respective sensor unit 11,12,13 mounted thereon. A fourth sensor unit 14 is mounted on the body 2 inside a control unit 15 which is described in detail below.

Each sensor unit 11,12,13,14 has the same structure. Schematic detail of sensor unit 11 is shown in FIG 2. Sensor unit 11 comprises an accelerometer 16 and conditioning element 17. The accelerometer 16 is a micromachined semiconductor device, such as Analog Devices ADXL05JH, that provides an analogue output signal on signal line 18. The output signal is proportional to the sine of the angle the accelerometer makes with

horizontal. Signal line 18 carries the analog signal to the control unit 15 for processing. The accelerometer is very robust and can withstand prolonge vibration and intense mechanical shock.

The conditioning element 17 includes power supply regulation, DC offset adjustment and a low pass filter to remove vibration effects. Power is supplie to the conditioning element 17 on power line 19. The conditioning element 17 ensures a stable voltage is supplie to the accelerometer 16 so that an accurate signal is provided to the control unit 15 on signal line 18.

The control unit 15 is mounted on the body 2. In diggers having a cabin 20, such as shown in FIG 1, it is convenient to mount the control unit within the cabin. In any event, the control unit 15 is mounted so as to be easily visible to the person operating the digger. Referring now to FIG 3, the control unit 15 consists of a processor means 21, such as a microcontroller, a power supply 22, a display means 23, associated memory means 24 and input means 25. As mentioned above, an accelerometer 14 is mounted in the control unit 15 to provide an absoute reference point on the machine 1 which is independent of the slope of the ground or the inflation of the tyres.

An input means 25 is provided for the operator to interface with the control means in the manner described below. A reset switch 26 may optionally be provided.

The memory means 24 is used to store calculated data from the processor means 21. It can also be used to store standard boom element lengths for a number of commercially available multi-element booms. The monitoring arrangement can be easily installed on a multi-element boom by

selecting the appropriate pre-stored setup information.

Cable 27 provides power to the accelerometers 11,12,13 and carries signals from the accelerometers back to the processor means 21. The <BR> <BR> processor means 21 includes a multiplexed analogue to digital converter that converts the analogue signals from the accelerometers to digital signals able to be processed by a microprocessor in the processor means.

A schematic of the front of the control unit 15 is shown in FIG 4. The display means 23 is a multi-element display providing a read-out of the current depth and slope. The depth and slope are calculated from a knowledge of the length of each boom element and the signals from each accelerometer.

In operation, the digger is manoeuvred and the bucket is positioned on the ground at the place digging is to commence. Power is applied to the control unit by working switch 25a and the processor means 21 sequentially steps through a set-up procedure. The position is zeroed by working the set switch 25b. The display then automatically steps to a depth selection mode.

The displayed depth is changed with change switch 25c. When the desired depth is displayed the set switch 25b is worked and the display steps to a slope selection mode. The displayed slope is changed with change switch 25c until the desired inclination of, say, a trench is displayed. The set switch 25b is again worked and the control unit 15 enters an operation mode in which the depth and slope is continuously displayed on the display 23.

A suitable alert can be displayed when the set position is reached.

For instance, the display may flash or an audible alarm may sound.

The calculations required for determining the depth and slope are best described by reference to FIG 5 which provides a vector diagram of the digger of FIG 1. It is conventional for angles below the horizontal to be considered as positive and angles above the horizontal to be considered as negative. The near arm 5 has a length of R1 and makes an angle X with the horizontal plane though pivot 8. The pivot 9 between the near arm 5 and the far arm 6 is at a distance L, and a height H, from the pivot point 8 and are calculated by: H, = R, Sin X L, = Ri Cos X The far arm 6 has a length of R2 and makes an angle of Y with the horizontal plane through pivot 9. The pivot 10 between far arm 6 and bucket 7 is at a distance L2 and height H2 from the pivot point 9 and are calculated by : <BR> H2 = R2 Sin Y<BR> L2 = R2 Cos Y Similarly, the end of the bucket 7 can be calculated by: <BR> H3 = R3 Sin Z<BR> L3= R3CosZ The end of the bucket can then be easily calculated as being at a depth D and position P being: D = H, + H2 + H3<BR> P = L, + L2 + L3

The depth D is displayed at the control unit. The slope S is calculated by S = P/D, and also displayed at the control unit. If desired the angle from the pivot point 8 to the end of the bucket can be calculated as tan-'S.

Although the above calculations can be made in the processor means 21, the inventor has found that the use of a look-up table is preferable to calculating the trigonometric functions. The look-up table is stored in memory means 24 and is programmed for each specific machine to which the monitoring arrangement is applied.

The use of a micromachined semiconductor device such as Analog Devices ADXL05JH facilitates the use of look-up tables because the sine of the angle is output directly. Since the lengths of each element of the multi- element boom are fixed and known a look-up table can be constructed that is specific to the particular multi-element boom and provides the depth and slope for each set of accelerometer readings. If other forms of inclinometer are used it may be necessary to calculate the trigonometric functions in the micro-controller thereby adding significantly to the computing power and time required.

As mentioned above, the accelerometer 14 provides an absoute reference. The reference signal R can be recorded and subtracted from the signals X, Y and Z to compensate for minor movements of the digging machine 1 while digging. It will be appreciated that during the digging process the digging machine will move slightly due to the load applied to the boom. In particular, the pivot 8 may get lower as the digging machine 1 sinks slightly into the ground. The reference signal R compensates for this

lowering of the pivot point 8. Such compensation is not available in known prior art systems.

The flowchart of FIG 6 summarises the operation of the monitoring arrangement for a multi-element boom. The unit is switched on by the switch 25a at the control unit 15. The display 23 of the control unit 15 shows the set depth and the message,"C for change; S for set". If the displayed depth is correct the operator presses the set button 25b, if not the depth is changed until it is correct and the set button is pushed. The set slope is then displayed and a similar procedure is followed to set the desired slope.

When the desired depth and slope have been set the monitoring arrangement is zeroed by placing the extremity of the multi-element boom at a start point and setting the position as zero. The processor means 21 then loops through a programme to read each sensor in turn and calculate the distance and height according to the method described above. When the calculated depth and slope reach the set points an alarm sounds to alert the operator of the multi-element boom. If the set slope and depth have not been reached the programme loops through a further set of readings and calculations.

Programming of set slope and depth information facilitates automatic operation of the multi-element boom. The main purpose of an operator is to ensure that the correct task is performed by, for example, the digger. The monitoring arrangement can be interfaced with a control system to remove the necessity of the operator.

It will be appreciated that the monitoring arrangement for a multi-

element boom has been described with reference to a digging machine for convenience and need not be limited in its application. Furthermore, the accelerometers have been described as operating in one plane although devices are available that provide signals indicative of movement in two or even three planes. The monitoring arrangement can therefore be implemented for determining the position of a multi-element boom in three dimensions. For implementation in three dimensions it is necessary to make two angle measurements using inclinometers as described. The measurement of angle in the horizontal plane may conveniently be made using, for example, a magnetic compass. As the look-up tables become more complex for the three dimensional case it is appropriate to calculate the trigonometric functions.

Throughout the specification the aim has been to describe the preferred embodiments of the invention without limiting the invention to any one embodiment or specific collection of features.