AU563897B2

AU563897B2 - Contact sensing device

Info

Publication number: AU563897B2
Application number: AU31508/84A
Authority: AU
Inventors: Michael David Crooke; J.P. Trevelyan
Original assignee: University of Western Australia
Current assignee: WESTERN AUSTRALIA THE, University of
Priority date: 1983-07-05
Filing date: 1984-07-05
Publication date: 1987-07-23
Anticipated expiration: 2004-07-05
Also published as: NZ208780A; EP0148910A1; WO1985000445A1; ZA845151B; AU3150884A

Description

CONTACT SENSING DEVICE .

BACKGROUND OF THE INVENTION

This invention relates to the sensing of contact between a mechanism or device, such as a cutter or grinder and the surface of an object, such as the skin of an animal, or a metallic object being ground. More particularly, but not exclusively, the invention is applicable to the sensing of contact between a shearing cutter and the skin of a sheep in an automated shearing operation, or between a cutter and the bone or other selected part as in cutting flesh from a bone in an automated butchering operation.

In controlling the movements of a shearing head in an automated shearing system, it is important that the controlling mechanism be aware of the initial contact between the comb of the cutter and the skin of the sheep, the so-called "point of cut", so that an adjusting movement of the cutter may be effected to avoid damage to the shee_p.

Experimental work has shown that the detection methods involving measurement of electrical resistance and capacitance, whilst being useful in providing proximity data for control of the shearing operation, are not altogether satisfactory for sensing the "point of cut" since they tend to be adversely affected by the condition of the wool, particularly when wet, salty or especially greasy. SUMMARY^" OF^" THE INVENTION

It is an object of the present invention to provide an alternative means of sensing contact between the cutter and the skin of the sheep. Although the present invention is particularly applicable to sensing of contact between the cutter and the skin of the sheep, it is equally applicable to other situations in which the point of contact between a mechanism or device and the selected part of an object must be detected. The invention provides a means for sensing contact between a mechanism or device and an object comprising means for generating a signal capable of being transmitted through.the^' body of the object on contact between the mechanism or device and the object, sensor means adapted to engage the object and capable of detecting the signal transmitted through the body of the object, and means for utilizing the detected signal to control the mechanism or device.

In one form of the invention applicable to sensing contact between a wool shearing cutter and the skin of a sheep, the signal capable of being transmitted through the body of the sheep is preferably generated by the cutter itself which typically generates an audio frequency signal capable of being transmitted through the sheep's body on contact between the cutter and the skin of the sheep. Although various forms of sensor have been considered, including acoustic detectors utilizing a stethoscope held against the animal, the most practical device so far investigated appears to be an accelerometer which detects cutter vibrations transmitted through the animal.

Although the use of the cutter as the source of the signal to be transmitted through, the body of the sheep is particularly convenient, it should be appreciated that factors that such, as variable cutter speed may make it desirable to mount a driven vibration source on the cutter so that the most suitable signal may be independently generated and controlled as desired.

In the above described preferred form, the accelerometer is preferably mounted on a vibration receiving plate which is preferably shaped in the form of a cradle for supporting the body of the sheep during the shearing operation. Alternatively, the sensor may be arranged on a separate support inserted under the sheep when supported for shearing, inserted in the sheep' mouth or otherwise held in contact with a suitable part of the sheep's body. However, for reasons described below, the use of a vibration receiving plate is preferre It is also preferred that the vibration receiving plate should be supported by an insulating medium which isolates the plate from extraneous sources of vibration. The use of the vibration receiving plate in the above form is preferred as it serves to increase the area of contact between the sheep and the detecting body so that attenuation of the cutter vibration caused by increases in separation between the point of shearing and the detector is reduced. BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will now be described with reference to the accompanying drawings in which:

Figure 1 is a schematic diagram of an experimental arrangement embodying the invention; Figure 2 is a perspective view of an experi¬ mental vibration receiving plate for use with the present invention; Figure 3 is a circuit diagram of a bandpass •filter suitable for use with the present invention; Figure 4 is a block diagram showing the bandpass filter in use; and Figure 5 is a schematic block diagram of an alternative signal conditioning arrangement. DESCRIPTION OF PREFERRED EMBODIMENTS

As a result of problems encountered with electrical methods of proximity detection, research was directed towards sonic techniques involving the transmission of sound waves through the sheep as a medium to sense comb/skin contact. Although the transmitted sound could have been an audio or ultra¬ sonic frequency generated artificially on the cutter or an audio frequency generated by the cutter mechanism itself, it was decided that the cutter mechanism was the logical choice as the transmitter of the sound.

Experiments were conducted with the direct detection of vibrations transmitted through a sheep using a piezo-electric accelerometer 1 connected to a support 2 for the sheep 3 as shown in the schematic block diagram of Figure 1 of the drawings. The accelerometer 1 such as a B. & K. 4367, was connected to an optional pre-amplifier 5, such as a B. & K. 4335, and the output from the pre-amplifier 5 was connected to a power amplifier 6 to which a display cathode ray oscilloscope 7 as attached. For comparison purposes, a resistor sensor 8 was attached to a mouth electrode 9 and the output was displayed both on a chart recorder 10 and, after being RMS to DC converted at 11, on the CR0.7. "The shearing handpiece 4 was found to be an adequate source of vibration and the vibration originatin from the cutter motion was about 80Hz. Choices for the mounting of the accelerometer 1 were direct attachment to the cradle which supports the sheep during shearing, or attachment to a separate plate which may be inserted under a sheep on the cradle.

Preliminary experiments showed various problem relating to the reception of extraneous vibrations and attenuation of the sonic signal in the sheep. The main source of extraneous vibrations included cutter vibrations transmitted through the structural members supporting the sheep, hydraulic system vibrations, support structure movements and sheep struggling. To reduce the hydraulic vibrations, an hydraulic attenuator was installed in series with the output of the hydraulic pump and was found to attenuate hydraulic vibtations up to 700Hz and since the piston pump pulsation frequency was 210Hz, this substantially reduced the hydraulic vibration problem.

In addition to the above,.the sonic signal received through the sheep was found to be more attenuate as the separation between the point of shearing and the accelerometer was increased. Attenuation was especiall prevalent when shearing,the neck or legs of the sheep.

In view of the above problems, a vibration receiving plate 12, of open V shaped configuration as shown in Figure 2 of the drawings, was designed to reduce the effect of extraneous vibrations and to increas the coupling between the sheep and the accelerometer 1 to reduce the problems of attenuation. The use of a large plate to detect vibrations was determined to be the best alternative after tests using a vibration

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O probe in direct contact with the skin of the sheep (a rod attached to the accelerometer) gave poor results. The vibration plate was designed to contact as much of the sheep's torso as practicable and foam padding 13 was glued to the underside of the plate to provide isolation from extraneous vibrations. A resonant beam 14 tuned to the frequency of the cutter was mounted on the vibration plate 12 and accelerometer mounting points (not shown) were provided on the beam 14 and on the plate 12 itself. Where the cutter speed is likely to be variable, the accelerometer 1 is preferably mounted on the plate 12. However where the source of vibration is likely to be constant, the accelerometer^'is preferably mounted on the beam 14. Hand shearing experiments produced chart recorder results which showed a satisfactory correlation between the sonic and resistive sensing signals as well as a substantial improvement in the sonic signal to noise ratio. -Nevertheless, sheep struggling generated low frequency vibrations so it was necessary to keep the sheep tightly constrained. In addition, the heart beat of the sheep caused regular spikes in the sonic sensor output and the tuned beam was found to have a relatively long response time (undamped: more to .2 sec, damped 0.1 sec.) .

Further investigation of the output signal showed that the fundamental frequency received was of substantially higher power than other extraneous vibrations and it was therefore decided to construct a bandpass filter to effectively extract the desired signa The circuit diagram of a suitable bandpass filter tunable from 60Hz to 160Hz is shown in Figure 3 of the drawings and will be well understood by a person skilled in the art without further description. The bandpass filter 15 was connected to the experimental apparatus 7 Figure 1 in the manner shown in Figure 4 of the drawings and chart recordings were made of the simultaneous sonic and resistive sensor outputs during hand-shearing. These recordings showed a substantially improved signal to noise ratio for sonic sensing with the heart beat and most other extraneous vibrations being rendered insignificant.

As mentioned above, it may be necessary in order to overcome problems relating to variable cutter speed to mount an oscillater driven vibration source on the cutter. Alternatively, signal conditioning techniques may be used to eliminate the problem of variable cutter speed and to decrease response times. For example, the sonic sensing signal may be multiplied or correlated by a signal derived directly from the cutter drive, as shown schematically in Figure 5 of the drawings.

Although the above description is restricted to detecting contact between a shearing cutter and a sheep's skin, the invention is equally applicable to other arrangements in which contact between a mechanism and an object is required to be sensed, for example, contact between a motor driven knife and bones in automated butchery, and_^contact between a grinder and an object being ground.

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Claims

The claims defining the invention are as follows:

1. A device for sensing contact between a mechanism or device and an object comprising means for generating a signal capable of being transmitted through the body of the object on contact between the mechanism or device and the object, sensor means adapted to engage the object and capable of detecting the signal transmitted through the body of the object, and means for utilizing the detected signal to control the mechanism or device.

2. The sensing device of Claim 1, wherein said sensor means comprises means for supporting the object and capable of transmitting said signal, accelerometer means attached to said supporting means and capable of detecting said signal transmitted through said object, and means for utilizing the output signal for said accelerometer to control the mechanism or device.

3. The sensing device of Claim 2, wherein said mechanism or device generates an audio signal which generates vibrations in said object or contact therewith, said vibrations being detected by said sensor means to produce an output signal indicative of contact between said mechanism and said object.

4. The sensing device of any preceding claim, wierein said object is a sheep and said mechanism is a shearing cutter or other wool removal device, said sheep being supported by means of a vibration receiving plate shaped to provide maximum contact between the plate and the sheep without interfering materially with the shearing process.

5. The sensing device of Claim- 4, wherein said plate is shaped in the form of a cradle for supporting the sheep during the shearing operation.

6. The sensing device of Claim 5, wherein said plate is of open V configuration and is supported by an insulating medium which isolates the p_ate from extraneous sources of vibration.

7. The sensing device of Claims 4, 5 or 6, wherein said plate has a resonant beam attached thereto and tuned to the vibration frequency of said signal associated with said shearing cutter or other wool removal device, said sensor means being attached to said resonant beam.

8. A device for sensing contact between a mechanism or device and an object substantially as described with reference to Figure 1, Figures 1 and

2, Figures, 2, 3 and 4 or Figure 5 of the accompanying drawings.