GB2208947A

GB2208947A - System for controlling the angular position of an implement

Info

Publication number: GB2208947A
Application number: GB8818954A
Authority: GB
Inventors: Robert David Weston
Original assignee: MCCONNEL F W Ltd; FW McConnel Ltd
Current assignee: MCCONNEL F W Ltd; FW McConnel Ltd
Priority date: 1987-08-19
Filing date: 1988-08-10
Publication date: 1989-04-19
Also published as: FR2620148A1; GB8818954D0; GB8719583D0

Abstract

The angular position (attitude) of an implement 25 e.g. a vegetation cutter, which is pivotable about an axis 26 at one end of an articulated arm 17, 18, 22 is controlled by a system which comprises a sensor 32 which generates a first electrical signal corresponding to the angular position of the implement relative to a datum plane, a device 38 for generating a second electrical signal which corresponds to a desired position of the implement 25, and a control circuit 34 operating an actuator 27 in accordance with a difference between the first and second signals. <IMAGE>

Description

"SYSTEM FOR CONTROLLING ANGULAR POSITION OF AN IMPLEMENT" This invention relates to a system for controlling the angular position of an implement which is pivotally mounted on an articulated arm, and in particular relates to control of a cutter for vegetation. It is known to mount vegetation cutters on an articulated arm which is operable by separate rams which can respectively extend the reach of the arm and raise or lower the cutter. A further ram is operable to pivot the cutter with respect to the arm.

In such arrangements altering the reach of the arm or changing the working height of the cutter will result in alteration of the attitude of the cutter relative to the work, and this alteration must be corrected by operation of the aforesaid further ram. An operator is thereby required to synchronize operation of all three rams in order to maintain the required attitude of the cutter.

An object of the invention is to provide a system by means of which, in an apparatus of the foregoing general kind, the number of controls necessary to maintain a required attitude of an implement is reduced, whereby operation of the apparatus is simplified.

According to the invention a system for controlling the angular position of an implement which is pivotally movable about a first axis t one end of an articulated arm, comprises a sensor for generating a first electrical signal corresponding to a sensed angular position of said implement relative to a datum plane, means for generating a second electrical signal implement relative to said datum plane, and an actuator responsive to a difference between said signals for moving said implement in a direction to reduce said difference.

In an alternative embodiment said sensor comprises a fibre optic element having its respective opposite ends secured to said implement and to a relatively fixed structure, a light source for applying light of a known wave length to one end of said fibre optic element and means for generating said first electrical signal in response to changes in wave length of said light during passage through said element, as a result of flexure thereof.

Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings in which: Figure 1 is an end view of a vegetation cutting apparatus mounted on a vehicle, Figure 2 is a block diagram of a control system forming part of the apparatus of Figure 1, Figure 3 is a diagram of an alternative form of control circuit forming part of Figure 2, Figure 4 is a diagram of a further alternative form of control circuit forming part of Figure 3, Figure 5 is a view corresponding to Figure 1 of an apparatus incorporating an alternative form of control system, and Figure 6 is a diagram of the control system of Figure 3.

As shown in Figure 1 a hedge-cutting apparatus, indicated generally at 10, is mounted on a support frame 11 carried by a tractor 12. The frame 11 includes a main upright support member 13 on whose upper end a carrier plate 14 is mounted for movement about a pivot 15 by a ram 16. Arm elements 17, 18 are mounted on the plate 14 for movement about respective pivots 19, 20. The arm element 17 is pivotally movable by a ram 21 which acts between the element 17 and the plate 14. The ends of the elements 17, 18 opposite to the pivots 19, 20 are connected to a further arm element 22 by means of pivots 23, 24. A cutter head 25 is mounted on the arm 22 by a pivot 26. Spacing of the pivots 19, 20 has the effect that extension of the ram 21 extends the reach of the arm formed by the elements 17, 18, 22 whilst keeping the height of the head substantially constant.

The cutter head 25 is movable about the pivot 26 by a ram 27 acting through a link 28 which enyages a crank 29 which is in turn fast with the cutter head 25 for pivotal movement therewith. A slave arm 30 constrains the connection between the link 28 and ram 27 to move arcuately about a pivot 31. A portion of the element 22 between the pivots 26, 31, the crank 29, the link 28 and the arm 30 form a four-bar chain such that the line of action of the ram 27 relative to the element 22 remains substantially constant throughout the full range of pivotal movement of the cutter head 25. The rate of this pivotal movement is thus substantially proportional to the rate of linear movement imparted by the ram 27.

Mounted on the cutter head 25 is a sensor 32 which provides, on a line 33, an electrical signal which corresponds to the angular position of the cutter head 25 with respect to a datum plane. In the present example the sensor 31 comprises a pendulum and an angular displacement transducer responsive to the relative positions of the pendulum and the head 25.

In this case the datum plane may be considered as that in which the transducer has zero output. In an alternative embodiment the sensor 32 may comprise a gyroscope and a transducer which may similarly be set to any desired datum plane.

As shown in Figure 2 a control circuit 34 includes a buffer amplifier 35 which receives signals on the line 33, a low-pass filter circuit 36 which eliminates spurious high frequency oscillations of the head 25 and a differential amplifier 37. One input of the amplifier 37 is supplied from the filter 36 and a further input is provided on a line 42 by a device 38 operable to select a required position of the cutter head 25. The amplifier 37 is also responsive to a limiting signal from a circuit 39 such that the system as a whole does not respond to minor angular variations of the head 25 and will thus operate in a stable fashion.An error signal corresponding to a difference between desired and sensed angular positions of the head 25 is supplied on a line 40 from the amplifier 37 to an electro-hydraulic valve 41 which selectively connects a pressure supply line P and a low pressure return line R to the ram 27. The arrangement is such that that the ram 27 is moved in a direction causing the sensed position signal on line 33 to correspond to the signals from the selector device 38.

The sensor 32, the device 38 and the circuit 39 are energised by a reference voltage V.

It is desirable that the system shall not be responsive to spurious oscillatory inputs originating at the sensor 32 and which may be lower than the cut-off frequency of the low-pass filter 36 described above.

It is also desirable that the cutter head 25 shall be movable, for example during parking or stowage, independently of signals from the sensor 32.

Figure 3 shows an alternative form of control circuit 50 which allows both of these requirements to be met.

The control circuit 50 is intended for use with a proportional electo-hydraulic valve in which a flow control element is positioned by axially aligned push-pull solenoids which are simultaneously energised by signals of equal magnitude and opposite polarity.

The circuit 50 has an input buffer amplifier 51 which is, by way of a switch 52, responsive either to the signal on line 33 from the sensor 32, or to a biassing voltage Vb. The output of the buffer amplifier is applied to a low pass filter 53 whose cut-off frequency is adjustable by a signal on line 54. The signal on line 54 is variable by means of an adjustable voltage Va which is applied through a summing junction 55. The summing junction 55 is also supplied with the output froma voltage generator 56 which provides an output signal in response to alternation of the signal level at the output of amplifier 51. In response to such alternation the cut-off frequency of the filter 53 may thereby ve reduced below that set by the voltage Va.

The output from the filter 53 is applied to the non-inverting and inverting inputs of respective difference amplifiers 57, 58. The respective inverting and non-inverting inputs of the amplifiers 57, 58 are connected to the line 42 from the selector device 38.

The outputs from the amplifiers 57, 58 are supplied to respective pulse-width modulator circuits 59, 60 which are also responsive to constant frequency square wave signals from a generator 61. Output signals on lines 61, 62 from the circuits 59, 60 respectively are applied to respective push-pull solenoids of the electohydraulic valve referred to above.

The frequency of the pulse width modulated signals on lines 61, 62 is about 27Hz and is adjusted to suit the operating time of the solenoid valve. Operation of the switch 52 to connect the bias voltage Vb to the buffer amplifier 51 has the effect that the implement 25 is positioned in accordance with a difference between the signal on line 42 and a reference voltage dervied from the bias Vb.

Figure 4 shows a further alternative form of control circuit 70, also for use with an electro-hydraulic valve having push-pull solenoids. In circuit 70 the signals on line 33, 42 are applied to respective A-D converters 71, 72 whose outputs are applied to a digital micro computer 73 which is programmed to provide the filtering and difference functions of the circuit 50 (Figure 3) and to permit operation in response to the signal on line 42 only.

Outputs from the computer 73 are applied through D-A converters 74, 75 to a pulse-width modulating arrangement 59, 60, 61 as described above.

The arrangement shown in Figure 5 corresponds mechanically to that shown in Figure 1, and the selector device 38 similarly provides a signal corresponding to a desired position of the cutter head 25. Mounted on a relatively fixed part of the tractor 12, for example on the frame 11, is a circuit 70 which is responsive to signals from the selector device 38.

As shown in Figure 6 the circuit 70 includes a source 71 of light of a knon wave length. Light from the source 71 is supplied to one end of a fibre optic element 72 whose other end is secured to the head 25 and is directed towards a reflector 73. An identical fibre optic element 74 runs parallel with the element 72 and receives light from the reflector 73. Light from the element 74 passes to a detector 75 which generates an electrical signal corresponding to the wave length of th received light. Though the wave length of light passing through optical fibres is changed by bends in those fibres, equal bends in opposite directions cancel out those changes, so that a change in wave length experienced over the whole length of a fibre is dependent only on an angle between the fibre ends. The electrical signal from the detector 75 thus corresponds to the angle between the cutter head 25 and a datum plane which is determined by the set-up of the circuit 70. A comparator arrangement, generally similar to the circuit 34 in Figure 2, is responsive to the desired and sensed position signals from the device 38 and detector 75 respectively to provide an error signal to an electro-hydraulic valve 41 which controls the ram 27 as described with reference to Figures 1 and 2.

It will be apparent that in an alternative arrangement the reflector 73 and fibre optic element 74 may be dispensed with and the detector 75 mounted on the cutter head 25, signals from the detector 75 being passed to the circuit 70 through a suitable extended conductor.

Claims

CLAIMS.

1. A system for controlling the angular position of an implement which is pivotally movable about a first axis at one end of an articulated arm, comprising a sensor for generating a first electrical signal corresponding to a sensed angular position of said implement relative to a datum plane, means for generating a second electrical signal corresponding to a desired angular position of said implement relative to said datum plane, and an actuator responsive to a difference between said signals for moving said implement in a direction to reduce said difference.

2. A system as claimed in claim 1 in which said sensor is a gravity responsive device mounted on said implement.

3. A system as claimed in claim 1 or claim 2 which includes filter means for eliminating surious oscillations from said first electrical signal.

4. A system as claimed in claim 3 in which said filter means is a low pass filter.

5. A system as claimed in claim 4 which includes means for adjusting the cut-off frequency of said low pass filter.

6. A system as claimed in claim 5 which includes means for generating a control voltage in response to an alternating component of said first electric signal, the cut-off frequency of said low-pass filter being responsive to said control voltage.

7. A system as claimed in any preceding claim which includes means for substituting a bias voltage for said first electrical signal.

8. A system as claimed in any preceding claim which includes means responsive to said difference between the first, and second electrical signals, for generating signals of constant frequency and of a pulse width which is modulated in dependence on he magnitude of said difference, said actuator being responsive to said pulse width modulated signals.

9. A system as claimed in claim 1 or claim 2 which includes a digital computer responsive to said difference between the first and second electrical signals, and means responsive to an output signal from said computer, for generating signals of constant frequency and of a pulse width which is modulated in dependence on the magnitude of said difference, said actuator being responsive to said pulse width modulated signal.

10. A system as claimed in claim 1 in which said sensor comprises a fibre optic element having its respective opposite ends secured to said implement and to a relatively fixed structure, a light source for applying a light of known wavelength to one end of the fibre optic element, and means for generating said first electrical signal in response to changes in wavelength of said light during its passage through said element as a result of flexure thereof.