GB2120419A - Control of agricultural instruments - Google Patents

Abstract

A draught force controller for an agricultural tractor includes transducers (6A, 7A) for generating a signal d representing the instantaneous value of the draught force applied to an earth working implement, and means (34) for generating an error signal e1 representing the difference between a desired value ds and the actual instantaneous value d of draught force. A control signal E is derived having a component K1e1 representing the instantaneous value of the error signal and a component <IMAGE> representing the time derivative thereof (or of the draught force). This control signal governs operation of hydraulic actuators for varying the depth of engagement of the implement in the soil. The control signal can be overridden if the actual depth of engagement falls outside a predetermined range, if the wheel slip exceeds a predetermined value, or if the position of the linkage relative to the tractor falls outside a predetermined range. <IMAGE>

Description

SPECIFICATION Control of agricultural implements This invention relates to the control of agricultural implements in particular to means for the control of the draught force acting on a soilengaging implement drawn through the soil.

In modern farming, as with other industrial activity, it is important that operations are carried out as efficiently as possible so as to make maximum use of manpower and equipment. In ploughing or other earth-working operations important factors are the depth to which the implement works the soil and the draught force applied to the implement. These two factors are closely related, since if the soil condition remains constant, draught is a function of depth.

By control of the tractor implement consistent working of the soil is made possible, and the tractor can be made to work near to its maximum power output at all times, so that a given task can be completed employing men and capital equipment for the minimum time. If a constant draught force can be maintained, the need for constantly changing gear is obviated.

Where the implement is mounted on a standard 3-point linkage of an agricultural tractor, one known method of control involves the sensing of the force in the top link of the 3-point linkage. It can readily be shown that this force is equal to a combination of the draught and vertical forces acting on the implement Systems have also been developed which sense and sum top and bottom link forces to produce a control signal proportional to the draught force only. In either case, a control system is provided in which a desired value of top link force or draught force (or by inference, depth) can be set. The control system compares the sensed value with the set value, and generates an error signal. If the error exceeds a predetermined value, the system activates some means by which the depth of the implement can be adjusted in the appropriate direction to reduce the error.

In systems of the kind described, the performance of the control system is influenced principally by four system, parameters, namely: i) deadband, which is the range of error which the system is designed to tolerate before any corrective action is instigated.

ii) the relationship between the rate of corrective movement and the magnitude of the developed error; iii) the maximum rate of corrective movement of which the system is capable; and iv) the delay time between sensing of an error of a magnitude which exceeds the deadband, and the instigation of corrective action.

The inventors have studied the effect of these parameters on top link and pure draught force sensing systems. It was found that reducing deadband and increasing rates of movement improved performance, but the possible degree of improvement with such systems was limited by the onset of instability in the control systems.

The inventors have found by investigation that pure draught sensing provides a basis for a control system which is more stable but less responsive than a top link sensing system. The inventors have further discovered, by mathematical analysis, that top link force includes a component related to the rate of change of draught force, i.e. to the time derivative thereof. This explains why a top link sensing system is more responsive, since the derivative component provides a control signal as soon as an error starts to form. The response is more or less immediate and there is no delay whilst an error forms of sufficient magnitude to initiate a control action. However, as already noted, a control system based on top link sensing as inherently less stable than one based on pure draught sensing.

The present invention seeks to make possible the provision of an improved control system which asserts closer control than prior art systems without correspondingly greater instability of control.

Accordingly, the present invention provides a control system for controlling the draught force acting on an agricultural soii-working implement, comprising means for sensing the instantaneous value of the draught force means for generating a control signal said control signal including a component representing the said instantaneous value and a component representing the time derivative of the said instantaneous value, means for setting a desired value of draught force or a quantity related thereto, means for generating a set signal representing the said desired value, means for generating a set signal representing the said desired value, means for generating a draught force error signal representing the difference between the control signal and the set signal and actuating means controlled by the value of the draught force error signal for altering the depth of engagement of the implement in the soil in a sense such as to reduce the magnitude of the error signal.

Preferably the control system includes means for controlling the variation in depth of engagement of the implement in the soil.

Desirably the control system includes means for limiting the slip of a driving wheel of a tractor by which the implement is drawn.

The invention will now be further described by way of example only, with reference to the accompanying drawings, of which: Figure 1 shows, in diagrammatic form, an agricultural tractor and implement, provided with a draught control system in accordance with the invention, and Figure 2 is a block diagram illustrating in greater detail the control system shown in Figure 1.

Figure 1 shows an agricultural tractor which draws a chisel plough 2 mounted on the tractor by means of a standard 3-point linkage 3. The 3point linkage comprises a nearly horizontal top link 4, and a pair of nearly horizontal bottom links 5.

The top link 4 is pivotally mounted on the structure of the tractor by means of a load pin 6 which includes a transducer 6A which can provide an electrical signal representing the force transmitted from the top link to the structure of the tractor, which is equal to the force acting longitudinally in the top link itself.

The bottom links 5 are pivotally mounted on the structure of the tractor by means of load pins 7 which similarly each include a transducer 7A which can provide an electric signal representing the force acting longitudinally of its respective bottom link.

The plough 2 includes a frame 8 having pivotal attachment points by which the top and bottom links are pivotally attached thereto. The implement can be raised and lowered by means of a pair of hydraulic ram devices 9, one acting on each bottom link through a lift arm 10 and lift rod 11.

Each arm 10 is nearly horizontal, and is pivoted to the structure of the tractor at 12, and to the rod 11 at 13. Each rod 11 is nearly vertical, and is pivoted to the bottom link at 14. The rams of the devices 9 can be extended and retracted under the influence of hydraulic fluid supplied thereto via hydraulic lines 1 5. Control of the hydraulic fluid supply is effected by means of a servo valve 1 6.

High pressure fluid can be supplied from a reservoir 20 to the valve 1 6 via a hydraulic line 17 by means of a pump 1 8 driven from the tractor engine. Fluid at low pressure can be returned from the valve 16 via low pressure line 21 to the reservoir 20.

Situated in the cab of the tractor is a control panel 30 containing electronic components for the control of the implement 2. Components mounted on the control panel are indicated within the dotted enclosure 30 in Figure 2.

As shown in Figure 2, the outputs of transducers 6A and 7A are transmitted by respective electrical lines 31, 32 to the control panel, where they are added together in a summing amplifier 33. It can readily be shown that output d of the summing amplifier 33 is an electrical signal representing the total horizontal draught force applied by the tractor 1 to the plough 2. The signal d is transmitted via electrical line 32 to a summing amplifier 34.

Also mounted on the control panel 30 is a potentiometer device 35 which can be set manually in accordance with a dial thereon, to provide an electrical output signal ds. The output signal ds represents a desired value of draught force as indicated on the dial of the device 35. The signal ds is also transmitted to the summing amplifier 34, which provides an output signal e, representing the difference between the signals ds and d. That is, e1 is an error signal representing the difference between the desired value of draught force as set on the device 35, and the actual value as sensed by the transducers 6A, 7A.

The error signal e, is fed to a proportional amplifier 36 which provides an output electrical signal K,e, where K1 is the constant gain provided by the amplifier. The signal e, is also fed to a derivative amplifier 37 which provides an output signal K2 de l dt' where K2 is the constant gain of the amplifier, and de dt1 is the time derivative of the error signal e1. These two output signals are added together in the summing amplifier 39 to give a combined output control signal E = K1e1 + K2 de dt1 which is fed to a priority switch 40. If the signal E exceeds a predetermined magnitude (deadband) the priority switch sends a command signal via an electrical line 41 which activates the electrically controlled servo valve 1 6.If the value of E is positive, the switch 40 actuates the valve 16 to connect the line 1 5 and hence the rams 9 with the low pressure line 21, so as to lower the implement and increase the draught force. If E is negative, the switch 40 actuates the valve 1 6 to connect the line 1 5 and hence the rams 9 with the high pressure line 17, so as to raise the implement and reduce the draught force. As soon as the value of E is restored to zero, the priority switch 40 activates the servo valve to close off all connections to the line 15, so that the rams 9 are locked in place.

By including in the control signal E a component dependent on the derivative de dt1 (equivalent to the derivative of the actual draught force), the system is made more responsive than a system controlled by a pure draught force signal to changes in draught force. That is to say, corrective action is initiated at an earlier stage, before any substantial error has developed between actual and desired values of draught force. However, unlike systems which rely upon top link sensing, by generating a separate derivative component the extent of its influence can be determined independently of the direct draught force component. The stability of the control system can thus be maintained by appropriate choice of the constants K1 and K2.

The control of the priority switch 40 by the control signal E is subject to over-ride by three over-ride circuits 50, 60, 70.

The over-ride circuit 50 is designed to ensure that the variation in the depth of engagement of the implement in the soil can be maintained within a certain band of values irrespective of the value of the draught force. This is plainly desirable to provide a minimum acceptable degree of soil preparation, and to avoid undesirably deep working.

The circuit 50 is provided with an electrical input signal from a transducer 51. As shown in Figure 1, the transducer 51 comprises a wheel 52 carried on the plough 2. The wheel runs on the surface of the ground as the plough shares are drawn therethrough. As the wheel rises and falls relative to the plough frame, it drives a potentiometer device which provides an electrical output signal h representing the depth of engagement of the ploughshares in the soil. A similar function might by fulfilled alternatively by an ultrasonic device, for example. The signal h is transmitted via an electrical line 53 to a summing amplifier 54 in the control panel 30. A desired mean depth can be set on a second potentiometer device 55 located on the control panel, and the device 55 sends a signal h, representing the set value to the summing amplifier 54.The amplifier 54 gives an output signal e2 representing the difference (hs - h). The signal e2 is transmitted to a discriminator device 56 which can be set by means of a dial on the control panel to a desired value e25 of the variation in depth which is to be tolerated. If le2l > e25 then the discriminator 56 sends an over-ride signal to the priority switch 40.

The effect of this is, irrespective of the value of signal E at the time, to cause the priority switch 40 to actuate the servo valve 1 6 to connect low pressure line 21 or the high pressure line 17 to the rams 9, as appropriate to increase or reduce the depth of the implement. When the rams have adjusted the height of the implement so that the signal e2 again falls within the set limits, the discriminator 56 stops sending, and control under signal E is re-asserted.

The circuit 60 is similar to the circuit 50, and is designed to obviate excessive slip of the tractor's driving wheels. For example, ground conditions may vary so that occasionally the set value of draught force leads to excessive wheel slip.

The circuit 60 includes a wheel slip sensor 61 (not shown in Figure 1) which can take one of a number of forms.

For example, the transducer may include a radar device for measuring the forward speed of the tractor. Alternatively, forward speed may be sensed by measuring the rotational speed of a non-driven wheel rolling with the tractor. This can be an extra, non-load bearing wheel, or in the case of two-wheel drive, can be one of the tractor's two non-driven wheels. The speed of the one or more driving wheels is readily sensed for comparison e.g. by means of a tachometer.

The slip sensor 61 sends an electrical output signal W to a summing amplifier 64, which also receives an input w5 representing a desired percentage slip which can be set on a potentiometer device 65 by means of a dial on the control box 30. The output e3 of the amplifier 64 representing the difference (WsW) is sent to a discriminator device 66, on which can be set, by means of a dial on the control panel 30, the maximum value e35 of variation in wheel slip which is to be tolerated, e.g. 20%. If e3 > e35, the discriminator 66 sends an over-ride signal to the priority switch 40.The switch 40 then controls servo-vaive 1 6 to supply high pressure fluid to the rams 1 9, irrespective of the valve of E, to reduce the depth of the implement until e3 falls below e35 in value. In the event of conflict between the demands of circuits 50 and 60, the priority switch 40 gives precedence to reduction of wheel slip.

The circuit 70 includes a transducer 71 in the form of a potentiometer device 72 providing an electrical output signal p representative of the position of the lift arm 10, and hence of the remainder of the 3-point linkage together with an attached implement, relative to the tractor body (or to a horizontal datum when the tractor stands on level ground). The output signal p is sent via a line 73 to a summing amplifier 74, which also receives an input p5 (representing a desired linkage position) from a potentiometer device 75 mounted on the control panel 30. The device 75 includes a graduated dial by means of which the desired value of p5 can be set.The summing amplifier generates an output signal e4 representing the difference (p,--p) which is transmitted to a discriminator device 76 on the control panel 40. The device 76 includes a dial by which can be set the maximum value e45 of error in linkage position which is to be tolerated. The value e45 can instead be built into the device 76 eliminating its dial. If le41 > e45 (i.e. if the numerical value of e4 is greater than e'45), the discriminator sends an over-ride signal to the priority switch 40, so that the servo-valve 1 6 supplies fluid to or takes fluid from the rams in a sense such as to reduce the numerical value of e4, irrespective of the value of E.In this case, the priority switch 40 connects the rams 9 to the high pressure line 17 if e4 is negative, indicating the linkage is lower than desired, and to the low pressure line 21 if e4 is positive.

In the event of a conflict, the priority switch 40 gives precedence to the demands of the circuit 70 over circuits 50 and 60.

The primary use of circuit 70 is likely to be for manoevering the linkage up or down under manual control. A switch 77 is therefore provided, by which the control circuit 70 can be rendered ineffective. Manual control is likely to be carried out conveniently from outside the cab at some times. The potentiometer device 75 (or a duplicate thereof) may therefore alternatively be provided in a form which can be moved independently of, and can act remotely from other parts of the control system, e.g. as unit 75A separate from the control panel, but connected to the remainder of the control system through a flexible cable 75B.

Other possible modifications will be apparent to those skilled in the art, and are also to be considered as within the scope of the invention.

For example, although the invention has been specifically described in relation to control of a chisel plough, it can be applied to a wide variety of tractor-drawn soil-working implements including mouldboard ploughs, cultivators af various kinds, and fully mounted, semi-mounted or trailed implements of various kinds.

Claims (20)

1. A control system for controlling the draught force acting on an agricultural soil-working implement, comprising means for sensing the instantaneous value of the draught force; means for generating a control signal, said control signal including a component representing the said instantaneous value and a component representing the time derivative of the said Instantaneous value; means for setting a desired value of draught force or a quantity related thereto; means for generating a set signal representing the said desired value; means for generating an error signal representing the difference between the control signal and the set signal; and actuating means controlled by the value of the error signal for altering the depth of engagement of the implement in the soil in a sense such as to reduce the magnitude of the error signal.

2. A control system according to claim 1, wherein the means for sensing the instantaneous value of draught force comprises a linkage for connecting the implement and a tractor, and load pins for inclusion in the linkage.

3. A control system according to claim 2 wherein the linkage is a standard agricultural 3point linkage, and the load pins are so located as to provide the pivotal connections between the tractor and the upper link, and between the tractor and the two lower links, the outputs of the three load pins being added to generate a signal representing draught force.

4. A controlling system according to any one of claims 1 to 3, including means for controlling the variation in depth of engagement of the implement in the soil.

5. A control system according to claim 4 including a depth transducer for generating an electrical signal representing the said actual depth of engagement.

6. A control system according to claim 5 wherein the depth transducer incorporates an ultrasonic device or incorporates a wheel mounted on the implement for up and down movement relative thereto and includes a potentiometer device which can generate the electrical signal representing the position of the wheel relative to the implement.

7. A control system according to claim 5 or claim 6, including means for setting a desired minimum or maximum value of depth of engagement; means for generating an electrical signal representing the set minimum or maximum depth; means for generating a depth error signal representing the difference between the electrical signals representing set minimum or maximum depth and actual depth; discriminator means for generating a depth over-ride signal when the depth error signal exceeds a preset value; and priority means for ensuring that a depth over-ride signal controls the actuating means in preference to the draught force error signal, in a sense such as to correct the depth of engagement of the implement.

8. A control system according to any one preceding claim including means for limiting the slip of a driving wheel of a tractor by which the implement is drawn.

9. A control system according to claim 8 including a transducer for generating an electrical signal representing slip.

1 0. A control system according to claim 9 wherein the transducer includes a radar device for measuring the forward speed of the tractor.

11. A control system according to claim 9 wherein the transducer includes means for measuring the rotational speed of a non-driven wheel.

12. A control system according to any one of claims 9 to 11 including means for setting a desired maximum value of wheel slip; means for generating a slip error signal representing the difference between the electrical signals representing set maximum slip and actual slip; discriminator means for generating a slip over-ride signal when the slip error signal exceeds a preset value; and priority means for ensuring that a slip over-ride signal controls the actuating means in preference to the draught force error signal, in a sense such as to reduce the depth of engagement of the implement.

13. A control system according to any one preceding claim, including linkage for connecting the implement and a tractor, and means for controlling the position of the linkage relative to the tractor.

14. A control system according to claim 13 including a transducer for generating an electrical signal representing the actual position of the linkage relative to the tractor.

1 5. A control system according to claim 14, including means for setting a desired linkage position;. means for generating an electrical signal representing the set linkage position; means for generating a position error signal representing the difference between the electrical signals representing set linkage position and actual linkage position; position discriminator means for generating a position over-ride signal when the position error signal exceeds a preset value; and priority means for ensuring that a position override signal controls the actuating means in preference to ali other signals, in a sense such as to reduce the magnitude of the position error signal.

1 6. A control system according to claim 1 5 including means by which the means for setting a desired linkage position can be rendered temporarily ineffective.

1 7. A control systems7 according to claim 15 or claim 16 wherein the means for setting a desired linkage position can be moved independently of, and act remotely from, other parts of the control system.

1 8. A control system according to claim 1 and substantially as hereinbefore described.

1 9. A control system substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.

20. Any novel feature or combination of features disclosed herein.