IMPLEMENT CONTROL SYSTEM

Abstract: A tractor implement draft force control system in which desired and actual implement draft force values are compared to provide a draft force error signal (DE). The system includes a gain control means (GC) which process the actual draft force signals so that at draft force signals above a predetermined level the increase in draft force signal (AS) per unit increase in draft force is reduced. Typically in the upper one-third of the draft force range the rate of increase in draft force is only one half of the rate of increase in the lower two-thirds of the draft force range. This has the effect of allocating significantly more of the total movement of the draft force setting control to the draft force range of implements such as moldboard and chisel plough which require more accurate control.

Description: This invention relates to systems for controlling the level of draft force experienced by an implement drawn by a tractor. Many such systems have been proposed in the past and although many of these proposals have proved adequate for certain implements or certain ground conditions difficulty is experienced in providing a system which can be used with a wide variety of implements and ground conditions. It is an object of the present invention to provide an improved form of implement draft force control system which mitigates the above difficulty. Thus according to the present invention there is provided a tractor implement draft force control system comprising:- means for setting a desired value of implement draft force, means for sensing the actual value of implement draft force, comparator means for comparing the desired and actual values of implement draft force to provide a draft force error signal and control means responsive to the error signal to control the working depth and hence the draft force of the implement, the system being characterised in that a gain control means is provided to process the actual draft force signals so that at draft force signals above a predetermined level the increase in draft force signal per unit increase in draft force is reduced. Typically in the upper one third of the draft force range say from 4 to 6 tons) the rate of increase in draft force is only one half of the rate of increase in the lower two thirds of the draft force range (i.e., below 4 tons). By controlling the gain of the draft force signal in the above manner account is taken of the different behaviour of different implements. For example, smaller and lighter implements produce relatively small changes in draft force for a given change in working depth whereas heavier and deeper working implements produce much larger changes in draft force for the same change in working depth. A significant result of the above gain control arrangement is that the system operator has significantly more of the total movement of the draft force setting control allocated to the draft force range of implements such as moldboard ploughs and chisel ploughs which require more accurate control by the operator and less of the setting control movement allocated to larger implements such as subsoilers which are more stable in their operation. This is clearly an advantage to the operator since it makes the draft force setting control less sensitive to adjustment in the operating range of moldboard and chisel ploughs. One embodiemnt of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a diagramatic representation of a control system embodying the present invention and the associated hydraulic valve components necessary to control a tractor implement hitch; Figure 2 shows the general form of the draft force signal from a moldboard plough and its relationship with the system deadband, Figure 3 shows two stages in the processing of the error signal in accordance with the invention, Figure 4 shows the gain curve for the draft force signal, and Figure 5 is a diagrammatic representation of the control dial used to set the desired draft force. Referring to Figure 1 a tractor hitch comprises a pair of draft links 10 (only one of which is shown in Figure 1) and a top link 10A which are pivotted on to the rear of a tractor main housing 11. The hitch supports a soil engaging implement which in the example chosen is a moldboard plough 50. The position of the draft links 10 relative to the housing 11 is controlled by a hydraulic ram 12 which is connected with the draft links 10 via a conventional rotatable cross-shaft 13 which carries a pair of lift arms 23 and lift rods 24. The supply of pressurised hydraulic fluid to the ram 12 and the venting of hydraulic fluid from the ram is effected by a solenoid operated spool valve 14 which is supplied with pressurised fluid by a pump 15 and which vents fluid to reservoir 16. The valve 14 is controlled by a draft force control system which includes a draft force setting means S may conveniently comprise a rotary potentiometer of the form shown in Figure 5 and referred to in greater detail below. This set value of draft force is fed to one input of a draft force comparator D. The other input of comparator D is fed with the actual draft force imposed on the draft links 10 by the plough 50 which is measured by a draft force sensor 18 and is relayed to comparator D via a gain control device GC referred to in more detail below. It will be appreciated that the draft force sensor 18 may be of any suitable type which is capable of providing an electrical signal proportional to the draft force imposed on the draft links. The output of comparator D is a draft force error signal which is fed to the remainder of the control system indicated in Figure 1 by the box Z via a lead/lag network LL and a deadband function DB. The remainder of the control system which is represented by the box Z in Figure 1 may be of any suitable form which can process the error signal emerging from deadband function DB and produce control signals to power the raise solenoid R and the lower solenoid L of valve 14 via line 21 and 22 respectively. An example of a suitable form of control system for controlling the solenoids R and L is disclosed in the Applicant's co-pending UK Patent Specification number 21887753. Since the constructional details of the remainder of the control system do not constitute part of the present invention no further description will be given in the present application. It will be appreciated that the control system Z simply interprets the error signal leaving deadband function DB and produces a signal to power either the raise solenoid R or the lower solenoid L as is appropriate depending on the sign of the error signal. The lead lag network LL is used in a known manner to speed up the response of the system and reduce hunting and the deadband function DB is used in the known manner to define a band of error signals which are ineffective to produce any control signals for the valve 14 thus preventing the system from being over sensitive. The draft force control system also includes a deadband control means in accordance with the Applicants co-pending European Specification No. 241168 from which the present application is divided. This deadband control means is represented diagrammatically in Figure 1 by the control loop 30. This loop includes a high pass filter 31, an absolute valve block 32, a peak detector 33, an arithmetic block 34 in which the signal may be multiplied by a constant, an upper limit control 35, and a lower limit control 36. The object of the control loop 30 is to produce a signal for varying the deadband applied by the deadband function DB so that the deadband width is made dependent on the high frequency components of the error signal emerging from the comparator D. For example, a typical variation with time of the draft force error signal DE emerging from comparator D is shown by curve 40 in Figure 2. This curve has a high frequency component shown by the portions 41, 42 and 43 of the curve in which the draft force signal fluctuations are of relatively short duration (i.e. less than 0.1 seconds) and a lower frequency component represented in the difference in the average levels of sections 41, 42 and 43 of curve 40. It will be observed that the duration of the fluctuations of the lower frequency components is significantly longer and is represented by, for example, the time period T in the Figure 2 which represents the time during which the draft force signal is at the generally higher level indicated by portion 42 of the curve. Typically the duration of the fluctuaitons of the low frequency component will be 1 or more seconds. As explained above it is the low frequency component of the draft force signal to which the system must attempt to respond. The error signal DE leaving comparator D is fed around loop 30 where the error signal is first passed through the high pass filter 31 which filters out the lower frequency components of the signal leaving a signal of the form shown at 40A in Figure 3. In a typical example the filter 31 operates at a cut-off frequency of 3Hz so that only those components of the signal whose frequencies are above 3Hz will be further processed by loop 30. After passing through filter 31 the signal then passes through the absolute value block 32 which rectifies the signal to produce an all positive signal as shown at 40B in Figure 3. The signal then passes through the peak detector 33 whose output is proportional to the peak to peak value of the signal 40B as indicated at P in Figure 3. The signal emerging from the peak detector 33 is then passed through the arithmetic block 34 which, in the particular example described, multiplies the signal by a constant which is greater than unity in order to ensure that the deadband applied by the deadband function DB is always slightly larger than the peak to peak value of the high frequency component of the draft error signal. Typically it has been found that a multiplication factor of 1.5 produces a stable control arrangement. The signal is then passed through the upper limit control 35 and the lower limit control 36 which ensure that the signal cannot be larger than an upper predetermined limit nor lower than a lower predetermined limit thus defining the maximum and minimum deadband widths. Typically the minimum deadband width is say 0.8% of the design draft force control range and the maximum deadband is say 6.0% of this range. Thus the loop 30 adjusts the width of the deadband applied by the deadband function DB to the slightly larger than the peak to peak value of the high frequency component of the draft error signal. Referring to Figure 2 the deadband width is represented by the band W and it will be observed that it has the effect of ensuring that the draft control system does not attempt to follow the high frequency variations 41 and 43 but will simply respond to that portion of the curve outside the deadband which is repesented by the time period T′ in Figure 2. In practice it will be found that due to the sudden large increase represented by portion 45 of curve 40 and decrease represented by portion of 46 of curve 40 the width of the deadband will temporarily enlarge as indicated at W′ and W″ in Figure 2. This does not effect the operation of the system which is described as above. It will be appreciated from the above the width of the deadband applied by deadband function DB is continually adjusted in accordance with the peak to peak value of the high frequency components of the draft force error signal. Thus the tractor operator is relieved of the onerous duty of having to adjust the sensitivity of the system to cater for different implements and varying ground conditions. In accordance with the present invention the draft force control system includes a gain control device GC. The operation of this device is best illustrated by reference to Figure 4 which shows the gain curve of device GC (i.e. the draft force transmitted signal TS plotted against the actual sensor signal AS). For lower draft force loads (that is say for loads up to point X in Figure 4 which is say 4 tons which is two-thirds of the maximum design draft froce load of say 6 tons) the gain of the draft force signal is represented as G in Figure 4. In the higher draft force load region (that is for example in the top one-third of the design draft force range i.e. between points X and Y) the gain of the draft force signal is reduced. In the example chosen the gain G is divided by 2 in the higher region. The gain of the draft force signal is controlled in the above manner in order to take account of the different behaviour of different implements. For example, smaller and lighter implements produce relatively small changes in draft force for a given change in working depth whereas heavier and deeper working implements produce much larger changes in draft force for the same change in working depth. A significant result of the above gain control arrangement is that the draft force setting dial DS shown in Figure 5 has significantly more of its rotary movement allocated to the draft force range of implements such as moldboard ploughs (see the dial position M) and chisel ploughs (see dial position C) which require more accurate control by the operator and less of the dial allocated to larger implements such as subsoilers (see dial position SS) which are more stable in their operation. This is clearly an advantage to the operator since it makes the draft force setting dial less sensitive to adjustment in the operating range of moldboard and chisel ploughs. It will be appreciated that the invention has been described above in relation to a so-called "bottom link sensing" type of draft force control system in which the forces applied to one or both of the bottom links of the tractor hitch by the implement are sensed and compared with the set draft force and that references above to "draft force" therefore refer to the force applied to the bottom link or links by the implement. The invention is, however, also applicable to the so-called "top link sensing" type of draft force control system in which the forces applied to the top link of the tractor hitch by the implement are sensed and compared with the set draft force. In such an application the references to "draft force" will be understood to refer to the force applied to the top link by the implement.

Claims: 1) A tractor implement draft force control system comprising:- means (S) for setting a desired value of implement draft force, means (18) for sensing the actual value of implement draft force, comparator means (D) for comparing the desired and actual values of implement draft force to provide a draft force error signal, and control means (z) responsive to the error signal to control the working depth and hence the draft force of the implement (50), the system being characterised in that a gain control means (GC) is provided to process the actual draft force signals so that at draft force signals above a predetermined level the increase in draft force signal (AS) per unit increase in draft force is reduced. 2) A control system according to claim 1 characterised in that in approximately the upper one-third of the draft force range the rate of increase in draft force is only one half of the rate of increase in approximately the lower two-thirds of the draft force range.

Inventors: BELLANGER, REGIS, GRAASKAMP, WILLIAM JAMES
Applicants: MASSEY-FERGUSON SERVICES N.V.
Application year: 1987
Main classification: A01B63
IPC codes: A01B63