Abstract:
A control system for a tractor that controls an operating condition of an attached implement and includes a pressure sensing device which senses the pressure in the hydraulic drive circuit and provides a signal indicative of the pull force necessary to pull the implement. The control system further includes a control device which receives the pressure signal and a second signal relating to the speed of the tractor and adjusts the position of the implement to a position within a pre-determined range in order to prevent damage to the tractor or implement.

Description:
BACKGROUND OF THE INVENTION 
     Field of Invention 
     The invention relates to a control system of a vehicle, especially a tractor, equipped with a Continuously Variable Transmission (CVT) of the hydrostatic-mechanical split type which includes a hydraulic drive circuit in which a hydraulic pump supplies pressurised fluid to a hydraulic motor. 
     Description of Related Art 
     A hitch, such as a three-point linkage is one known arrangement used to attach implements to a drawing vehicle, for example an agricultural tractor. The implement may be fully mounted or semi-mounted on the tractor whereby a semi-mounted implement has a wheel engaging with the ground during soil operation while a fully-mounted implement puts all its load on the three-point linkage. 
     Three point linkages most frequently consist of two lower lifting arms to which an implement is attached. The lower lifting arms can be pivoted by respective hydraulic actuating cylinders to adjust the height position of the implement relative to the tractor. Furthermore, these lower lifting arms may be manually adjusted by length to be appropriate for an implement to be attached. An additional top link connects the implement to the tractor above the lower lifting arms. This top link is used to pivot the implement about a horizontal transverse axis and is adjustable by means of a threaded connection, or a hydraulic cylinder. 
     Alternative designs of three-point linkages are known, such as the arrangements shown in U.S. Pat. No. 6,321,851, US 2003/217852 and U.S. Pat. No. 5,997,024 in which the lower links are replaced by two or four variable length hydraulic rams. This variable length ram arrangement enables multi axis movement of any implement attached to the linkage. 
     To control the three-point linkage, modern tractors are mainly equipped with electronic linkage control systems to improve work quality and operator comfort during operation. 
     Such electronic linkage control systems operate in three well known modes: 
     Position Control: In general, the tractor speed is kept constant by a speed control system and the position of the lower lifting arms is sensed directly or indirectly so that the working depth of the implement in the soil can be adjusted whilst the speed of the tractor is kept constant. 
     Draft Control: The implement is raised and lowered in the soil depending on the draft force applied by the implement to reduce fuel consumption, avoid engine stall or avoid damage of the implement or tractor. Again, vehicle speed is kept constant. If the implement is lowered into the ground an initial draft is applied defining a zero level. The operator can then set a value representing a force increase which means that the operator can decide how fast the implement is lifted when a small force increase or a large force increase occurs. The value of the force entered by the operator does not represent an exact value of the force applied, e.g. 5 kN, but defines the responsiveness of the draft control. The objective of this function is to move the implement while avoiding excessive draft or pull force variations. Therefore, a draft force sensor, typically in the form of a draft force sensing pin which connects the lower lifting arms to the tractor chassis is used to measure the horizontal load applied to the tractor by the implement. 
     Intermix of Position/Draft Control: This control arrangement, as its name implies, is a mixture of position and draft control in which a draft control system can only lift the implement within a limited range of positions. This function is provided to avoid excessive movement of the implement in the soil resulting in poor working quality. Again, vehicle speed is kept constant by a speed control system. 
     Only the draft control and intermix mode (both referred to as drag modes) operate under measurement of the drag force. Generally, deactivating the drag modes results in that the system enters the position mode with no drag force influencing the lifting heights. It may however be difficult to install a draft force sensing pin due to the complex three-dimensional geometry of a linkage. Further, the sensing pins may become dirty or damaged and thus may not function properly. Accordingly, a control system which does not rely on sensing pins is preferred. 
     A linkage control based on CVT parameters can result in that the control system moves the position of the linkage over a wide vertical displacement range as a reaction to the drag force. Various situations have been identified in which the movement of the linkage should be limited in drag mode, since otherwise the draft control will cause the implement to crash to the ground, or cause the linkage to collide with the wheels of the tractor, or the drawbar of an trailer. 
     For example, if the draft force rises continuously because a plough in the ground has hit a rock, the draft control will move the linkage up until the highest end position is reached. Other situations have been identified where an increasing, or decreasing draft force will cause vertical displacement of the linkage and any attached implement to the lowest or highest position. Detecting these situations can be quite difficult. Some implements are simply towed by attachment to a ball hitch rather than being mounted to the linkage, with actuators on the implement controlling operating conditions of the implement based on information received from the tractor, for example via a CAN-BUS link, or ISOBUS. 
     In the case where an implement is attached to a ball hitch on the tractor, or a when a tow bar is attached to the tractor, that is, the linkage is not used, a drag force determined by the CVT would deliver a significant change of drag signal when the roll of the tractor changes, or the vehicle travels uphill, or downhill or during acceleration. Under normal circumstances, this would cause the linkage to move, and thus when a tow bar is attached would cause it, or the linkage to collide with the tow bar. 
     In the case where an implement is being transported in a lifted position, the operator is ordinarily responsible for deactivating the draft or intermix mode manually when travelling along a road with an implement held in a lifted position. If this is not done the drag force determined by the CVT delivers a significant change of drag signal when the roll of the tractor changes, or the vehicle travels uphill, or downhill or during acceleration. This could result in the implement to be lowered and crashing to the ground. 
     In the case where an implement is attached which is not in contact with the ground during operation, for example fertiliser spreaders and sprayers, the CVT delivers a significant change of drag signal when the roll of the tractor changes, or the vehicle travels uphill, or downhill or during acceleration. This would ordinarily result in the implement being highered which is not intended, or lowered which is also not intended and may be dangerous if not expected. 
     In the case of acceleration to a new speed regardless of the position of the implement, the change of drag signal may result in unintentional movement of the linkage. The faster the vehicle is going, the greater is the risk of damage through unintentional movement of the linkage. 
     OVERVIEW OF THE INVENTION 
     It is an aim of the invention to provide a safer draft control function on a tractor which limits the movement of the linkage as a result of a change in draft signal. It is a further object of the invention to provide a control system which avoids the use of draft force sensing pins. 
     In accordance with the invention there is provided a control system for a tractor having a transmission with a hydraulic drive circuit in which a hydraulic pump supplies pressurised fluid to a hydraulic motor, wherein said control system controls an operating condition of an implement attached to the tractor and said control system comprises a pressure sensing means which senses the pressure in the hydraulic drive circuit and provides a signal which is indicative of the current pull force necessary to pull the implement, the system further comprising a control means which receives the pressure signal and a second signal relating to the speed of the tractor and adjusts the current position of the implement to a new position when said pressure signal varies, characterised in that the new position lies within a pre-determined position range dependent on the second signal in order to maintain an optimal, yet safe operating condition of the implement. 
     By limiting the vertical displacement of the linkage, even though the draft force is changing, the control system prevents damage to the implement, or the tractor. 
     The higher the speed of the tractor, the narrower the pre-determined position range is. This thus improves safety at higher speeds. 
     Preferably, the pre-determined position range is effected when attachment of the implement is detected. 
     Preferably, the new position of the implement is effected by controlling movement of a linkage to which the implement is attached. 
     More preferably, the linkage is a three point linkage. 
     For linkage mounted implements, a control system can control the position of an implement attachment linkage relative to the tractor via an actuator means which raises and lowers the implement linkage relative to the tractor. Such a control system can be used to control a linkage which is attached to the rear and/or front of the tractor. No additional draft force sensors are involved as the variation of the pull force of the linkage is determined from the variation in the hydraulic drive circuit pressure. Also the need to use existing pin sensors is avoided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, by way of example only, with reference to the accompanying drawings in which:— 
         FIG. 1  diagrammatically shows a side view of a tractor with a linkage control system embodying the present invention, 
         FIG. 2  diagrammatically shows a driveline of a tractor with a hydrostatic mechanical CVT and having a linkage control system embodying the present invention, 
         FIG. 3  diagrammatically shows in more detail the hydrostatic mechanical CVT portion of the driveline of  FIG. 2 , and 
         FIG. 4  shows the forces acting between a tractor wheel and the ground. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Referring to the drawings, an agricultural tractor  1  has a driveline  2  having a combustion engine  3 , a flywheel  4 , a continuously variable transmission, or CVT, T of the hydrostatic-mechanical split type and a rear axle housing  300 . Combustion engine  3  is connected to the CVT, T by chassis part  310 . 
     A three-point linkage  400  is attached to the rear axle housing  300  and mainly consists of two lower lifting arms  401  to which an implement is attached. A plough  500  with ground engaging means  501  is attached to lower lifting arms  401 . An additional top link  402  connects the implement  500  to the tractor  1 . The top link  402  is of a hydraulic type adjustable in length to adjust the inclination of the plough  500  with the ground. The lower lifting arms  401  can be pivoted about axis A by respective hydraulic actuating cylinders  403  which move rocker arm  404  and lift rod  405 . The height of the lifting arms can thus be changed by pivoting the lifting arms about axis A and this movement is hereafter referred to as the vertical displacement of the lifting arms. The hydraulic actuating cylinders  403  are supplied with an actuating fluid by a control valve  406 . Control valve  204  controls which chamber  403   a  (to lift the implement) or chamber  403   b  (to lower the implement) of the hydraulic actuating cylinders  403  is charged with fluid. Control valve  406  is connected to a pump  407  which is driven by combustion engine  3  and connected with a fluid tank  408 . 
     The position of the lower lift arms  401  is indirectly measured by a position sensor  409  which senses the position of a cam  410  attached to rocker arm  404 . 
     An additional pressure sensor  411  is provided to measure the fluid pressure in the chamber  403   a  of the hydraulic actuating cylinders  403 . The fluid in chamber  403   a  is compressed when the implement weight is fully taken up by the three-point linkage  400  and therefore a pressure increase indicates movement of the implement to a high position for transportation. 
     A tractor control unit  13  is provided to control various functions of the vehicle. The control unit  13  is electronically connected to various components via CAN-BUS, for example, the transmission and display and input devices. The control unit  13  also contains software to drive the electronic linkage control system. The control unit  13  is connected to an input and display device  13   a  in the tractor cab to receive input from the operator and to show information to the operator. 
     Position sensor  409 , control valve  406  and pressure sensor  411  are connected to the control unit  13 . 
       FIG. 2  shows the driveline  2  of the tractor  1  in more detail. The torque supplied by combustion engine  3  via a flywheel  4  is distributed to a mechanical branch  6  and a hydrostatic branch  7  of the transmission T via a planetary drive  5 . The hydrostatic branch  7  mainly consists of hydrostats  200 ,  210 , wherein hereafter the hydrostat  200  is designated as the hydraulic pump  200  and the hydrostat  210  as the hydraulic motor  210 . Both hydraulic pump  200  and hydraulic motor  210  can be pivoted by an adjustment unit, also referred to as an ADU to change delivery/intake volume as described in  FIG. 3 . 
     Both the mechanical branch  6  and the hydrostatic branch  7  of the transmission are driven and brought together on a CVT output shaft  10  at the end of CVT, T. The CVT output shaft  10  delivers an output torque to the respective driveline front and rear axles  11  and  12 . 
     CVT output shaft  10  drives a rear axle differential  12   a  splitting the torque to a left rear axle portion  12   b  and a right rear axle portion  12   c . Both rear axle portions  12   b ,  12   c  are provided with brakes  12   d , final reduction gears  12   e  and wheels  12   f.    
     CVT output shaft  10  also drives a front axle drive gear pair  11   a  followed by a front wheel drive clutch  11   b  to disengage and engage front axle driveline. In addition a front brake  11   c  is provided which is connected to a cardan shaft  11   d  which ends in a front axle differential  11   e  splitting the torque to a left front axle portion  11   f  and a right front axle portion  11   g . Both front axle portions  11   f ,  11   g  are provided with final reduction gears  12   h  and wheels  12   i . Wheels  12   i  are steerable about a substantially vertical axis using a hydraulic steering cylinder  11   j  mounted on the front axle. 
     The driveline  2  is also equipped with an anti-skid system  15  which mainly consists of an anti-skid control unit  15   a  integrated in the tractor control unit  13  of the tractor  1 , speed sensors  15   b  for each wheel  11   i ,  12   f  and a further anti-skid sensor  15   c . The anti-skid sensor  15   c  provides parameters to control the brake function, for example acceleration in various axes, or inclinations of the vehicle. Anti-skid control unit  15   a  may be separate from tractor control unit  13 . 
     Alternatively, a GPS system may also deliver parameters such as the acceleration or the inclination of the vehicle. 
       FIG. 3  shows a diagrammatic sketch of the hydrostatic mechanical split type transmission T having an adjustment unit ADU defined by the broken line. The components outside the broken line belong to the power unit of the transmission. 
     The hydrostats  200 ,  210  illustrated in  FIGS. 2 and 3  are an axial piston pump and an axial piston motor of an oblique-axis design, in which the delivery/intake volume is changed by the pivoting of the axis of rotation of the pistons to an axle drive shaft, not shown. 
     By means of a first valve unit  30  allocated to hydraulic pump  200  and by means of a second valve unit  31  allocated to the hydraulic motor  210 , the individual pivot angle of the hydraulic pump  200  and/or of the hydraulic motor  210  can be adjusted. 
     Depending on the specified revolution speed transmission ratio iT set by the operator via control unit  13  an actuator element  20  is rotated by means of an actuator motor  21 . The actuator motor  21  is in this case controlled by the control unit  13 . Because the valve units  30 ,  31  are coupled to the actuator element  20 , these valve units  30 ,  31  are displaced corresponding to the actuator element  20 . As a result, oil present in a line  32  can flow into a cylinder  33 ,  33 ′,  34 ,  34 ′ allocated to the valve unit  30 ,  31 . 
     Due to the displacement of the actuator element  20 , the oil flow is accordingly directed out of line  32  and into the cylinders  33 ,  33 ′,  34 ,  34 ′. Thereby the pivot angle of the hydraulic pump  200  and of the hydraulic motor  210  is adjusted. The pivot angle, and therefore the delivery volume of the hydraulic pump  200  and the intake volume of the hydraulic motor  210  can be changed accordingly. This makes it possible for the revolution speed of the axle drive shaft (not shown in  FIGS. 2 and 3 ) to be adjusted, and with it the revolution speed transmission ratio of the transmission T. 
     The hydraulic pump  200  is connected by fluid circuit HC to the hydraulic motor  210 . The fluid circuit HC in has an upper circuit UHC and a lower circuit LHC. The direction of the arrow F represents a flow direction of the fluid located inside the hydraulic circuit HC during forwards travel of the tractor and the direction of the arrow R represents a flow direction of the fluid during reverse travel of the tractor. 
     By means of a first measuring unit  110 , the pressure value pUHC prevailing in the upper circuit UHC can be measured. This pressure value pUHC is then sent to the control unit  13  represented in  FIG. 1 . Moreover, both the pressure in the upper circuit UHC as well as the pressure in the lower circuit LHC is conducted by means of a shuttle valve  120  to a second measuring unit  100  in order to measure the pressure value pHCmax. This pressure value pHCmax is also sent to the control unit  13 . 
     The shuttle valve in the transmission T is designed in such a way so as to communicate to the second measuring unit  100  the greater of the two pressures present in the upper circuit UHC or the lower circuit LHC as a pressure value pHCmax. When the tractor is stationary, the second measuring unit  100  issues a system pressure of the upper circuit UHC or the lower circuit LHC as pressure value pHCmax. A rotation sensor, not visible in  FIG. 2 , is arranged at the hydraulic motor  210  with which the direction of the rotation of the hydraulic motor  210  is determined and the direction of travel of the vehicle can be concluded. 
     When the vehicle is stationary a system pressure of about 15 bar is set in the fluid circuit HC. This system pressure of 15 bar results from the fact that, by means of a supply line  130 , the fluid circuit HC is supplied with a constant system pressure by means of a constant hydraulic pump, not shown, driven by the combustion engine. Two check valves  140  prevent oil from flowing back into the supply line. As soon as the utility vehicle moves or the transmission is no longer stationary, the pressure inside the fluid circuit rises, depending on the drive torque, to a high-pressure value of over 15 bar. With an average loading of the transmission, a high-pressure value of between 250-350 bar is provided. A limit of 500 bar must not be exceeded to avoid over stressing of the transmission and its components. 
     Pressure pHCmax, transmission ratio iT, or the pivot angle of the hydraulic motor  210 , or alternatively the intake volume V of the hydraulic motor  210  represent parameters which determine the output torque Mhydr of the hydraulic branch  7 . As the transmission ratio iT is known, the pivot angle and intake volume parameters of the hydraulic motor  210  are can be determined by look-up tables or characteristic maps. 
     As described in relation to  FIG. 1 , the torque supplied by combustion engine  3  is distributed to a mechanical branch  6  and a hydrostatic branch  7  of the hydrostatic mechanical split type transmission T in which the fraction of torque transmitted by both branches depends on the transmission ratio iT. So if the fraction of the hydrostatic branch  7  is determined as described above, the fraction Mmech transmitted by the mechanical branch  6  can also be determined depending on the current transmission ratio iT. 
     The overall output torque MOT of the transmission can then be calculated from 
     
       
         
           
             
               
                 
                   MOT 
                   = 
                   
                     
                       Mhydr 
                       + 
                       Mmech 
                     
                     = 
                     
                       
                         
                           pHC 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           max 
                           * 
                           V 
                         
                         
                           2 
                           ⁢ 
                           π 
                         
                       
                       + 
                       Mmech 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
     The pressure pHCmax is measured as described above and the intake volume V of the hydraulic motor  210  is determined by characteristic maps depending on the transmission ratio iT. 
     The output torque MOT of the transmission is supplied to the wheels resulting in a wheel torque MW:
 
 MW=MOT*iTW    (Equation 2)
 
     In this equation iTW represents the overall gear ratio between the transmission and wheel and is the product of the gear ratio of the rear axle differential  12   a  and the final reduction gears  12   e  in rear wheel mode, for example:
 
 ITW= 9.2(for the rear axle differential 12 a )×3.58(for the final reduction gears 12 e )=32.97 overall.
 
     Knowing the wheel torque MW, the pull force FP can be calculated by using the known relationship of the forces on a wheel as shown in the diagram in  FIG. 4 .
 
 MW=FP*r+FV*f=FC*r    (Equation 3)
 
In which:
         r represents the effective wheel radius depending on tyre pressure and wheel size provided by the wheel manufacturer in respective tables   f represents the offset of the point of application of the wheel vertical force (see  FIG. 4 ) caused by roll resistance and sinking of the wheels       

     The circumferential force FC is a theoretical value achieved by converting equation (3): 
     
       
         
           
             
               
                 
                   FC 
                   = 
                   
                     
                       
                         M 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         W 
                       
                       r 
                     
                     = 
                     
                       FP 
                       + 
                       
                         FV 
                         * 
                         
                           f 
                           r 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     4 
                   
                   ) 
                 
               
             
           
         
       
     
     As the linkage control only needs an indication of an increase in pull force FP, FV (which remains constant) can be ignored and so the equation can be simplified to: 
     
       
         
           
             
               
                 
                   
                     
                       M 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       W 
                     
                     r 
                   
                   = 
                   FP 
                 
               
               
                 
                   ( 
                   
                     Simplified 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       Eq 
                       . 
                       
                           
                       
                       ⁢ 
                       4 
                     
                   
                   ) 
                 
               
             
           
         
       
     
     Thus an increase of the pull force ΔFP would result in an increase of the torque demand ΔMW and therefore an increase of pHCmax. 
     As pHCmax is constantly measured in the system, this parameter can be used to control the linkage based on an increased draft force applied by the implement. 
     So by monitoring pHCmax which is already done for transmission control and protection purposes, an increase or decrease of the draft or pull force can be detected and processed in the electronic linkage control system to provide functions like draft control and intermix position/draft control. 
     The change in drag force is fed into a tractor control unit which is programmed to higher, or lower the linkage in response to the change as programmed. In accordance with the invention, the vertical displacement of the linkage is limited. 
     This limit prevents the implement or tractor being damaged unnecessarily by a downwards or upwards movement of the linkage in response to a change in drag force. 
     There are four cases where the movement of the linkage in response to a change in drag force may cause problems when in drag mode. In each case, the implement is in an operating condition be it attached to a ball hitch and not the linkage, or stowed for transportation, or attached for operation without contacting the ground, or semi mounted or fully mounted. The term implement covers all tools, attachments and equipment which can be attached to a tractor including the following which is not an exhaustive list: ploughs, tow bars, sprayers, mowers, drills and planters. 
     In the case where an implement is attached to a ball hitch and not the linkage, or a trailer is connected by a tow bar to the tractor. A decrease in drag force may result in the linkage being highered which may cause the linkage to collide with the wheels of the tractor or the tow bar which is dangerous. Similarly, if an increase in drag force is detected, for example if a tractor is travelling uphill, the linkage may be lowered which may result in an attached tow bar colliding with the tractor wheels. 
     So, in a first step, an implement is detected (for example, via the electric supply/light connector). Alternatively fluid couplings may be used to detect the attachment of an implement. But this connector does not provide information as to where the implement is attached (that is whether it is attached to the linkage or to the ball hitch). 
     Moreover, the pressure in the lifting cylinders cannot always be used to detect whether an implement is not attached or semi mounted to the linkage. 
     For example, an unloaded linkage may result in the lifting cylinders indicating a pressure of 11 bar which represents a mass of the linkage as being around 600 kg. When a plough is mounted to the linkage and engages the ground (semi mounted) the pressure may change to about 15 bar as the ground supports some of the weight of the plough. On the headland, when fully lifting the plough from the ground for rotation, the pressure may increase to about 45 bar. The difference in pressure between the implement being semi mounted and an unloaded linkage is small: 4 bar+/−a tolerance of 1 to 2 bar which is difficult to detect. This may result in that the condition of a plough being semi mounted is not detectable by the system and as a result the drag mode will remain in an active mode because the pressure limit has not been reached. If this happens, then the linkage may be moved to its highest or lowest position in response to the drag force which can have serious ramifications if an implement, such as a plough is attached. If the pressure limit is set too high by the operator, the drag mode is deactivated while an implement is attached. 
     Therefore, in accordance with the invention the vertical displacement of the linkage is limited in the drag mode when an implement is attached to a ball hitch of the tractor and not the linkage, or when a tow bar is attached to the tractor to avoid the implement or tow bar being dropped to the ground, or raised high unexpectedly. 
     In the case where an implement is stowed for transportation in a fully lifted position. In this situation the implement is held high in a stowed position for transportation on the road. Using the cylinder pressure to detect the lifted position is not suitable. As described above, the pressure of the lifting cylinders of a lifted linkage can be too similar to the cylinder pressures when the implement is engaging the ground. It is therefore difficult to safely distinguish between these two different situations. 
     Therefore, in accordance with the invention the vertical displacement of the linkage is limited when an implement is being transported to avoid the case of an implement being dropped onto the road. 
     In the case where an implement is not contacting the ground during operation, for example an attached spreader. If the operator forgets to deactivate the drag mode, the sprayer may be lowered or highered unintentionally when a change in drag is sensed. Again, if vertical displacement of the linkage is limited in response to a change in drag force, the damage to an attached implement which does not contact the ground during normal operation is avoided or limited. 
     In the case where an implement is fully mounted, semi mounted, stowed for transportation or attached for operating above the ground, the risk of damage caused by an unintentional movement of the linkage is greater the faster the tractor is travelling. Accordingly, the range of the vertical displacement of the linkage is limited in relation to the speed of the tractor. The higher the speed of travel of the tractor, the narrower the vertical displacement range of the linkage. For example, if the tractor is travelling between 0-10 km/hr, the linkage may have a limited range of movement of 20 cm. If the speed is between 11-20 km/hr, the range of movement of the linkage may be limited to 10 cm, thus reducing the risk. 
     The linkage may also be equipped with means to detect if an implement is attached thereto, for example a sensor in the attachment hooks of the lower links  401 . The range of vertical displacement of the linkage can be limited accordingly as described in the above situations. 
     In accordance with the invention, the vertical displacement of the linkage is limited for the drag mode by pre-determined settings defined by the linkage or tractor manufacturer or by the operator. 
     As the difference of the pull force FP cannot be used as a control parameter for damping the implement linkage when the linkage is stowed high in a position for transportation, the pressure sensor  411  is provided to measure the fluid pressure in the chamber  403   a  of the hydraulic actuating cylinders  403 . A variation of the measured pressure signal from sensor  411  indicates that the implement is oscillating which can also result in the weight on the front axle varying which can cause the tractor to oscillate which impacts on the steering and stability of the tractor. So the signal from pressure sensor  411  is forwarded to the control unit  13  to adjust the damping characteristics of the implement lifting circuit. The damping characteristics can then be adjusted by the linkage control system by adjusting the control valve  406  to reduce oscillation by allowing the implement to move relative to the tractor and thus increase driving steerability and stability. 
     As described previously, there are different modes of linkage operation: Position control mode, Draft Control mode and Position/Draft intermix mode in which the height of the linkage is continually supported by the tractor. But the linkage can also be set to a “float position” mode in which implements, such as seed drills, which have a ground contacting wheel or wheels can follow the ground contours with part of the weight of the implement supported by the implement wheel or wheels) and part of the weight supported by the linkage as the implement is drawn along behind the tractor. 
     Alternatively, the damping may be adjusted by means of wheel load sensors which indicate the variation in wheel load which is symptomatic of tractor oscillation. These wheel load sensors may be pressure sensors measuring pressure variations in the suspension cylinder of the front axle, or strain gauges in the rigid rear axle housing. 
     Although the foregoing examples have concentrated on linkage control systems, as indicated previously the invention is not so limited, being applicable to any tractor control system for the controlled operation of an implement where a draft force measurement is required.