Abstract:
A vehicle includes a continuously variable transmission by means of which torque is transmitted from an engine to drive wheels of the vehicle, a braking system for applying a braking force to the drive wheels and a controller for setting the transmission ratio of the transmission. Additionally, means are provided for applying to the controller a signal indicative of the engine applied torque, and means for supplying to the controller a signal to indicate application of wheel brakes. The controller is operative when the brakes are applied to discontinue setting the transmission ratio in dependence upon the speed of the drive wheels and to set the transmission ratio instead in such a manner as to minimize the absolute value of the engine output torque or the torque transmitted through the transmission or to follow a desired torque setpoint.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an agricultural vehicle, such as a tractor, in which the drive train between the engine and the driven wheels includes a continuously variable transmission (CVT). 
     BACKGROUND OF THE INVENTION 
     It is known for the transmission of an agricultural vehicle, such as a tractor or a harvester, to incorporate a CVT acting in conjunction with a change speed gearbox offering multiple drive ratios. One known form of CVT is a hydrostatic transmission which comprises a variable displacement pump connected by hydraulic lines to a hydraulic motor. The torque applied to the wheels is set in this case by varying the displacement of the pump. The ensuing description will be made by reference to such a hydrostatic transmission but it should be mentioned that other forms of continuously variable transmission, such as a belt, chain, or electrically driven CVT can be used in place of a hydrostatic transmission, and reference to changing the displacement of the pump should be construed as being equivalent to varying the transmission ratio of the CVT by other means, such as by varying the geometry of the cone sheaves of a belt driven CVT. 
     To set the torque applied to the drive wheels by the engine, a controller is provided which varies the displacement of the pump in dependence upon the difference between the desired and actual values of the wheel speed. Thus, if the vehicle speed or the engine speed drops below the desired value, the pump displacement is adapted to match the desired vehicle or engine speed. 
     It is sometimes necessary to apply drive torque to the wheels at the same time as the brakes are applied. For example, at the commencement of a hill start, the engine may provide torque to counteract the force of gravity at the same time as the brakes are applied to hold the vehicle still. When the brakes are then released, the vehicle moves forward gradually. Also when driving downhill, the engine may be used in conjunction with the brakes to retard the vehicle. 
     If the control system setting the transmission ratio of the CVT takes no account of the applied braking torque, it cannot distinguish between applied braking torque and the reaction torque of the vehicle wheels when standing on an incline. The CVT transmission ratio will therefore be varied in order to counteract the effect of the brakes. 
     For this reason, it has previously been proposed to detect when the brakes are applied and, under such conditions, the transmission ratio is open loop controlled. This is not entirely satisfactory as it leads to unnecessary brake wear, it makes it difficult to achieve smooth hill starts and it risks overheating of the brakes during prolonged driving downhill. 
     SUMMARY OF THE INVENTION 
     A vehicle has a continuously variable transmission for transmitting torque from an engine to drive wheels of the vehicle, a braking system for applying a braking force to the drive wheels and a controller for setting the transmission ratio of the transmission in such a manner as to maintain a desired speed of the drive wheels or of the engine. The controller receives a signal indicative of the engine applied torque, and a signal to indicate application of wheel brakes. When the brakes are applied, the controller discontinues setting the transmission ratio in dependence upon the speed of the drive wheels and sets the transmission ratio instead in such a manner as to minimize the absolute value of the engine output torque or the torque transmitted through the transmission or to follow a desired torque setpoint. 
     The controller can switch between two feedback strategies depending on whether or not the brakes are applied. In the case where the CVT is a hydrostatic transmission, for example, when the brakes are not applied, the controller sets the displacement of the pump to achieve the desired wheel speed, in the conventional manner. However, when the brakes are sensed to be applied, the pump displacement is not merely set to zero. Instead, the controller varies the displacement of the pump while monitoring the load torque on the transmission. If a change in the displacement in one sense causes a reduction in the applied transmission torque, then a further displacement is made in the same sense. When a change in the pump displacement results in an increase in the transmission supplied torque then a further change is made in the opposite sense. In this way, when the brakes are applied, the transmission output torque is minimized. Thus, when the brakes are not applied, the controller acts to maintain the desired wheel or engine speed whereas, when the brakes are applied, control of the wheel speed is left to the brakes and the controller acts to minimize the absolute value of the transmission output torque. The term “absolute” value refers to the fact that it is only the magnitude of the transmission torque that is taken into consideration, without regard to the direction in which it acts. It is clear that the same control strategy may be applied to follow a given engine torque setpoint, as an example to have an additional “engine braking”. In this situation the engine appears to be a “load” of the transmission. 
     The signal indicative to the applied engine torque may conveniently be measured by apparatus, such as that described in U.S. Pat. No. 5,596,153 which directly measures the torque transmitted through a flywheel. Alternatively, the engine applied torque may be deduced in the case of a hydrostatic transmission from the pressure in the hydraulic lines connecting the pump to the motor of the hydrostatic transmission. 
     The signal indicative of brake application may either be provided by an electrical switch associated with a brake pedal or it may be derived by sensing pressure in the brake lines if the brake system is operated hydraulically. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described further, by way of example, with reference to the accompanying drawings, in which the single FIGURE is a diagrammatic representation of the drive train of an agricultural vehicle. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the drawings, the thick lines represent lines along which torque is transmitted, continuous lines indicate electrical lines carrying control signals and dotted lines represent electrical lines carrying sensor signals. 
     The drive train comprises an engine  10  connected to an engine controller (ECU)  30  by a control cable  68 . Normally, the engine is set to operate by the ECU at a constant speed selected for optimum engine performance and efficiency. 
     Typically a tractor is equipped with a number of different auxiliary consumers, all of which have to be provided with energy by engine  10 . Implements attached to the tractor may require mechanical power from the tractor PTO and/or hydraulic energy from a hydraulic pump. Other auxiliary consumers such as trailer brake valves, steering valves, remote valve, etc. May also require hydraulic energy. The transmission however requires a large amount of torque for the propulsion of the tractor. Hence, the engine output torque T E  is divided between the transmission torque T T  and torque for auxiliary consumers T S . 
     The transmission torque T T  is supplied to a multi-speed planetary gear set  20  via two separate torque lines, the first T G  via the gear shaft directly from the engine and the second from a CVT, in this case a hydrostatic transmission comprising a variable displacement pump  12  driven by the motor and a hydraulic motor  18  connected to the pump  12  by supply and return lines  14  and  16 . The design of the gear set  20  and its layout in parallel with a hydrostatic transmission are described in detail in European Patent Publication No. EP 1541898, which is incorporated herein by reference. 
     The planetary gear set  20  drives an end reduction gear  22  at a speed dependent upon the speeds of its two input shafts and the prevailing setting of the transmission ratio of the planetary gear set  20 . The reduction gear  22 , which may be a differential, drives the wheels  24 . A braking system  40  is also provided to apply torque to the drive wheels  24 . 
     Under normal driving conditions with no brakes applied, the displacement of the pump  12  is set by a vehicle speed controller  32 . The latter controller  32  receives a control signal over a line  64  from a multi-function handle which indicated the wheel speed desired by the vehicle operator. The controller  32  also receives over a line  58  a sensor signal indicative of the actual wheel speed. The desired and actual speeds are compared to generate an error signal which is relayed over a line  60  to a pump controller  34 . In this mode of operation, the pump controller transmits over a control line  56  a signal for changing the position of the swash plate of the variable displacement pump in the correct sense to reduce the error signal received over the line  60 . Thus, when more speed is required, the displacement is increased to increase the torque applied to the drive wheels and conversely when the desired speed is exceeded the displacement is reduced. 
     In operation, when accelerating from a standstill, a first gear ratio is selected in the gear set  20 . The displacement of the pump  12  is then increased to increase the vehicle speed until the displacement attains a certain value. The gear ratio is then changed to the second ratio while the displacement is set to a low value. To continue accelerating, the displacement is once again increased. This process of changing gear ratios and increasing the speed by changing the pump displacement is repeated until the desired wheel speed is reached. For deceleration with engine braking, the process is essentially reversed, the pump displacement being reduced in each gear and then the next lower gear ratio is selected. 
     As so far described, the drive train is conventional and the mode of operation described above, that is to say closed loop control of the speed of the drive wheels  24 , would normally be adopted regardless of whether or not the brakes are applied. As the control system cannot distinguish between increased gravitational resistance and braking torque, the controller  32  will attempt to increase the applied engine torque not only when the vehicle is being driven uphill but also when the brakes are applied. This is clearly undesirable as the engine and the brakes would be fighting each other, resulting in poorer braking, increased brake wear and reduced engine efficiency. 
     In the present invention, this problem is overcome by adopting a different control strategy when the brakes are sensed to be applied. When the brakes are on, a signal is received over a line  66  by the pump controller  34 . On sensing this signal, the controller  34  disregards the speed error signal received over the line  60  from the speed controller  32  and instead sets the displacement of the pump in dependence upon the torque output of the entire transmission T T  or the torque transmitted through the hydrostatic transmission. 
     The drawing shows signal lines  62  and  63  that can be used in the present invention. The first is a line  62  that may be connected to a transducer mounted on the engine flywheel or the engine output shaft, before the auxiliary consumers have been split off, to measure the total engine torque T E . A suitable apparatus for measuring the engine output torque is described in the Applicants&#39; earlier patent U.S. Pat. No. 5,596,153, which is incorporated herein by reference. 
     A second signal line  63  may be connected to a twist sensor, which measures the angular deflection of the shaft supplying torque T S  to the auxiliary consumers. This angular deflection represents the transmitted torque T S . 
     Based on the input of both signal lines  62  and  63  representing engine torque T E  and torque split off to auxiliary consumers T S , the pump controller  34  is able to filter the torque T T  supplied to the transmission. The calculated value obtained this way allows the pump controller to generate a signal for changing the position of the swash plate of the variable displacement pump in order to minimise transmission torque T T . 
     The alternative shown in the drawing is to measure the torque transmitted through the hydrostatic part of the transmission. Pressure sensors  42  and  44  are provided in the supply and return lines  14  and  16  connecting the variable displacement pump  12  of the hydrostatic transmission to the motor  18 . The difference between the measured pressures, transmitted to the pump controller  34  over lines  52  and  54  is a measure of the torque transmitted through the hydrostatic transmission. The pressure difference in the hydraulic lines is representative for the torque T T  transmitted through the transmission taken into consideration the speed ratio between the input and output shaft of the hydrostatic unit. 
     Another alternative is to measure the torque T G  on the gear shaft, e.g. by measuring the angular deflection of the gear shaft. This torque T G  is also representative for the torque T T  through the transmission when the speed ratio between the input and output shaft of the hydrostatic unit is taken into consideration. 
     Of course, it is only necessary to use one or other of the approaches described above. 
     The aim of the pump controller  34 , when the brakes are sensed to be on, is to minimise the transmission output or to follow a desired torque setpoint. If transmission torque T T  is to be minimised, there is no single value of output torque that the transmission seeks to achieve, as this value will depend on the driving conditions. Instead the control strategy adopted within the pump controller  34  is one of continual trial and error. If one were to plot a graph of transmission output torque T T  against pump displacement, one would find a minimum turning point and it is at this point that the controller  34  seeks to set the pump displacement. This it does by changing the displacement in one sense and monitoring the resulting change in engine output torque. If a decrease is sensed then the displacement is changed further in the same sense until the minimum turning point is reached. Continuing to change the displacement in the same sense will result in overshooting the minimum turning point and when this occurs the sense in which the displacement is changed is reversed. 
     If a desired torque setpoint is to be achieved, in other words if the hydrostatic unit is to act as a load on the transmission, then the transmission ratio of the CVT would be set in dependence on a torque error signal indicating the difference between the desired setpoint and the prevailing magnitude of the transmitted torque. This has the effect that the hydrostatic unit adds a constant braking torque to that of the braking system itself. 
     The drawing shows two distinct controllers  32  and  34 . The two control functions have been notionally separated to highlight the fact that two different control strategies are adopted depending on whether or not the brakes are sensed to be applied. Hence the controller  32  can be considered as implementing wheel speed control when the brakes are off while the controller  34  implements torque control when the brakes are sensed to be on. It will be appreciated that in practice both control functions can be performed by suitable programming of a single microprocessor.