Abstract:
A PTO clutch of an agricultural vehicle connects an input driveline to an output driveline for coupling to an attached implement. A method and system for controlling the PTO clutch includes sensors for sensing rotational speeds on both sides of the clutch. Clutch slip is determined from the sensed speeds. A controller receives an actual slip signal and a desired slip signal and controls pressure in the clutch to maintain a constant desired clutch slip in order to avoid overload conditions. The torque transmitted by the clutch is determined as a function of the slip in the clutch and the clutch pressure, and a signal representing this torque is displayed to an operator.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a system and method for controlling a clutch, particularly a power take-off (PTO) shaft clutch. 
     Various systems and methods are known for controlling a torque transmitting clutch, such as a PTO clutch for transmitting power to an attached implement. There are, for example, control systems which use sensed rotational speed to determine operating conditions of a PTO shaft clutch. Published patent DE-A-40 01 398 describes a power take-off shaft clutch which is controlled by an electronic evaluation unit and thereby can react to critical operating conditions. In particular, slip of the power take-off shaft clutch is to be avoided in order to prevent increased wear or destruction of the clutch. Sensors sense engine specific data, such as rotational speed and torque, so that the evaluation unit can react to certain limit values. If a certain engine limit rotational speed value is not reached, the power take-off shaft clutch is disengaged and the load on the driveline is removed. 
     The system of DE-A-40 01 398 also senses the rotational speed values at the inlet and the outlet of the power take-off shaft clutch, and monitors the clutch slip by comparison of these values. When pre-determined values of slip are exceeded, the electronic evaluation unit disengages the power take-off shaft clutch by means of a control valve. However, disengaging the clutch when the slip limit values are exceeded leads to an interruption of the operating process that can only be resumed after a renewed clutch engagement process. A similar condition occurs when an engine rotational speed limit is not reached and the PTO shaft clutch is disengaged, in order to reduce the load on the engine driveline. In this case, the clutch can be re-engaged only under restricted operating conditions. 
     It would be desirable to provide a method and a system for controlling a PTO clutch and which overcomes the aforementioned problems. In particular, it would be desirable to monitor the load on the clutch during the operation of attached implements, so that overload conditions on the driveline as well as on the attached implement and the components connected to it can be avoided. 
     SUMMARY 
     Accordingly, an object of this invention is to provide a PTO clutch control system which maintains a constant slip in the clutch. 
     This and other objects are achieved by the present invention, wherein a pressure operated PTO clutch of an agricultural vehicle connects an input driveline to an output driveline for coupling to an attached implement. A method and system for controlling the PTO clutch includes sensors for sensing rotational speeds on both sides of the clutch. Clutch slip is determined from the sensed speeds. A controller receives an actual slip signal and a desired slip signal and controls pressure in the clutch to maintain a constant desired clutch slip in order to avoid overload conditions on the input driveline or the output driveline of agricultural machines and their attached implements. A signal representing the torque transmitted by the clutch is displayed to an operator. The torque transmitted by the clutch is determined as a function of the slip in the clutch and the clutch pressure. 
     A controller maintains the slip at a constant value independent of the torque transmitted, by actively controlling the clutch pressure. Since the torque transmitted by the clutch has an approximately linear relationship with the clutch pressure and the valve current, these parameters can be utilized to determine the torque transmitted by the clutch. The higher the valve current, and therewith the pressure level at which the clutch can be operated at the desired slip, the higher is the torque transmitted by the clutch. The load or torque transmitted by the clutch is determined as a function of the constant slip value and the clutch pressure. 
     Detecting load by electronically controlling slip has been shown to be useful in PTO shaft drives. The control can react to changes in the load so rapidly that a stable operation with relatively constant slip is possible. During testing on a PTO clutch brake with a defined load it could be shown that the clutch pressure and therewith the valve control electrical current are representative of the torque in the PTO shaft and that it is possible to determine load during operation. A further advantage of the slip control is the protective function against overload. Shock loads and related torque peaks in the PTO shaft driveline during operation are intercepted and damped by short term peaks in the slip of the PTO shaft clutch. 
     The clutch slip is preferably maintained at a predetermined standard slip value, such as between 0.1% to 2.0%. The most appropriate value has been found to be a standard slip value of approximately 0.5%. 
     Slip is maintained constant by varying the clutch pressure with a valve, preferably a proportional pressure control valve. Valve electrical current is utilized as control magnitude for the control of the slip. Preferably, the control magnitude is limited by an input of a maximum control magnitude so that a maximum torque cannot be exceeded in the PTO shaft, thus protecting the vehicle driveline, the PTO shaft gearbox and the drive for the attached implement. 
     The maximum control magnitude can be inputted manually or automatically by an identification system on the attached implement which can be plugged into a CAN, ISO, LBS or a similar interface in a “Plug-and-Play” manner. 
     The PTO shaft clutch control system includes sensors and an evaluation system. The sensors detect a rotational speed on each side of the clutch. The evaluation system, which is part of an electronic control system, determines the slip in the clutch considering the gear ratio of the clutch system. The control system continuously senses clutch pressure. Clutch pressure is controlled to maintain clutch slip at a constant value. The torque transmitted by the clutch is determined from the constant value of the slip and the clutch pressure. Preferably, the electronic control is an integrated controller and is configured corresponding to DIN 19226. 
     The invention determines the torque transmitted by the clutch using simple sensors, and displays this information continuously to the operator. With this invention the torque transmitted by the clutch can be limited to protect the driveline and the attached implement against overloads. In addition, sudden changes in the torque are prevented by short-term increases in the clutch slip. The invention is extremely economical because no significant increase in sensor capability is required. 
     This control system may be applied to agricultural machines which have a PTO shaft connected to an attached implement. The PTO shaft clutch is preferably a wet multi-disk clutch such as used on John Deere series 6010 to 6910 agricultural tractors. Such clutches have a very high durability, even when subjected to slipping operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a control system for controlling the slip of a PTO shaft clutch. 
     FIG. 2 is a control system diagram of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, an internal combustion engine  10  drives a drive shaft  12 . A rotation speed sensing gear  14  is coupled to drive shaft  12 . A hydraulic PTO shaft clutch  16  connects the drive shaft  12  with a two-stage PTO shaft gearbox  18  that transmits torque to a PTO shaft  20 . An implement  54  can be connected to the PTO shaft  20  or the PTO stub shaft. As is well known, the engine  10  drives the vehicle drive wheels (not shown) through a vehicle gearbox (not shown). 
     The input stage  22  of the PTO shaft drive gearbox  18  is connected to a hydraulic brake  24  which can brake and stop the entire PTO shaft output driveline. A gear  26  is mounted on the PTO shaft  20  for sensing the output shaft rotational speed. Rotational speed sensors  28  and  30  sense the rotational speeds of the drive shaft  12  and the PTO shaft  20  and supply speed signal n 1  and n 2  to an evaluation unit  34  which is integrated into a control unit  32 . The evaluation unit  34  determines the slip X of the clutch  16  from speeds n 1  and n 2  and receives a gear ratio signal of the PTO shaft gearbox  18 , from an appropriate sensor (not shown). 
     The clutch  16  and the brake  24  are controlled by a electrohydraulic proportional valve  42 , which in a first position, as shown, connects the clutch  16  with a hydraulic pump  44  and connects the brake  24  with an unpressurized reservoir  46 . In a second position the proportional valve  42  connects the brake  24  with the hydraulic pump  44  and connects the clutch  16  with the reservoir  46 . Proportional valve  42  controls the pressure in clutch  16  and maintains the pressure in clutch  16  proportional to the magnitude of the electrical current applied to the solenoid of valve  42 . 
     A pressure sensor  36  transmits a clutch pressure signal P to the evaluation unit  34 . Unit  34  determines the torque transmitted by the clutch  16  as a function of the clutch slip and the clutch pressure, and supplies a torque signal to display  48 . A target slip value input unit  38  provides a target slip value Xs to the control unit  32  so that the controlled value of the slip can be adjusted. Evaluation unit  34  receives the target slip value Xs, compares it to the actual slip X, and provides a differential slip value Xd as an input to controller  40 . Controller  40  operates as shown FIGS. 2 and 3 and provides a solenoid control electrical current to the solenoid of proportional valve  42  to maintain the slip in clutch  16  at the desired target slip. 
     A manual input unit  50 , such as a rotary potentiometer placed in the vehicle cab (not shown), can be used to set a limit value for the valve current and thereby the pressure in the clutch or the maximum torque that can be transmitted by the clutch  16 . The control unit  32  uses this limit value to avoid overload conditions on the input driveline as well as the output driveline. 
     An implement connected to the PTO shaft  20  can be identified to the control unit  32  by an interface  52 , such as a CAN, ISO, LBS or similar interface, to which can be coupled a connector (not shown) in a “Plug and Play” manner. For each type of attached implement a maximum torque value can be stored in the control unit  32 , so that torque can be limited to a maximum value specific to the particular attached implement. 
     Referring now to FIG. 2, the engine  10  rotates at a rotational speed of n 1  and delivers torque M 1 , which is transmitted by the clutch  16  and gearbox  18  to a PTO shaft  20 . The PTO shaft  20  rotates at a rotational speed of n 2  and transmits the output torque MA to the attached implement  54 . An actual slip value X is a function of rotational speeds n 1  and n 2 , of the transmission ratio of the PTO shaft gearbox  18 , of the disturbance magnitude Z 2 , which depends on the friction coefficient or wear condition of the clutch, and of disturbance magnitude Z 3 , which depends on the load of the attached implement  54 . The actual slip value X is compared with a predetermined slip target value Xs of, for example, 0.5%. 
     The resulting slip differential value Xd is an input to the controller  40 . Preferably, the response of controller  40  varies depending upon the range of the input value Xd. In response to the slip differential value Xd, the controller  40  supplies to valve  42  a valve current control signal Y. In response to signal Y, valve  42  controls the pressure in clutch  16  and or in the brake  24 . Clutch pressure P is also a function of pressure variations represented by disturbance magnitude Z 1 . Controller  40  is designed to maintain slip difference Xd as small as possible and preferably equal to zero, and to thereby maintain the slip of clutch  16  at the desired constant slip target value Xs. 
     Since at a constant controlled slip, a known relationship exists between the drive torque MA operating at the PTO shaft  20  and the current in the valve  42 , which can be determined by tests or by theoretical calculations, the output torque MA can be determined from the existing slip value and the current in the valve  42 . 
     Preferably, the controller is optimized with respect to its response to disturbances, and to prevent increased slip. If, however, a sudden increase in the torque occurs during operation at the PTO stub shaft and as a result the slip exceeds the predetermined value, for example, of 0.5%, then the controller reacts accordingly and increases the current to the valve  42  and increases the clutch pressure. During very rapid changes in the power requirement of the attached implement very high undesirable slip can occur for brief periods, so that the control must react sufficiently fast, in order to maintain the slip as constant as possible. 
     In order to assure an optimum and rapid control response, the response of controller  40  varies depending upon the magnitude of the actual slip X. For example, controller  40  may have three different sets of control parameters, each for one of three corresponding ranges of actual slip X. Such control parameters may include a proportional amplification parameter, Kp(1-3) and a response time parameter Tn(1-3), such as defined by DIN 19266, so that the controller will have a proportional and integral performance and will perform dynamically. Preferably, the response of the controller will be faster and more aggressive for higher actual slip values, so that the proportion of time at increased slip values is reduced. Preferably, an operator may manually adjust the controller  40  to optimize its performance in response to sudden disturbances. 
     While the present invention has been described in conjunction with a specific embodiment, it is understood that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, this invention is intended to embrace all such alternatives, modifications and variations which fall within the spirit and scope of the claims.