Abstract:
A tracked vehicle has an electrohydraulic drive/steering system having an engine driven variable displacement hydraulic steering pump which drives a hydraulic steering motor which is coupled to a differential track drive mechanism via a transmission with multiple gear ratios. A steering wheel is coupled to a variable friction device which produces a variable friction force which resists rotation of the steering wheel. A control system is responsive to a position of the steering wheel and controls the steering pump displacement and controls the friction device. The control system sets the variable friction device to its high friction level when a limit of the steering pump displacement is reached when the transmission is in a higher one of its gear ratios.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a tracked vehicle steering system, and more particularly, to a tracked vehicle steering system which has a steering wheel coupled to a variable friction device. 
     There are commercially available tracked vehicles with hydro-mechanical drive/steering systems which include an engine driven hydraulic steering pump which is responsive to steering pump control signals provided from a spring-centered steering wheel, and which drives a hydraulic steering motor. The steering motor drives a differential track drive mechanism which drives left and right tracks, and turns the vehicle at turning rates which depend on the magnitude of the steering pump control signals and engine speed. With such systems, the steering wheel position provides a direct indication of the position of swash plate of the pump, so the limit of pump capacity is coincidental with the limit of steering wheel rotation. While functional, such systems lack various features, and vehicles with such systems do not drive like cars or other familiar vehicles. For instance, though the position of the steering wheel indicates the swash plate position, the actual turn radius of the vehicle will depend on both the engine speed and ground speed. 
     A proposed electrohydraulic tracked vehicle drive/steering system has been described in U.S. Pat. 6,039,132, issued Mar. 21, 2000, and assigned to the assignee of this application. As described in U.S. Pat. No. 6,000,490, issued Dec. 14, 1999, and also assigned to the assignee of this application, there has also been proposed an electrohydraulic tracked vehicle drive/steering system which has a non-spring centered steering wheel coupled to a variable friction steering input device. The steering input device produces a variable, two-level friction force which resists turning of the steering wheel, provides feedback to the operator, simulates“end stops” on the steering wheel motion, and thus allows the tracked vehicles to drive more like wheeled vehicles. In this proposed system the higher friction level is turned on when a certain amount of steering wheel rotation is reached, similar to what occurs with a wheeled vehicle. But in higher gears of the vehicle transmission, the limit of steering pump stroke is reached before the corresponding amount of steering wheel rotation is reached. Thus, additional rotation of the steering wheel cannot cause a tighter turn. It would be desirable to provide the operator with an indication of when this condition is about to be reached. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide an electrohydraulic tracked vehicle steering system which provides the operator with an indication of when the limit of the steering pump stroke is reached when the transmission is in higher gear ratios. 
     These and other objects are achieved by the present invention, wherein a ratio of the desired or commanded steering motor speed to the engine speed is generated and compared to a limit value. When the ratio value reaches the limit, the limit of the steering pump is assumed to be imminent, and a variable friction steering input device coupled to the steering wheel is set to a high friction level. This indicates to the operator that the end of steering capability has been attained, and that a tighter turn radius should not be expected under such conditions. This is especially useful in a tracked vehicle with an electrohydraulic drive/steering system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified schematic diagram of a tracked vehicle drive/steering control system for use with the present invention; and 
     FIG. 2 is a logic flow diagram of an algorithm executed by a microprocessor-based control unit of the control system of FIG.  1  and which implements an embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, an engine  10  of a tracked vehicle has an output shaft  12  which drives a right angle gear  14  and a transmission  16 , such as a 16-speed powershift transmission which is available on production John Deere 8000T tractors. The transmission  16  includes hydraulically operated clutches and brakes (not shown), various ones of which will operate as a main clutch  18  in response to a conventional clutch pedal and linkage (not shown). The engine  10  is controlled by an electronic engine control unit  11 . The electronic engine control unit  11  is communicated with a steering system unit (SSU)  13  via a bus  15 . 
     The transmission  16  drives a final or right angle drive  20 , which drives a left track drive wheel  22  via left steering planetary drive  24 , and a right track drive wheel  26  via right steering planetary drive  28 . The steering planetary drives  24  and  28  are preferably such as described in U.S. Pat. No. 5,390,751, issued Feb. 21, 1995 to Puetz et al., and assigned to the assignee of this application. Additional outboard planetaries (not shown), as provided on John Deere 8000T tractors, are mounted between the steering planetaries and the respective drive wheels, but are not further described because they are not directly involved in the subject matter of this application. A parking brake  30  is coupled to the output shaft of transmission  16 , and left and right service brakes  32 ,  34  are coupled to the left and right drive wheels  22 ,  26 , respectively. 
     The right angle gear  14  drives a variable displacement steering pump  40 , such as a 75 cc, 90 series pump made by Sauer-Sundstrand. The pump  40 , in turn, powers a hydraulic fixed displacement steering motor  42 , such as a 75 cc, 90 series motor, also made by Sauer-Sundstrand. The steering motor  42  drives, via a cross shaft  44  and gear  46 , a ring gear  47  of left planetary drive  24 , and via cross shaft  44 , gear  48  and reverser gear  50 , a ring gear  52  of right planetary drive  28 . 
     The steering pump  40  has a swashplate (not shown), the position of which is controlled by a swashplate control valve or electronic displacement control (EDC)  60 . The EDC is preferably a two stage device with first stage including a flapper type valve operated by a pair of solenoids  59 ,  61 , and a second stage including a boost stage to the pump, such as is used on the production John Deere 8000T Series tracked tractor. 
     An operator presence switch  51  provides an operator seat presence signal to the SSU  13  via the bus  15 . An engine speed sensor  62 , such as a commercially available mag pickup, provides an engine speed signal to the SSU  13 . The solenoids  59 ,  61  of valve  60  are controlled by pulse-width-modulated (PWM) pump control signals generated by SSU  13 . 
     An operator controlled steering wheel  74  is preferably connected to a non-spring centered input mechanism  72 , such as described in U.S. Pat. No. 6,000,490, issued Dec. 14, 1999, and assigned to the assignee of the present application. The input mechanism  72  includes an electromagnetically controlled friction device or brake  75  and a rotary position transducer or incremental encoder  77 , such as a commercially available Grayhill Series 63R encoder or an OakGrigsby 900 Optical Encoder. The encoder  77  provides to SSU  13  a steering wheel position signal representing the position of operator controlled steering wheel  74 . The encoder  77  generates a plurality, preferably 128, of pulses per each revolution of the steering wheel  74 . The SSU  13  then repeatedly generates and updates a COUNT value representing the number of optical encoder pulses corresponding to the movement of the steering wheel  74  relative to the position of the steering wheel  74  at center. For example, a negative COUNT value will be generated when the steering wheel  74  is rotated counterclockwise from its center position, and a positive COUNT value will be generated when the steering wheel  74  is rotated clockwise from its center position. Thus, COUNT has a magnitude which is proportional to its angular displacement from its center position, and a sign representing the direction (clockwise or counterclockwise) from its center position. 
     The SSU  13  also receives gear shift command signals from gear shift lever mechanism  73 , such as described in U.S. Pat. No. 5,406,860, issued Apr. 18, 1995 to Easton et al., and such as used on production John Deere 8000 Series tractors. The gear shift lever mechanism  73  includes a shift lever  53  which is movable to forward upshift and downshift, reverse upshift and downshift, neutral and park positions within a guide  55 . 
     A drive line rotation speed sensor  76 , preferably a differential Hall-effect speed sensor such as used on production John Deere 8000T tractors, is mounted in proximity to the final drive  20 , and provides to the SSU  13  a variable frequency final drive speed or wheel speed signal. A magnetic ring  78  is mounted for rotation with the motor  42 , and a Hall-effect transducer  80  mounted near the magnetic ring  78  provides to the SSU  13  an incremental motor position signal and a motor direction signal. A pair of clutch status switches  82  are located within the transmission  16  and are operatively associated with the linkage (not shown) between the clutch pedal (not shown) and the main clutch  18 , and provide a clutch status signal to the SSU  13 . 
     The SSU  13  includes a commercially available microprocessor (not shown) which generates the pump control signals which are communicated to the solenoids  59 ,  61  of valve  60 . Preferably the pump control signals are generated as a function of the COUNT value as a result of the SSU executing a main control algorithm (not shown), such as described in pending U.S. patent application Ser. No. 09/456,702, filed Dec. 9, 1999, and assigned to assignee of the present application, and which is incorporated herein by reference. 
     According to the present invention, preferably every 20 milliseconds, the SSU  13  also executes a subroutine or algorithm  100  which is illustrated by FIG.  2 . The algorithm  100  starts at step  102 . Step  104  reads the engine speed (rpm) signal from sensor  62 . Step  106  reads the steering wheel position signal (COUNT) from encoder  77 , and reads the vehicle speed signal (VEHSPD, in Hz) from sensor  76 . Step  108  calculates a desired or commanded steering motor position increment (DSMINC) according to the following equation: 
     
       
         DSMINC=(COUNT×VEHSPD)/500. 
       
     
     “Step  110  calculates a ratio value (RATIO) by dividing the engine speed by the desired steering motor position increment. If RATIO is greater than a threshold or limit, such as  11 , step  112  directs the routine to step  114 . Step  114  activates the friction device or brake  75  of the steering input device  72  so that the operator can feel that it is more difficult to turn the steering wheel  74 , and so that the operator will be given an indication that the limit of steering pump stroke is being reached while the transmission  16  is in a higher gear ratio. 
     If in step  112 , RATIO is not greater than the limit, then step  112  directs the routine to step  116  which deactivates the friction device or brake  75 .” 
     “Because the routine is periodically repeated, the routine operates to effectively convert the position increment value to a speed value. Thus, the routine effectively calculates a commanded steering motor speed, and generates a ratio value representing a ratio of the engine speed to the desired steering motor speed. Thus, the routine operates to generate an engine speed signal, to generate a commanded steering motor speed value, to generate a ratio value representing a ratio of the engine speed to the desired steering motor speed, to compares the ratio value to a limit value, and to control the variable steering friction device as a function of a relationship between the ratio value and the limit. Specifcally, the variable steering friction device is set to its high friction condition when the ratio value is above the limit, and is turned off when the ratio value is not above the limit.” 
     Preferably, the friction device  75  is put into its high friction condition only when the steering wheel is being manipulated in an attempt to achieve a tighter turn (absolute value of COUNT increasing). Conversely, whenever the absolute value of COUNT is decreasing, the friction device or brake  75  is turned off or placed in its low friction condition. 
     The conversion of this flow chart into a standard language for implementing the algorithm described by the flow chart in a digital computer or microprocessor, will be evident to one with ordinary skill in the art. 
     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 appended claims.