Abstract:
A control system for a utility vehicle, particularly a compact utility tractor, that is speed-controlled by a hydrostatic transmission, prevents engine stalling by automatically reducing the stroke of a pump of the hydrostatic transmission, and thus reducing the vehicle speed, when the unloaded engine speed drops below a predetermined threshold. The apparatus includes a servo controlled hydrostatic transmission, an engine speed sensor, a throttle position sensor and a controller. The unloaded engine speed is determined by either an engine throttle lever position or by an engine speed capture algorithm that continuously monitors engine speed and records engine speed occurring when the transmission control pedals are positioned in neutral.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The invention relates to vehicles for industrial and agricultural use, such as utility tractors. Particularly, the invention relates to engine anti-stall transmission control of a utility vehicle that incorporates a hydrostatic transmission as an operator-controlled speed-adjusting component of the vehicle drive train. 
     BACKGROUND OF THE INVENTION 
     Compact utility tractors having hydrostatic transmissions are commonly purchased by customers that are not experienced tractor operators and are used for jobs, such as material handling with a front loader, that subject the tractor to sudden load application. With inexperienced drivers, engine stall may occur, leading to operator frustration and a perception that the tractor lacks sufficient engine power. This problem is exacerbated with foot pedal control of the hydrostatic transmission because in order to prevent engine stalling, the operator must actually reduce the pedal actuation, to decrease the stroke of the hydrostatic pump. This however is contrary to the action usually applied to foot pedal accelerators in cars and trucks to prevent engine stalling under load. 
     SUMMARY OF THE INVENTION 
     The invention provides an apparatus and method to prevent engine stalling, in a utility vehicle having a hydrostatic transmission, by automatically reducing the stroke of the hydrostatic transmission pump, and thus the vehicle speed, when engine speed drops below a predetermined threshold. 
     The apparatus and methods of the invention effectively prevent engine stall in a tractor having hydrostatic transmission speed control. The apparatus and methods can prevent engine stall when the vehicle is operating in either forward or reverse direction. 
     The apparatus includes a hydrostatic transmission, an engine speed sensor, a throttle position sensor and a controller, such as a microcontroller. A servo piston operating against a centering spring is moved to adjust the capacity of the pump in the hydrostatic transmission. The position of the servo piston is controlled by electro-hydraulic proportional pressure reducing valves that modulate the pressure applied to the servo piston. In normal vehicle operation, the operator depresses a foot pedal. A potentiometer senses the position of the foot pedal and sends a voltage signal to the controller. The controller software calculates a command current from the signal and drives the pressure reducing valves with the current. The greater the current, the greater the pump stroke and the faster the tractor wheels are turned. 
     As the tractor comes under load, the engine speed begins to drop. The microcontroller software continuously monitors the engine speed from a pulse pickup unit, and compares the engine speed to the estimated unloaded engine speed which is based on the position of the throttle lever as measured by the throttle position sensor. The engine speed is allowed to drop an amount specified by a software parameter. When the engine speed drop exceeds a threshold, then the microcontroller responds by reducing the current command to the pressure reducing valves, thus reducing the pump capacity and the tractor wheel speed. The amount of current reduction is calculated using a PID algorithm using the error between the unloaded speed, estimated from the throttle position sensor, and the actual engine speed. 
     As a further aspect of the invention, a method is provided for reducing the cost of implementing engine stall prevention in utility vehicles. The inventive system eliminates the need for a throttle position sensor, thus reducing the overall cost of the system. 
     The further aspect of the invention establishes a method for predicting the unloaded engine speed for anti-stall control from a measurement of engine speed while the transmission controls of the vehicle are in a neutral position. 
     The further aspect of the invention uses an engine speed sensor such as a pulse pickup unit, speed control foot pedals or other manual direction control, and a microcontroller with software. The software monitors the engine speed and the foot pedal speed controls continuously. When the foot pedals are in neutral, i.e., neither the forward or the reverse pedal is depressed, and the vehicle is not moving, the software captures and stores the engine speed in a microcontroller memory. This is an accurate estimate of the unloaded engine speed. As the operator commands the vehicle into motion, the engine speed will drop depending on the level of load. When the engine speed drop exceeds a specified value compared to the unloaded engine speed, a control command is sent from the microcontroller to the transmission to reduce the vehicle speed proportional to the amount of engine drop, and thus the power required. When the driver changes direction via foot pedal, the engine speed recovers rapidly, thus permitting an updated measurement of unloaded engine speed to be captured by the microcontroller memory. 
     This process works best when the engine throttle position is left unchanged during vehicle operation, but does work successfully if the operator changes throttle position while the vehicle is not moving. 
     This further aspect of the invention is particularly applicable to vehicle operations that involve regular changes in direction, such as moving materials with a front-end loader. Because the driver regularly shifts the transmission through neutral when changing direction, the microcontroller can regularly and accurately update its measurement of the unloaded engine speed without the need for a throttle position sensor. This further aspect of the invention permits engine stall prevention to be implemented at a reduced cost. 
     Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the embodiments thereof, from the claims and from the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the method of the utility vehicle speed control system of the present invention; 
     FIG. 2 is a schematic sectional view of the servo control system used in a hydrostatic transmission of FIG. 1; 
     FIG. 3 is an exploded, fragmentary perspective view of the servo control system of FIG. 2; 
     FIG. 3A is a schematic sectional view of a proportional pressure reducing valve of the system of FIG. 3; 
     FIG. 4 is a schematic sectional view of a hydrostatic transmission; 
     FIG. 5 is a block diagram of the speed control algorithm steps of the present invention; 
     FIG. 6 is a block diagram of an engine speed control algorithm routine incorporated into the present invention; and 
     FIG. 7 is a proportional valve control diagram demonstrating the operation of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While this invention is susceptible of embodiment in many different forms, there are shown in the drawings, and will be described herein in detail, specific embodiments thereof with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiments illustrated. 
     FIG. 1 illustrates in block diagram form, a vehicle  20  incorporating a preferred embodiment speed control system  24  of the present invention. The vehicle incorporates a hydrostatic transmission  26  and a range transmission  27 , such as a multi-speed gear transmission for transmitting power through a differential (not shown) to one or more driven wheels  28 . 
     The hydrostatic transmission  26  includes a variable displacement pump  30 , and a hydraulic motor  34 . An engine  35  rotationally drives the variable displacement pump  30 . The hydraulic motor drives the multi-gear transmission drive  27  interposed between the hydraulic motor  34  and the driven wheel  28 . 
     The control system  24  includes a controller  52 , such as a microprocessor-based microcontroller, in signal-communication with an engine throttle position sensor  56  connected to an engine throttle lever  57 . The microcontroller  52  is also in signal-communication with an engine speed sensor  64  located in close proximity to a rotating part of the engine  35 , such as rotating teeth or targets on the engine flywheel  65 . Preferably, the sensor  64  is a Hall effect sensor. The sensor  64  is configured to either send a speed signal to the microcontroller or to send a stream of pulses to the microcontroller, or to an associated component, wherein the microcontroller correlates engine speed to the frequency of the pulses. 
     The control system  24  includes a forward pedal  72 , and a reverse pedal  74 . The forward pedal  72  is operatively engaged with a potentiometer  82  to produce a forward pedal position signal, and a reverse pedal  74  is operatively engaged with a potentiometer  84  to produce a reverse pedal position signal. The potentiometers  82 ,  84  are signal-connected to the controller  52 . 
     The controller  52  is signal-connected, through appropriate signal conditioning or amplifying circuitry (not shown), to a solenoid  106   a  of a forward drive proportional control valve  106 , and to a solenoid  108   a  of a reverse drive proportional control valve  108 . The output current to energize the forward or reverse control valve solenoids  106   a ,  108   a  is substantially proportional to the corresponding pedal position signal. 
     FIGS. 2 and 3 illustrate the hydrostatic transmission servo control in more detail. Given an engine drive speed and a range transmission or final drive gear selection, the hydrostatic transmission provides infinitely variable speed control, forward and reverse, by operation of the foot pedals  72 ,  74 . Each valve  106 ,  108  is connected to a source of pressurized hydraulic fluid S and a return channel R that is at a reduced hydraulic pressure. Preferably, the return channel R recirculates hydraulic fluid back to hydraulic reservoir of the vehicle. 
     Depressing the forward foot pedal  72  causes an electrical output signal or voltage of the potentiometer  82  to be transmitted to the controller  52 . The controller  52 , through software, generates a pre-selected current ramp output, to energize the solenoid  106   a  of the forward drive proportional valve  106 . The proportional valve  106  is opened according to the ramp output, allowing pressurized hydraulic fluid, fed from the source S into the inlet  107  of the valve  106 , to flow through the valve  106 . The pressurized hydraulic fluid is communicated into, and pressurizes, a servo cylinder  114  on one side of a servo piston  112  that is slidably housed in the cylinder  114 . The other valve  108  allows fluid to flow from within the cylinder  114 , from an opposite side of the servo piston  112 , to the return channel R. 
     The piston  112  has a notch  115  that holds a piston follower  116  (FIG.  3 ). The piston follower  116  controls movement of a variable displacement pump cam plate or swashplate  118 . Movement of the piston  112  causes the swashplate  118  in the hydraulic pump to rotate out of the neutral position. Maximum displacement of the pump  30  is attained when the servo piston  112  is moved to its extreme position. The swashplate  118  can be positioned in a range of positions selected by the forward foot pedal  72 . 
     When the reverse pedal  74  is pressed, the potentiometer  84  sends an electrical output signal or voltage to the controller  52 . The controller  52 , through software, generates a pre-selected current output ramp to energize the solenoid driver  108   a  of the reverse drive proportional valve  108 . The reverse drive proportional valve  108  is opened, according to the ramp output, to allow pressurized hydraulic fluid, fed into an inlet  119  of the valve  108  from the source S, to flow through the valve  108 . The pressurized hydraulic fluid is communicated into, and pressurizes the servo cylinder  114  on an opposite side of the servo piston  112  within the cylinder  114 . The other valve  106  is allows fluid to flow from within the cylinder  114 , from the one side of the servo piston  112 , to the return channel R. 
     Preferably, the valve solenoids  106   a ,  108   a  are driven by pulse width modulation type currents that modulate output pressure proportionally according to the controlled width of step pulses of current applied to the driver. While the frequency of the pulses remains substantially the same, the pulse widths are changed to modulate the valves. 
     The hydrostatic system is preferably a closed loop fluid power system that consists of a charge pump (not shown), and the variable displacement pump  30 , which is driven by a flex plate/dampener assembly (not shown) connected to the engine flywheel. The charge pump provides pressurized fluid to the proportional valve inlets  107 , 119 . Return fluid from the servo control unit is routed to the reservoir of the tractor&#39;s hydraulic system. 
     An exemplary example of a control valve, such as the control valve  106 , is illustrated in FIG.  3 A. The solenoid  106   a  includes a plunger  120  (shown schematically) driven by the solenoid coil  121  (shown schematically). The plunger  120  drives a valve spool  122  within a housing  123 . The housing provides the pressurized hydraulic fluid inlet  107 , in the form of plural openings, and an outlet  124 , in the form of plural openings, to the hydraulic fluid reservoir. A control pressure outlet  125  communicates hydraulic fluid at a modulated pressure to the servo cylinder  114  as shown in FIG.  2 . The solenoid coil  121  drives the plunger  120  downward (in FIG. 3A) to open the inlet  107  to the outlet  125  through an annular channel  122   a.    
     The channel  122   a  is open to an oblong orifice  122   b  through the spool  122  to communicate fluid into an interior  122   c  of the spool. The interior of the spool is open to the outlet  125 . The pressure of the hydraulic fluid at the control outlet  125  is substantially proportional to the force applied to the spool by the plunger, ranging between reservoir pressure, the pressure at the outlet  125  with the inlet  107  closed, as shown in FIG. 3A, to pressurized hydraulic fluid supply pressure, the spool  122  moved down to close the outlet  124  and open the inlet  107 . 
     An annular screen  107   a  and a circular screen  125   a  can be supplied to the inlet  107  and to the outlet  125  respectively. 
     The control valve  108  can be identically configured as described above for the control valve  106 . 
     FIG. 4 illustrates the hydrostatic transmission  26  in more detail. The hydrostatic pump  30  illustrated is an axial piston, servo controlled, variable displacement piston pump. Input shaft splines  126  are driven via a flex plate (not shown) bolted onto the engine flywheel (not shown). 
     Fluid flow through the pump  30  is controlled by changing the angle of the swashplate  118 . The location, off center, of the swashplate controls the distance the pistons  130  travel inside the piston bores  132  of the rotating assembly. The direction that the swashplate is rotated from center determines the direction of fluid flow, forward or reverse. The number of degrees the swashplate is deflected determines how much fluid will be displaced which controls transmission speed. 
     The hydrostatic pump  30  provides hydraulic fluid to the hydrostatic motor  34  through the back plate  138 . Hydraulic fluid in the power train circulates in a closed loop. Fluid leaves the hydrostatic pump  30 , flows through the hydrostatic motor  34 , and is returned to the hydrostatic pump. Fluid that leaves this closed loop circuit, such as to the case drain, is replenished by fluid from the charge pump. 
     The hydrostatic motor  34  is a high torque axial piston motor. The motor is located on the rear of the back plate. The hydrostatic motor drives an output shaft coupled to the range transmission  27  that transfers power to the wheels. The range transmission  27  can be a multi-speed range gear transmission, such as a three-speed or four-speed gearbox. 
     A method of preventing engine stall in a utility vehicle is set forth in FIG.  5 . The method includes the steps of: step  300 , continuously sensing the rotational speed of a rotating part in an engine of the vehicle; step  304 , operating the vehicle in forward and reverse; step  306 , recording the rotational speed of the part when the transmission is in neutral as the unloaded engine speed, or alternatively, continuously monitoring the position of the throttle lever; step  310 , establishing a speed drop threshold; step  316 , comparing the rotational speed of the part to the unloaded engine speed; step  318 , determining a difference between the unloaded engine speed and the speed of the part; step  320 , whenever the engine speed drops below the threshold, proportionally reducing the energizing current to the proportional control valves to reduce hydrostatic transmission output, to unload the engine. 
     The unloaded engine speed can be determined by the throttle lever position sensor  56  or alternatively by recording the engine speed each time the transmission passes through the neutral position, i.e., neither forward nor reverse pedals being depressed and the vehicle being stationary. The threshold engine drop amount can be a preselected amount or can be calculated based on a percentage of the engine unloaded speed. 
     FIG. 6 illustrates a control algorithm routine of the software of the microcontroller  52  which compares the engine speed as sensed by the Hall effect pickup unit  64  to the unloaded engine speed, which uses PID (proportional, integral, derivative) feedback control mathematics to diminish the difference by controlling the speed output of the hydrostatic transmission. The routine changes the output signal from the microcontroller to the proportional control valves proportionally, to reduce or increase the hydrostatic transmission speed output. 
     FIG. 7 illustrates the proportional relationship between the engine speed drops and the proportional valve current signal from the microcontroller. The speed drop is defined as the unloaded actual engine speed minus the actual measured engine speed. To operate effectively, some engine speed drop under load is necessary. This is indicated as a speed drop threshold ST. In the example shown in FIG. 7, the engine speed can drop 200 rpm before the proportional control valve current from the microcontroller is changed. After the threshold ST is reached, further drop in engine rpm decreases the proportional valve current substantially linearly during a ramp down phase RDP until, at a certain engine speed drop, no further current reduction is required to prevent engine stalling. 
     From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims.