Patent Application
20070130938
This invention relates to controls for operating hydrostatic transmissions in tractors and utility vehicles for agriculture, lawn care or recreational use.
Tractors and utility vehicles used for agriculture, lawn care or recreational use may have a hydrostatic transmission that drives a final drive transmission or range transmission of the vehicle. The final drive transmission or range transmission may drive at least one wheel. The vehicle speed may be operator modulated by changing the drive ratio of the hydrostatic transmission, as well as the range gears. The drive ratio may be changed by moving the angle of a swashplate of a variable displacement pump of the hydrostatic transmission.
Hydrostatic transmissions in tractors and utility vehicles may be operated using foot pedals to control the direction and speed of the vehicle. For example, a first foot pedal may pivot a swash plate in the hydraulic pump to an angular alignment in which hydraulic fluid provided to the hydraulic motors propels the vehicle forward, and a second foot pedal may pivot the swash plate to move the vehicle in reverse. If neither foot pedal is applied, the swash plate may be in a neutral position.
Advantages of hydrostatic transmissions include infinite speed control for a given engine speed, powered creep, and anti-stall control. It is highly desirable for a tractor or utility vehicle to have these operational features that are available in hydrostatic transmissions. However, some operators are not accustomed to hydrostatic transmission controls, and/or prefer using a gear shift to control travel of the tractor or utility vehicle, and others would prefer using only an accelerator pedal like an automatic transmission. A control apparatus is needed for a hydrostatic transmission that can simulate a gear shift, or automatic transmission, to control movement of the tractor or utility vehicle. A transmission control is needed for a hydrostatic transmission that will enable operators to select between different modes of operation.
Additionally, manufacturing costs are higher for tractors or utility vehicles that are produced with several different transmissions available. A transmission control is needed for a hydrostatic transmission that can reduce manufacturing costs by allowing a single transmission to simulate more than one operational mode.
The invention provides a transmission control for a hydrostatic transmission of a tractor or utility vehicle that includes alternative control interfaces and transmission responses. The transmission control includes a microcontroller to provide an output current to the proportional control valves for a hydrostatic transmission. The microcontroller may function using a different control algorithm for each operational mode.
As shown in the block diagram of
In one embodiment, hydrostatic transmission 101 may include a variable displacement pump 108 providing variable volume flow rate of pressurized hydraulic fluid. Pump 108 may have an angularly adjustable swashplate, and the pump displacement may be set by the angle of the swashplate. At least one proportional control valve 106 may be operated by servo controls 107 connected to the swashplate, the control valve(s) being signal-connected to the coil current output of microcontroller 102. Hydraulic motors 104 may receive the pressurized hydraulic fluid from pump 108. The hydraulic motors may be operatively connected to the rotating part of a drive train, preferably a gear or set of range gears 113.
In one embodiment, microcontroller 102 may be linked electronically to transmission controls 103 and may provide a current output to hydrostatic transmission 101 in one or more operational modes.
In one embodiment, transmission controls 103 may be used in one or more operational modes to operate the hydrostatic transmission or HST. A first operational mode of transmission controls 103 may enable the hydrostatic transmission to simulate an automatic transmission. The first mode also may be referred to as the “automatic” mode. To place the transmission controls in the first “automatic” mode, switch 109 may be placed in an “Automatic” position. If a position sensor detects the switch in the “Automatic” position, a specified analog output voltage may be provided to microcontroller 102 enabling the transmission to simulate an automatic transmission.
In one embodiment, a second operational mode of transmission controls 103 may simulate a gear drive transmission. The second operational mode may be referred to as a “gear” mode. To operate the transmission controls in the second “gear” mode, switch 109 may be turned to one of several positions representing and simulating discrete and distinct “gears” or speeds.
In one embodiment, switch 109 may be used to select operation of the hydrostatic transmission in either of the “automatic” or “gear” operational modes. For example, switch 109 may be a rotary switch or lever that may be turned or rotated to shift between different operational modes. Switch 109 may have two or more distinct positions. The position of switch 109 may be sensed and provided as a shift lever sensor analog voltage.
Alternatively, switch 109 may be a shift lever with a non-rotary shift configuration. For example, switch 109 may be a shift lever that can be moved in a linear pattern or other pattern to each of several positions. Alternatively, switch 109 may include push button controls to shift between modes or within modes.
In one embodiment as shown in
Additionally, in one embodiment, shift lever 120 may be swung through an arc around the base of the lever shaft to a forward drive command position, a neutral position, and a reverse drive command position. The shift lever may be located anywhere in the operator area of the tractor or vehicle. The shift lever may have a discrete position to command forward movement, a discrete position to indicate neutral, and a discrete position to command reverse movement. Detents may be included to provide points of reference and markings may be included to signify command position. The position of the shift lever may be sensed with either an analog electrical device or with electrical switch contacts. The shift lever position may be used by the microcontroller to provide a current command to set the swash plate in the HST to operate the tractor or vehicle in forward, neutral or reverse.
In one embodiment, there may be a distinct step and/or detent between each “gear” or position of switch 109. Alternatively, the switch may provide continuous shifting without distinct steps between each position, or may provide a bump between positions. For example, the switch may provide continuous or infinitely variable speeds without distinct positions or steps. Instead of a rotary switch or shift lever, switch 109 may have push button controls to step up or down between each “gear” or speed, and between the forward, neutral and reverse positions.
The position of the shift lever may be sensed by one or more position sensors to provide an analog rotary position output to the microcontroller. In one embodiment, a variety of different position sensors may be used to determine the position of switch 109 and provide a shift lever sensor voltage. The position sensors include but are not limited to a potentiometer, a hall effect sensor, a pressure transducer, or a series of switches. For example, as shown in
In one embodiment, if the transmission controls are operated in the first “automatic” mode, the position of “accelerator” pedal 110 may be used to control HST motor speed. In the “auomatic” mode, HST motor speed may be controlled independent of engine speed. The “accelerator” pedal position may be sensed by a position sensor such as a potentiometer, a hall effect sensor, or a pressure transducer. The sensor may provide an analog voltage signal output based on pedal position.
In one embodiment, in the “automatic” mode, the microcontroller may convert the pedal position sensor voltage to an HST motor speed command between zero and 100%. For example, as shown in
In one embodiment, the speed command from the accelerator pedal position sensor may be subject to an optional profile modification step. Profile modification may be used to adjust performance of the transmission controls, or to provide finer control at lower speeds.
In one embodiment, the speed command from the accelerator pedal position sensor may be converted to a current value. For example, as shown in
In one embodiment, the microcontroller may include an algorithm that compares a throttle position input to to an actual engine speed input. The throttle position input indicates a predicted engine speed. If the actual engine speed is less than the predicted engine speed, an adjustment can be made to an output to the proportional pressure reducing valve which controls the swashplate position in the hydrostatic pump. The swashplate adjustment can de-stroke the swashplate to reduce load from the engine and bring the actual engine speed back up to the predicted engine speed.
In a second embodiment, if the transmission controls are operated in the first “automatic” mode, the speed command from the accelerator pedal position sensor may be converted to a current value as shown in
In either the first or second embodiments, the microcontroller 102 may provide a current to proportional valve(s) 106 to provide a control pressure to a servo piston to control the hydrostatic motor swash plate angle. The speed of the hydrostatic motor may be increased or decreased as a result. For a given control pressure, the actual hydrostatic motor speed may depend on the load and/or pressure of the system.
As shown in
In one embodiment of the transmission controls in a “gear” mode, the analog voltage signals from the shift lever sensor may be converted to desired speed commands between zero and 100%.
In one embodiment, the transmission controls optionally may include “clutch” pedal 105 to provide a modulator for either or both operational modes. The position of the “clutch” pedal may be sensed using any position sensor including a potentiometer, a hall effect sensor, or a pressure transducer. The sensor may produce an analog voltage signal that may be converted to a clutch pedal position command between zero and 100%. For example, if “clutch” pedal 105 is let all the way out (released) by the operator, the full or 100% command may be provided. As “clutch” pedal 105 is depressed, the clutch pedal command may be reduced proportionally to the amount of pedal travel. If “clutch” pedal 105 is fully depressed to the bottom of travel, the command may be reduced to zero to stop movement of the tractor or utility vehicle. Alternatively, the “clutch” pedal may increase the deceleration rate when the bottom of travel is reached. Optionally, the “clutch” pedal may need to be fully depressed to change between the two operational modes.
In one embodiment, the transmission controls also may include a throttle position sensor. Throttle 112 may be used to control the speed of engine 100, and a throttle control position sensor also may provide a throttle sensor analog voltage signal to microcontroller 102. As shown in
In one embodiment, microcontroller 102 may process the transmission control commands to provide a set point for a hydrostatic motor speed percent command. As shown in
Maximum possible HST motorspeed. For example, the maximum possible HST motor speed may be a fixed value of about 2000 rpm for a typical HST motor.
Throttle position ratio. This is the ratio of predicted engine speed (based on throttle position) to maximum engine speed. For example, the ratio may be between zero and 100% depending on the sensed position of the throttle.
Speed command. In the first “automatic” mode, the speed command may be a continuous variable between zero and 100% depending on the “accelerator” pedal position. In the second “gear” mode, the speed command may be a stepped variable having a value for each “gear” selected by shift lever or switch 103. For example, the speed command may be 25% if the shifter is placed in a first position to simulate first gear, 50% if the shifter is in a second position to simulate second gear, 75% if the shifter is in a third position simulating third gear, and 100% if the shifter is in a fourth position simulating fourth gear.
“Clutch” command. This optional input may be a variable between zero and 100% based on the sensed “clutch” pedal position.
In one embodiment, the Maximum possible HST motor speed, Throttle position ratio, Speed command and optional “Clutch” command may be used to determine a set point for the desired HST motor speed. Each of these inputs may be multiplied together. For example, if Maximum possible HST motor speed is 2000 rpm, Throttle position ratio is 80%, Speed command is 75%, and “Clutch” command is 100%, the set point for desired HST motor speed will be 1200 rpm.
In one embodiment, microcontroller 102 may process the inputs schematically represented in
In one embodiment, the PID result may be converted to a current output. The current may be provided to proportional valve(s), which may provide control pressure to a servo piston to control a hydrostatic motor swash plate angle. For a given control pressure, the hydrostatic motor speed may depend on the load or pressure of the system.
In one embodiment, the tractor also may have several range gears 113 between the hydrostatic motor and the drive wheels. With three range gears, and four hydrostatic transmission step input speeds, twelve distinct speeds may be employed. Additionally, a range gear position/final drive speed sensor 114 may be provided.
Having described the preferred embodiment, it will become apparent that various modifications can be made without departing from the scope of the invention as defined in the accompanying claims.
