Abstract:
An automatic control system for a hydrostatic transmission having a control circuit receiving a control flow from a control pump which represents a speed of a prime mover and valve components including a single adjustable valve which creates a delta pressure signal to control the transmission ratio which controls the speed of the prime mover.

Description:
BACKGROUND OF THE INVENTION 
     The invention describes an automatic control system for a hydrostatic transmission, especially as a propulsion system for vehicles, in which the control of the transmission ratio relates to the speed of the prime mover and the sum of loads on the prime mover is automatically matched to the power limit of the prime mover. 
     Hydrostatic transmissions for vehicles with a control system related to the speed of the prime mover (normally combustion engine) exist in various types. All existing systems are of similar principle, which is described as follows: 
     Displacement of the pump of the hydrostatic transmission (closed circuit type) is controlled by a hydrostatic cylinder which is actuated by a delta pressure signal. The delta pressure signal is generated using a flow across a fixed throttle. The flow itself is generated by a measuring pump which is running at the speed of the prime mover, or a certain proportion of that speed. The generated delta pressure signal to displace the pump is approximately proportional to the speed squared. The described system gives a certain relation between the speed of the prime mover and the ratio of the hydrostatic transmission. 
     The operator of the vehicle controls the speed of the prime mover with a pedal. 
     The described system doesn&#39;t provide a control of transmission ratio to cause the combustion engine to operate always in optimum condition, especially an optimum control is impossible, if there are auxiliary systems driven by the same prime mover as the propulsion system. Due to the insufficient control system, the prime mover shows severe variations in speed depending on load; in maximum speed drops to 50% of the rated speed and in extreme situations the engine stalls. 
     To get a better performance, systems are known, where the orifice of the speed measuring throttle is changed with the position of the prime mover control pedal. This feature changes the proportion between speed of the prime mover and transmission ratio, depending on the position of the control pedal for the prime mover. Other systems use, for instance, the position of an injection pump, in case of a Diesel engine, to influence hydrostatic transmission ratio. Other systems also use a centrifugal force governor to generate a control signal which influences the transmission ratio to reduce engine stall. 
     The described control systems are insufficient due to the requirements of optimum power control, optimum acceleration control (usage of full prime mover power), and optimum braking performance (defined deceleration). 
     The described controls become more complicated as they more nearly fulfill the requirements. For advantageous performance in the braking condition of a vehicle, well known systems use a so-called inch-valve (which is used for other purposes, too). Actuation of the inch-pedal reduces the delta pressure signal which controls pump displacement. 
     SUMMARY OF THE INVENTION 
     A primary object of the invention is the design of a simple control system which incorporates the advantages of known control systems (combination of prime mover speed and transmission ratio) and avoids the described disadvantages of those systems (problem of power control, of optimum braking, and of complication of a system). 
     The invention describes purely hydraulic elements which change the basic principle of the control from the previously known operation sequence in which the driver control pedal controls the speed of the prime mover and the speed of the prime mover controls the transmission ratio, to the new sequence wherein the driver control pedal controls the transmission ratio and transmission ratio controls the speed of prime mover. 
     An important feature of the new system is the fact that the driver&#39;s control pedal influences one single control element (control console), which generates the signals to operate all necessary actuators to control the prime mover, hydrostatic pump, optionally, the hydrostatic motor, and the power in the transmission system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic circuit showing the basic principle of an automatic control system for propulsion transmissions, which are known today; 
     FIG. 2 is a circuit diagram, which shows the basic principle of the invention; 
     FIG. 3 is a circuit diagram of a possible variation of FIG. 2; 
     FIG. 4 is a circuit diagram of another variation according to FIG. 2; 
     FIG. 5 is a circuit diagram of a variation of the control according to FIG. 2; 
     FIG. 6 is an enlarged circuit diagram of the basic principle, which shows in addition the control of the hydrostatic motor; 
     FIG. 7 is a circuit diagram of the integration of an optional motor control into the principle of the invention; 
     FIG. 8 is a circuit diagram of an alternative solution to the circuit of FIG. 7; and 
     FIG. 9 is a circuit diagram of an alternative solution to the circuit of FIG. 8. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Control systems according to the circuit diagram in FIG. 1 are produced today in at least five different designs. 
     A variable displacement over centre pump 4 acts with a hydromotor 16, normally in a closed circuit to form a hydrostatic transmission, with an option of the motor being of a variable displacement. The displacement cylinder 5 for the pump is actuated by a delta pressure signal. A directional control valve 6 is used to control the direction of motion of the vehicle. A metering pump 7, driven by the prime mover 1, delivers a flow proportional to the speed of the prime mover. This flow generates, passing the throttle or flow restricting orifice 8, a delta pressure signal, which is used to actuate the cylinder 5. There is approximately a quadratic relation between speed and pump displacement. 
     Well-known are designs, where the delta pressure signal at throttle 8 is amplified by hydraulic means to get a high level signal actuating displacing cylinder 5. By that means, the pressure signal at throttle 8 can be kept small. 
     A driver&#39;s pedal 11 normally controls the pretension of a spring (=required r p m) at a centrifugal force governor 3 of the combustion engine. The injection pump 2 provides maximum fuel feed for the Diesel engine 1, until the force of the spring of the governor is compensated by the centrifugal forces acting on the governor 3. This control by means of the control pedal of a driver keeps the speed of the Diesel engine at a certain value as long as the motor is not overloaded. Depending on the position of the pedal 11 which sets the required speed of the combustion engine, the flow of pump 7 generates a delta pressure signal across throttle 8, which generates a certain displacement of the pump 4 via cylinder 5. The mechanically or hydraulically operated inch-valve 9 can reduce the delta pressure at the resistance 8 independently of the delivery of the pump 7. As a result, as the main pump reduces displacement, the speed of the motor 16 decreases. The power limiting feature can be improved by adding the actuators 10 and 11, but it is impossible to come to an optimum power control using these types of systems. 
     The orifice 8 connects to line sections 12 and 13 leading to drain and the inch-valve connects to line sections 14 and 15 leading to drain. 
     FIG. 2 shows the basic form of the invention with the elements which are the same as those shown in the prior art system of FIG. 1 being given the same reference numeral. 
     The hydrostatic transmission has the variable displacement unit 4, such as an axial piston pump, connected in closed circuit by lines 4a and 4b, with a displacement unit, such as an axial piston motor 16. The control pump 7 is shown as rotatable with the pump 4 and prime mover 1 by shaft members 7a and delivers a control flow through a line 7b to a three-way control valve 3a. A new driver control pedal 11a is connected to the flow control valve for connecting the rate of flow therethrough to a line 12 having the throttle 8 and which by line section 13 connects to drain. The engine does not have the driver-operated pedal 11, as shown in FIG. 1. 
     An outlet line 17a from the flow control valve has branch lines 17b and 17c and a drain line with a throttle 17. The branch line 17b leads to a control cylinder 20 having a piston spring-urged in a direction to act against the pressure in branch line 17b. The branch line 17c extends to a cylinder 19 having a piston spring-urged to act against the pressure in the branch line. The cylinder 19 controls an adjustable throttle 18 connected into a line 18a which connects flow control valve 3a and throttle 8. The throttle 18 connects to drain by line sections 24 and 25. 
     The driver control pedal 11a sets the adjustment of the three-way flow control valve 3a, which in this control system acts as a hydraulic governor (same function as a mechanical centrifugal force governor). The three-way flow control valve 3a can be placed between pump 7 and throttle 8 (as shown in FIG. 2), or between line sections 12 and 13 downstream of throttle 8. 
     If the control flow of pump 7 (which is proportional to the speed of the Diesel engine 1) exceeds the set flow value, which is set by the position of the pedal 11a, valve 3a bypasses the overshooting flow Δ q of pump 7 to throttle 17, which flow generates a delta pressure signal (proportional to Δ q 2 ). This delta pressure (pressure difference) signal actuates the actuating piston of the cylinder 20 which displaces the injection pump from maximum fuel feed (where it stands before the actuator starts to work by a pretension of the spring) to a smaller fuel feed, and in that way reduces the rpm of the Diesel engine 1 to the speed which is required by the set of the valve 3a, and the pedal 11a. The delta pressure signal at throttle 17 acts in addition on the cylinder 19, which with increasing delta pressure signal closes the orifice of throttle 18 of a power limiting valve. If the Diesel engine is going to be overloaded, this can be a load caused by the propulsion system, or a load caused by any other system which is driven in addition by the Diesel engine, then the Diesel engine speed becomes lower than the speed which is required by the setting of pedal 11a. As a result of the difference between set speed and actual speed, the bypass flow across throttle 17 decreases, and, according to this, the delta pressure signal at throttle 17 decreases. First reaction of the control system is to increase fuel feed to the Diesel engine by action of cylinder 20. When the maximum fuel feed is reached, then cylinder 19 starts to open the orifice of valve 18; this lowers the delta pressure signal at the throttle 8 (this is an automatic inch function). By this means, the displacement of pump 4 is reduced and the power of the hydro-motor 16 is matched to the available power of Diesel engine 1. This provides a power limiting control system. 
     The function of throttle 8 in FIG. 2 is the same in the system, as that shown in FIG. 1. In both systems, the throttle 8 controls the relation between prime mover speed and displacement of the pump. The new system, shown in FIG. 2, normally would not need an inch throttle 9, 10 for braking, but in many applications this inch valve is incorporated for other purposes and it can be used for braking, too. 
     The elements 3a, 9, 8, 10, 11a, 17, 18, 19, and the element 6 can be combined in one control block by design. The throttles 8, 9, and 18 can be combined in one element, also. 
     By adjustment of throttle orifice of throttle 17, the speed drop of the Diesel engine 1 can be adjusted very accurately. 
     The simultaneous operation of fuel feed for the Diesel engine 1, and of the power limiting valve 18 by the delta pressure signal at throttle 17 gives unlimited freedom for the design of the characteristics of the propulsion system (displacement of pump 4, orifice of throttle 8, displacement characteristic of cylinder 5). 
     FIG. 3 relates to another embodiment of the invention and shows, that the control signal for cylinders 19, and 20 can be generated as well by using an adjustable two-way flow control valve 3b with a pressure relief valve 21 connected into line 17b, together with the throttle 17 downstream thereof, being in parallel with the flow control valve 3b. The control of the propulsion system is done by the pedal 11b acting on the flow control valve 3b. 
     FIG. 4 shows another embodiment of the basic idea of the invention, i.e., to generate a pressure signal to control the combustion engine 1 and displacement of pump 4 by use of a hydraulic flow limiting device. This is accomplished by an adjustable throttle 3c, actuated by a pedal 11c. The cylinders 20 and 19 react on a certain pressure signal, the level of the signal is given by the position of pedal 11c, and delivery of pump 7. 
     FIG. 5 illustrates another embodiment wherein the delta pressure signal at throttle 17 can also be used to actuate the prior art lever 11 of the centrifugal force governor (FIG. 1), using a cylinder responsive to the signal. 
     The circuit diagram in FIG. 6 shows an extension of the basic idea of the invention disclosed in FIG. 2. A displacement control 23 of the variable displacement hydromotor 16 is controlled by the delta pressure signal at throttle 17 (the motor may increase displacement with increasing pressure signal) through a line 23a. This system includes the variable displacement motor 16 in the power limiting regulation system and it fulfills the basic idea that only one control signal influences all actuators to control the position and performance of the different members of the transmission system (prime mover, hydro pump 4, hydro-motor 16). It may be required that the hydro-motor 16 not reduce its displacement from maximum when the Diesel engine 1 is working at low speed (this gives maximum tractive force at low speeds of prime mover). To achieve this performance, a valve 29 interrupts flow in the line 23a to displacement cylinder 23, when the delta pressure signal at throttle 8 is low. A control orifice 38 is positioned in line 23a. 
     FIG. 7 illustrates another embodiment wherein the cylinder 23 can be controlled directly by the delta pressure signal at throttle 8. The performance of a control of this type, which is already known, can be improved by use of an additional control flow pump 22, which is connected to the hydro-motor 16. This pump 22 delivers an additional flow to the flow of pump 7, and both flows pass through the throttle 8. The use of both control flows allows a very accurate adjustment of the control system with control of both pump and motor displacement. 
     FIG. 8 shows an alternate version of FIG. 7 wherein a control motor 22a replaces the control pump 22 and subtracts from the flow of pump 7 prior to its flow passing through the throttle 8. 
     A separate control for pump displacement and motor displacement is shown in FIG. 9. The displacement signal for motor cylinder 23 is generated by the control pump 22 and the delta pressure signal at the throttle 27. A separate adjustable power limiting valve 31 for the hydro-motor 16 is positioned by the delta pressure signal at throttle 17, using a cylinder 30. A junction between the two regulation systems for pump and motor (which normally stabilizes the operation) can be done by the pipes 35, 32, 33, 34, using in addition an orifice 26 which, optionally may be adjustable. If this is done, it may be possible to omit one of the two power limiting valves 18, 19, or 30, 31. 
     Finally, it must be mentioned, that all circuits, which show the basic idea of the invention (FIG. 2 to FIG. 9), can be operated without the throttles 8 and 27, as systems to control a speed, or a power. In that case, the flow control valves 3a, 3b, 3c would be placed in the charge flow or discharge flow line of the closed circuit. In that case, it would be necessary to generate by external means the delta pressure signal, which is necessary to control displacement of the pump 4, which in addition is needed at the power limiting valve 18, 19. 
     Instead of the control throttle 3c, 11c in FIG. 4, it is possible to install between lines 12 and 13 a two-way valve 3b, 11b or a three-way flow control valve 3a, 11a having a throttle 17 in parallel therewith. 
     The use of a hydraulically operated valve 6a to control direction of vehicle motion instead of an electrical operated valve 6 allows use of this valve 6a as a power limiting valve, too, controlled by the delta pressure signal at the throttle 17.