Field of the Invention
This invention relates to a hydrostatic variable displacement pump, in particular an axial piston pump employing a swashplate construction, the displacement volume of which can be varied in opposite displacement directions starting from a neutral position in which the displacement volume is zero. The displacement volume can be varied by an electro-hydraulic actuator device, which is provided with a safety function to ensure that the variable displacement pump is set in the neutral position in the event of a fault scenario.
Description of Related Art
Hydrostatic variable displacement pumps that can be varied in both directions of displacement from a neutral position in which the displacement volume is zero and operated in a closed circuit can be used, for example, as pumps in hydrostatic transmissions to function as traction drives for vehicles, such as mobile machines. The variable displacement pump, which can be continuously varied in both directions of displacement, is connected in a closed circuit with one or more hydrostatic motors, such as one or more constant displacement motors with a constant intake volume or one or more variable displacement motors with a variable intake volume. By varying the displacement volume of the variable displacement pump, and in the case of variable displacement motors by varying the intake volume of the variable displacement motor, the translation ratio of the hydrostatic transmission formed by the closed circuit can be continuously varied. Thus, the speed of travel of the vehicle can be continuously varied. The direction of displacement of the variable displacement pumps in closed circuit hydrostatic transmissions of this type determines the direction of travel of the machine. By varying the displacement volume of the variable displacement pump, the acceleration and deceleration of the vehicle can be controlled both in the forward direction of travel and in the reverse direction of travel.
In these hydrostatic transmissions, the adjustment characteristics (behavior) of the displacement volume of the variable displacement pump characterizes the behavior of the hydrostatic transmission and, in the case of a traction drive, the traction response of the vehicle. Properties such as precision control and dynamic response can be achieved effectively with the variable displacement pump by an electro-hydraulic actuator device to vary and set the displacement volume. The electrical input signal of the electro-hydraulic actuator device is a current generated by an electronic control device. As a result of which, in a hydrostatic traction drive, the traction characteristics of the vehicle can be freely programmed and stored in the software of the electronic control device.
The ability to vary the displacement volume of the variable displacement pump represents an additional safety feature if people are present in the immediate vicinity of the vehicle. To guarantee safe conditions when the hydrostatic transmission is a traction drive, it is necessary to be able to reliably reset the variable displacement pump into the neutral position where the displacement volume is zero to bring the vehicle to a stop and keep it stationary in the event of a fault scenario, e.g., a system error or the failure of an electronic or electrical component.
For this purpose, electro-hydraulic actuator devices of variable displacement pumps are provided with a safety function which, in the event of a fault scenario, ensures that the variable displacement pump is reset into the neutral position.
A hydrostatic traction device with a variable displacement pump and an electro-hydraulic actuator device provided with a safety function of the general type described above is disclosed in DE 43 35 086 C1. To set the displacement of the variable displacement pump in either of two directions, the electro-hydraulic actuator device of DE 43 35 086 C1 has two identically constructed electro-hydraulic setpoint encoders in the form of electrically actuated pressure reducing valves, each of which specifies a setpoint for the displacement volume in the form of a control pressure that is applied to a slave piston for each direction of displacement of the variable displacement pump. The slave cylinder has two control pressure chambers that act in opposition to each other. A particular control pressure chamber of the slave piston, and thus a direction of displacement of the variable displacement pump, is associated with each pressure reducing valve. Each pressure reducing valve is actuated by a magnet to generate a control pressure that is applied to the associated control pressure chamber of the slave piston. The safety function is provided by a depressurization valve that is electrically actuated by an additional magnet, which actuates spring-loaded safety valves on the pressure reducing valves to deactivate them.
In the event of a fault scenario, the current feed to the magnet of the depressurization valve is terminated so that the spring-loaded safety valves displace the pressure reducing valves toward a reduction of the control pressure in order to deactivate the pressure reducing valves. The control pressure chambers of the slave piston are depressurized to a tank in the event of the fault scenario. As a result of which, the spring-centered slave piston is actuated by the springs into the center position and, thus, the variable displacement pump is set in the neutral position where the displacement volume is zero and the vehicle is brought to a stop.
On variable displacement pumps of this type, the electro-hydraulic actuator device has two identically constructed electro-hydraulic setpoint encoders, each of which is actuated by a magnet, to set the variable displacement pump in either of two directions of displacement, plus a depressurization valve that is electrically actuated by an additional magnet for the safety function to deactivate the two setpoint encoders in the event of a failure scenario. This arrangement entails a great deal of construction effort and expense on account of the large number of valves and magnets required. Moreover, because the two electro-hydraulic setpoint encoders each require a valve axis and the electrically actuated depressurization valve requires an additional third valve axis, an electro-hydraulic actuator device with a safety function of this type requires a great deal of space.