Patent Application
20110030360
The invention is directed to a hydraulic system.
Hydraulic systems of that type include a hydrostatic piston machine, the volumetric flow of which can be steplessly adjusted using a valve control. The hydrostatic piston machine includes a plurality of pistons that can move in a reciprocating manner in cylinders, each piston limiting a working space having a volume that changes with the stroke of a piston and that can be connected via a low-pressure valve to a low-pressure connection, and via a high-pressure valve to a high-pressure connection. The low-pressure valves, at the least, are actuated using an actuator which, in turn, is activated by a control unit. It must be possible to switch the valves highly dynamically so that the working space can be blocked very rapidly or released for through-flow. The functionality, the essential properties such as volumetric flow, pulsation, torque output, and leakage of the piston machine that includes such a valve are highly dependent on the closing points of the valves. The functionality of the unit is influenced considerably by varying operating conditions.
Since the operating conditions of the piston machine are not known, however, the exact switching point at which the valves must be actuated is difficult to find. Faulty states of the piston machine result, which are caused by inept actuation of the valves. For example, the cylinders may not be decompressed if the high-pressure valves are not deactivated at the right time.
By determining the exact switching points, it is possible to avoid supplying current to the valves unnecessarily and for an excessive duration. An additional electrical loss caused by supplying current for an excessive duration can thereby be prevented.
The problem addressed by the invention is that of further developing a hydraulic system in a manner such that the expected functionality of the hydrostatic piston machine is ensured under different operating conditions.
In a hydraulic system according to the invention, the switching points at which the valves are actuated are dependent on operating conditions. Switching the valves in a manner that is controllable depending on the operating conditions ensures that the hydrostatic piston machine will function correctly. The valves achieve specific states at certain working points of the piston. Adapting the switching of the valves on the basis of the particular operating condition prevents faulty states of the piston machine from occurring and fundamentally improves efficiency.
The switching points are preferably dependent on the rotational speed of the drive shaft. The sensing of the rotational speed can be realized particularly easily using a velocity sensor. The exact dependencies of the switching points of the valves on the rotational speed of the drive shaft are determined on the basis of physical models of the system.
According to a particularly preferred embodiment of the present invention, the switching points for actuating the valves are dependent on temperature. The working conditions of a hydrostatic piston machine change with the temperature. At higher temperatures, the viscosity of the hydraulic fluid decreases, thereby making it more fluid and thinner. As a result, the valves can switch more rapidly, thereby enabling the switching points for actuating the valves to occur slightly later. Analogously, when viscosity is high, the switching point can occur slightly earlier. The temperature of the hydraulic fluid can be sensed in a simple, cost-effective manner using a measuring device that outputs signals to the control unit. The exact dependencies of the switching points of the valves on the temperature are determined on the basis of physical models of the system.
It can be advantageous to make the switching points dependent on the pressure in the high-pressure line or the low-pressure line. Pressure can then be measured using a pressure-measuring device. The pressure-measuring device can be realized easily and cost-effectively, and outputs signals to the control unit which, in turn, activate the valves as a function of pressure. The dependence of the switching time of the valves on pressure is determined on the basis of physical models of the system.
The switching points are preferably dependent on error functions. This means that targeted error detection and counteractive control can be realized by adaptively adjusting the switching points.
According to a particularly preferred embodiment of the present invention, the switching points for actuating the valves are determined using a calculation using physical models of the system. In so doing, the entire system, including the relevant properties, are simulated in a simulation model under varying operating conditions. The various operating-condition scenarios are investigated in the simulation to determine the optimal switching points for actuating the valves (offline calculation). The models can also be programmed in the control unit, and the switching points for the actual operating point can be calculated directly by the control unit (online calculation).
It can be advantageous to use an observer that is disposed in the control unit. It is then possible to determine the system state via observation on the basis of a few system variables detected using measurement technology in order to form a model. The model is used as the basis for calculating the exact switching times for actuating the valves. By comparing actual states with the expected states calculated by the observer, more exact statements regarding the switching point can be attained.
The state observer includes a model of the system from which the expected switching points are obtained, and a regulator that changes the switching points depending on the measurable variables.
An embodiment of a hydraulic system according to the present invention is shown in the drawings. The invention will now be explained in greater detail with reference to these drawings.
In the following, the operating method of a valve-controlled piston machine 1 having a digitally adjustable pump capacity/intake volume will be explained with reference to
According to the schematic depiction in
Moreover, each working space 8 is connected via a drain valve 22 to a drain line 24, which is likewise common to all working spaces 8, and which leads into a tank 26. In the embodiment shown, drain valves 22 and inlet valves 16 are designed as electrically releasable and blockable non-return valves. Inlet valve 16 is preloaded in its home position shown into a closed position via a not-shown spring, and can be moved into an opened position by applying current to a solenoid actuator 28, thereby allowing the pressure medium to flow out of inlet line 18 into particular working space 8. In its home position shown, drain valve 22 is preloaded into the opened position using a spring. By supplying current to solenoid actuator 30, drain valve 22 is moved into a blocking position in which the pressure medium cannot flow out of working space 8. Solenoid actuator 28, 30 is activated by a control unit which is used to set the different modes (full mode, partial mode, idle mode), and so the intake volume of hydraulic motor 1 is steplessly adjustable, wherein the pulsation can also be reduced to a minimum by activating valves 16, 22 in a suitable manner. In the embodiment shown, valves 16, 22 are activated depending on the rotational speed of output shaft 12, the rotational speed being detected using a rotational speed sensor 36 and reported via a signal line to control unit 34. In principle, other characteristic data such as the torque acting on output shaft 12, the intake volume of hydraulic motor 1, or the angle of rotation of the swash plate can be taken into account, of course, in the activation of valves 16, 22.
In piston stroke graph A, given operating conditions BA, switching point SPA results from the setpoint assignment by the control unit in that activation of current supply IA to valve A at time t, which corresponds to the time for magnetization plus the travel time of the valve, occurs earlier. Switching point SPA for actuating valve A takes place at the desired instant.
If operating conditions BB occur, however, time t increases by Δt due to the changed operating conditions BB, and switching point SPB FOR actuating valve B is delayed by Δt since current is supplied to valve IB at the same time Analogously, current is supplied by Δt too soon if time t shortens.
In the hydraulic system according to the invention, switching point SPE, at which valve B is actuated is dependent on operating condition BB, and therefore current supply IB′ to valve B starts at a point in time which results from a physical model having operating condition BB, and switching point SPB starts at the desired point in time.
It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of constructions differing from the types described above.
While the invention has been illustrated and described as embodied in a device having a torque-limiting unit, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.
Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention.
What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2009 036 346.7 | Aug 2009 | DE | national |
