Method for controlling a hydrostatic drive

Abstract

A method is used to control a hydrostatic drive including a drive machine, a hydraulic pump coupled to the drive machine, and a hydraulic motor coupled to the hydraulic pump via a hydraulic working circuit. In the method, at least one of multiple manipulated variables of the hydrostatic drive is ascertained and set, in the course of a precontrol, from a specified setpoint value for at least one of the controlled variables, including a pressure in the hydraulic working circuit, a speed of the hydraulic pump, and an output variable of the hydrostatic drive. In the method, the remaining controlled variables and/or manipulated variables are automatically updated.

Claims

1. A method for controlling a hydrostatic drive, which includes a drive machine operably connected to a hydraulic pump and a hydraulic motor operably connected to the hydraulic pump via a hydraulic working circuit, the method comprising: performing a precontrol operation, which comprises: based on a specified setpoint target value for at least one controlled variable of a plurality of controlled variables, determining trajectories of all remaining controlled variables of the plurality of controlled variables, the plurality of controlled variables including a pressure in the hydraulic working circuit, a speed of the hydraulic pump, and an output variable of the hydrostatic drive; and based on the specified setpoint target value for the at least one controlled variable and the determined trajectories all remaining controlled variables, determining dynamic paths for a plurality of manipulated variables, the plurality of manipulated variables being operating variables that achieve the specified setpoint target value for the at least one controlled variable; and after the precontrol operation, operating the hydrostatic drive based on the determined plurality of manipulated variables.

2. The method according to claim 1, wherein the output variable of the hydrostatic drive is one of a rotational angle of the hydraulic motor, a speed of the hydraulic motor, and a torque of the hydraulic motor.

3. The method according to claim 1, wherein: the hydraulic motor is coupled to a propulsive drive, and the output variable of the hydrostatic drive is one of a driving distance of the propulsive drive, a driving speed of the propulsive drive, and a tractive force of the propulsive drive.

4. The method according to claim 1, wherein the at least one controlled variable is at least one of the output variable of the hydrostatic drive and the speed of the hydraulic pump, the precontrol operation further comprising: receiving the specified setpoint target value from one of a machine driven by the hydrostatic drive and a vehicle driven by the hydrostatic drive.

5. The method according to claim 1, wherein the at least one controlled variable is at least one of the pressure in the hydraulic working circuit and the speed of the hydraulic pump, the precontrol operation further comprising: recalling the specified setpoint target value from data stored in the hydrostatic drive.

6. The method according to claim 5, wherein the data stored in the hydrostatic drive includes at least one of a maximum permissible pressure in the hydrostatic drive, an efficiency of the hydrostatic drive, a time behavior of components of the hydrostatic drive, and protection of components of the hydrostatic drive.

7. The method according to claim 1, wherein the plurality of manipulated variables of the hydrostatic drive includes (i) a torque of the drive machine, (ii) a volumetric displacement of the hydraulic pump or a first control pressure that sets said volumetric displacement, and (iii) a displacement volume of the hydraulic motor or a second control pressure that sets said displacement volume.

8. The method according to claim 7, wherein the determining of the trajectories of all remaining controlled variables includes: determining filtered setpoint values for each of the controlled variables and at least one time derivative of the determined filtered setpoint values based on the specified target setpoint value of the at least one controlled variable.

9. The method according to claim 8, wherein the determining of the dynamic paths for the plurality of manipulated variables includes determining the dynamic paths based on actual values of the plurality of controlled variables and the plurality of manipulated variables, on the determined filtered setpoint values and on the at least one time derivative of the determined filtered setpoint values, and on the specified setpoint target value.

10. The method according to claim 1, wherein the drive machine comprises an internal combustion engine, an electric machine, or a turbomachine.

11. The method of claim 1, wherein the method is carried out by a processing unit.

12. The method of claim 11, wherein the processing unit is triggered to carry out the method by a computer program.

13. The method of claim 12, wherein the computer program is stored on a non-transitory machine-readable storage medium.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematically shows a hydrostatic drive which is suitable for carrying out a method according to the disclosure.

(2) FIG. 2 shows a block diagram of an integration of a method according to the disclosure into a device which is driven by a hydrostatic drive.

(3) FIG. 3 schematically shows a sequence of a preferred embodiment of a method according to the disclosure.

(4) FIGS. 4(a), 4(b), 5(a), 5(b), 6(a), 6(b), 7(a), 7(b), 8(a), and 8(b) show the curves over time of controlled and manipulated variables upon specification of a setpoint value of a controlled variable in each case when a preferred embodiment of a method according to the disclosure is utilized.

DETAILED DESCRIPTION

(5) FIG. 1 schematically shows, by way of example, a hydrostatic drive 100 comprising the components which are relevant for carrying out a method according to the disclosure. For the sake of clarity, the representation of further components is omitted here. A variable hydraulic pump 120 is connected to a variable hydraulic motor 130 by a closed hydraulic working circuit 140.

(6) The hydraulic pump 120 is driven by a drive machine 110 and the hydraulic motor 130, in turn, drives a propulsive drive 150. In the example shown, this can be, for example, a hydrostatic drive for a vehicle. When the hydrostatic drive is used for a stationary machine, the propulsive drive 150 is therefore replaced by another suitable component, for example, a grinding mechanism.

(7) The drive machine 110 will not be specified in greater detail in this example, but possible drive machines are internal combustion engines such as, for example, an internal combustion engine (ICE), i.e., internal combustion engines according to the Diesel cycle, the Otto cycle, or the Seilinger process. Engines having an external heat supply are likewise conceivable. Instead of an internal combustion engine, an electric machine or a turbomachine (turbine) can also be used.

(8) The drive machine 110 can drive the hydraulic pump 120 either directly or by means of a transmission. A method according to the disclosure can be used both with open as well as closed hydraulic circuits, as shown in FIG. 1. It can also be used with fixed displacement motors or shift motors, and not only with variable hydraulic motors, as shown in FIG. 1.

(9) The manipulated variables which are relevant for a method according to the disclosure are the pressure p.sub.H in the hydraulic working circuit, the speed n.sub.P of the hydraulic pump, and an output variable of the hydrostatic drive. This output variable can be either an output variable of the hydraulic motor or also a variable of the propulsive drive, if one is provided.

(10) In the case of direct output variables at the hydraulic motor, the relevant controlled variable is one of the variables rotational angle .sub.M, speed n.sub.M, and torque M.sub.M of the hydraulic motor. In this regard it is noted that only one of these three variables is considered to be the controlled variable in each case; the two others therefore arise on their own.

(11) In the case of derived output variables for a propulsive drive, the relevant controlled variable is one of the variables driving distance s.sub.veh, driving speed v.sub.veh, and tractive force F.sub.Z of the propulsive drive. Only one of these three variables is considered to be the controlled variable in this case as well. In addition, it is noted that these three variables can be converted into the three aforementioned variables, for example, with consideration for the gear ratio and the wheel circumference.

(12) The manipulated variables which are relevant for a method according to the disclosure are a torque M.sub.M,St of the drive machine, a control pressure p.sub.P,St for changing the volumetric displacement (in this case, for example, by outwardly pivoting a swash plate of the hydraulic pump, which therefore corresponds to a volumetric displacement of the hydraulic pump), as well as a displacement volume v.sub.M,St of the hydraulic motor. The specification of the volumetric displacement of the pump or of the displacement volume of the hydraulic motor can take place in different ways in each case (for example, directly by specifying the position or indirectly by specifying the pressure), as mentioned above.

(13) FIG. 2 shows a block diagram of an integration of a method according to the disclosure into a device, for example, a vehicle, which is driven by a hydrostatic drive. In this case, a distinction can be made between two domains, namely vehicle 210 and drive 220.

(14) In the vehicle 210 domain, vehicle functions 211 are set or specified, for example, by an operator or by safety systems. In the drive domain 220, data 221 for the drive train or the hydrostatic drive are set and/or specified. In addition, a precontrol 225 is carried out in the drive domain 220, by means of which manipulated variables can be ascertained from specified controlled variables. Reference is made here to FIG. 3 and the associated description with respect to the mode of operation of the precontrol. The manipulated variables are subsequently transferred by the precontrol 225 to the hydraulic drive 100 or are set there.

(15) The controlled variables which can be specified by the vehicle functions 211 include, in this case, rotational angle .sub.M, speed n.sub.M, and torque M.sub.M (or also, therefore, driving distance s.sub.veh, driving speed v.sub.veh, and tractive force F.sub.Z), as well as speed n.sub.P.

(16) The data 221 for the hydrostatic drive include, in this case, the pressure p.sub.H in the hydraulic working circuit. Specifications, in particular, of the hydrostatic drive are taken into consideration here, such as, for example, protection of the individual components of the drive, a maximum permissible pressure in the working circuit, an efficiency of the hydrostatic drive, or a time behavior of the components. In addition, the speed n.sub.P of the hydraulic pump can also be specified here, provided a request therefor from the vehicle functions is not present. An optimization, for example, with respect to an overall efficiency of the hydrostatic drive, including the working machine, can be carried out in this case.

(17) FIG. 3 schematically shows a sequence of a preferred embodiment of a method according to the disclosure, in particular a precontrol. Initially, filtered setpoint values y and their time derivatives {dot over (y)}, , . . . of the first and higher orders are ascertained from the controlled variables pressure p.sub.H, speed n.sub.P, and one of the variables rotational angle .sub.M, speed n.sub.M, and torque M.sub.M within the scope of trajectory planning 310. The orders to be utilized depend on the selected model of the controlled system. At least one of the controlled variables is specified as a target variable in this case; the others are then automatically adapted or updated accordingly. For the sake of completeness, it is also noted that, instead of the three variables rotational angle .sub.M, speed n.sub.M, and torque M.sub.M, it is also possible to use the three variables driving distance s.sub.veh, driving speed v.sub.veh, and tractive force F.sub.Z, depending on the application.

(18) In general, the target variables from the vehicle functions or the data for the hydrostatic drive can have any type of time behavior, and can also have jumps, in particular. The trajectory planning 310 transforms the specifications into a time behavior which can be implemented by the controlled system. Low-pass filters are preferably utilized for the trajectory planning 310, the order of which is oriented toward the orders of the required time derivatives of the filter variable.

(19) In the further course, the manipulated variables torque M.sub.A,St, control pressure p.sub.P,St, and displacement volume V.sub.St are ascertained from the filtered setpoint values or time derivatives y, {dot over (y)} and within the scope of a dynamic path model 320.

(20) The dynamic path model is derived, for example, from a physically based system of differential equations of the drive train. In this case, a system of differential equations having the following form is set up for the drive train:
{dot over (x)}=f(x,u),
y=g(x,u)
wherein u is the vector of the manipulated variables, x is the vector of the state variables, and y is the vector of the controlled variables.

(21) In this case, the selected system of differential equations must satisfy the system property of so-called flatness. The system of differential equations can then be transformed, for example, into a representation according to the following equation:
u=h(y,{dot over (y)},. . . ).

(22) The vector function h in this case is an implementation of the aforementioned dynamic path model 230. As mentioned above, the orders of the derivative of the filtered setpoint values y depend on the model which is used.

(23) The system of differential equations for the exemplary drive train is as follows:

(24) Hydraulic motor:
{dot over (V)}.sub.M,St=f.sub.1(V.sub.M,V.sub.M,St,p.sub.H)
{dot over (n)}.sub.M=f.sub.2(M.sub.M,M.sub.ZM)
{dot over ()}.sub.M=f.sub.3(n.sub.M).
Hydraulic pump:
{dot over (V)}.sub.P=f.sub.4(V.sub.P,n.sub.P,p.sub.H,p.sub.P,St)
{dot over (n)}.sub.P=f.sub.5(M.sub.P,M.sub.A,M.sub.ZP).
Pressure in the working circuit:
{dot over (p)}.sub.H=f.sub.6,1(p.sub.H,V.sub.P,n.sub.P,V.sub.M,n.sub.M,q.sub.Z)
p.sub.H=f.sub.6,2(V.sub.P,n.sub.P,V.sub.M,n.sub.M,q.sub.Z).
and torque of the drive machine:
{dot over (M)}.sub.A=f.sub.7(M.sub.A,M.sub.A,St).

(25) There are two alternative modelings for the pressure in the working circuit. In the case of short, stiff working lines, the time behavior of the pressure is a great deal faster, for example, than the activating times of the hydraulic components. The pressure in the working circuit can then be described analytically according to f.sub.6,2. If the time behavior of the pressure in the working circuit is in the same order of magnitude or is greater than the time behavior of the other components (for example, by a memory), however, the modeling f.sub.6,1 must be selected.

(26) The precontrol can be improved by accounting for disturbance variables which act on the drive. These options are M.sub.ZP (additional load moment at the hydraulic pump, for example, power take-off shafts), M.sub.ZM (total running resistance), and q.sub.Z (leakage in the working circuit). These and further variables can be ascertained via measurement or calculation and can be incorporated into the calculation of the precontrol values.

(27) In addition to a physically based modeling of the drive train, a data-based modeling is also possible. Examples thereof are transfer functions (polynomials) from a path identification or also models based on neural networks.

(28) FIGS. 4(a), 4(b), 5(a), 5(b), 6(a), 6(b), 7(a), 7(b), 8(a), and 8(b) show examples of courses of controlled variables (in part (a) of each figure) and manipulated variables (in part (b) of each of the figures) over time t upon specification of a setpoint value of a controlled variable in each case when a preferred embodiment of a method according to the disclosure is utilized. In each case, setpoint values and actual values of the controlled variables are shown with indices setpoint and actual, respectively, and are shown for the manipulated variables with indices St and actual, respectively.

(29) The y-axes show the displacement s in m, torque M in Nm, speed n in l/min, pressure p in bar, and displacement volume V in cm.sup.3.

(30) In order to facilitate understanding, the examples show the guidance when a setpoint value of a controlled variable is changed in each case, while each of the two further setpoint values of the controlled variables remains unchanged. It becomes clear in this case that, as a rule, all three manipulated variables must be changed in order to implement these specifications. In addition, it is apparent that the controlled system (drive train) follows the setpoint values quasi ideally (in this context, when the setpoint values are constant, the scale of the y-axis must also be taken into consideration). In practical use, changes in the setpoint values also occur simultaneously, for example, a higher torque at the hydraulic motor and, simultaneously, a higher pressure in the working circuit can be demanded. This can also be implemented using a method according to the disclosure.

(31) FIGS. 4(a) and 4(b) show the guidance upon specification of a driving distance s.sub.veh. Potential applications are, for example, agricultural engineering or material handling. When used for harvesting, the grain tank of combine harvesters is unloaded, during the working travel, into a trailer (towed by a tractor) traveling adjacent thereto. In order to load the trailer preferably evenly, the relative position of the combine harvester and the tractor must be varied. This application can be mapped onto the control of a driving path.

(32) When container ships are unloaded, the container transporters must receive the container at a precisely specified position. A spreader of the crane simultaneously deposits four containers, for example, onto the correspondingly positioned transporters. The approach by the transporters to this position can likewise be implemented in the manner shown.

(33) FIGS. 5(a) and 5(b) show the guidance upon specification of a speed n.sub.M of the hydraulic motor. Potential applications are airport devices, construction machines, or agricultural engineering. Machines of the GSE (ground support equipment) are used on the apron and, in some cases, also within the terminals. A reduced maximum speed must be adhered to in the terminal, for safety reasons. The entry into and exit out of these safety zones can be implemented by means of a method according to the disclosure.

(34) The grinding mechanisms of stone crushers are speed-controlled depending on the material to be crushed, etc. The precontrol can be utilized when the speed specification changes.

(35) Some telescopic handlers are equipped with shift-on-fly transmissions. During the shifting process, the hydraulic motor (primary shaft) is separated from the secondary shaft and is coupled in again with a new transmission ratio. The speeds must be synchronized for this purpose. This can be easily mapped by means of the speed guidance shown.

(36) FIGS. 6(a) and 6(b) show the guidance upon specification of a torque M.sub.M of the hydraulic motor. Potential applications are construction machines or municipal machines and street sweepers. An essential criterion for a wheeled loader is, for example, the controllable implementation of the propulsive force. The driver requires feedback from the machine on the resistances which just occurred. These requests are ideally solved by the interpretation of the gas pedal as a request for tractive force and an implementation of the same by a method according to the disclosure.

(37) Municipal machines and street sweepers are supposed to have a continuously increasing driving speed and therefore require brake control systems. Upon engagement of a brake control system (ESP) for stabilizing the vehicle, the propulsive drive must implement a specified drive torque. This application corresponds to the case which is shown.

(38) FIGS. 7(a) and 7(b) show the guidance upon specification or change of a pressure p.sub.H in the working circuit. This concept is generally usable. The pressure or the high pressure in the working circuit should be guided, for example, in order to implement component protection, set the optimal efficiency of the hydrostatic drive or, in the case of a secondary control, to provide the necessary working pressure.

(39) FIGS. 8(a) and 8(b) show the guidance upon specification or change of a speed n.sub.P of the hydraulic pump (or the working machine coupled thereto). Potential applications in this case are agricultural technology or material handling. Agricultural and municipal machines are often equipped with mechanical power take-off shafts for driving mounted implements or towed working machines. The power take-off shaft has a transmission ratio, which is fixed or variable in a few stages, with respect to the drive machine (usually an internal combustion engine). An exact speed must be set in order to achieve a desired working result. This is easily mapped using the algorithm shown.

(40) Fork lifters are typically equipped with constant-delivery pumps (gear pumps) for the working hydraulics. In order to achieve a rapid working motion, for example, a lifting of the load, the speed of the working machine must be dynamically adapted. The algorithm shown ideally accomplishes this without influencing the propulsive drive.