Control device for a hydrostatic drive for at least two actuators

A control device for a hydrostatic drive for at least two actuators, having a variable displacement pump whose adjusting device is acted on in accordance with the displacement and/or the flow pressure, wherein each actuator has associated with it, in its hydraulic supply line, a control valve and a preceding displacement controller in the form of a pressure compensator, is simplified while ensuring accurate control. This is achieved by associating with the control device an electronic regulating device which, depending on a measuring element that detects the opening position of the pressure compensator, controls, by means of an adjusting member associated with the adjusting device, the displacement setting of the pump, increasing it as the opening of the pressure compensator(s) becomes larger and decreasing as the opening becomes smaller.

TECHNICAL FIELD OF THE INVENTION 
The invention relates to a control device for a hydrostatic drive for at 
least two actuators 
BACKGROUND OF THE INVENTION AND PRIOR ART 
A control device of this kind is a so-called "load sensing" control in 
which the fraction of the delivery flow which reaches each actuator is 
determined by means of a delivery flow controller that is pressurized as a 
function of the opening area of the associated control valve. Thus the 
displacement setting of the pump is set in accordance with the actuator 
loads which are transmitted to the adjusting device of the pump by means 
of load pressure feed back lines. 
The flow controller usually comprises so-called pressure compensators which 
perform the .DELTA.P.sub.2 delivery flow regulation by ensuring a constant 
pressure difference of about 8 to 20 bar. With this "loadsensing" 
principle the displacement adjustment thus occurs with the above-mentioned 
pressure difference from the load pressures, which can result in 
oscillation problems. 
OBJECT OF THE INVENTION 
The object of the invention is to simplify a control device of the kind 
mentioned above and still ensure accurate control. 
BRIEF DESCRIPTION OF THE INVENTION 
This object is achieved by a control device for a hydrostatic drive for at 
least two actuators, comprising a variable displacement pump whose 
adjusting device can be acted on as a function of the displacement and/or 
of the flow pressure, wherein each actuator has associated with it, in its 
hydraulic supply line, a control valve and a preceding displacement 
controller in the form of a pressure compensator, wherein the controller 
is associated with an electronic regulating device which, depending on a 
measuring element that detects the opening position of the pressure 
compensator, controls, by means of an adjusting member associated with the 
adjusting device, the displacement setting of the pump, increasing it as 
the opening of at least one of the pressure compensators becomes larger 
and decreasing it as the opening becomes smaller. 
In the arrangement according to the invention the displacement setting is 
electronically controlled as a function of the automatically regulated 
pressure compensator or compensators. Not only are hydraulic feed-back 
lines made unnecessary, but accurate or stable control, insensitive to 
oscillation, is also achieved, which is particularly suitable for 
applications where oscillations inevitably occur, as is particularly the 
case with vehicles and building machinery such as cranes, wheel loaders, 
bulldozer and excavators. Slight over- or undersupply to the actuator 
during the control process is consciously accepted. 
The invention is based on the fact that the pressure compensator of the 
actuator with the greatest load is in a specific state, namely completely 
open, so long as the pump is producing too little pressure. If the pump 
delivers a higher pressure than is required by the actuator with the 
greatest load, the inlet pressure compensator is released from the fully 
open state and takes up an intermediate position. Under operating 
conditions where the actuator requirements remain the same, with the 
control device according to the invention the pump will always be 
regulated so that the displacement is increased as the opening becomes 
larger and is reduced as it becomes smaller. 
Advantageous results have for example been obtained with the embodiment in 
which the measuring element comprises a limit switch. In this embodiment 
the pump is always controlled in such a way that at the limit stop and 
with contact of the limit switch (pressure compensator completely open) it 
is adjusted to give greater displacement, and in intermediate (partially 
open) positions of the pressure compensator give less displacement. 
When there are several actuators, the pump is swung out or held swung out 
until all the pressure. compensators have changed from their completely 
open end position to an intermediate position. After this the pump is 
swung back or held swung back until one of the pressure compensators 
returns to its completely open end position. This is the sign for the 
control system according to the invention that there is not enough 
pressure and that the pump should be readjusted for more displacement. 
Advantageous results have however also obtained, for example, with the 
embodiment in which the measuring element comprises a position pickup. 
Advantageous results also occur when the set point value for the control 
process corresponds to a specified position of a pressure compensator of 
the actuator with the highest load pressure. 
Substantially improved control response is obtained by replacing the 
two-point control principle with a control principle which continuously 
registers the deviation of the pressure compenstor of the actuator with 
the greatest load pressure by means of a position pickup and supplies it 
to the pump proportionally, integrally and/or differentially via a 
controller. A suitable controller for this is an ordinary PID control. 
In the embodiment wherein the set point value for the control process 
corresponds to a specified position of a pressure compensator of the 
actuator with the highest load pressure, it is also advantageous for the 
specified pressure compensator position to correspond to the widest 
possible opening of the pressure compensator (opened as wide as possible).

DETAILED DESCRIPTION OF THE INVENTION 
Of the hydrostatic drive, generally denoted by 1 in the drawing, only the 
pump 2 and a part of the open circuit, namely the main line 3, are shown. 
For simplicity, the actuators are not shown. Associated with each actuator 
is a main line section 5 in which a control valve 7 in the form of a 
proportional directional control valve is installed. These are each 5/3 
way directional control valves with a middle blocked position and lateral 
alternating positions for reversing the function of the hydraulic motor 
(actuator), should this be necessary. Located in the main line sections 5 
are throttles 10, which are integrated in the control valves 7 in the 
present exemplary embodiment. 
The pump 2 is one with variable displacement which can be set by an 
electro-hydraulic adjusting device 9 which is connected to the main line 3 
of the pump 2 by a hydraulic line 11 and is thus adjustable by the feed 
pressure of the pump 2. The adjusting device 9 comprises a hydraulic 
cylinder 12 whose adjustment piston 13 is hydraulically adjustable against 
a spring 14, and engages the displacement setting member of the pump 2. 
Arranged in each main line section 5 leading to the associated control 
valve 7 is a flow control valve in the form of a pressure compensator 15. 
The pressure compensator 15 is acted on by the feed pressure prevailing in 
the respective main line section 5 and, by reason of a connecting line 18, 
by the load pressure prevailing behind the control valve 7. At the 
pressure compensators, pressure drops .DELTA.P.sub.1 of up to several 
hundred bar can occur, whilst at the measuring throttle 10 a constant 
.DELTA.P.sub.2 of about 8 to 9 bar is maintained, due to the volume flow 
control. There is an exception to this since with an operational 
requirement of zero displacement the pressure compensator will carry out a 
micro-movement around the completely closed position. 
Associated with the adjusting device 9 is a setting member, generally 
denoted by 19, which acts on the amount of the displacement in accordance 
with a pilot flow which is produced from the position of the pressure 
compensator. The setting member 19 comprises an electromagnetic 
proportional valve 21 which is arranged in the line 11 connecting the main 
line 3 to a working chamber 22 of the cylinder 12 and which controls the 
passage to this working chamber 22 or to a return flow 23. In the present 
exemplary embodiment the other working chamber 24 of the cylinder 12 can 
also be acted on by the load pressure via a line section 25. The spring 14 
is arranged in the working chamber 24 which is formed by the piston ring 
chamber of the cylinder 12. 
A control line 26 which transmits an auxiliary pressure can be connected to 
the line 11 and is safeguarded against the main line 3 on both sides of 
its connection point 27 by check valves. 
The control device is associated with an electronic control device 31 which 
is connected via electric lines 32 to 34 to the pressure compensators. The 
electronic control device 31 is also connected to the setting member 19 or 
the proportional valve 21 via an electric line 35. 
Each pressure compensator 15 has associated with it a limit switch 36 which 
is actuated when the pressure compensator 15 is completely open and is 
thus able to signal that the pressure compensator 15 is in the completely 
open position. 
Instead of the limit switch 36 a position pickup in the form of a measuring 
element for recording any desired opening positions as well as 
intermediate positions of the pressure compensator 15 can be provided. 
Numerals 38 and 39 indicate hydraulic or electric lines leading to the 
control valves 7, by means of which the control valves 7 can be adjusted 
or set at will for the purpose of pre-setting a selected volume flow for 
the respective actuator or for the actuators. 
The functioning of the control device will now be described in more detail. 
It is assumed that the pressure compensator 15 of the actuator with the 
greatest load occupies a specific state (completely open) as long as the 
pump 2 is producing too little pressure. If the pump 2 transmits a higher 
pressure than is required by the actuator with the greatest load the 
pressure compensator 15 will be released from its completely open state 
and will take up an intermediate position. These two considerations show 
that there is a connection between the position of the pressure 
compensator 15 of the actuator with the greatest load and the adjusting 
signal required at the pump 2, such that the pump 2 must give more 
pressure and more displacement (more swivel angle) until the pressure 
compensator 15 begins to respond and is released from its completely open 
position. 
As soon as the pressure compensator 15 of the actuator with the greatest 
load is released from the completely open end position or from the limit 
stop, this means that the pressure produced at the pump 2 and the volume 
flow are sufficient for the requirements of the actuator. The pump 2 does 
not need to swing out further, but can be swung back a little until the 
pressure compenstor reaches its open state again. 
In a stationary operational state with a constant actuator requirement the 
pump 2 would thus always be regulated in such a way that when the pressure 
compensator is positioned in its end position, namely at the limit switch 
36 (pressure compensator completely open), it is controlled to give 
greater displacement, and at intermediate positions of the pressure 
compensator 15 it is controlled to give less displacement. 
When there are several actuators the pump 2 would have to be swung out 
until all the pressure compensators have changed from their completely 
open end position to an intermediate position. The pump 2 can then be 
swung back until one of the pressure compensators 15 reaches its 
completely open end position again. This is the signal for the control 
system that there is not enough pressure and that the pump has to be set 
at greater displacement again, etc.. 
The clear connection described between the pump setting required and the 
state of the pressure compensator 15 can very simply be used, with the aid 
of the electronic logic circuit 31 (analog or digital), the limit switches 
36 (preferably non-contact inductive or optical) and the setting member 
19, for load sensing control. 
A possible variation of the control system can take the form that an 
electronic circuit (logic circuit 31) resets the pump via a PID controller 
to the pressure compensator setting of the actuator which at that point in 
time has the greatest pressure load. 
In the stationary state the pump has only to make a small differential 
adjustment movement, whereby the .DELTA.P.sub.1 is essentially by-passed. 
Such an electronic control is very simple to carry out and can readily be 
effected by oscillation technology methods by the simple adjustment of the 
dynamic behavior of the control system that is then possible (electronic 
damping, should it be necessary). The advantages of electronic control are 
numerous: 
Simple arrangement (hydraulic and electrical). 
No LS-feed back (load pressure feed back) necessary (no LS line) as at the 
actuator with the highest volume flow requirement the pressure compensator 
is completely open and this is detected by the associated measuring 
element (limit switch or position pickup). 
Simple functional connection between actual value pickups (measuring 
element) and adjusting device (simple electric proportional adjustment). 
Elimination of the pressure difference .DELTA.P.sub.1 for the actuator with 
the greatest volume flow requirement. 
No problems with oscillations as no direct hydraulic load pressure feed 
back occurs, but is provided by pressure compensator positions detected by 
measuring elements and by electric signal reception and evaluation. 
Electric damping is easily possible (should it be necessary). 
No power loss at the actuator with the highest operating pressure. 
When there are no actuator requirements the standby pressure can be reduced 
to the region of 0 bar. 
Little heating in standby operation. 
Regulated operation of the pump; hence automatic adaptation to changed 
desired value-actual value constellations in the valves and in the pump. 
The actuator with the greatest load pressure is provided with the volume 
flow required. 
Because of simple, economical construction it is also of interest for 
equipment, especially building machinery (excavators, etc.) in the lower 
power groups. 
No special load sensing pumps necessary, only an EP pump 
(electronic-proportional adjustment). 
Uniform control response, since peformance curve adjustments are 
acknowledged and controlled at the valves and at the pumps.