Abstract:
Improved control of a hyrostatic drive machine at very small speeds of rotation. The rotation speed control signal in the creep rotation speed region is modified in dependence upon the actual value of the rotation angle and possibly the efficiency characteristic diagram so as to produce a regulated constant torque.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a circuit arrangement for rotation speed regulation of a hydrostatic machine connected to a conduit with impressed pressure. 
     In the known electrical speed regulation of such a machine (German Patentschrift No. 3441185, Aug. 13, 1987) the rotation speed desired value and the rotation speed actual value generated in a tachogenerator or other pickup are supplied to the rotation speed regulator. Connected to the speed regulator is a pivot angle adjustment means for the machine. The pivot angle of the machine working as motor for driving a load defines the absorption (displacement) volume. This generates a torque for driving the load, a corresponding speed of rotation resulting. 
     With very small adsorption (displacement) volumes of the machine corresponding to relatively small rotation speeds of regulation behavior of the machine is greatly impaired because in the region of very small speeds there is no proportionality between the pivot angle of the machine and the torque generated by the machine. There are several reasons for this: with reciprocating piston machines during a revolution the number of acting operating pistons changes, this also changing the torque generated. Furthermore, the frictional torque arising at the machine and the leakage losses depend on the rotation angle so that this also reduces the uniformity of the torque. 
     The consequence is that at small rotation speeds the drive turns jerkily and irregularly and can even stop altogether under load. Under such operating conditions the control operation carried out by the speed regulator takes place too late and as a result due to the phase displacement between rotation speed deviation and correction no uniform speed can be set. 
     The problem underlying the invention resides in improving the circuit arrangement for the rotation speed regulation of the type outlined at the beginning so that even very small speeds can be reliably controlled. 
     SUMMARY OF THE INVENTION 
     According to the invention the speed control signal present as output of the speed regulator and representing the pivot angle desired value for the absorption setting of the machine is varied in dependence upon the rotation angle actual value of the hydrostatic machine in the range of small speeds in such a manner that the torque delivered by the machine remains constant. Thus, the pivot angle desired value is corrected in dependence upon the rotation angle signal. The circuit arrangement according to the invention offers particular advantages when the machine operates as positioning drive, i.e. for example a tool or the like is to be moved by very low rotation speed into an exact position after disconnection of a fast feed. Usually, after switching off the fast feed the rotation speed of the drive diminishes corresponding to a function, for example a ramp, until the end point is reached. It is particularly advantgeous when the positioning can take place down to very small creep speeds of rotation. 
     A further development of the invention resides in that the rotation speed control signal is corrected in dependence upon the characteristic diagram of the hydraulic-mechanical and volumetric efficiency of the hydrostatic machine in such a manner that the torque generated by the machine remains constant. The efficiency is thus determined in the range of the low rotation speeds and corresponding correction factors generated so that in dependence upon the rotation speed the stored correction factors can be called up and linked to the speed control signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An example of embodiment of the invention will be explained in detail hereinafter with the aid of the drawings in which a circuit arrangement for rotation speed control of a hydrostatic machine is schematically illustrated. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A hydrostatic machine 1 with variable volume and pivotal beyond the zero point into both adjustment ranges for pump and motor operation is connected to a pressure conduit 2 with impressed pressure and coupled to a load 3 and a tachogenerator 4 which supplies an actual value signal of the rotation speed. The pivot angle of the machine 1 is set by the piston 5 of the actuating cylinder 6. For this purpose the piston 5 is subjected via a servovalve 7 to fluid. The servovalve 7 is actuated via a servo amplifier 8 which is connected to a pivot angle regulator 9 to which the difference between the pivot angle actual value α 2i  and the pivot angle desired value α 2k3  is supplied. The difference is formed in a comparison stage 11. The pivot angle actual value is generated in a displacement pickup 10. The value α 2k3  is equal to α 2k2 , when the comparison stage is connected via the switch 28 directly to the correction circuit 18. 
     In comparison stage 14 the speed difference is formed from the speed actual value n 2i  and the speed desired value n 2s  and is supplied to the speed regulator 15. The rotation speed control output signal α 2s  represents the pivot angle desired value for setting the absorption capacity of the machine 1 which at a predetermined load generates a predetermined rotation speed. 
     The rotation speed control signal is corrected in a transfer element 16 in dependence upon the rotation angle actual value ρ 2i . The rotation angle actual value of the machine is generated in a pulse generator 17 which is coupled to the machine 1. 
     In dependence upon the digital rotation angle signal increment generated in the pulse generator 17 the non-linear transfer or transmission element 16 changes the pivot angle desired value in accordance with the following equation: ##EQU1## 
     The rhythm of the rotation angle signal is governed by ##EQU2## wherein n is the (total number of) pistons in the machine. 
     Proceeding from a value zero at the start of the first regulating operation for the rotation angle step width ρ 2R  in accordance with the above equation a counter z is incremented, it being assumed that the machine 1 and the pulse generator 17 are synchronized. To avoid exceeding the counting range z is reset when a predetermined limit is exceeded. 
     For odd z ((Z/2)≠integer (Z/2)) the pivot angle correction which applies is then ##EQU3## 
     For even z ((Z/2)=integer (Z/2)) even the following equation applies ##EQU4## 
     In a further non-linear transmission or transfer element 18 the corrected pivot angle desired value α 2k1  is corrected in dependence upon the efficiency so that at the output the twice-corrected pivot angle desired value α 2k2  is present. This makes it possible to improve the result of the first correction from the transfer element 16. The correction is carried out in accordance with 
     
         α.sub.2k2 =α.sub.2k1 ·η(Δp.sub.HD, n.sub.2, α.sub.2).sup.-1 
    
     In dependence upon the efficiency for the measured characteristics of the high pressure difference ΔP HD  in the conduit 2 with impressed pressure, the speed n 2  and the pivot angle α 2 , a factor is determined which depends on the efficiency and by which the pivot angle desired value α 2k1  is multiplied. 
     The values for the efficiency characteristic diagram with the associated multiplication factors are preferably contained in a memory. 
     Preferably, above a predetermined limit rotation speed the two correction circuits 16 and 18 are disabled. For this purpose for example a switch 19 is provided which opens in this case so that the transfer element 16 is bridged and to pivot angle desired value α 2s  of the rotation speed regulator 15 is applied directly to the transfer element 18 whose multiplication factor in this case is set to one. 
     In the example of embodiment the speed regulator 15 is also preceded by a rotation angle regulator 20 to which the difference Δρ 2  of the rotation angle actual value ρ 2i  and a rotation angle desired value ρ 2s  is supplied and which provides the speed desired value signal n 2s . Via a switch 21 the rotation angle actual value ρ 2i  is supplied only beneath the aforementioned limit rotation speed. The rotation angle regulator 20 is thus effective only at creep speed, in particular to permit a rotational position drive; it may however also remain activated above the creep speeds. The speed desired value signal n 2s  furnished by the rotation angle regulator 20 can have a ramp-like form, in particular decrease in ramp manner from a predetermined high speed to a creep speed and can in particular reach the speed zero after a predetermined number of increment steps of the rotation angle ρ 2  to stop the drive in a predetermined position. 
     Above the limit rotation speed the switch 21 is preferably open so that the speed desired value n 2s  corresponds to the rotation angle desired value ρ 2s . 
     The rotation angle controller or regulator 20, the speed regulator 15 and the pivot angle regulator 9 are provided with limiting circuits which ensure that the respective output signals of the regulators cannot exceed a predetermined amplitude. 
     Furthermore, the pulse generator 17 and the tachogenerator 4 may be combined in a unit. 
     In particular in drives having a low total inertia of hydromotor 1 and load 3 the relatively poor control behaviour of the control circuit (slow response, large rotation speed changes, slow correction) regulating from the low amplification of the rotation speed control circuit can be improved in that a correction value α 2k3  is formed proportional to the quotient of the measured actual value of the torque M i  and the measured actual value of the high-pressure difference Δp HD  : ##EQU5## 
     This advantageously prevents a temporary speed difference due to the action of the load at the moment of its rising by adapting the torque M 2  of the hydro unit 1 to the load 3. 
     Thus, by controlling the rotation speed desired value n 2s  it is ensured that said correction algorithm on a time variation thereof has no effect. 
     It should be ensured with a suitable circuit that this correction algorithm is of no effect stationarily. 
     It is apparent from the drawing that for this high-pressure-dependent load correction the torque M i  of the motor 1 is determined in a torque measuring means 25 and the pressure gradient of the motor from high pressure p HD  to low pressure p ND  determined in each case in a pressure sensor 23, 24. The values Δp HD  and M i  are combined in a transfer element 26 in accordance with the aforementioned equation and a factor K is generated which is supplied to a further correction circuit 27 which when the switch 28 is actuated furnishes a corrected pivot angle α 2K3  dependent on the high pressure and torque. 
     The time behaviour of the total speed regulator circuit corresponds to a delay member of at least the 3rd order. The time constants can be determined from simulation data and results or from an on-line parameter identification. By breaking down into partial fractions the real roots of the three delay members of the 1st order or the mixed real-conjugate complex roots of the delay member of the 2nd order and the real roots of the delay member of the 1st order are determined. 
     By a suitable compensation circuit of two time members in the rotation speed regulator the two dominant time constants of the nominator broken down into partial fractions of the transfer element are approximately eliminated and thus the oscillatable overall system of the 3rd order converted to a non-oscillatable system of approximately the 1st order. With such a pole compensation a time constant compensation for the speed regultor is achieved.