Circuit pressure control system for hydrostatic power transmission

A circuit pressure control system for a hydrostatic power transmission having a variable-displacement hydraulic pump driven by a prime mover, a hydraulic actuator for actuating a load and a displacement adjusting mechanism for the hydraulic pump. The hydraulic pump and actuator are connected together in closed or semi-closed circuit, and the displacement adjusting mechanism is actuated by a signal indicative of the operating lever manipulated variable and a signal indicative of the actual displacement of the hydraulic pump to control the speed of the hydraulic actuator. The circuit pressure control system is provided with a sensor for sensing the circuit pressure of the hydrostatic power transmission and generating a signal indicative of the sensed circuit pressure, a device for calculating based on the manipulated variable and circuit pressure signal a hydraulic pump displacement command that causes the displacement of the hydraulic pump to be changed at a maximum rate when the circuit pressure is below a predetermined value and causes the changing rate of the displacement to be reduced when the predetermined value is exceeded thereby, and a device for comparing the displacement command with the actual displacement of the hydraulic pump and producing a signal for decreasing the difference between them and supplying such signal to the displacement adjusting mechanism.

BACKGROUND OF THE INVENTION 
This invention relates to circuit pressure control systems for hydrostatic 
power transmissions, and more particularly, it is concerned with a circuit 
pressure control system for a hydrostatic power transmission of a 
hydraulically operated machine, such as a bulldozer, hydraulic shovel, 
hydraulic crane, etc. 
In one type of control system known in the art for a hydrostatic power 
transmission of a hydraulically operated machine, such as a bulldozer, 
hydraulic shovel, hydraulic crane, etc., a variable-displacement hydraulic 
pump driven by a prime mover is connected to a hydraulic actuator for 
actuating a load in closed or semi-closed circuit and the speed of the 
hydraulic actuator is controlled by varying the displacement of the 
hydraulic pump. A swash-plate pump of the reversible tilt type is used, 
for example, as a variable-displacement hydraulic pump. A displacement 
adjusting mechanism connected to a hydraulic pressure source via a servo 
valve is used as means for varying the hydraulic pump displacement. When 
the servo valve is supplied with an operating current commensurate in 
value with the deviation of a hydraulic pump swash-plate tilt (hydraulic 
pump displacement) signal Y from an operating lever manipulated variable 
signal X.sub.L, it operates to bring the displacement adjusting mechanism 
into communication with the hydraulic pressure source to thereby control 
the hydraulic pump swash-plate tilt to render the swash-plate tilt Y equal 
to the operating lever manipulated variable signal X.sub.L. 
In a closed circuit hydrostatic power transmission, a hydraulic motor is 
usually employed as a hydraulic actuator, and an auxiliary pump for merely 
supplying the hydraulic fluid to compensate for leaks from the main 
circuit is provided. 
In a semi-closed circuit hydrostatic power transmission, a hydraulic 
cylinder is usually employed as a hydraulic actuator, and when the 
hydraulic cylinder is actuated the difference between the supply and 
discharge of the working fluid due to the difference in volume between the 
supply side and the discharge side of the cylinder is released through a 
flushing valve from the main circuit. 
In such hydrostatic power transmission, abrupt actuation of the operating 
lever would cause a sudden increase in the delivery of the hydraulic pump, 
and the circuit pressure would become inordinately high due to the inertia 
of the load driven by the hydraulic actuator. This tendency would be 
marked when the inertia of the load is high. To avoid this phenomenon, 
conduits of the main circuit have mounted thereacross a crossover relief 
valve for releasing the difference between the delivery by the hydraulic 
pump and the suction by the hydraulic actuator. The working fluid thus 
released represents a loss of energy. 
In order to avoid the loss of energy referred to hereinabove, proposals 
have been made to use circuit pressure control means. One of such 
proposals involves a circuit pressure control system described in "MACHINE 
DESIGN", pages 114-116, issued on Oct. 7, 1976. This system includes a 
three-way change-over valve mounted between the hydraulic fluid inlet of 
the servo valve connected to the hydraulic pump displacement adjusting 
mechanism and the hydraulic pressure source. The servo valve has a spring 
mounted in one pilot section thereof, and the circuit pressure of the 
hydrostatic power transmission is caused to act on the other pilot section 
thereof, so that when the hydraulic actuator is accelerated the three-way 
change-over valve is actuated to decrease the volume of hydraulic fluid 
supplied through the servo valve to the displacement adjusting mechanism 
as the circuit pressure rises above the value set by the spring, to 
thereby decrease the rate of increase of the delivery by the hydraulic 
pump and avoid the circuit pressure rising to an inordinately higher value 
than the value set by the spring. Thus it is possible to avoid a loss of 
energy occurring when the excess fluid in the main circuit is released 
through the crossover relief valve. 
As described hereinabove, the aforesaid type of circuit pressure control 
system is capable of performing the desired pressure control function to 
avoid an inordinate rise in circuit pressure, when the hydraulic actuator 
is accelerated. However, a rise in circuit pressure occurs not only when 
the hydraulic actuator is accelerated but also in other operating 
conditions in which the hydraulic actuator functions as a hydraulic pump. 
In such operating conditions, it is desired that the energy produced by 
the operation of the hydraulic actuator as a hydraulic pump be recovered 
by the prime mover through the hydraulic pump. The circuit pressure 
control system of the type described hereinabove has been unable to effect 
control as desired in such operating conditions, with a result that the 
recovery of the energy has not been effected as desired. 
More specifically, when the hydraulic actuator or motor is accelerated in 
the positive direction, for example, a circuit pressure on the discharge 
side of the hydraulic pump would rise. If, thereafter, the operating lever 
is restored to obtain deceleration, a circuit pressure on the suction side 
of the hydraulic pump would rise since the hydraulic actuator functions as 
a pump when the hydraulic actuator is decelerated. It is generally desired 
that the kinetic energy of the hydraulic motor and the load be recovered 
by the prime mover in the form of power recovery through the hydraulic 
pump. To realize the power recovery, the rate of a reduction in the pump 
displacement or swash-plate tilt is required to be restricted to avoid a 
sudden reduction in the delivery by the hydraulic pump. In the aforesaid 
circuit pressure control system, however, the three-way changeover valve 
is actuated with a rise in circuit pressure, and the pressure applied to 
the hydraulic fluid supply port of the servo valve communicating with the 
hydraulic pressure source through the three-way change-over valve is 
decreased. Thus no hydraulic fluid is supplied to the displacement 
adjusting mechanism and the pump swash-plate is moved toward a neutral 
position by the action of swash-plate neutral restoration springs of the 
displacement adjusting mechanism. As a result, it is impossible to 
effectively control the circuit pressure and to achieve power recovery. 
Not only when the hydraulic actuator is actuated but also in case an 
external force is exerted on the output shaft of the hydraulic actuator to 
forcedly actuate same when the hydraulic actuator is operated at a 
constant speed or when it is accelerated, the hydraulic actuator would 
function as a pump and the circuit pressure would show an inordinate rise 
in the event that the external force is excessively high in magnitude. In 
such a case, since the aforesaid circuit pressure control system has no 
function of increasing the hydraulic pump swash-plate tilt to cope with a 
rise in its circuit pressure, it is impossible for the system to increase 
the suction by the hydraulic pump, thereby making it impossible to achieve 
effective power recovery. 
SUMMARY OF THE INVENTION 
This invention has been developed for the purpose of obviating the 
aforesaid disadvantages of the prior art. Accordingly the invention has as 
its object the provision of a novel circuit pressure control system for a 
hydrostatic power transmission capable of effectively controlling the 
circuit pressure and achieving power recovery by the prime mover even in 
any operating condition in which the hydraulic motor performs a pumping 
action. 
According to the invention, there is provided a circuit pressure control 
system for a hydrostatic power transmission including a 
variable-displacement hydraulic pump driven by a prime mover, a hydraulic 
actuator for actuating a load and a displacement adjusting mechanism for 
the hydraulic pump, the hydraulic pump and actuator being connected 
together in closed or semi-closed circuit, and the displacement adjusting 
mechanism being actuated by a signal indicative of the operating lever 
manipulated variable and a signal indicative of the actual displacement of 
the hydraulic pump to control the speed of the hydraulic actuator, such 
circuit pressure control system comprising means for sensing a circuit 
pressure of the hydrostatic power transmission and generating a signal 
indicative of the sensed circuit pressures, means for calculating based on 
the manipulated variable and circuit pressure signals a hydraulic pump 
displacement command which is determined to cause the displacement of the 
hydraulic pump to be changed at a maximum rate when the circuit pressure 
is below a predetermined value and cause the rate of change of the 
displacement to be reduced when the predetermined value is exceeded 
thereby, and means for comparing the displacement command with the actual 
displacement of the hydraulic pump and producing a signal for decreasing 
the difference between them and supplying such signal to the displacement 
adjusting mechanism. 
In one embodiment of the invention, the means for calculating the hydraulic 
pump displacement command comprises a circuit including a function 
generator for producing as its output, when the circuit pressure is below 
a preset value, a constant maximum value and, when the preset value is 
exceeded by the circuit pressure, a value which decreases in proportion to 
the amount by which the preset value is exceeded, an adder for producing 
the difference between the operating lever manipulated variable and the 
hydraulic pump displacement command, a comparator for producing an output 
`1` when the difference is positive and an output `-1` when it is 
negative, a multiplier for producing the product of the output of the 
function generator and the output of the comparator, and an integrator for 
integrating the output of the multiplier. 
In another embodiment, the means for calculating the hydraulic pump 
displacement command comprises a computer in which the operating lever 
manipulated variable and circuit pressure signals are read therein, an 
increment of the hydraulic pump displacement is determined based on the 
circuit pressure signal by a function stored in the memory beforehand in 
such a manner that when the circuit pressure is below a preset value the 
increment is a constant maximum value and when the preset value is 
exceeded thereby the increment is reduced in proportion to the amount by 
which the preset value is exceeded, and the increment determined is added 
to or deducted from the hydraulic pump displacement command produced in 
the preceding cycle, depending on whether the deviation of the operating 
lever manipulated variable from the hydraulic pump displacement command of 
the preceding cycle is positive or negative, so as to produce the 
displacement command for the current cycle. 
Preferably, the means for sensing the circuit pressure comprises a first 
sensing means for sensing a pressure in the discharge side conduit for the 
hydraulic pump and generating a signal indicative of such pressure, and a 
second sensing means for sensing a pressure in the suction side conduit of 
the hydraulic pump and generating a signal indicative of such pressure, 
and the means for calculating the hydraulic pump displacement command 
comprises means for selecting one of the output signals of the two sensing 
means depending on whether the deviation of the operating lever 
manipulated variable from the pump displacement command is positive or 
negative, the calculating means being operative to calculate the pump 
displacement command based on the operating lever manipulated variable and 
the output signal selected by the selecting means. 
Preferably, the means for calculating the pump displacement command 
comprises function means for generating as its output a value `1` when an 
output RPM of the prime mover or an RPM command for the prime mover is 
below a preset value and a value which decreases substantially in inverse 
proportion to the prime mover output RPM or prime mover RPM command when 
the preset value is exceeded thereby, the calculating means being 
operative to calculate the pump displacement command based on the output 
signal of the function means in addition to the operating lever 
manipulated variable and circuit pressure signals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferred embodiments of the invention will now be described by referring 
to the accompanying drawings. FIG. 1 shows a closed circuit hydrostatic 
power transmission incorporating therein one embodiment of the circuit 
pressure control system in conformity with the invention. As shown, a 
prime mover 1 as of an internal combustion engine is connected to a 
reversible tilt type variable-displacement hydraulic pump 2 for driving 
same. A hydraulic motor 3 is connected to a load 4 for driving same, which 
may be an inertia load, such as a swinging member of a hydraulic shovel. 
The hydraulic pump 2 and the hydraulic motor 3 are interconnected at their 
discharge ports and suction ports by hydraulic conduits 5a and 5b 
respectively, to constitute a closed circuit. A charge pump 6 is provided 
for supplying hydraulic fluid to the closed circuit to compensate for 
leaks therefrom. The numeral 7 designates a low pressure relief valve of 
the charge circuit which is connected to the main circuit through a 
conduit mounting check valves 8a and 8b. The main circuit mounts crossover 
relief valves 9a and 9b. 
The displacement of the hydraulic pump 2 is controlled by a displacement 
adjusting mechanism 2a which is generally in the form of a hydraulic 
piston for operating a swash-plate of the hydraulic pump 2. The 
displacement adjusting mechanism 2a is controlled by a servo valve 10 of 
the electrohydraulic type which controls the flow rate and direction of 
the hydraulic fluid supplied from a pilot hydraulic pressure source 11 to 
the displacement adjusting mechanism 2a by means of an operating current 
i. The operating current i is supplied from a control unit 12 to the servo 
valve 10. 
The control unit 12 includes a pressure control circuit 13 and a 
swash-plate tilt control circuit 14. The pressure control circuit 13 
produces a swash-plate tilt command X by calculation from a lever 
manipulated variable signal X.sub.L from a manipulated variable detector 
15a for an operating lever 15 and a circuit pressure control signal P from 
a pressure sensor 17 which senses circuit pressure Pa or Pb through check 
valve 16a and 16b mounted across the main circuit and produces an 
electrical signal indicative of the higher circuit pressure. The command X 
is supplied to the swash-plate tilt control circuit 14, which compares the 
command X with a swash-plate tilt signal Y from a pump swash-plate tilt 
detector 18 mounted in the displacement adjusting mechanism 2a, and 
produces and passes to the servo valve 10 the operating current i 
indicative of the difference between the command X and the swash-plate 
tilt signal Y. 
The detailed construction of the control unit 12 will be described by 
referring to one embodiment thereof shown in FIG. 2, which shows the 
control unit 12 in analog representation in a block diagram. Parts shown 
in FIG. 2 similar to those shown in FIG. 1 are designated by like 
reference characters. The pressure control circuit 13 will first be 
described. The numeral 19 is an adder which produces by calculation the 
difference .epsilon. between the lever manipulated variable signal X.sub.L 
and the swash-plate tilt command signal X. The numeral 20 is a comparator 
which compares the difference .epsilon. with 0 and produces an output S 
which is 1 when .epsilon..gtoreq.0 and an output S which is -1 when 
.epsilon.&lt;0. The numeral 21 is a function generator which produces an 
output V based on the value of the pressure signal P. More specifically, 
the function generator 21 produces an output Vo of a constant value when 
the circuit pressure signal P and the value Po are in the relation P&lt;Po 
and an output of V=Vo-k(P-Po) when P.gtoreq.Po. That is, when the circuit 
pressure signal P exceeds the preset value Po, the output V of the 
function generator 21 has its value reduced in proportion to the amount by 
which the preset value Po is exceeded. The numeral 22 is a multiplier 
which produces the product .DELTA.X of the output V of the function 
generator 21 with the output S of the comparator 20. More specifically, 
when the difference .epsilon. is .epsilon..gtoreq.0, .DELTA.X=V, and when 
.epsilon.&lt;0, the sign is inverted and .DELTA.X=-V. The value of .DELTA.X 
indicates a swash-plate tilting speed. The numeral 23 is an integrator 
which produces the tilt command X by integrating the output .DELTA.X of 
the multiplier 22. The tilt command X is supplied to the swash-plate tilt 
control circuit 14 and at the same time fed back to the adder 19 while 
changing its sign. 
The swash-plate tilt control circuit 14 includes an adder 24 and a servo 
amplifier 25. The adder 24 produces the difference between the output of 
the pressure control circuit 13 or the tilt command X and the swash-plate 
tilt signal Y and supplies its output to the servo amplifier 25 which 
amplifies the output of the adder 24 and supplies same to the servo valve 
10 as the operating current i. 
The control unit 12 of the aforesaid construction operates as follows. 
First, acceleration of the hydraulic motor 3 in the positive direction of 
rotation will be described. When the operating lever 15 is abruptly 
manipulated from neutral in the positive direction, the lever manipulated 
variable signal X.sub.L is X.sub.L &gt;X or .epsilon.&gt;0 and the output S of 
the comparator 20 is 1, because the tilt command X shows no sudden change 
due to influences exerted by the integrator 23. Meanwhile the circuit 
pressure P is low in initial operating condition, so that P&lt;Po and the 
output of the function generator 21 is Vo. Thus the output .DELTA.X of the 
multiplier 22 is .DELTA.X=Vo. That is, the differentiated value (dX/dt) of 
the swash-plate tilt command X becomes Vo. Thus the swash-plate tilt Y of 
the hydraulic pump 2 increases at maximum speed due to the action of the 
swash-plate tilt control circuit 14, servo valve 10 and displacement 
adjusting mechanism 2a. Since the hydraulic pump 2 has a delivery Qp which 
is in proportion to the swash-plate tilt Y, its changing rate dQp/dt is 
also maximized. Consequently the circuit pressure Pa suddenly increases 
and the pressure signal P exceeds the preset value Po in a short period of 
time. When the pressure signal P is P&gt;Po, the output V of the function 
generator 21 is V=Vo-K(P-Po), so that the differentiated value (dX/dt) of 
the tilt command X also follows the change in the value of V. Thus the 
swash-plate tilting speed is reduced with a rise in circuit pressure, so 
that the speed at which the circuit pressure rises is slowed down and the 
hydraulic motor 3 is accelerated while the circuit pressure Pa settles at 
a constant value in the vicinity of the preset value Po. If the value of 
Vo is set at a high value or the load 4 has high inertia, then the preset 
value Po may be far exceeded by the pressure signal P. In this case, the 
output V of the function generator 21 which is the differentiated value 
(dX/dt) of the tilt command X is V&lt;0. Thus the swash-plate of the pump 2 
has a negative tilting speed and supply of power that might be wasted can 
be avoided. 
Deceleration of the hydraulic motor 3 rotating in the positive direction 
until it stops will be described. This case involves a sudden return of 
the operating lever 15 from the positive position to neutral. At this 
time, the difference (X.sub.L -X) between the lever manipulated variable 
signal X.sub.L and the tilt command X is .epsilon.&lt;0, so that the output S 
of the comparator 20 is -1 and the output .DELTA.X of the multiplier 22 
is -V. If the pressure signal P is P&lt;Po when deceleration is initiated, 
then the output .DELTA.X of the multiplier 22 is .DELTA.X=dX/dt=-Vo, so 
that the swash-plate of the hydraulic pump 2 shifts to neutral at a 
maximum negative tilting speed. At this time, the flow rate of the fluid 
drawn by the hydraulic pump 2 decreases suddenly, so that the circuit 
pressure Pb suddenly rises due to the action of the hydraulic motor 3. 
When the pressure signal P exceeds the preset value Po, the output 
.DELTA.X of the multiplier 22 or dX/dt changes into 
dX/dt=-[Vo-K(P-Po)]=-Vo+K(P-Po), because the output V of the function 
generator 21 changes to V=Vo-K(P-Po). Stated differently, an increase in 
circuit pressure causes a reduction in the absolute value of the negative 
swash-plate tilting speed, and the hydraulic motor 3 is decelerated while 
the circuit pressure Pb settles at a constant value in the vicinity of the 
set value Po, as is the case with the operation for acceleration. When the 
preset value Po is far exceeded by the circuit pressure Pb for various 
reasons, the output of the function generator 21 has a negative value and 
dX/dt&gt;0. Thus control is effected to increase the swash-plate tilt while 
the recovered power is increased. 
To accelerate the hydraulic motor 3 rotating in the negative direction, the 
circuit pressure Pb is controlled while avoiding the supply of power that 
might be wasted, in the same manner as the motor 3 rotating in the 
positive direction is decelerated. To decelerate the motor 3 rotating in 
the negative direction, the circuit pressure Pa is controlled while 
effectively recovering power in the same manner as described by referring 
to acceleration of the motor 3 rotating in the positive direction. 
An embodiment of the control unit 12 in the form of a computer, such as a 
microcomputer, will now be described. FIG. 3 is a flow chart showing the 
operation of the control unit 12 shown in FIG. 2 as it is constructed as a 
computer. The operation procedures shown in FIG. 3 are repeatedly followed 
at a rate of once for each cycle time .DELTA.T. 
First of all, the lever manipulated variable signal X.sub.L and the 
pressure signal P are read in. Then, an increment .DELTA.X of the 
swash-plate tilt X corresponding to the pressure signal P which is stored 
in the memory beforehand is determined based on the pressure signal P. The 
relation between the pressure signal P and the increment .DELTA.X of the 
swash-plate tilt X has a characteristic as shown in FIG. 4. That is, when 
the pressure signal P is lower than the preset value Po, the increment 
.DELTA.X is .DELTA.Xo which is constant; when the pressure signal P is 
higher than the set value Po, the increment .DELTA.X has a relation 
.DELTA.X=.DELTA.Xo-K(P-Po). 
Thereafter, the deviation Z of the lever manipulated variable signal 
X.sub.L from the swash-plate tilt command signal X produced as an output 
in the preceding cycle is produced by calculation. When Z.gtoreq.0, the 
command signal produced in the preceding cycle is added with the value of 
.DELTA.X to produce a new tilt command signal X which is supplied to the 
tilt control routine (or the tilt control circuit 14 shown in FIG. 2). 
When Z&lt;0, the command X produced in the preceding cycle is changed into a 
new command X by deducting the increment .DELTA.X therefrom, and the new 
command X is supplied to the tilt control routine. When the value of the 
increment .DELTA.X is negative, the command X decreases even if the 
negative increment is added and increases even if it is deducted. The 
control procedures shown in FIG. 4 are followed once for each .DELTA.T, so 
that the changing rate with time of the swash-plate tilt control 
.DELTA.X/.DELTA.Y is .DELTA.X/.DELTA.Y when Z.gtoreq.0 and 
-.DELTA.X/.DELTA.Y when Z&lt;0. Thus it will be appreciated that the control 
unit 12 in the form of a computer can achieve the same results as the 
control unit 12 shown in FIG. 2. 
From the foregoing description, it will be appreciated that the circuit 
pressure control system according to the invention is capable of 
effectively controlling the circuit pressure of a closed or semi-closed 
circuit hydrostatic power transmission even if it is in operating 
condition in which the hydraulic motor performs a pumping operation, so 
that power recovery by the prime mover can be achieved effectively. 
Another embodiment of the invention will be described by referring to FIG. 
5. Like the first embodiment shown in FIG. 1, this embodiment is applied 
to a closed circuit hydrostatic power transmission, and parts shown in 
FIG. 5 similar to those shown in FIG. 1 are designated by like reference 
characters and their description will be omitted. 
The hydrostatic power transmission shown in FIG. 5 additionally includes a 
flushing valve 30 and a low pressure relief valve 31 constituting a 
flushing circuit connected to the main circuit. 
The numeral 32 designates a control unit which is distinct from the control 
unit 12 shown in FIG. 1 in the construction of a pressure control circuit 
33. More specifically, the pressure control circuit 33 produces a 
swash-plate tilt command X by calculation from a lever manipulated 
variable signal X.sub.L from a manipulated variable detector 15a for the 
operating lever 15 and one of the circuit pressure signals Pa and Pb 
produced as electrical signals by pressure sensors 34a and 34b sensing the 
pressures Pa and Pb in hydraulic fluid conduits 5a and 5b, and supplies 
the command X to the swash-plate tilt control circuit 14. 
One example of the detailed construction of the control unit 32 will be 
described by referring to FIG. 6 which is a block diagram showing the 
control unit 32 in analog representation. Parts shown in FIG. 6 similar to 
those of the first embodiment shown in FIG. 2 are designated by like 
reference characters and their description will be omitted. As shown, the 
pressure control circuit 33 includes an analog switch 35 for selecting one 
of the circuit pressure signals Pa and Pb supplied to the function 
generator 21. The analog switch 35 switches the function generator between 
the circuit pressure signals Pa and Pb by the output S of the comparator 
20. More specifically, when .epsilon..gtoreq.0 and S=1, the circuit 
pressure Pa is selected as a circuit pressure signal, and when 
.epsilon..gtoreq.0 and S=-1, the circuit pressure Pb is selected as a 
circuit pressure signal. 
The control unit 32 of the aforesaid construction operates as follows. When 
the operating lever 15 is suddenly manipulated from neutral in the 
positive direction to accelerate the hydraulic motor 3 in the positive 
direction of rotation, the output S of the comparator 20 is 1 as described 
previously by referring to the embodiment shown in FIG. 1. Thus the analog 
switch 35 selects the circuit pressure Pa as a circuit pressure signal P. 
The procedures followed thereafter are the same as those described by 
referring to the first embodiment. Thus the control unit 32 controls the 
circuit pressure Pa which is the pressure in the higher pressure side 
conduit 5a and essentially functions in the same manner as the first 
embodiment shown in FIG. 2. 
When the operating lever 15 is abruptly returned to neutral from positive 
to decelerate and stop the hydraulic motor 3 rotating in the positive 
direction, the difference .epsilon. between the lever manipulated variable 
signal X.sub.L and the swash-plate tilt command X is .epsilon.&lt;0, so that 
the output S of the comparator 20 is -1. Thus the analog switch 35 selects 
the circuit pressure Pb as a circuit pressure signal. The procedures 
followed thereafter are the same as those described by referring to the 
first embodiment. In this operating condition, the conduit 5b is the 
higher pressure side due to the pumping action of the hydraulic motor 3. 
Thus the control unit 32 controls the circuit pressure Pb in the conduit 
5b of the higher pressure side and essentially functions in the same 
manner as the first embodiment shown in FIG. 2. 
The control unit 32 operates in the same manner as described by referring 
to the first embodiment when the hydraulic motor 3 is accelerated in the 
negative direction of rotation and when it is decelerated during rotation 
in the negative direction. 
The embodiment shown in FIG. 5 has the additional function of avoiding the 
phenomenon that the swash-plate tilt of the hydraulic pump increases due 
to the existence of a positive feedback condition in the pressure control 
loop inspite of the operating lever being suddenly returned to its 
original position, when the operating lever is immediately returned as an 
external force acting in a direction opposite to the direction of 
operation of the hydraulic actuator 3 is exerted thereon. 
To enable the aforesaid function of the control unit 32 to be clearly 
understood, the control unit 32 will be described by referring to FIG. 7 
in which it is assumed that the hydraulic motor 3 is a hydraulic cylinder 
for better understanding. In FIGS. 7, a, b, c and d show the lever 
manipulated variable X.sub.L, the swash-plate tilt Y, the stroke of a 
hydraulic cylinder and the circuit pressure respectively. Assume that the 
operating lever 15 is manipulated from neutral at a time t.sub.o in full 
stroke as shown in FIG. 7a, that the hydraulic cylinder reaches the end of 
its stroke at a time t.sub.1 as shown in FIG. 7c and that the operating 
lever 15 is abruptly returned to neutral at a time t.sub.2. When the speed 
at which the operating lever 15 is returned to neutral is higher than the 
speed at which the swash-plate tilt Y decreases as shown in FIG. 7b, the 
deviation .epsilon.(=X.sub.L -X) shown in FIG. 6 has a negative value and 
the output S of the comparator 20 is -1. At this time, the swash-plate 
tilt Y of the hydraulic pump 2 is not zero yet, so that the circuit 
pressure or the pressure Pa on the discharge side of the hydraulic pump 2 
rises to a relief pressure level. Thus the output V of the function 
generator 21 shown in FIG. 2 has a negative value in the first embodiment. 
This makes the output .DELTA.X of the multiplier 22 .DELTA.X=S.times.V&gt;0, 
so that the swash-plate tilt Y increases after a time t.sub.3 as shown in 
FIG. 7b in spite of the operating lever being returned to its original 
position. In the embodiment shown in FIG. 6, however, the analog switch 35 
selects the lower circuit pressure Pb as a circuit pressure signal P upon 
receipt of the output S of the comparator 20. Thus the function generator 
21 produces an output V which is V&lt;0, so that the output .DELTA.X of the 
multiplier 22 is .DELTA.X=S.times.V&lt;0. Accordingly, further increase in 
the swash-plate tilt Y can be avoided. 
An embodiment of the control unit 32 in the form of a computer, such as a 
microcomputer, will now be described. FIG. 8 is a flow chart showing the 
operation of the control unit 32 shown in FIG. 5 as it is constructed as a 
computer. The operation procedures shown in FIG. 8 are repeatedly followed 
at a rate of once for each cycle time .DELTA.T. 
First of all, the lever manipulated variable signal X.sub.L and the circuit 
pressures Pa and Pb are read in, and the deviation Z of the lever 
manipulated variable signal X.sub.L from the swash-plate tilt command 
signal X produced as an output in the preceding cycle is produced by 
calculation. Then, one of the circuit pressures Pa and Pb is selected as a 
circuit pressure signal P depending on whether the value of the deviation 
Z is positive or negative. That is, when Z.gtoreq.0, the circuit pressure 
Pa is selected as a circuit pressure signal P, and an increment .DELTA.X 
of the swash-plate tilt X corresponding to the pressure signal P which is 
stored in the memory beforehand is determined based on the pressure signal 
P. The relation beteen the pressure signal P and the increment .DELTA.X of 
the swash-plate tilt X has a characteristic as shown in the function 
generator 21 shown in FIG. 6. That is, when the increment .DELTA.X is 
lower than the preset value P.sub.o, the increment .DELTA.X has a constant 
value .DELTA.X.sub.o ; and when the pressure signal P is higher than the 
preset value P.sub.o, the increment .DELTA.X has a relation 
.DELTA.X=.DELTA.X.sub.o -K (P-P.sub.o). 
The value of the increment .DELTA.X is added to the swash-plate tilt 
command signal X produced as an output in the preceding cycle to produce a 
new swash-plate tilt command X signal which is supplied to the tilt 
control routine (or the tilt control circuit 14 shown in FIG. 6) as an 
output. When the deviation Z is Z&lt;0, the circuit pressure Pb is selected 
as a circuit pressure signal P, and an increment .DELTA.X of the 
swash-plate tilt corresponding to the pressure signal P is determined. The 
increment .DELTA.X is deducted from the tilt command X produced as an 
output in the preceding cycle to produce a new tilt command X which is 
supplied as an output to the tilt control routine. If the increment 
.DELTA.X has a negative value, then the tilt command X decreases when the 
increment .DELTA.X is added and the tilt command X increases even when it 
is deducted. The control procedures are followed once for each .DELTA.T, 
the changing rate with time of the swash-plate tilt command 
.DELTA.X/.DELTA.T is .DELTA.X/.DELTA.T when Z.gtoreq.0 and 
-.DELTA.X/.DELTA.T when Z&lt;0. Thus the control unit 32 can achieve the same 
effects as these achieved by the control unit 12 shown in FIG. 2. 
From the foregoing description, it will be appreciated that the second 
embodiment of the circuit pressure control system in conformity with the 
invention is capable of effectively controlling the circuit pressure of a 
closed or semi-closed circuit hydrostatic power transmission even if it is 
in operating condition in which the hydraulic motor performs a pumping 
operation, so that power recovery by the prime mover can be achieved 
effectively. Moreover, the phenomenon of positive feedback can be avoided 
even if the operating lever is suddenly returned to its original position 
when the hydraulic actuator reached the end of its stroke or when the 
parts driven by the hydraulic actuator strike each other. This is 
conducive to increased safety in operation. 
A further embodiment of the circuit pressure control system in conformity 
with the invention will be described by referring to FIG. 9. 
In this embodiment, the invention is incorporated in a closed circuit 
hydrostatic power transmission as is the case with the first embodiment. 
In FIG. 9, parts similar to those shown in FIG. 1 are designated by like 
reference characters, and their description will be omitted. 
This embodiment includes an output RPM sensor 40 of the prime mover 1, an 
accelerator lever 41, an acceleration sensor 41a and an electronic 
governor 42. The electronic governor 42 effects control in such a manner 
that it receives an RPM command signal .omega..sub.r from the acceleration 
sensor 41a and an output RPM .omega. from the output RPM sensor 40 and 
operates a rack, not shown, of a fuel injection pump in a direction in 
which the deviation of the output RPM .omega. from the RPM command signal 
.omega..sub.r can be reduced, so as to let the output RPM .omega. of the 
prime mover 1 follow the RPM command signal .omega..sub.r. 
A control unit 43 is distinct from the control unit 12 of the first 
embodiment in that the pressure control circuit 44 is distinct in 
construction from the pressure control circuit 13. More specifically, the 
pressure control circuit 43 receives an output RPM signal .omega. from the 
output RPM sensor 40, in addition to a lever manipulated variable signal 
X.sub.L from the manipulated variable detector 15a for the operating lever 
15 and a circuit pressure signal P from the pressure sensor 17, and 
produces a swash-plate tilt command X by calculation from these signals, 
to supply the tilt command X to the swash-plate tilt control circuit 14 as 
an output. In place of the output RPM signal .omega., an RPM command 
signal .omega..sub.r may be supplied to the pressure control circuit 44. 
One example of the detailed construction of the control system 43 will be 
described by referring to FIG. 10, which is a block diagram showing the 
control unit in analog representation. In FIG. 10, parts similar to those 
shown in FIG. 2 are designated by like reference characters and their 
description will be omitted. 
As shown, the pressure control circuit 44 further includes a second 
function generator 45 receiving the supply of an output RPM signal .omega. 
and producing an output W indicative of an approximate inverse number of 
the output RPM .omega., and a second multiplier 46 producing by 
calculation the product of the output .DELTA.X.sub.1 of the multiplier 22 
and the output W of the second function generator 45 and generating an 
output .DELTA.X.sub.2. The second function generator 45 generates an 
output W=1 when the output RPM signal .omega. is below .omega..sub.o which 
is the idling RPM of the prime mover 1, and generates, as the output RPM 
signal .omega. increases beyond .omega..sub.o, an output W=.omega..sub.o 
/.omega. which decreases substantially in inverse proportion to the output 
RPM .omega.. That is, as the output RPM signal .omega. increases by 
exceeding .omega..sub.o, the output gradually becomes smaller than unity 
while remaining in the positive range. The output .DELTA.X.sub.2 of the 
second multiplier 46 is supplied to the integrator 23. 
The control unit 43 of the aforesaid construction operates as follows. It 
will be understood that with the output RPM of the prime mover 1 being 
constant, the control unit 43 functions in the same manner as described by 
referring to the first embodiment, when the hydraulic motor 3 is 
accelerated in the positive or negative direction of rotation and when the 
rotating in the positive or negative direction is decelerated. 
Moreover, the control unit 43 has the specific function of controlling the 
circuit pressure with an optimum pressure control characteristic at all 
times without having the characteristic being essentially affected by a 
change in the output RPM of the prime mover 1. 
Generally, the delivery Q by a hydraulic pump is proportional to the 
product of the swash-plate tilt Y of the hydraulic pump and the RPM 
.omega. of the prime mover. Thus, even if the changes .DELTA.Y in the 
swash-plate tilt Y are constant, the change .DELTA.Q in the delivery Q by 
the hydraulic pump becomes larger with an increase in the RPM of the prime 
mover. Thus in the first embodiment shown in FIG. 1, when a change in the 
swash-plate tilt is produced by the control function of the control unit, 
the change .DELTA.Q in the delivery by the hydraulic pump becomes larger 
with an increase in the RPM of the prime mover, even if the change 
.DELTA.Y in the swash-plate tilt remains constant. This causes a reduction 
in stability although the response and control accuracy can be increased, 
with a result that the system may vibrate. Conversely, when the RPM of the 
prime mover is low, response and control accuracy will become worse. 
In the embodiment shown in FIGS. 9 and 10, the control unit 43 receives the 
output RPM signal .omega. from the prime mover and produces the output W 
which is multiplied by the output .DELTA.X.sub.1 of the multiplier. Thus 
with the output RPM being in a range above the idling RPM .omega..sub.o, 
the output .DELTA.X.sub.1 of the multiplier is corrected so that it 
becomes smaller in inverse proportion to an increase in the RPM, to use a 
corrected value .DELTA.X.sub.2 in place of .DELTA.X.sub.1. Consequently, 
even if the RPM of the prime mover shows a change, there is substantially 
no change in the delivery by the hydraulic pump and the control system 
shows a stable control characteristic. 
One example of the detailed construction of the control unit 43 shown in 
FIG. 9 in which an analog circuit is used will be described. For this 
purpose, it is possible to use a computer, such as a microcomputer. The 
operation of the control unit 32 using a computer will be described by 
referring to FIG. 11. 
In effecting control by using a computer, the control procedures are 
repeated at a rate of once for each cycle time .DELTA.T. First of all, the 
manipulated variable signal X.sub.L for the operating lever 15, the 
pressure signal P and the output RPM signal .omega. of the prime mover 1 
are read in. A tentative increment .DELTA.X.sub.1 of the swash-plate tilt 
corresponding to the pressure signal P which is written beforehand in the 
memory (corresponding to the first function generator 24 shown in FIG. 10) 
is determined based on the value of the pressure signal P. The relation 
between the pressure signal P and the tentative increment .DELTA.X.sub.1 
of the swash-plate tilt is as shown in FIG. 12. That is, when the pressure 
signal P is lower than P.sub.o, .DELTA.X.sub.1 has a constant value 
.DELTA.X.sub.o, and when P&gt;P.sub.o, .DELTA.X.sub.1 =.DELTA.X.sub.o 
-K(P-P.sub.o). 
Then, the value W of an approximate inverse number of the output RPM 
.omega. which is written beforehand in the memory (corresponding to the 
second function generator 45 shown in FIG. 10) is determined based on the 
output RPM .omega.. The relation between the output RPM .omega. and the 
approximate inverse number W is as shown in FIG. 5. That is, when the 
output RPM .omega. of the prime mover 1 is below the idling RPM signal 
.omega..sub.o, W=1, and when .omega.&gt;.omega..sub.o, W=.omega..sub.o 
/.omega.. An increment .DELTA.X.sub.2 of the swash-plate tilt is produced 
by calculation from the tentative increment .DELTA.X.sub.1 of the 
swash-plate tilt and the approximate inverse number W of the output RPM 
.omega.. 
Thereafter, a deviation Z (corresponding to the difference .epsilon. shown 
in FIG. 2) of the lever manipulated variable signal X.sub.L from the 
swash-plate tilt command signal X produced in the preceding cycle is 
obtained by calculation. When Z.gtoreq.0, the value of .DELTA.X.sub.2 is 
added to the command signal produced in the preceding cycle to produce a 
new swash-plate tilt command X which is supplied to the swash-plate tilt 
control routine (or the swash-plate tilt control circuit 20 shown in FIG. 
10). When Z&lt;0, the value of .DELTA.X.sub.2 is deducted from the command X 
produced in the preceding cycle to produce a new command X which is 
supplied to the swash-plate tilt control routine. 
The control procedures are followed once for each cycle time .DELTA.T, so 
that the swash-plate tilt command has a changing rate with time of 
.DELTA.X.sub.2 /.DELTA.T or -.DELTA.X.sub.2 /.DELTA.T. Thus the same 
effects as described by referring to the control circuit shown in FIG. 10 
can be achieved. 
From the foregoing description, it will be appeciated that the third 
embodiment of the invention enables, in a closed or semi-closed circuit 
hydrostatic power transmission, the circuit pressure to be effectively 
controlled while letting the power be recovered by the prime mover, even 
in the operating condition in which the hydraulic motor performs a pumping 
action. In addition, the changing rate of the swash-plate tilt with time 
is in inverse proportion to the output RPM of the prime mover, so that a 
change in the output RPM of the prime mover is prevented from influencing 
the pressure control characteristic. Thus the control system according to 
the invention can perform excellent control function at all times. 
In the embodiments of the invention shown and described hereinabove, the 
invention is incorporated in a closed circuit hydrostatic power 
transmission including a hydraulic pump and a hydraulic motor. However, it 
will be understood that the invention can achieve the same results when 
incorporated in a semi-closed circuit hydrostatic power transmission 
wherein a hydraulic cylinder is used in place of the hydraulic motor and a 
flushing valve is provided.