Hydraulic drive system

A hydraulic drive system of the invention comprises a plurality of flow rate sensors (10A, 10B) for detecting respective flow rates supplied to a plurality of hydraulic actuators (3A, 3B), valve controllers (11A, 11B) for respectively controlling a plurality of flow control valves (40A, 40B) such that the flow rates detected by the plurality of flow rate sensors are coincident with the flow rates instructed by a plurality of control levers (5A, 5B), and a pump tilting controller (12; 12A-12F) for controlling a delivery rate of a hydraulic pump (1) such that the delivery rate of the hydraulic pump is smaller by a predetermined flow rate (.DELTA.Q.sub.ref ; X.sub.ref) than the total of the flow rates instructed by the plurality of control levers. The pump tilting controller controls the delivery rate of the hydraulic pump (1) by using flow rate deviations (.DELTA.Q.sub.1, .DELTA.Q.sub.2) resulted from respectively subtracting the flow rates detected by the flow rate sensors (10A, 10B) from the flow rates instructed by the control levers (5A, 5B).

TECHNICAL FIELD 
The present invention relates to a hydraulic drive system for driving a 
plurality of hydraulic actuators by a single variable displacement 
hydraulic pump, and more particularly to a hydraulic drive system for 
driving a plurality of hydraulic actuators while controlling a delivery 
rate of a hydraulic pump depending on a demanded flow rate. 
BACKGROUND ART 
As to a hydraulic drive system for driving a plurality of hydraulic 
actuators by a single variable displacement hydraulic pump, there is known 
a so-called load sensing control system in which a delivery rate of the 
hydraulic pump is controlled in such a manner as to supply only the flow 
rate demanded by the hydraulic actuators. The load sensing control system 
is described in, for example, West German Patent No. 3,321,483, JP, A, 
60-11706 which are equivalent to U.S. Pat. No. 4,617,854) and JP, A, 
2-261902. 
The load sensing control system (hereinafter referred to as an LS control 
system) comprises a variable displacement hydraulic pump, a plurality of 
hydraulic actuators connected to the hydraulic pump in parallel, a 
plurality of flow control valves for respectively driving the plurality of 
hydraulic actuators, a plurality of control levers for instructing 
respective flow rates to the plurality of flow control valves, a circuit 
for detecting a maximum one of the load pressures of the plurality of 
hydraulic actuators, and a pump regulator for controlling a delivery rate 
of the hydraulic pump so that a delivery pressure of the hydraulic pump is 
held higher by a fixed value than the maximum load pressure. 
When any one of the control levers is operated, the associated flow control 
valve is opened with an opening corresponding to an input amount from the 
control lever (i.e., a demanded flow rate), whereby a hydraulic fluid from 
the hydraulic pump is supplied to the associated hydraulic actuator 
through a pressure compensating valve and the flow control valve. 
Simultaneously, a load pressure of that hydraulic actuator is introduced 
as the maximum load pressure to the pump regulator which controls the pump 
delivery rate so that the pump delivery pressure is held higher by a fixed 
value than the maximum load pressure. At this time, when the input amount 
from the control lever (i.e., the demanded flow rate) is small, the 
opening of the flow control valve is also small and so is a flow rate of 
the hydraulic fluid passing through the flow control valve, so that the 
pump delivery pressure is held higher by a fixed value than the maximum 
load pressure at the small pump delivery rate. When the input amount from 
the control lever (i.e., the demanded flow rate) is increased, the opening 
of the flow control valve is also increased and so does the flow rate of 
the hydraulic fluid passing through the flow control valve, whereupon the 
pump delivery rate is increased to keep the pump delivery pressure higher 
a fixed value than the maximum load pressure. 
Meanwhile, in the system making control of the pump delivery rate in that 
way, when plural hydraulic actuators are simultaneously driven by 
operating plural control levers, the flow control valve associated with 
the hydraulic actuator on the lower load side produces a larger 
differential pressure across the same than that on the higher load side, 
and the hydraulic fluid is supplied at a larger flow rate to the hydraulic 
actuator on the lower load side. The combined operation of those plural 
hydraulic actuators can no longer be performed in accordance with an 
opening ratio between the flow control valves (i.e., a demanded flow rate 
ratio). To prevent such a disadvantage, the LS control system includes a 
pressure compensating valve disposed upstream of the flow control valve 
for controlling a differential pressure across the flow control valve. 
When the differential pressure across the flow control valve associated 
with the hydraulic actuator on the lower load side becomes large during 
the combined operation, the upstream pressure compensating valve is 
operated in a valve-closing direction to restrict the flow rate, thereby 
reducing the differential pressure across that flow control valve. As a 
result, the differential pressures across the flow control valves on both 
the higher and lower load sides are maintained at substantially the same 
value, enabling the associated plural actuators to be simultaneously 
driven in accordance with the opening ratio between the flow control 
valves (i.e., the demanded flow rate ratio). 
With the LS control system, as mentioned above, since the delivery rate of 
the hydraulic pump is controlled depending on the demanded flow rate, a 
part of the pump delivery rate which is wastefully consumed can be reduced 
to make economical operation possible. In order to surely perform the 
combined operation, the pressure compensating valve is required for 
controlling the differential pressure across the associated flow control 
valve. 
Relating to the LS control system, particularly, there is also known U.S. 
Pat. No. 4,712,376 which discloses a system wherein the total of input 
amounts from all the control levers (i.e., demanded flow rates) is 
calculated for the purpose of controlling respective openings of the flow 
control valves. This disclosed system is intended to cope with a lack of 
the pump delivery rate during combined operation of driving plural 
actuators, by restricting the respective openings of the flow control 
valves depending on the amount of such a deficiency, so that the combined 
operation is performed in accordance with a demanded flow rate ratio. In 
addition, though not directly related to the LS control, JP, A, 52-76585 
discloses a system in which a flow rate of the hydraulic fluid supplied to 
a hydraulic actuator is detected for controlling an opening of an 
associated flow control valve so that the flow rate is held in match with 
a demanded flow rate. 
DISCLOSURE OF THE INVENTION 
However, the above-mentioned LS control system has had the following 
problems. 
In a hydraulic drive system of the type adopting LS control, as explained 
above, there produces a differential pressure across the flow control 
valve. Given the differential pressure across the flow control valve being 
.DELTA.P.sub.1, the differential pressure .DELTA.P.sub.1 is determined by 
a rated flow rate and size of the flow control valve. If the flow control 
valve used has a large size relative to its rated flow rate, the 
differential pressure .DELTA.P.sub.1 can be set to a small value. On the 
contrary, if the flow control valve used has a small size relative to its 
rated flow rate, the differential pressure .DELTA.P.sub.1 must be set to a 
large value. Also, the differential pressure .DELTA.P.sub.1 must be set to 
a value which is produced when the hydraulic fluid flows at the rated flow 
rate with the input amount from the control lever maximized to make the 
opening of the flow control valve maximum. Therefore, in the case of using 
a flow control valve having a size that is small relative to its rated 
flow rate for reducing the system size, the differential pressure .DELTA. 
P.sub.1 necessarily becomes a large value. 
Additionally, the differential pressure .DELTA.P.sub.1 is not determined by 
the above conditions only. More specifically, the viscosity of the working 
oil (hydraulic fluid) is changed to a large extent depending on 
temperatures and becomes large at a low temperature. To enable the 
hydraulic fluid to flow at a rated flow rate even under a low temperature, 
therefore, it is required that the differential pressure .DELTA.P.sub.1 be 
set to a higher value with a margin. Accordingly, the value of the 
differential pressure .DELTA.P.sub.1 must be larger than the value 
determined by the foregoing conditions. In particular, when the hydraulic 
drive system is used with a hydraulic machine such as a hydraulic 
excavator, there is a substantial possibility that the construction 
machine is used in an outdoor environment at an extremely low temperature, 
which requires the margin to be relatively large and hence renders the 
differential pressure .DELTA.P.sub.1 to be increased. 
Thus, the differential pressure .DELTA.P.sub.1 across the flow control 
valve is usually set to a large value and a pressure loss in the hydraulic 
circuit also becomes large correspondingly. 
Furthermore, the LS control system generally includes the pressure 
compensating valve as mentioned above. The pressure compensating valve 
also produces a pressure loss .DELTA.P.sub.2 besides the differential 
pressure .DELTA.P.sub.1 across the flow control valve. The pressure 
loss.DELTA.P.sub.2 comprises a pressure loss produced by the pressure 
compensating valve itself (i.e., a pressure loss produced when the 
pressure compensating valve is maximally opened), and a pressure loss 
produced due to that the pressure compensating valve associated with the 
actuator on the lower load side is restricted. 
In the LS control system, therefore, the pump delivery rate must be 
controlled in consideration of the differential pressure .DELTA.P.sub.1 
and the pressure loss .DELTA.P.sub.2 so that the pump delivery pressure is 
held higher a fixed value than the maximum load pressure. Stated 
otherwise, assuming that the fixed value in the LS control is a target 
differential pressure .DELTA.P.sub.0, this target differential pressure 
.DELTA.P.sub.0 must be set to a value larger than the sum of the 
differential pressure .DELTA.P.sub.1 and the pressure loss .DELTA.P.sub.2 
and, in practice, it is set to a still larger value in consideration of a 
pressure through lines and so on. The target differential pressure 
.DELTA.P.sub.0 is usually in a range of 15 to 30 bar and this value cannot 
be said to be small relative to a usual rated value of the hydraulic 
circuit in a range of 250 to 350 bar. 
Another problem experienced in the LS control system is as follows. As 
explained above, the flow rate of the hydraulic fluid supplied to the 
hydraulic actuator is adjusted on condition that the differential pressure 
across the flow control valve is held constant by the pressure 
compensating valve. In practice, however, a flow of the hydraulic fluid 
(working oil) passing through the flow control valve is always affected by 
viscosity of the working oil. Particularly, when the working oil has high 
viscosity at a low temperature, the flow rate of the hydraulic fluid 
supplied to the hydraulic actuator becomes smaller than that instructed by 
the input amount from the control lever (i.e., the demanded flow rate). 
An object of the present invention is to provide a hydraulic drive system 
which has a function of controlling a delivery rate of a hydraulic pump in 
accordance with a demanded flow rate, produces a small pressure loss, and 
can perform high-accurate flow control regardless of the temperatures of 
the working oil. 
To achieve the above object, according to the present invention, there is 
provided a hydraulic drive system comprising a variable displacement 
hydraulic pump, a plurality of hydraulic actuators connected to said 
hydraulic pump in parallel, a plurality of flow control valves for 
respectively driving said plurality of hydraulic actuators, and a 
plurality of flow rate instructing means for instructing respective flow 
rates to said plurality of flow control valves, wherein said system 
further comprises a plurality of flow rate sensor means for detecting 
respective flow rates supplied to said plurality of hydraulic actuators, 
first control means for respectively controlling said plurality of flow 
control valves so that the flow rates detected by said plurality of flow 
rate sensor means are coincident with the flow rates instructed by said 
plurality of flow rate instructing means, and second control means for 
controlling a delivery rate of said hydraulic pump such that the delivery 
rate of said hydraulic pump is smaller by a predetermined flow rate than 
the total of the flow rates instructed by said plurality of flow rate 
instructing means. 
In the above hydraulic drive system, preferably, said second control means 
controls a displacement volume of said hydraulic pump such that the total 
of the flow rates detected by said plurality of flow rate sensor means is 
smaller by said predetermined flow rate than the total of the flow rates 
instructed by said plurality of flow rate instructing means. 
Also, in the above hydraulic drive system, preferably, said second control 
means controls the delivery rate of said hydraulic pump by using flow rate 
deviations resulting from respectively subtracting the flow rates detected 
by said plurality of flow rate sensor means from the flow rates instructed 
by said plurality of flow rate instructing means. 
Further, in the above hydraulic drive system, preferably, said second 
control means comprises first calculation means for calculating the total 
of flow rate deviations resulted from respectively subtracting the flow 
rates detected by said plurality of flow rate sensor means from the flow 
rates instructed by said plurality of flow rate instructing means, 
deviation output means for outputting a value corresponding to said 
predetermined flow rate as a reference deviation, second calculation means 
for calculating a difference between the total of the flow rate deviations 
obtained by said first calculation means and the reference deviation 
output from said deviation output means, and third calculation means for 
determining a target displacement volume of said hydraulic pump based on 
the difference obtained by said second calculation means. In this case, 
said first calculation means preferably comprises means for adding said 
flow rate deviations. Said first calculation means may comprise means for 
selecting a maximum value of said flow rate deviations. 
Moreover, in the above hydraulic drive system, preferably, said second 
control means comprises first calculation means for calculating the total 
of the flow rates instructed by said plurality of flow rate instructing 
means, deviation output means for outputting a value corresponding to said 
predetermined flow rate as a reference deviation, second calculation means 
for calculating a difference between the total of the instructed flow 
rates obtained by said first calculation means and the reference deviation 
output from said deviation output means, and third calculation means for 
determining a target displacement volume of said hydraulic pump based on 
the difference obtained by said second calculation means. 
Additionally, in the above hydraulic drive system, preferably, said second 
control means includes deviation output means for outputting a value 
corresponding to said predetermined flow rate as a reference deviation. 
Said deviation output means preferably stores said reference deviation as 
a constant beforehand. Said deviation output means may include means for 
determining said reference deviation depending on the total of the flow 
rates instructed by said plurality of flow rate instructing means. Also, 
said deviation output means may include means for determining one of said 
plurality of hydraulic actuators which is subjected to a maximum load 
pressure, means for selecting one of the flow rates instructed by said 
flow rate instructing means which corresponds to said hydraulic actuator 
subjected to the maximum load pressure, and means for determining said 
reference deviation depending on said selected instructed flow rate. 
Furthermore, in the above hydraulic drive system, preferably, said second 
control means comprises integration means for calculating a target 
displacement volume of said hydraulic pump adapted to make the delivery 
rate of said hydraulic pump smaller by said predetermined flow rate than 
the total of the flow rates instructed by said plurality of flow rate 
instructing means, means for calculating the total of the flow rates 
instructed by said plurality of flow rate instructing means, means for 
calculating a modification value for said target displacement volume based 
on the total of said instructed flow rates, and means for adding said 
modification value to the target displacement volume calculated by said 
integration means and calculating a final target displacement volume. 
In the present invention thus arranged, the first control means performs 
flow servo control such that the flow rates detected by the flow rate 
sensor means are coincident with the flow rates instructed by the flow 
rate instructing means. Through this flow servo control, the hydraulic 
actuators are always supplied with the hydraulic fluid (working oil) at 
respective flow rates corresponding to the instruction values from the 
flow rate instructing means in spite of changes in temperatures of the 
working oil, etc. The second control means controls the delivery rate of 
the variable displacement hydraulic pump such that the delivery rate of 
the hydraulic pump is smaller by the predetermined flow rate than the 
total of the flow rates instructed by the flow rate instructing means. By 
so controlling the pump delivery rate to become smaller by the 
predetermined flow rate, it is possible with the above flow servo control 
that the flow control valve associated with the hydraulic actuator 
producing the maximum load pressure is controlled to be maximized in its 
opening, and hence a pressure loss produced by that flow control valve can 
be reduced. 
By effecting the above control of the pump delivery rate by the second 
control means using flow rate deviations resulted from respectively 
subtracting the flow rates detected by the flow rate sensor means from the 
flow rates instructed by the flow rate instructing means, an influence of 
errors in the flow rate sensor means, control equipment for the hydraulic 
pump and so on can be eliminated and the aforesaid predetermined flow rate 
can be set to a small value when the pump delivery rate is to be 
controlled in accordance with demanded flow rates in parallel to the above 
flow servo control. As a result, an amount of deficiency in the flow rate 
supplied to the hydraulic actuator producing the maximum load pressure can 
be made smaller to enable accurate flow control. 
By effecting the above control of the pump delivery rate by the second 
control means using the calculated total of the flow rates instructed by 
the flow rate instructing means, the pump delivery rate can be controlled 
independently of the flow servo control, which enables stable control free 
from hunting.

BEST MODE FOR CARRYING OUT THE INVENTION 
Hereinafter, the present invention will be described in conjunction with 
illustrated embodiments. 
First Embodiment 
A first embodiment of the present invention will be explained with 
reference to FIGS. 1 to 4. 
In FIG. 1, a hydraulic drive system according to this embodiment comprises 
a variable displacement hydraulic pump 1 driven by a prime mover (not 
shown) and having a displacement volume varying mechanism (hereinafter 
represented by a swash plate), a plurality of hydraulic cylinders or 
actuators 3A, 3B . . . (hereinafter represented by 3A, 3B) connected to 
the hydraulic pump 1 in parallel and driven by a hydraulic fluid delivered 
from the hydraulic pump 1, a plurality of flow control valves 40A, 40B . . 
. (hereinafter represented by 40A, 40B) for respectively controlling flow 
rates of the hydraulic fluid supplied to the plurality of hydraulic 
cylinders and controlling driving of these hydraulic cylinders, a 
plurality of control levers 5A, 5B . . . (hereinafter represented by 5A, 
5B) for instructing respective flow rates to the plurality of flow control 
valves, input amount sensors 50A, 50B . . . (hereinafter represented by 
50A, 50B) for outputting electric signals proportional to respective input 
amounts from the control levers, flow rate sensors 10A, 10B . . . 
(hereinafter represented by 10A, 10B) for detecting respective flow rates 
of the hydraulic fluid supplied to the hydraulic cylinders, valve 
controllers 11A, 11B . . . (hereinafter represented by 11A, 11B) for 
respectively controlling driving of the flow control valves 40A, 40b based 
on signals from the input amount sensors 50A, 50B and the flow rate 
sensors 10A, 10B, a pump tilting controller 12 for calculating a tilting 
command value (target displacement volume) of the swash plate of the 
hydraulic pump 1 based on signals from the valve controllers 11A, 11B, and 
a regulator 20 for driving the swash plate 1a of the hydraulic pump 1 
based on a signal from the pump tilting controller 12. 
The flow control valves 40A, 40B are of solenoid actuated valves 
electromagnetically driven with respective control signals from the valve 
controllers 11A, 11B. As the input amount sensors 50A, 50B, potentiometers 
are used by which operation of the control levers 5A, 5B in one direction 
from their neutral positions is given with a - sign and their operation in 
the other direction is given with a "+" sign. The flow rate sensors 10A, 
10B can be of, for example, the turbine flow type, the volume type or the 
Doppler type. The regulator 20 has a solenoid valve operated in response 
to the signal from the pump tilting controller 12, and the swash plate la 
is driven through operation of that solenoid valve. The valve controllers 
11A, 11B and the pump tilting controller 12 each comprise a microcomputer. 
Alternatively, these controllers may be constituted by one common 
microcomputer. 
The valve controllers 11A, 11B and the pump tilting controller 12 have 
control functions shown in block diagrams of FIGS. 2 to 4. These control 
functions will be apparent from the following description of operation of 
this embodiment. 
Now, when the control lever 5A, for example, is operated, its input amount 
is detected by the input amount sensor 50A and applied to the valve 
controller 11A. As shown in FIG. 2, the valve controller 11A calculates a 
deviation .DELTA.Q.sub.1 between a detected input amount X.sub.1 and a 
flow rate Y.sub.1 detected by the flow rate sensor 10A in a subtracter 
110, integrates the deviation .DELTA.Q.sub.1 in an integrator 111, and 
further calculates an opening command value K.sub.1 by multiplying by a 
gain K.sub.i. In this embodiment, taking into account that the flow rate 
sensor 10A always produces a positive output, an absolute value circuit 
114 takes an absolute value of the input amount X.sub.1, the absolute 
value being compared with the detected flow rate Y.sub.1. A switching 
control unit 112 outputs a digital value "1" when the sign of the input 
amount X.sub.1 (i.e., the direction in which the control lever 5A is 
operated) is "+", and a digital value "0" when it is "-". Thus, the 
opening command value K.sub.1 is output to one side of the flow control 
valve 40A in correspondence with the operating direction of the control 
lever 5A through a switch 113 under control of the switching control unit 
112. When the input amount (instructed flow rate) X.sub.1 becomes equal to 
the detected flow rate (actual flow rate) Y.sub.1, the opening command 
value K.sub.1 comes into a steady state. 
Through the foregoing feedback control, the opening degree of the flow 
control valve 40A is controlled depending on the input amount from the 
control lover in such a manner that, even with change in viscosity of the 
working oil and other factors, the flow control valve 40A is precisely 
controlled to such an opening as adapted to provide the instructed flow 
rate. Hereinafter, that control of the flow control valve will be referred 
to as flow servo control. 
Also, when the control lever 5B is operated, the flow servo control is 
performed by the valve controller 11B in exactly the same manner as 
mentioned above. When the control lever 5A and the control lever 5B are 
both operated the valve controllers 11A, 11B implement the same flow servo 
control independently of each other. Note that status amounts and 
calculated values relating to the valve controller 11B are indicated by 
adding a suffix 2. 
FIG. 3 shows a modification in which another function is added to the 
functions shown in FIG. 2. In FIG. 3, the same components as those in FIG. 
2 are denoted by the same reference numerals. Denoted by 116 is a 
proportional element Kp for the deviation .DELTA.Q used to improve 
responsivity of the control, and 117 is a differentiation element 
Kd.multidot.S for the deviation .DELTA.Q used to provide stability in the 
control. The remaining functions are the same as shown in FIG. 2. 
In parallel to the foregoing flow servo control by the valve controller 
11A, the pump tilting controller 12 makes control as shown in FIG. 4. More 
specifically, in FIG. 4, the pump tilting controller 12 receives the 
deviations (hereinafter referred to as flow rate deviations) 
.DELTA.Q.sub.1, .DELTA.Q.sub.2 calculated by the subtracters 110 of the 
valve controllers 11A, 11B shown in FIG. 2. Note that the pump tilting 
controller 12 receives the flow rate deviations .DELTA.Q.sub.1 to 
.DELTA.Q.sub.n in FIG. 4 on an assumption that the hydraulic actuators, 
the flow control valves, the valve controllers, etc. are each provided in 
number of n. The pump tilting controller 12 calculates the total 
.DELTA..SIGMA.Q of those flow rate deviations .DELTA.Q.sub.1 to 
.DELTA.Q.sub.n in an adder 120. An output .SIGMA..DELTA.Q of the adder 120 
is compared in a subtracter 122 with a reference deviation 
.DELTA.Q.sub.ref which is set as a constant in a deviation setting unit 
121 beforehand, thereby calculating a value equal to a result of 
subtracting the latter from the former. The value obtained by the 
subtracter 122 is further subjected to calculation in an integrator 123 
which has the same function as the integrator 111 shown in FIG. 2, and the 
calculated result is output as a tilting command value L to the regulator 
20. In accordance with the tilting command value L, the regulator 20 
controls tilting of the swash plate 1a of the hydraulic pump 1 for 
controlling the delivery rate of the hydraulic pump 1. 
Operation of the pump tilting controller 12 will now be considered. As 
explained above, the valve controllers 11A, 11B implement the flow servo 
control for the flow control valves 40A, 40B so that the deviations 
.DELTA.Q.sub.1, .DELTA.Q.sub.2 between the instructed flow rates (demanded 
flow rates) corresponding to the input amounts X.sub.1, X.sub.2 and the 
detected flow rates (actual flow rates) Y.sub.1, Y.sub.2 each become zero. 
In contrast, the pump tilting controller 12 controls the delivery rate of 
the hydraulic pump 1 based on the integrated value of the value resulted 
by subtracting the reference deviation .DELTA.Q.sub.ref from the total 
.DELTA..SIGMA.Q of the flow rate deviations. This implies that the pump 
delivery rate is controlled so that the total of the detected flow rates 
Y.sub.1, Y.sub.2 becomes smaller than the total of the demanded flow rates 
by a predetermined flow rate corresponding to the reference deviation 
.DELTA.Q.sub.ref. Thus, the delivery rate of the hydraulic pump 1 is 
controlled to a flow rate smaller than the total demanded flow rate by a 
predetermined flow rate corresponding to the reference deviation 
.DELTA.Q.sub.ref. 
Accordingly, when only the control lever 5A is operated, the hydraulic 
cylinder 3A is supplied with the hydraulic fluid at a flow rate smaller by 
the reference deviation .DELTA.Q.sub.ref than that corresponding to the 
input amount from the control lever 5A, although the valve controller 11A 
performs the flow servo control for the flow control valve 40A. Therefore, 
the opening of the flow control valve 40A is controlled to its maximum 
value and the resulting smaller pressure loss by the flow control valve 
40A makes it possible to suppress the delivery pressure of the hydraulic 
pump 1 at a lower level. A reduction in the supply flow rate by the amount 
of .DELTA.Q.sub.ref will not give rise to any trouble in practical use if 
the reference deviation .DELTA.Q.sub.ref is set to a value as small as 
possible while achieving the intended function. 
While the above explanation is concerned with the case of driving the 
hydraulic actuator 3A only, it similarly applies to the case of 
simultaneously driving the plural hydraulic actuators. More specifically, 
those hydraulic actuators other than that producing the maximum load 
pressure are supplied with the hydraulic fluid at respective demanded flow 
rates through the flow servo control by the associated valve controllers, 
but the hydraulic actuator producing the maximum load pressure is supplied 
with the hydraulic fluid at a flow rate smaller than the reference 
deviation .DELTA.Q.sub.ref than the demanded flow rate and the associated 
flow control valve is maximized in its opening through the flow servo 
control. 
From the standpoint of saving in energy, the delivery pressure of the 
hydraulic pump is desirably the same as the maximum one of the load 
pressures produced by the plural hydraulic actuators. However, since the 
hydraulic fluid is supplied via the flow control valve to the hydraulic 
actuator producing the maximum load pressure, it is inevitable that the 
delivery pressure of the hydraulic pump is raised by an amount of the 
pressure loss produced by the flow control valve. Conversely, this means 
that by making the above pressure loss smaller, the delivery pressure of 
the hydraulic pump can be ideally suppressed to a necessary lowest value. 
In this embodiment, because the flow control valve associated with the 
hydraulic actuator producing the maximum load pressure is maximized in its 
opening, as mentioned above, the pressure loss produced by the flow 
control valve is minimized, enabling the delivery pressure of the 
hydraulic pump to be ideally suppressed to a necessary lowest value. 
Also, the fact that the delivery rate of the hydraulic pump 1 is controlled 
to a value smaller by the reference deviation .DELTA.Q.sub.ref than the 
demanded flow rate has an important meaning in this embodiment as set 
fourth below. 
Let it be supposed that the reference deviation .DELTA.Q.sub.ref is not set 
in this embodiment. This corresponds to the case that the hydraulic drive 
system shown in FIG. 1 has the pump tilting controller not provided with 
the components 121, 122 in the block diagram of FIG. 4. Let it be also 
supposed that the delivery rate of the hydraulic pump happens to become 
larger than the demanded flow rate in the above arrangement. This 
condition may occur, for example, if the flow servo control functions, 
prior to a reduction in the delivery rate of the hydraulic pump, for 
restricting the opening of the flow control valve to achieve the target 
flow rate, when the input amount from the control lever is reduced. In 
such a case, the surplus hydraulic fluid is returned to a reservoir via a 
relief valve provided, though not shown in FIG. 1, near a pump delivery 
port for the purpose of safety. Thus, the pump delivery pressure is raised 
up to a set pressure of the relief valve no matter how light the actuator 
load may be. At this time, because of being kept under the flow servo 
control by the valve controllers 11A, 11B, the flow control valves are 
controlled such that their openings are reduced to supply the hydraulic 
fluids at respective predetermined flow rates even with the associated 
actuators having light loads. Accordingly, the total flow rate deviation 
.SIGMA..DELTA.Q becomes 0 and the output of the integrator 123 is not 
changed, meaning that the pump tilting amount remains the same and the 
above relief condition is maintained in such case. In other words, the 
hydraulic pump cannot generate the required flow rate and pressure only, 
making the system fail to function as a practical one. 
In contrast, with this embodiment, even if the system comes into the relief 
condition and the total flow rate deviation .SIGMA..DELTA.Q becomes 0, the 
tilting amount of the hydraulic pump is gradually reduced with the 
presence of .DELTA.Q.sub.ref, enabling the system to escape from the 
relief condition. As a result, the hydraulic pump can be efficiently 
operated while generating the required flow rate and pressure only. Thus, 
the presence of the reference deviation .DELTA.Q.sub.ref makes it first 
possible with, in parallel to the flow servo control, implement control of 
the pump delivery rate in accordance with the demanded flow rate. 
Furthermore, this embodiment uses the total flow rate deviation 
.SIGMA..DELTA.Q, rather the input amounts X.sub.1, X.sub.2 from the 
control levers, for controlling the pump delivery rate in accordance with 
the demanded flow rate, and this feature provides the following important 
action. 
Consider first the case that the delivery rate of the hydraulic pump is 
controlled by receiving the input amounts X.sub.1, X.sub.2 from the 
control levers without introducing the reference deviation 
.DELTA.Q.sub.ref. In this case, if there exist no errors in the flow rate 
sensors 10A, 10B, the regulator 20 and so forth, no problems occur. Stated 
otherwise, if so, the pump delivery rate can be controlled to be 
coincident with the demanded flow rate in parallel to the flow servo 
control. Generally, however, the sensors contain errors in terms of 
detection accuracy. Accordingly, it is supposed that while the total of 
the input amounts X.sub.1, X.sub.2 from the control levers is recognized 
as 100 l/min, for example, and the hydraulic pump actually delivers the 
hydraulic fluid at a flow rate of 100 l/min, the hydraulic fluid is 
supplied to the actuators only at an actual flow rate of 99 l/min in a 
steady state and the flow control valves are subjected to the flow servo 
control independently of each other. This case happens, for example, if 
one flow rate sensor detects a flow rate of 51 l/min despite the actual 
flow rate being 50 l/min. In such a case, the hydraulic fluid is delivered 
from the hydraulic pump at 100 l/min, whereas the actuators are supplied 
with it at only 99 l/min, resulting in the problem that there occurs a 
surplus flow rate of 1 l/min which is released similarly to the 
above-mentioned case. Accordingly, the hydraulic pump requires power 
greater than necessary and the efficiency of the entire system is lowered. 
A first method for avoiding the above drawback is to set the pump delivery 
rate at a relatively small value such that the delivery rate of the 
hydraulic pump is still insufficient or smaller than the value obtained by 
subtracting all of the accumulated errors possibly occurring in the 
sensors, the regulator and so forth from the required pump delivery rate. 
This can be realized by providing a reference deviation .DELTA.Q.sub.ref 
as with this embodiment. Note that the first method will be described in 
detail later as another embodiment (see FIGS. 11 and 12). In that case, 
the reference deviation .DELTA.Q.sub.ref is given by approximately 1 to 5% 
of the maximum delivery rate of the hydraulic pump x N (where N is the 
number of hydraulic actuators). Assuming now that accuracy of the flow 
rate sensors 10A, 10B are each .+-.2 /min, there are three hydraulic 
actuators, and the delivery rate accuracy of the hydraulic pump is 3 /min, 
by way of example, the reference deviation must be set as follows: 
.DELTA.Q.sub.ref .gtoreq.2 (l/min ).times.3+3 (l/min)=9 (l/min) 
A second method for avoiding the above drawback is to use the total flow 
rate deviation .SIGMA..DELTA.Q as practiced in this embodiment. More 
specifically, using the total flow rate deviation .SIGMA..DELTA.Q is 
equivalent to informing the hydraulic pump of whether the flow rates are 
sufficient or deficient, based on the result of the flow servo control on 
the hydraulic actuator side and, therefore, the aforesaid relief condition 
will not occur due to the accuracy of the flow rate sensors 10A, 10B. 
Also, since the tilting amount of the hydraulic pump is only increased and 
decreased based on information about sufficiency or deficiency in the flow 
rates from the hydraulic actuator side by using the integrator 123 rather 
than specifying an absolute value of the tilting amount, accuracy on the 
pump control side will never be affected. 
However, in the case of using the total flow rate deviation 
.SIGMA..DELTA.Q, the relief condition may occur for another reason as 
mentioned above in the absence of the reference deviation 
.DELTA.Q.sub.ref, making the system fail to function as a practical one. 
Because .DELTA.Q.sub.ref used in this case is not affected by accuracy of 
the sensors and the pump control side, it can be set to a very small value 
in consideration of, strictly speaking, an error possibly occurred in 
calculation by the controllers which generally comprise microcomputers. 
The reference deviation .DELTA.Q.sub.ref is approximately 0.1 to 3% of the 
maximum delivery rate of the hydraulic pump. Accordingly, it is possible 
to minimize a lack of the flow rate for the hydraulic actuator producing 
the maximum load pressure and to achieve the accurate flow control. It 
should be understood that when a response becomes slow in the transient 
region because the reference deviation .DELTA.Q.sub.ref is too small, the 
reference deviation .DELTA.Q.sub.ref is actually determined, taking into 
account responsivity as well. 
With this embodiment, as explained above, since the flow servo control is 
performed so as to make the opening of the flow control valve match the 
demanded flow rate, the hydraulic actuator driven through the flow control 
valve can be operated with high accuracy without being affected by oil 
temperatures, etc. Also, since the flow control valve associated with the 
hydraulic actuator producing the maximum load pressure is maximized in its 
opening, the pressure loss can be suppressed to a small value. 
Further, with this embodiment, since the delivery rate of the hydraulic 
pump is controlled by using the total flow rate deviation .SIGMA..DELTA.Q, 
the pump delivery rate can be controlled by setting a small value of the 
reference deviation .DELTA.Q.sub.ref without causing the relief condition, 
and an influence of the reference deviation upon the flow control is 
minimized to enable the accurate flow control. 
Second Embodiment 
A second embodiment of the present invention will be described with 
reference to FIG. 5. In this embodiment, a pump tilting controller 12A has 
functions different from those shown in FIG. 4 only in that a maximum 
value selector 124 is provided instead of the adder 120, the remaining 
functions are the same. The maximum value selector 124 selects a maximum 
one of the deviations .DELTA.Q.sub.1, .DELTA.Q.sub.2 . . . .DELTA.Q.sub.n 
and outputs it to the subtracter 122. Selecting the maximum flow rate 
deviation by the maximum value selector 124 in this embodiment implies 
that the tilting control of the hydraulic pump is performed by using 
information about the actuator input flow rate that is most insufficient, 
whereby a transient response is improved. 
Referring back to FIG. 1, when the hydraulic cylinder 3A is driven by 
operating only the control lever 5A, the valve controller 11A implements 
the flow servo control for the flow control valve 40A in such a manner as 
explained above. In the case of sole operation of one hydraulic actuator, 
because the total flow rate deviation .SIGMA..DELTA.Q and the maximum flow 
rate deviation have the same value, the pump tilting controller 12A 
implements the control with the same functions as those of the first 
embodiment shown in FIG. 4. Specifically, the flow rate deviation 
.DELTA.Q.sub.1 as a deviation between the input amount X.sub.1 and the 
detected flow rate Y.sub.1 is selected as the maximum flow rate deviation 
by the maximum value selector 124, and the pump delivery rate is 
controlled to become smaller by the reference deviation .DELTA.Q.sub.ref 
than the demanded flow rate. Also, the flow control valve 40A is 
controlled to have its maximum opening. 
Let it be supposed that, under the above condition, the control lever 5B is 
operated to drive the hydraulic cylinder 3B and the hydraulic cylinder 3B 
produces a higher load pressure than the hydraulic cylinder 3A. In this 
case, the delivery pressure of the hydraulic pump 1 is raised and, at the 
same time, the tilting amount of the swash plate 1a of the hydraulic pump 
1 must be increased, thereby giving rise to a transient phenomenon below. 
For the flow control valve 40A, since the pressure is raised in a maximum 
opening condition, the flow rate becomes too large and the flow rate 
deviation .DELTA.Q.sub.1 takes a negative value. On the other hand, for 
the flow control valve 40B, since the pressure is raised in a maximum 
opening condition, the flow rate becomes insufficient until the tilting 
amount of the hydraulic pump 1 is increased, and the flow rate deviation 
.DELTA.Q.sub.2 takes a positive value. 
In such a condition, (.DELTA.Q.sub.2 -.vertline..DELTA.Q.sub.1 
.vertline.)-.DELTA.Q.sub.ref is applied to the integrator 123 in the first 
embodiment having the functions shown in FIG. 4. Meanwhile, .DELTA.Q.sub.2 
is selected by the maximum value selector 124 and .DELTA.Q.sub.2 
-.DELTA.Q.sub.ref is applied to the integrator 123 in the this embodiment 
having the functions shown in FIG. 5. Thus, the value (absolute value) 
applied to the integrator 123 is larger in this embodiment of FIG. 5 than 
in the first embodiment of FIG. 4. Accordingly, the tilting command value 
L can be increased at a higher speed and responsivity of the tilting in 
the transient region can be improved. 
In a steady state, the flow rate supplied to only the hydraulic cylinder 3B 
as the hydraulic actuator producing the maximum load pressure becomes 
insufficient by an amount of the reference deviation .DELTA.Q.sub.ref and 
the flow control valve 40B is controlled to be maximized in its opening. 
Also, the flow rate deviation .DELTA.Q.sub.2 (=+.DELTA.Q.sub.ref) for the 
hydraulic cylinder 3B is selected as the maximum flow rate deviation by 
the maximum value selector 124 and the input to the integrator 123 becomes 
0, thereby keeping the pump tilting amount constant. At this time, because 
of the flow rate deviation .DELTA.Q.sub.1 for hydraulic cylinder 3A being 
0, there is obtained the same result as the case that the total flow rate 
deviation .SIGMA..DELTA.Q is calculated and output the integrator 123 in 
the first embodiment having the functions shown in FIG. 4. In other words, 
the maximum value selector 124 functions as means for calculating the 
total flow rate deviation .SIGMA..DELTA.Q in a steady state. 
As a result, with this embodiment, it is possible to not only obtain the 
same advantage as that of the first embodiment, but also achieve the pump 
tilting control with a good response since the tilting control of the 
hydraulic pump is performed by using the maximum flow rate deviation as 
information about the actuator input flow rate that is most insufficient. 
Third Embodiment 
A third embodiment of the present invention will be described with 
reference to FIG. 6. In the foregoing embodiments, the reference deviation 
.DELTA.Q.sub.ref has been described as a preset constant. It has also been 
stated that the satisfactory operation can be achieved by setting the 
reference deviation .DELTA.Q.sub.ref to be approximately 0.1 to 3% of the 
maximum delivery rate of the hydraulic pump in consideration of 
responsivity in the transient region. However, because the hydraulic 
actuator operated under the maximum load pressure is always supplied with 
the hydraulic fluid only at a flow rate smaller the deviation 
.DELTA.Q.sub.ref than the demanded flow rate, the deviation 
.DELTA.Q.sub.ref is desirably made as small as practicable in fine 
operation requiring higher accuracy. This embodiment includes a function 
to meet such a requirement. 
In FIG. 6, a pump tilting controller 12B receives, in addition to the 
signals of the flow rate deviations .DELTA.Q.sub.1, .DELTA.Q.sub.2 . . . 
.DELTA.Q.sub.n from the valve controllers 11A, 11B, the signals of 
absolute values of the input amounts X.sub.1, X.sub.2 . . . X.sub.n from 
the control levers and calculates the tilting command value L based on 
these signals. Specifically, the pump tilting controller 12B has an adder 
126 for adding the absolute values of the input amounts X.sub.1, X.sub.2 . 
. . X.sub.n, and a multiplier 127 for multiplying the total of these 
absolute values of the input amounts by a constant Kx. An output of the 
multiplier 127 becomes the deviation .DELTA.Q.sub.ref. The remaining 
functions are the same as those shown in FIG. 4. 
With this embodiment thus arranged, the total of the demanded flow rates is 
calculated by the adder 126 and the deviation .DELTA.Q.sub.ref is 
determined by multiplying the total demanded flow rate by the proper 
constant Kx. Thus, the deviation .DELTA.Q.sub.ref is determined in 
proportion to the total demanded flow rate, with the result that when the 
total demanded flow rate is small, a control error in the flow rate 
supplied to the hydraulic actuator producing the maximum load pressure can 
be made smaller. On the contrary, when the total demanded flow rate is 
large, the deviation .DELTA.Q.sub.ref also becomes large to permit the 
control with a good response in the transient region. 
Fourth Embodiment 
A fourth embodiment of the present invention will be described with 
reference to FIGS. 7 and 8. This embodiment is intended to provide another 
method of determining the reference deviation .DELTA.Q.sub.ref. In FIG. 7, 
the same components as those in FIG. 1 are denoted by the same reference 
numerals. 
In FIG. 7, a hydraulic drive system of this embodiment includes shuttle 
valves 13A, 13B . . . (hereinafter represented by 13A, 13B), pressure 
sensors 14A, 14B . . . (hereinafter represented by 14A, 14B), and a 
maximum load pressure selector 15. The pressure sensors 14A, 14B 
respectively output, through the shuttle valves 13A, 13B, electric signals 
V.sub.1, V.sub.2 proportional to load pressures of the hydraulic cylinders 
3A, 3B. The maximum load pressure selector 15 receives the signals from 
the pressure sensors 14A, 14B and outputs a signal N corresponding to the 
hydraulic actuator which produces a maximum load pressure. A pump tilting 
controller 12C has the same functions as those of the pump tilting 
controller 12 shown in FIG. 1 except for its part. 
FIG. 8 is a block diagram for explaining functions of the pump tilting 
controller 12C. The pump tilting controller 12C receives, in addition to 
the signals of the flow rate deviations .DELTA.Q.sub.1, .DELTA.Q.sub.2 . . 
. .DELTA.Q.sub.n from the valve controllers 11A, 11B, the signals of 
absolute values of the input amounts X.sub.1, X.sub.2 . . . X.sub.n from 
the control levers and the signal N from the maximum load pressure 
selector 15. The pump tilting controller 12C has a switching unit 129 for 
receiving the absolute values of the input amounts X.sub.1, X.sub.2 . . . 
X.sub.n and the signal N from the maximum load pressure selector 15 and 
selecting the absolute value of the input amount corresponding to the 
hydraulic actuator which produces the maximum load pressure, and a 
multiplier 127 for multiplying the selected absolute values of the input 
amount by a constant Kx. An output of the multiplier 127 becomes the 
deviation .DELTA.Q.sub.ref. The remaining functions are the same as those 
shown in FIG. 4. 
In this embodiment, as mentioned before, the hydraulic actuator producing 
the maximum load pressure is always supplied with the hydraulic fluid at a 
flow rate smaller by the reference deviation .DELTA.Q.sub.ref than the 
demanded flow rate. Therefore, by changing the reference deviation 
.DELTA.Q.sub.ref depending on the instructed flow rate for that hydraulic 
actuator, control accuracy can be further increased. The pressure sensors 
14A, 14B and the maximum load pressure selector 15 shown in FIG. 7 are 
provided for the above purpose. More specifically, the maximum load 
pressure selector 15 functions as means for detecting the hydraulic 
actuator producing the maximum load pressure; i.e., it selects the 
hydraulic actuator producing the maximum load pressure based on the 
pressure signals applied thereto and outputs the signal N corresponding to 
that hydraulic actuator. The pump tilting controller 12C receives the 
signal N at the switching unit 129, selects one of the absolute values of 
the input amounts from the control levers corresponding to that hydraulic 
actuator, and outputs it to the multiplier 127. As a result, the hydraulic 
actuator producing the maximum load pressure is surely supplied with the 
hydraulic fluid at a flow rate smaller than the demanded flow rate by a 
value equal to the product of the demanded flow rate and the constant Kx. 
GiwBn the value Kx being 0.01, by way of example, the deviation 
.DELTA.Q.sub.ref is 1% of the instructed flow rate for the hydraulic 
actuator. 
With this embodiment, since the reference deviation is determined depending 
on the demanded flow rate for the hydraulic actuator producing the maximum 
load pressure, a control error in the flow rate supplied to that hydraulic 
actuator can be made smaller when the demanded flow rate is small. On the 
contrary, when the demanded flow rate is large, the deviation 
.DELTA.Q.sub.ref also becomes large to permit the control with a good 
response in the transient region. 
Fifth Embodiment 
A fifth embodiment of the present invention will be described with 
reference to FIG. 9. While the above fourth embodiment uses the maximum 
load pressure selector as means for detecting the hydraulic actuator 
producing the maximum load pressure, this embodiment adopts another method 
in this respect. 
In FIG. 9, a pump tilting controller 12D of this embodiment has a maximum 
value selector 13 which receives the opening command values K.sub.1, 
K.sub.2 . . . K.sub.n calculated by the respective valve controllers, 
selects the hydraulic actuator corresponding to the maximum opening 
command value as the hydraulic actuator producing the maximum load 
pressure, and then outputs the corresponding signal N. Since the hydraulic 
actuator producing the maximum load pressure is controlled with the 
maximum opening, the hydraulic actuator producing the maximum load 
pressure can be also detected in this embodiment by selecting the 
hydraulic actuator corresponding to the maximum opening command value. In 
response to the signal N from the maximum value selector 130, the 
switching unit 129 selects one of the absolute values of the input amounts 
from the control levers corresponding to that hydraulic actuator, and 
outputs it to the multiplier 127. The remaining functions are the same as 
those shown in FIG. 4. 
This embodiment can also provide advantages similar to the fourth 
embodiment shown in FIGS. 7 and 8. 
Sixth Embodiment 
A sixth embodiment of the present invention will be described with 
reference to FIG. 10. This embodiment is intended to improve responsivity 
of the pump tilting control. 
In FIG. 10, a pump tilting controller 12E receives the signals of the flow 
rate deviations .DELTA.Q.sub.1, .DELTA.Q.sub.2 . . . .DELTA.Q.sub.n from 
the valve controllers 11A, 11B and the signals of absolute values of the 
input amounts X.sub.1, X.sub.2 . . . X.sub.n from the control levers, and 
calculates the tilting command value L based on these signals. 
Specifically, the pump tilting controller 12E has an adder 131 for adding 
the absolute values of the input amounts X.sub.1, X.sub.2 . . . X.sub.n, a 
multiplier 132 for multiplying the total of these absolute values of the 
input amounts by a constant Ky, and an adder 133 for adding an output of 
the multiplier 132 to the output of the integrator 123. An output of the 
multiplier 132 is used as a modification value for the tilting command 
value and an output of the adder 133 becomes the final tilting command 
value L. The remaining functions are the same as those shown in FIG. 4. 
With this embodiment thus arranged, since the modification value 
proportional to the total of the absolute values of the input amounts 
X.sub.1, X.sub.2 . . . X.sub.n is added in the adder 133 to the tilting 
command value obtained as an integrated value, there can be provided an 
advantage of improving responsivity in the transient region. Note that for 
the same reason as stated in connection with the second embodiment of FIG. 
5, a maximum value selector may be used instead of the adder 131. 
Seventh Embodiment 
A seventh embodiment of the present invention will be described with 
reference to FIGS. 11 and 12. In this embodiment, the delivery rate of the 
hydraulic pump is controlled in accordance with the demanded flow rate by 
using the total of the input amounts from the control levers rather than 
the total .SIGMA..DELTA.Q of the flow rate deviations. 
In FIG. 11, a hydraulic drive system of this embodiment includes a pump 
tilting controller 12F for receiving the signals of the input amounts 
X.sub.1, X.sub.2 from the control levers 5A, 5B detected by the input 
amount sensors 50A, 50B, and calculating the tilting command value. 
In the pump tilting controller 12F, as shown in FIG. 12, absolute values of 
the input amounts X.sub.1, X.sub.2 from the control levers 5A, 5B in an 
absolute value circuit 140 and these absolute values are added in an adder 
141 to determine the total .SIGMA.X of the input amounts. An output 
.SIGMA.X of the adder 141 is compared in a subtracter 142 with a reference 
deviation X.sub.ref set as a constant in a deviation setting unit 143 
beforehand, thereby calculating a value equal to a result of subtracting 
the latter from the former. The value obtained by the subtracter 142 is 
further subjected to calculation in a proportion unit 144 and the 
calculated result is output as a tilting command value L to the regulator 
20. In accordance with the tilting command value L, the regulator 20 
controls tilting of the swash plate 1a of the hydraulic pump 1 for 
controlling the delivery rate of the hydraulic pump 1. 
As stated before, when the delivery rate of the hydraulic pump is 
controlled by using the total .SIGMA.X of the input amounts from the 
control levers without introducing the reference deviation X.sub.ref, the 
delivery rate of the hydraulic pump may become larger than the flow rate 
actually passing through the flow control valve due to errors in the flow 
rate sensors 10A, 10B, the regulator 20 and so forth, which results in the 
problem that the surplus flow rate may be released. Setting of the 
reference deviation X.sub.ref makes it possible to eliminate that problem 
and achieve economical operation. In this embodiment, the reference 
deviation X.sub.ref is given by approximately 1 to 5% of the maximum 
delivery rate of the hydraulic pump .times.N (where N is the number of 
hydraulic actuators). 
Further, as with the case of using the total flow rate deviation 
.SIGMA..DELTA.Q, since the pump delivery rate is kept smaller than the 
demanded flow rate, the flow control valve associated with the hydraulic 
actuator producing the maximum load pressure is controlled to be maximized 
in its opening, whereby the pressure loss can be suppressed to a small 
value. 
Additionally, with this embodiment, since the pump tilting is controlled 
through an open loop independently of the flow servo control for the valve 
controllers 11A, 11B, it is possible to ensure stable delivery rate 
control of the hydraulic pump without causing hunting. 
INDUSTRIAL APPLICABILITY 
According to the present invention, as described above, since the flow 
servo control is performed so as to make the opening of the flow control 
valve match with the demanded flow rate, the hydraulic actuator driven 
through the flow control valve can be operated with high accuracy without 
being affected by oil temperatures, etc. Also, since the flow control 
valve associated with the hydraulic actuator producing the maximum load 
pressure is maximized in its opening, the pressure loss can be suppressed 
to a small value. Further, in the case that the delivery rate of the 
hydraulic pump is controlled by using the total flow rate deviation 
.SIGMA..DELTA.Q, the pump delivery rate can be controlled by setting a 
small value of the reference deviation .DELTA.Q.sub.ref without causing 
the relief condition. In addition, accurate flow control can be enabled. 
Alternatively, in the case that the delivery rate of the hydraulic pump is 
controlled by using the total input amount .SIGMA.X, the pump delivery 
rate can be controlled not only in a reliable manner without causing the 
relief condition, but also in a stable manner without causing hunting.