JPS6049102A - Electrohydraulic servo mechanism - Google Patents

Abstract

PURPOSE:To improve response and control accuracy by comparing the deviation of an operational instruction signal of a hydraulic actuator and a positional signal with two set values different in size and by controlling the hydraulic actuator by the result of the comparison. CONSTITUTION:A mechanism performing an operational control by driving a delivery variable mechanism 2 of a variable delivery pump 1 by a servo mechanism 4 through two solenoid valves 6a, 6b is provided with a control device 10B which receives signals X, Y from a discharge rate instruction signal generator 9 and a position detecting device 5. The control device 10B obtains a deviation Z of the signals X and Y and compares it with values of positive and negative, different in size, DELTA1, -DELTA1, DELTA2, -DELTA2(DELTA2>DELTA1, -DELTA2<-DELTA1). When Z<-DELTA2 the solenoid valves 6a, 6b are turned on and off respectively and when -DELTA2<=Z <=-DELTA1, both solenoid valves 6a, 6b are turned off to stop the servo mechanism 4.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the control of hydraulic actuators in various hydraulic systems, and particularly to an electro-hydraulic servomechanism suitable for driving an oil outlet actuator using a solenoid valve in accordance with an operation command thereof.

In order to drive a hydraulic actuator of a hydraulic system in accordance with an operation command, there is an electro-hydraulic on/off servo mechanism that uses an inexpensive and rigid solenoid valve. Such a conventional servo machine +14 is shown in FIGS. 1 to 3.

In FIG. 1, 1 is a hydraulic pump, 2 is a variable displacement mechanism for the hydraulic pump 1 which includes a swash plate, an oblique shaft, etc., 3 is a servo piston connected to the variable displacement mechanism 2 and drives it, and 4 is a servo piston. Servo cylinder housing piston 3,
4a and 4b are a left chamber and a right chamber within the servo cylinder 4 separated by the servo piston 3. Here, the servo piston 3 has a pressure receiving area on the left chamber 4a side that is equal to that on the right chamber 4b.
+14 is made so that it is twice the pressure receiving area of iitll.

Reference numeral 5 denotes a position detection device combined with the variable displacement mechanism 2 or the servo piston 3, which uses a variable resistor, a differential transformer, etc., and outputs an electric signal according to the position of the variable displacement mechanism 2 or the servo piston 3. do.

Reference numerals 6a and 6b are electrically operated B valves, and both electrically operated valves 6a16b are configured to cut off the pressure oil when not energized (off) and conduct the pressure oil when energized (on).

7 is a hydraulic power source connected to the solenoid valve 6a and the right chamber 4b of the servo cylinder 4, and 8 is an oil tank connected to the solenoid valve 6b.

The servo piston 3 operates as follows by turning on and off the solenoid valves 6a and 6b. First, when the % solenoid valve 6a is turned on and the solenoid valve 6b is turned off, the pressure oil from the hydraulic source 7 is transferred to the right ventricle 4b.
and is led to the left ventricle 4a via the solenoid valve 6a. As a result, the same pressure is applied to both end surfaces of the servo piston 3, and the servo piston 3 is pushed to the right due to the difference in pressure receiving area. In response to this, each displacement mechanism 2 is also driven. Next, electricity? When both the a-valves 6a and 6b are turned off, the left chamber 4a of the servo cylinder 4 is cut off from communicating with the hydraulic power source 7 and the oil tank 8, and the cylinder 3 is held at the current position and becomes stopped.

Also, when the solenoid valve 6a is turned off and the solenoid valve 6b is turned on, pressure oil from the hydraulic source 7 enters the right chamber 4b of the servo cylinder 4, while the left chamber 4b flows into the oil tank 8 via the solenoid valve 6b.
As a result, the servo piston 3 is pushed to the left, and the variable displacement mechanism 2 is also driven accordingly. In this way, by appropriately turning on and off the solenoid valves 6a and 6b, the servo piston 3 is arbitrarily driven, and the hydraulic pump 1
The discharge flow rate can be controlled.

FIG. 2 is a system configuration diagram of a conventional control device for the electro-hydraulic servomechanism shown in FIG. 1. In the figure, 5 is the position detection device. Reference numeral 9 denotes a discharge amount command signal generating device that commands the discharge flow rate of pressure oil from the hydraulic pump 1. According to this command signal, the servo piston 3 is operated to drive the displacement volume variable mechanism 2, and eventually the oil discharge pump 1 to 1 ga of pressure oil is discharged according to the command signal. The discharge amount command signal generating device 9 uses a potentiometer, a differential transformer, or the like. LOA is a control device configured using a microcomputer. The control devices J and OA control the signal y of the position IJ・j detection device 5 and the discharge amount command signal generator l:L.
The multiplexer 10 switches the signal
a, A/D converter 10 that converts the input signals x, y into corresponding digital values X, Y; b, these values X;
, Y are taken in, and the CPU 7L performs the calculation and control of the storage space (
Central processing unit) io c, CPU 10 c calculation, control hand 1114'! z memorized 170 M (
+7-de-only memory) it) d. RAM (random access memory) 10e for storing the results of calculations and control.

and an output interface 11 that outputs the results of calculation and control. It is composed of l f. lla and llb are amplifiers for driving the electromagnetic valves 6a and 6b, respectively, and amplify the output signals 8a and Sbi from the output interface 10f of the control device 10. In addition, the output signal 8a%S
b is at either the station level H or the low level L, and when the output (No. 8 5a1Sb is at the high level H, the solenoid valve 6a16 is
b7J″--energized.

Next, the operation of this control device 1 (IA) will be explained with reference to the flowchart shown in 131d and the characteristics 1 of the output a of the 1-piece tray of the sword 4 (a) and 4 (b). −
Ru.

According to the signal stored in the OM10d, the signal X from the discharge amount command signal generator 9 is sent to the multiplexer 10a and the '0' A/D converter 1. It is taken into CP TJ IQ c as value X via Ob (
In step St), it is stored in A M 10 e.

In a similar operation, the signal y of the position detection device 5 is converted to CP as the value Y.
U J, taken into Oc (step SZ), RAM
10e. Next, Ci' U 10 C performs the operation H-Z = X-Y to calculate the deviation Z between the value X and the value Y according to the command from O~110d (step SS). If the deviation Z is positive (Z>0), is the command value 4 This means that the tilt amount is larger than the current tilt amount Y of the variable mechanism 2, so control must be performed to increase the tilt iY so that it approaches the command value X.On the contrary, the bias fi
When Z is negative (Z<0), the tilting amount Y is larger than the command value X.
Since it is larger, control must be performed to reduce the tilt iY. In addition, when the deviation Z is O, since the tilting iY matches the command value X, control is performed to keep the variable displacement mechanism 2 in a stopped state.

By the way, as mentioned above, the command value X and the tilting amount YIJ are always
''- If control is performed to make them equal, the deviation Z will repeatedly become positive or negative due to slight fluctuations in the tilting amount Y or command value X, resulting in extremely poor stability and instability. In order to prevent this, a dead zone in the range of the set value ±Δ is provided in advance, and when the deviation Z is in the range of -Δ1≦Z≦Δ1, it is assumed that the tilt iY is equal to the command value XK (deviation is O). , when the deviation Z is larger than the value Δ1 (Z>
Δ, ), there is a means for controlling to increase the tilting iY, and a means for controlling to decrease the tilting amount Y when the deviation Z is smaller than the value -Δ (Z<-Δ1). It is taken. Although the stability of the control operation can be ensured by setting such a dead zone, the larger the dead zone is, the smaller the allowable range of deviation Z (-Δ1≦Z≦Δ1) becomes and the error increases. This is obvious, and the control viscosity cannot be avoided.

IL) and control device f6. Contains an explanation of its operation. Step S
3, the deviation Z claimed next has the value Δ1. -Δ1 (step 84).

As a result of this comparison, when the deviation Z is smaller than the value -Δ1 (Z
<-Δ, ), the output signal δat for the solenoid valve 6a is set to a low level L, as shown in FIG. Dekai No. 8 No. 5 against
Control is performed to set b(z high level 11) (step S5). Therefore, the solenoid valve 6a is turned off, the solenoid valve 6b is turned on, the servo piston 3 moves to the left, and the tilting amount Y decreases.Step s ,, St, 8s, s,, S
, WOH OVER-'J-RU I11 control is repeated until the deviation Z reaches a value within the dead zone. When it is determined in step S4 that the deviation Z is within the dead zone, that is, when -Δ1≦2≦Δ1, as shown in FIG. 4(a),
As shown in (b), the output (! “l’; Sa , S
Both of them become low level (step S), and the servo piston 3 is brought to a stopped state.

Also, in step S, the deviation Z is larger than the value Δ1 (Z>Δ
When I) and η' are disconnected, as shown in FIGS. 4(a) and 4(b), control is performed to set the output signal Sa'6 to a high level and the output signal 5, 1d, Sb to a low level L (step 8
7). Through this control, the servo piston 3 moves to the right, and the tilting amount Y increases. Step bI.

The control that passes through bl, b3, S4, +57 is repeated until the deviation Z is less than 1% (jj). Through the above operations, the tilting amount y is determined by the discharge point command. The value corresponds to the signal X, and pressure oil is discharged from the hydraulic pump 1 in accordance with the signal X.

In such a conventional device, the output signal 8a of the fi II control device ioA, the output signal Sb and the electromagnetic valve 5a.

There is a response delay between the 6b(1) operation and the 6b(1) operation. For example, when the signal Sa changes from the low level L to the crystal level L(), the solenoid valve 6a cannot immediately change from OFF to ON, and there is a dead time until it turns ON.
Ru. Similarly, when the signal Sa changes from the high level H to the low level LK, the ah valve 6a cannot immediately change from on to off, and there is a dead time before the ah valve 6a turns off. The same holds true for the relationship between the number Sb and the solenoid valve 6b. Therefore, the relationship between the dead time (hereinafter referred to as dead time τd) and the dead zone Δ will be discussed with reference to FIGS. 5(a), (b), and (C). The following discussion will be made only regarding the dead time that occurs when the signal Sb outputs a low level L and the signal Sa changes from the high level 1 to the low level. The same is true. In FIG. 5, since both the signals Sa and Sb are at the low level L at time t1 (signal Sb
is not shown because it continues to be a low level output. ), the deviation Z is based on the value within the dead zone. That is, when the discharge amount command signal X is the value X, the value of the signal y during this period is
If I, the value y1 is x, -Δ1≦y1≦x1+Δ1
is within the range of , and the servo piston 3 is at rest.

When the time t is reached and the command signal X changes from the value x1 to the value x2 as shown in FIG. 5(a), the operation q is performed in step S3 in the flowchart of FIG.

In step S4, it is determined that the deviation Z is a dog based on Δ1, and in step S, the output signal S a
becomes a high level h as shown in FIG. 5(b), and the solenoid valve 6a is turned on as shown in FIG. 5(c). Therefore, the servo piston 3 moves to the right, and the tilting amount I6 y increases as shown in FIG. ), the tlli difference Z
is a value within the amorphous zone, and this is determined in step S4, and the control in step S is performed, and the output signal Sa becomes a low level L as shown in FIG. 5(b). but,
The solenoid valve 6a does not turn off immediately, but turns off after the dead time τd has elapsed. Therefore, during the dead time τd, the servo piston 3 moves further to the right, and the tilting amount signal y continues to increase. When the solenoid valve 6a is turned off at time t3 after the dead time τd has elapsed, the servo piston 3 stops and the signal y also becomes a constant value y.

By the way, the moving speed of the servo piston 3 (the rate of change of the signal y) vO is determined by the output of the hydraulic power source 7, + the opening degree of the valve 6a (or l torque vii; tii 4*characteristic). Yes, the pressure of the hydraulic source 7 is increased (
However, if the pressure loss of the 1L nasal valve 6a is made smaller, the speed 2o becomes larger. Increasing the speed Vnf makes it possible to follow changes in the output signal Sa in a short time,
Servo device i8. - The responsiveness of the system will be improved. On the other hand, the moving sword of the servo piston 3 during the dead time of 16, (4
The change in No. 6 y) is not shown in No. 5 lad (a), and the value Δy
This value Δy is the speed ■o of Zerbovist/3 and the dead time τd(/:), that is, Δy=τd·■o. Therefore, in order to improve responsiveness, the speed Vo'
, 3) If it is increased, Δy will inevitably become thicker. However, in order to stabilize the servo device u1, the value Δy must not exceed the dead zone (-Δ1≦Z≦ΔI), that is, Δy≦2.
・The condition is that Δ is satisfied.

From the above, the relationship between dead zone (2・ΔIL, dead time τd and speed vo is 2・Δ1≧■o・τd).

Here, since the dead time τd is an inadmissible value as mentioned above, in the conventional control device 16, the speed Vo is increased in order to improve the response (then the insensitivity becomes 1'i
becomes larger and the control accuracy decreases.On the other hand, if the dead band is made smaller to improve the control accuracy, the stability condition is not met, so the speed ■o has to be made smaller, and the response deteriorates. That means doing so. As described above, in the conventional servo mechanism, it was extremely difficult to realize a mechanism with high precision and high responsiveness.

The present invention has been made in view of these circumstances, and it is an object of the present invention to solve the above problems and provide an electro-hydraulic servo mechanism with good precision and fast response speed.

In order to achieve this object, the present invention interposes a N1m valve between a hydraulic power source and a hydraulic actuator, and a signal from a position detection device of the hydraulic actuator and a command signal from a hydraulic actuator operation command signal generator. Means f for controlling the electromagnetic valve so that the hydraulic actuator is driven in a direction that eliminates the deviation value according to the deviation value that is the difference from the deviation value.
and define a positive or negative first set value for the deviation value and a positive or negative second set value whose absolute value is larger than the first set value, and set the absolute value of the deviation value to be the second set value. When the absolute value is smaller than the absolute value and is greater than or equal to the first set absolute value, the driving speed of the hydraulic actuator is reduced as a % sign.

Hereinafter, "misfire" will be explained based on the illustrated embodiment.

FIG. 6 is a system configuration diagram of a control device for an air-hydraulic servo mechanism according to an embodiment of the present invention. In the figure, 10B is the control device of this embodiment, which includes the same multiplexer lOa, A/IJy converter 1b, CPU], O
C, has an AM10e and an output interface 21Qf. 10 h Cf' U 1. OC(7) rh
It is an RC OM that stores calculation and control procedures, and it is a RC OM that stores calculation and control procedures.
], U d has different storage contents.

Here, before explaining the operation of this embodiment, an outline of means for solving the conventional problems will be explained.

As mentioned in the explanation of the conventional device, the servo machine (・(
In order to achieve the high degree of $+'j of '9, it is necessary to slow down the speed vo of the servo cylinder 3; There are 8 narrows that make the speed ■o faster. In order to solve this contradictory problem, in this embodiment, in addition to the value Δ1.-Δ, which is set in the conventional servo machine +1"4, another value Δ1.-Δ is set. .

This is to set. As shown in FIG. 7, the value Δ is larger than the value Δ (Δ2>Δ1), and the value −Δ.

is the value −Δ, smaller than (−Δ2<−Δl). In addition, N indicates a dead zone, D is an area between the value -Δ and the value -Δ1,
1 indicates the area between the value Δ and the value Δ. In this way, the value Δ7. -Δ, and when the deviation Z is less than the value -26 or exceeds the value Δ2, the servo piston 3 is driven at high speed, and when the deviation 2 enters the area DI or area 1], the servo piston By controlling 1- to reduce the speed of 3, the above-mentioned trade-off problem can be solved. That is,
deviation Z is area L).

IJ! Since the servo piston 3 is driven at high speed before it enters the dead zone, it is possible to obtain good response, and the servo piston 3 that has been driven at high speed approaches the dead zone N and reaches the area , D, Once inside, the speed reaction is decelerated and the speed and dead time tTt (V, Xτd) become smaller,
For this reason, the stone can be made smaller and more expensive.
You can get degrees.

Next, the operation of this embodiment will be explained with reference to the flowchart shown in FIG. 8 and the characteristic diagrams of the output signal of the control device shown in FIGS. 9(a) and 9(b).

First, according to the procedure of )(, OM 10 h K memorized), the highest ejection command (b) is sent to the output signal (X) of the generator 9 and the signal (y) of the position detection device 5 to the multiplexer IUa and the A/D converter 10 (b ix). via II as value X and value Y
Next captured (step SIO+S11), RA
Stored in M to e. Then CP 010 C
& Calculation that lengthens the deviation Z between the imported value X and value Y Z-
The individual difference B requested for X-Y row l is compared with a predetermined value (step 513).

This predetermined value is the value Δ2. Δ8. -Δ1. −Δ.

As a result of the comparison, if the deviation Z is smaller than the value -Δ2 (Z<-Δ2), the signal Sa output from the output interface 10f is at a low level L, as shown in FIG. 9(a). As shown in FIG. 9(b), the signal Sb is set to a high level fiyH (step 814). Therefore, the 1111 valve 6a is turned off, the solenoid valve 6b is turned on, and the A servo piston 3 moves to the left at high speed, decreasing the discharge SL. Step S+o, SI l s S12, SIs
, S14 are repeated, and the absolute value of the deviation Z becomes smaller.Finally, as a result of comparison in step S+s, the deviation Z falls within the range shown in FIG.

When the signal Sa and the signal Sb are set to high level 11 as shown in FIGS. 9(a) and 9(b) (step 8: i+5). This results in
Both the solenoid valves 6a and 6b are turned on, the pressure oil from the oil outlet σ and 17 is divided into the chamber 4a and the cylinder 8, and the servo piston 3 is decelerated. Due to the deceleration shift of the servo piston 3 to the left,
When the absolute value of deviation 2 becomes smaller and enters the region shown in Fig. 7 (-Δ15Z≦ΔI), No. 13 8a, Sb
As shown in FIGS. 9(a) and 9(b), both become low level L (step 516), both solenoid valves 6a16b are turned off, and the servo piston 3 stops at 8.

Also, if the deviation Z is larger than the value Δ2 (Z>Δ2), as shown in FIG. 9(a), (i'r 4 S a is set to high level I, and the signal is changed as shown in FIG. 9(b). 811 is set to low level L (step 81g). Therefore, the solenoid valve 6a is turned on, the solenoid valve 6b is turned off, the servo piston 3 moves to the right at high speed, and the discharge 'r'r-'Ff: increases. Teyu (. Step b 10% 811 %5lts 5l
The fljlj control of lq Sts is repeatedly illuminated, the deviation B becomes smaller, and when it finally enters the area D't ('1JK = Figure 7) (Δ1×Z≦Δ2), No. A Sa and Sb become N.
11a (a) and (b) are both high as shown in 1) L
/ l-1 (step S, 7), and the solenoid valves 6a, 6
When b is both on and 1, the servo piston 3 is decelerated.

As the servo piston 3 decelerates to the right, the value of the deviation Z becomes smaller and enters the range (-Δ1≦Z≦Δ1).
A flill jill step 816 is performed, and the servo piston 3 is stopped as described above.

FIGS. 10(a) to 10(f) are time charts of the operation of this embodiment. Time 1. Previously, the discharge ll1l
The command im number ``Low level due to the control that passes through step S+a'',
, solenoid valve 6a16b&! The servo piston 3 is turned off and the servo piston 3 is brought to a stopped state. Now, at time t1, when the signal X changes to the value x2, the deviation Z (Z = X,
−y, )Δ,) occurs, and step S,. ,S++,S
Hs S13, by the control that passes S+s, Haku! ,7
Sa becomes a high level H as shown in FIG. 10(b).

As a result, the solenoid valve 6a is turned on, as shown in FIGS. 10(d) and 10(f), and the servo piston 3 moves to the left at high speed ■o. At time t4, the value of E y becomes the value (X2-
Δ2), and the deviation Z is the area jgAD shown in FIG.
When entering, the control goes through steps S and 7, and the signal Sb also goes to the 10th level and becomes high level as shown in (C). As a result, as shown in FIGS. 10(e) and 10(f), the 'i4i and rA valves 6b are turned on, and the servo piston 3 is decelerated to the low speed Vo'. The servo piston 3 is decelerated and decreases (.), and when the value of the signal y reaches the value (x2 - ΔI) at time t (enters the zero to band),
The control becomes step S+af: red filter control, and the 10th
As shown in FIGS. (b) and (C), both signals Sa and Sb are turned off. Then, as mentioned above, the dead time τd passes and reaches time t6, and FIGS. 10(d), (e), (
As shown in f), the electric hook valve 6a16b is turned off, and the servo piston 3, which was moving to the left in response to the deceleration f:, stops. Here, the difference Δy between the value of +jy when it enters the dead zone and the value of the signal y after the dead time τd has passed is (■0′・τd
), which is clearly smaller than the difference Δy in the conventional example, which is (■o·τd). This means that the insensitivity @ can be reduced.

In this way, the present embodiment calculates the deviation between the discharge M command signal and the tilting amount signal, and uses this deviation as the first set value that defines the dead zone and the second set value that defines the speed range. Compare and when the absolute value of the individual difference is larger than the absolute value of the second set value, move the servo piston at high speed, make it 7 strokes, and 1(,!'
When the absolute value of the + difference is less than the absolute value of the second set value and larger than the absolute value of the set value of ^ and 1, the servo piston 3 is decelerated, so the servo mechanism has good response and high viscosity. can be configured. On the one hand, this means that for applications that do not require a high degree of control, but instead require high responsiveness, the speed of the servo piston can be increased without increasing the dead zone;
On the other hand, for applications that do not require such high response but rather high precision, it is possible to achieve high precision by reducing the speed of the servo piston (by reducing the dead zone). also means

In addition, in the description of the above embodiment, as a means for decelerating the servo piston, a means for setting both signals to high level 11 and turning on each solenoid valve was exemplified, but it is not necessarily limited to such a means. Alternatively, one signal can be set to a high level, and the other signal No. 43 can be set to a high level and a low level alternately. In this case, by appropriately selecting the high-level time and the low-level time, the corresponding Gishyu valve can be placed in an intermediate state between the OFF state and the ON state, and the speed of the servo piston in the deceleration region is can be freely controlled, and the magnitude of υ and failure AL% Yir and the maximum speed of the servo piston can also be freely selected.

As described above, in the present invention, the deviation between the discharge amount command No. 48 and the position signal is determined, this deviation is compared with the positive and negative first setting values, and the positive and negative second setting values, and the absolute value of the deviation is is the second
The drive speed of the hydraulic actuator is reduced when the absolute value of the set value is less than or equal to the absolute value of the first set value, so the response is good and the servo machine +14 with a high 3"+'4 degree Can be configured.

[Brief explanation of the drawing]

Figure 1 shows the discharge direction of the hydraulic bonder and the CL air used for control.
2 is a system diagram of the conventional control device for the electro-hydraulic servomechanism shown in FIG. 1, and FIG. 3 explains the operation of the control device shown in FIG. 2. The flowchart, Figures 4(a) and (b), shows the characteristics of the output signal of the control device shown in Figure 8I1.2, Figure 5(a).
, (b) and (C) are shown in FIGS. 1 and 2. 2
Figure 7 is a system configuration diagram of a control device for an electro-hydraulic servomechanism according to an actual example of the present invention, and Figure 7 shows the settings used in the control device shown in Figure 6. A diagram to explain, FIG. 78, a flowchart to explain the operation of the control device shown in FIG. 6, and FIG.
a), (b) are characteristic diagrams of the output signal of the control device shown in FIG.
(C) and (f) are time charts illustrating the operation of the electro-hydraulic servo mechanism using the straight metal manufactured by Seiwa 1 shown in FIG. 6. 1...Hydraulic pump, 2...Od displacement variable displacement machine 48j, 3...Servo piston, 4
... Servo cylinder, 5 ... Position detection device, 6a, 6b ... Solenoid valve, 7 ...
Oil output υ1.9... Discharge amount command signal generator, 1
0B...control device, 10a...multiplexer, 101)...Al1) converter, 10
C...CPU, 10e...RAM,
1 (If... Output interface, 10h...
...ROM 0 Figure 1...2Z 1□--□ l1a 9=-rob fOc IOf :Sa χ Y :'s, 11b 1 9th 3rd section m4 animals S. Figure 5 Figure 6 fR7 Figure 8 Figure 9 a

Claims (1)

[Claims] a hydraulic source, a hydraulic actuator, a solenoid valve interposed between the hydraulic pressure and the hydraulic actuator to control the driving sound of the hydraulic actuator, a position detection device for the hydraulic actuator, and an operation command signal generation for the hydraulic actuator. The difference between the device, the command signal of this operation command signal generator, and the position signal detected by the position detection device
An electro-hydraulic servomechanism comprising means for obtaining a difference, and means for controlling the electromagnetic valve so that the hydraulic actuator is driven in a direction to eliminate the deviation according to the value of the deviation. positive and negative set values and positive and negative second set values whose absolute values are larger than the first set value are determined, and the absolute value of the deviation value is set to the second set value. An electro-hydraulic servo mechanism, characterized in that the electro-hydraulic servo mechanism is provided with means for switching the electromagnetic valve so as to reduce the driving speed of the hydraulic actuator when the absolute value of the first set value is smaller than the absolute value of the first set value.