JPH1164191A - Material testing machine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform a control according to the state change of a sample by estimating the spring constant of the sample from the relation between detected load and displacement, and correcting the control gain to be set for a hydraulic servo system. SOLUTION: The operation of an actuator 3 is controlled, and a displacement caused in a sample S is monitored through a displacement gauge 6 while gradually increasing the load applied to the sample S. When a minute displacement of the degree exceeding the minimum resolution of the displacement gauge 6 is detected, the operation of the actuator 3 is stopped, and the load applied to the sample at that time is held as it is. The displacement is stabilized in this state, and the spring constant of the sample S is estimated from the relation between the load and displacement detected in the stabilized state to determine the optimum control gain of a hydraulic servo control system. Thus, even if the control gain is shifted from the optimum value by the change of rigidity of the sample S during the test, it is corrected with the determined optimum control gain, whereby a highly precise test can be performed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a material testing machine for measuring the properties of a sample while controlling a load applied to the sample via a hydraulic servo system, such as a load, a displacement or a strain, by feedback control. More particularly, the present invention relates to an electro-hydraulic servo type material testing machine capable of performing control corresponding to a change in the state of a sample during a test.

[0002]

[Related Background Art] In recent years, since the range of application to various samples is wide and control with high accuracy is possible, FIG.
A closed-loop electrohydraulic servo-type material testing machine configured as shown in FIG. This material testing machine generally holds test pieces S, which are various samples, between a frame 2 and an actuator 3 composed of a hydraulic cylinder via a load cell 1 and supplies the specimen S from a hydraulic source 4 via a servo valve 5. The actuator 3 is driven using the applied pressure oil (oil pressure), thereby applying a load to the test piece S.

The load applied to the test piece S by the load is detected by the load cell 1, and the displacement generated in the test piece S by the application of the load is measured by a displacement meter 6.
The strain is further detected by the strain gauges 7 attached to the test piece S. The control unit 8 constituted by a microcomputer or the like inputs the load, displacement, and strain detected as described above, for example, so that the deviation between the load detected by the load cell 1 and the target load becomes zero (0). To the servo valve 5 via the servo amplifier 9
Is feedback controlled. The actuator 3 driven hydraulically by such a feedback control system
Is servo-controlled to adjust the load (load) applied to the test piece S.

The electro-hydraulic servo control system is expressed as a closed loop block diagram constituting a feedback control system as shown in FIG. That is, the control system obtains a deviation Δ between the control target value for the load (or displacement) and the change (load or displacement) of the test piece S detected via the detection amplifier 8a via the deviation device 8b. The above-mentioned deviation Δ is obtained under the controller 8c in which the control gain is set.
The actuator 3 is hydraulically driven via the servo valve 5 by controlling the operation of the servo amplifier 9 so that is set to zero (0), whereby the load (load or displacement) applied to the test piece S is adjusted. It can be expressed as a control loop.

[0005]

However, in the electrohydraulic servo type material testing machine configured as described above,
It is important to perform the test under a control gain optimally set according to the specification (rigidity) of the test piece S. By the way, if the control gain is insufficient, the control response will be poor,
If the control gain is too large, hunting may occur,
Problems such as applying an excessive load (load) to the sample occur. Therefore, conventionally, generally, the stiffness of the test piece S is estimated based on the experience of the operator (tester) to temporarily set the control gain, and trial and error is performed under the preliminary load / displacement control. The control gain is optimized.

However, even if a material test is performed after setting an appropriate control gain by trial and error as described above,
During the test, the state (rigidity) of the test piece (sample) S changes, and the control gain may deviate from the optimum condition.
In particular, when the test piece (sample) S is in the elastic range with respect to the load applied to the test piece (sample) S, the control state gradually changes, and as a result, highly accurate measurement (test) is eventually performed. There is a problem that it will not be possible.

The present invention has been made in view of such circumstances, and has as its object to provide a feedback control system for controlling the state of a sample even when the rigidity of the sample changes during a test. An object of the present invention is to provide a material testing machine which can optimally set a control gain according to a change and can always stably perform a high-precision material test.

[0008]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides feedback control of a load applied to a sample via a hydraulic servo system in accordance with a change in the sample, for example, load, displacement or strain. While testing the properties of the sample, a so-called electro-hydraulic servo type material testing machine,
In particular, during a test period in which a load is applied to the sample under a predetermined control gain set for the hydraulic servo system to test the sample, the load applied to the sample and the displacement generated in the sample. At a predetermined cycle, and means for estimating the spring constant of the sample from the relationship between the detected load and displacement, and a control gain set for the hydraulic servo system according to the estimated spring constant. Is corrected.

That is, according to the present invention, the spring constant of a sample is estimated from the relationship between the load and the displacement of the sample periodically detected during a test, and optimal control for the hydraulic servo system is performed based on the estimated spring constant. By obtaining a gain and correcting the control gain set for the hydraulic servo system according to the optimum control gain, the control system for the hydraulic servo system is always stabilized regardless of a change in the state of the sample. It is characterized by performing tests with high accuracy.

[0010] Further, the present invention provides,
Prior to the start of the test, the control gain for the hydraulic servo system is gradually increased from its minimum value to gradually increase the load applied to the sample, and the sample is determined based on the relationship between the load and the displacement when the displacement of the sample is detected. After initializing the control gain for the hydraulic servo system by estimating the spring constant of the above, the test on the sample is started.

Further, the present invention provides, as described in claim 3,
Regarding the control gain for the hydraulic servo system, the load applied to the sample and its displacement can be determined by referring to a table showing the relationship between the spring constant previously obtained for various samples and the optimum control gain specific to the hydraulic servo system. It is characterized in that it is determined according to the spring constant estimated from the relationship.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, a material tester according to an embodiment of the present invention will be described below. In particular, the load applied to a sample (test piece) S, specifically, the load and its displacement amount are described. The function for optimally setting the control gain for the hydraulic servo system to be controlled will be described. The material testing machine according to this embodiment basically includes, as shown in FIG. 1 described above, an actuator 3 that is hydraulically driven via a servo valve 5 and a control unit 8 that controls the operation of the servo valve 5. Be composed. One of the features of the material testing machine is that the state of the sample S (stiffness) during the test of the sample (test piece S) executed under a predetermined control gain is one of the functions of the control unit 8. 3) The function of monitoring the change and correcting the control gain according to the change in the state to optimize and set the control gain for the hydraulic servo system.

This function is performed when the actuator 3 is driven under a feedback control system in which a predetermined control gain is set and a load is applied to the sample S to perform a fatigue test. Means for detecting the displacement occurring in the sample S at a predetermined cycle (detection means), and means for estimating the spring constant of the sample S from the relationship between the measured load and the displacement of the sample S. (Spring constant estimation means) and means (control gain correction means) for correcting and optimizing the control gain set for the hydraulic servo control system according to the estimated spring constant.

In particular, in the material testing machine according to this embodiment, as a characteristic inherent to the hydraulic servo control system, the relationship between the spring constant of each sample and the optimum control gain is obtained in advance in a table and obtained as described above. It is characterized in that an optimum control gain for the control system is obtained by referring to the table according to the spring constant of the sample estimated from the obtained relationship between the load and the displacement, and this is set.

Next, for the hydraulic servo control system of the material testing machine, a test of the sample (test piece S) under a control gain set according to the rigidity of the sample (test piece S) and the test The correction of the control gain accompanying the change in the state (rigidity) of the sample (test piece S) in the medium will be described. Initial setting of the control gain for the hydraulic servo control system is performed, for example, by chucking the sample S between the frame 2 and the actuator 3 as shown in FIG. The operation starts by gradually increasing the load (load) applied to the sample S by controlling the operation of the actuator 3 while gradually increasing the control gain for the system from its minimum value. At this time, a minute displacement of the sample S, for example, a slight displacement exceeding the minimum resolution of the displacement meter 6 is detected while monitoring the displacement generated in the sample S by the application of the load (load) via the displacement meter 6. Until the load is increased. Note that the maximum load applied to the sample S at the time of the initial setting is, for example, three times the target maximum load value to be applied to the sample S in the fatigue test.
It is preferable to limit 0 to 50% as a guide so that an excessive load (load) is not applied to the sample S before the test is started.

If a small displacement of the sample S is detected, the operation state of the actuator 3 is frozen, and the load state (load value) applied to the sample S at that time is kept as it is. That is, the load state on the sample S is frozen as it is. The load state is maintained (frozen) for a predetermined time (time T2) until the displacement of the sample S is stabilized. At this time, when the rigidity of the sample S is high and the spring constant KL is large, the displacement is stabilized in a relatively short time as shown by the characteristic A in FIG. However, when the rigidity of the sample S is low and the spring constant KL is small, FIG.
As shown by the characteristic B, the displacement stabilizes after a certain degree of displacement fluctuation occurs. Waiting time T2 for holding the status
Is set in consideration of the time required for stabilizing the displacement due to such a difference in the spring constant.

After waiting for the stabilization of the displacement in this way, the load and the displacement in the state where the sample S is stabilized are respectively detected over a plurality of samples. That is, the detection of the load and the displacement is performed by, for example, 10
It is repeated over about 0 to 200 samples. Thereafter, the average of the detected load value and the average of the displacement amount are obtained, and the spring constant KL of the sample S is estimated by the calculation of [average load value ÷ average displacement amount]. Then, according to the estimated spring constant KL, for example, according to a table showing the relationship between the spring constant of various samples previously determined and the optimum control gain unique to the hydraulic servo control system, the optimal control to be set in the hydraulic servo control system. Find the gain and initialize it.

The relationship between the spring constant and the optimum control gain is determined, for example, with respect to various samples S whose spring constants are known under the optimization control of the hydraulic servo control system in the material testing machine in advance. By obtaining control gains and arranging these data, they are obtained in advance as table data unique to the hydraulic servo control system. In addition, each time a spring constant and an optimum control gain at that time are newly obtained with the execution of the fatigue test, the table data is updated by learning the actual data.

Here, a spring constant KL indicating the rigidity of the sample S
The relationship between the above and the optimum control gain in the hydraulic servo control system of the material testing machine for testing the sample S will be briefly described. The hydraulic servo control system that drives the actuator 3 that applies a load to the sample S forms a feedback control system as described above. The optimal control gain for stably operating the feedback control system changes depending on the rigidity (spring constant KL) of the sample S, and takes a value specific to the control system.

That is, the hydraulic servo control system of the material testing machine is as follows:
As shown in the block diagram in FIG. 2, the servo amplifier 9, the servo valve 5, the actuator 3, and the sample S (load) are represented as a closed loop transmission system. Therefore, to realize stable operation of the servo control system, the servo amplifier 9
The transmission system (transfer function) inherent to the material testing machine such as
It is necessary to set an optimum control gain in the controller 8c, for example, in consideration of a load transfer system (transfer function) depending on the nature of the load.

Incidentally, optimal control gains (proportional gain KP, integral gain KI) corresponding to the spring constant KL of the sample S in a so-called displacement control system for controlling the operation of the actuator 3 by focusing on the displacement of the sample S. Is, for example, FIG.
As shown in FIG. In addition, an optimal control gain corresponding to the spring constant KL of the sample S in a so-called load control system that servo-controls the operation of the actuator 3 by focusing on the load applied to the sample S is, for example, as shown in FIG. become.

Therefore, in the present invention, as a preparatory process, a plurality of samples S whose spring constant KL is known in advance within the range of the spring constant KL covering the test range in the material testing machine.
Is used to tune the proportional gain KP and the integral gain KI in the displacement control system and the load control system, respectively, to optimize the control system, and obtain the optimum control gains KP, KI for various spring constants KL. . The optimal control gains KP, KI for these various spring constants KL
Is expressed as data having a table structure as shown in FIG. 4C, and this is prepared in advance as a characteristic unique to the material testing machine. By referring to such table data, the optimum control gains KP and KI unique to the material testing machine shown in FIGS. 4A and 4B are obtained according to the spring constant KL of the sample S as described later. It has become.

On the other hand, in order to estimate the spring constant KL of the sample S, the sample S
A load is gradually applied to the sample S until a minute displacement is detected. The control of the load (load) applied to the sample S is performed by utilizing the relationship between the spring constant KL and the proportional gain KP at the time of step response in a load control system as shown in FIG. As illustrated in (b), this is realized by gradually increasing the proportional gain KP over time. More specifically, for example, under a target load pattern assuming a ramp-hold waveform in which the maximum load is limited as shown in FIG. 3, the proportional gain KP (the proportional gain ), The load control is performed while gradually increasing the gain value. At this time, the integral gain KI is set to, for example, zero (0).

The relationship between the spring constant KL and the proportional gain KP at the time of step response in the displacement control system is shown in FIG. 6 (a). Accordingly, by utilizing this relationship, a load (displacement) may be applied to the sample S under displacement control while gradually increasing the proportional gain KP with time as shown in FIG. 6B, for example. good. Even when a load (displacement) is applied to the sample S under the control of such a displacement control system, the load state is naturally frozen when a minute displacement of the sample S is detected. It is.

As described above, a load is gradually applied to the sample S, and when a minute displacement of the sample S is detected, for example, if the proportional gain value and the target value at that time are kept constant, the sample S Without applying unnecessary load (load) to
The load can be frozen and the displacement can be stabilized as described above. The time required for the stabilization of the displacement generally differs depending on the spring constant KL as shown in FIG. Therefore, as to the freezing time in the load state, as described above, the time required for the displacement to be stabilized is expected in the range of the spring constant KL covering the test range in the material testing machine, particularly under the condition where the spring constant KL is the smallest. It is good to set in.

After the displacement of the sample S is stabilized after waiting for such a lapse of time, the displacement amount and the load value are periodically collected over a plurality of samples, and, for example, the average thereof is obtained.
Absorbs variations in detected values due to disturbances and other factors. Thereafter, the calculation of [average load value / average displacement amount] is performed to estimate the spring constant KL of the sample S.
That is, by applying a light load (load) that does not impose a load on the sample S, a minute displacement is generated in the sample S, and the sample S is determined based on the relationship between the load and the displacement generated in the sample S at that time. Is estimated.

After the spring constant KL of the sample S has been estimated in this manner, the control gain corresponding to the estimated spring constant KL is calculated back according to the table described above. Thereby, the optimum control gains KP and KI to be set in the control system are obtained.
Can be obtained, and the initial setting of the optimized control gain when testing the sample S with unknown spring constant KL can be realized.

After the control gain for the hydraulic servo control system has been initialized as described above, the test of the sample S is started under the control gain. Specifically, the target value given to the control system is variably controlled under the signal generator, so that the fatigue test, creep test,
A reclamation test, a tensile test, and a compression test are selectively performed.

For example, as shown in FIG. 8, a signal generator generates a test waveform that changes continuously and repeatedly, such as a sine wave, a triangular wave, a rectangular wave, and a ramp wave. Then, by inputting this test waveform as a control target value for the hydraulic servo control system, the load applied to the sample S is repeatedly changed, thereby executing a fatigue test of the sample S. In addition, pull
In the case of a compression test, the test is executed by giving a ramp waveform as a target value, and in the case of a creep reclamation test, the test is executed by giving a ramp-hold waveform as a target value. You.

When the test is performed on the sample S in this way, a test waveform that changes periodically and repeatedly is given as a target value, and the load applied to the sample S is periodically changed to execute a fatigue test. In the case of a dynamic test,
The control gain set in the hydraulic servo control system may deviate from its optimum value due to a change in the state (rigidity) of the sample S with the progress of the test. Such a deviation of the control gain from the optimum value causes a change in the load actually applied to the sample, and it is inevitable that the test accuracy is reduced.

That is, a load (load) with a certain variation width (amplitude)
When the fatigue test is performed while repeatedly changing the load, the change width of the load (load) actually applied to the sample S may change, or the level of the load (average load) itself may change. Such a change in the load condition (test condition) for the sample S naturally causes an error in the test result. This error causes a decrease in test accuracy and, consequently, a decrease in reliability of the fatigue test.

Therefore, in the material testing machine according to the present invention, for example, as described above, a predetermined control gain is set for the hydraulic servo control system and a fatigue test of the sample S is performed, and a predetermined period is set. The load applied to the sample S and the displacement occurring in the sample S at that time are measured, and the spring constant KL of the sample S is constantly estimated from the relationship between the measured load and the displacement. And its spring constant K
The change in L is monitored. When a change in the spring constant KL of the sample S is detected, an optimal control gain corresponding to a new spring constant KL associated with a change in the state of the sample S is obtained by, for example, referring to the above-described table. By optimizing the control gain set in the control system, the optimization is achieved.

More specifically, as shown in FIG. 9, the processing procedure of the control unit 8 is performed at a cycle sufficiently faster than the change cycle of the load applied to the sample S, as schematically shown in FIG. The load applied to the sample S and the displacement caused by the load are measured every cycle (for example, 100 μsec) (Step S1). Then, it is determined whether or not data (load and displacement) of a predetermined number of samples has been collected (step S2). If data of a sufficient number of samples has been obtained for estimating the spring constant KL, the average of the loads and displacements is obtained. The respective values are obtained, and the spring constant KL of the sample S is estimated based on each of these average values (step S3).

Thereafter, the difference ΔKL between the estimated spring constant KL and the spring constant corresponding to the control gain currently set in the hydraulic servo control system, that is, the amount of change in the spring constant due to the change in the state of the sample S is calculated. Find (Step S
4). Then, in correcting the control gain, it is determined whether or not the change (difference) ΔKL exceeds a predetermined threshold value (Step S5). The determination threshold is for preventing the control gain from being excessively corrected due to a slight change in the spring constant. Then, when the variation ΔKL of the spring constant exceeds the determination threshold, an optimum control gain corresponding to the new spring constant KL is obtained (step S6). The calculation of the optimum control gain corresponding to the new spring constant KL is performed by referring to, for example, the above-mentioned table showing the relationship between the spring constant and the optimum control gain.

As described above, if the optimum control gain corresponding to the new spring constant KL changed according to the state change of the sample S is obtained by referring to the table,
By setting the optimal control gain in the hydraulic servo control system, the control gain set in the hydraulic servo control system is corrected (step S7). The correction of the control gain is performed, for example, at a timing synchronized with a change cycle of a target value (load) given to the hydraulic servo control system as described above.

Thus, according to the material testing machine of the present invention, during the test of the sample S as described above, the load and the displacement of the sample S caused by the load actually applied to the sample S are determined. Detection is performed at a predetermined cycle, and the spring constant KL of the sample S is sequentially estimated according to the load and the displacement.
When the state of the sample S, that is, its spring constant KL changes with the progress of the test, the changed spring constant KL
By obtaining the optimum control gain according to the above, and resetting it in the hydraulic servo control system, the control gain of the servo control system is optimized and corrected.

Therefore, according to such a material testing machine having the function of optimizing the control gain of the hydraulic servo control system, the control gain of the hydraulic servo control system to be set according to the spring constant of the sample S is Even if the error of the control gain initially set by trial and error is large, the sample S
It is possible to optimize and set the control gain of the hydraulic servo control system according to the spring constant of the sample S estimated in the test process. Although the test was started with the control gain for the hydraulic servo control system optimized,
As the test proceeds, the state of the sample S (spring constant KL)
When the control gain of the hydraulic servo control system deviates from the optimization condition due to the change, the control gain of the hydraulic servo control system is changed according to the spring constant of the sample S estimated in the test process of the sample S. Is optimized.

Therefore, according to the control gain optimizing function, the control gain of the hydraulic servo control system can always be optimized, and the test of the sample S can be executed while keeping the control system stable. The test accuracy can be sufficiently improved. In addition, as described above, the spring constant K of the sample S
Only when L changes beyond the above-described determination threshold,
That is, the control gain is optimized and corrected only when the error of the control gain cannot be ignored. Therefore, the control gain is corrected in response to a change in the spring constant caused by a slight change in the state of the sample S. There is no. Therefore, while maintaining the stable state of the control system, it is possible to execute the fatigue test with high accuracy by setting the control gain optimally.

The present invention is not limited to the above embodiment. In the embodiment, the optimal control gain for the spring constant KL of each sample is determined in advance as a table, and by referring to this table, the optimal control gain corresponding to the estimated spring constant is determined. The relationship between the spring constant and the optimal control gain is given in the form of a several order approximation, and the spring constant K is calculated according to this approximation.
The optimum control gain according to L may be obtained by an approximate calculation process.

The control gain to be initially set for the hydraulic servo control system may be determined while varying the control gain by trial and error under the empirically estimated spring constant. You may make it set based on the known data (spring constant etc.) about the sample S.
Further, with the optimization correction of the control gain, for example, it is also possible to correct the level of the signal waveform given as the control target value for the load and the amplitude thereof. In addition, the present invention can be variously modified and implemented without departing from the gist thereof.

[0041]

As described above, according to the present invention, during a test period in which a load is applied to a sample under a predetermined control gain initially set for the hydraulic servo system to test the sample, The load applied to the sample and the displacement generated in the sample are detected at a predetermined cycle, and the spring constant of the sample is estimated from the relationship between the detected load and the displacement to the hydraulic servo system. The control gain to be set is corrected.

Therefore, according to the present invention, when there is an error in the initially set control gain or when the state of the sample changes during the test, the control gain of the control system deviates from its optimum value with the change of the spring constant. Even if it is the case, since the control gain is optimized and corrected according to the state (rigidity) of the sample at that time,
It is possible to test the properties and characteristics of a sample with high accuracy while always applying an appropriate load to the sample under a stable control system.

Further, as described in claim 2, the load applied to the sample is gradually increased by gradually increasing the control gain for the hydraulic servo system from its minimum value prior to the start of the test of the sample, whereby the minute displacement of the sample is increased. Since the spring constant of the sample is estimated from the relationship between the load and the displacement at the time when is detected, and the control gain for the hydraulic servo system is initially set according to the estimated spring constant, excessive load on the sample is prevented. The characteristics can be measured stably and accurately without adding to the intention.

Further, as set forth in claim 3, a table showing the relationship between the spring constant inherent to the hydraulic servo system and the optimum control gain is prepared, and by referring to this table, the optimum value for the hydraulic servo system is obtained. Since the control gain is obtained and set, it is possible to obtain an effect that the test can be advanced while simplifying and effectively stabilizing the control system.

[Brief description of the drawings]

FIG. 1 is a diagram showing a schematic configuration of a material testing machine according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of a control system in the electrohydraulic servo type material testing machine according to the present invention.

FIG. 3 is a diagram showing a concept of an automatic setting process of a control gain in the present invention as a time course.

FIG. 4 is a diagram illustrating a relationship between a spring constant of a sample unique to a control system and an optimum control gain.

FIG. 5 is a diagram showing a relationship between a spring constant and a proportional gain at the time of a step response in a load control system, and a temporal increase characteristic of the proportional gain.

FIG. 6 is a diagram illustrating a relationship between a spring constant and a proportional gain at the time of a step response in a displacement control system, and a temporal increase characteristic of the proportional gain.

FIG. 7 is a diagram showing a change in a time required for displacement stabilization during a step response with respect to a spring constant.

FIG. 8 is a diagram illustrating an example of a signal waveform that defines a change characteristic of a load applied to a sample according to a test condition.

FIG. 9 is a diagram showing an example of a control gain correction processing procedure executed in the control unit of the material testing machine according to one embodiment of the present invention.

[Explanation of Signs] S Sample (test piece) 1 Load cell 2 Frame 3 Actuator 4 Hydraulic source 5 Servo valve 6 Displacement gauge 7 Strain gauge 8 Control unit 9 Servo amplifier

Claims (3)

[Claims] 1. A material tester for measuring a load, displacement or strain generated on a sample by feedback-controlling a load applied to the sample via a hydraulic servo system, wherein a predetermined control gain set for the hydraulic servo system is provided. Test means for applying a load to the sample under the condition to test the sample, and detecting a load applied to the sample and a displacement generated in the sample during a test period of the sample by the test means at a predetermined cycle. Detecting means, spring constant estimating means for estimating the spring constant of the sample from the relationship between the detected load and displacement, and control gain correction for correcting a control gain set for the hydraulic servo system according to the estimated spring constant. And a means for testing the material. 2. The test means gradually increases the control gain for the hydraulic servo system from its minimum value before starting the test to gradually increase the load applied to the sample, and the control means detects the displacement at the time when the displacement of the sample is detected. 2. The test for the sample is started after estimating a spring constant of the sample from the relationship between the load and the displacement, and initially setting a control gain for the hydraulic servo system according to the estimated spring constant. Material testing machine according to 1. 3. The control gain according to claim 1, wherein the control gain for the hydraulic servo system is obtained by referring to a table showing a relationship between a spring constant unique to the hydraulic servo system and an optimum control gain. Material testing machine.