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1. Foreword

In a general mechanical system, a motor used for replacing

human force drives a mechanical device to finish many tasks.

Depending on the power transmission method, a mechanical system

can possess different characteristics. For instance, take systems using

a belt or gears as a power transmission medium. Although the system

has accurate positioning characteristics, its power output is restricted

by the motor properties and the complicated mechanical structure,

thus failing to meet the requirements of high power from a small size.

A hydraulic system uses hydraulic oil as the power transmission

medium, which is ideal for small size with high power output and

high response scenarios.

The hydraulic system was gradually developed from the

hydrostatic transmission theory was first proposed by Blasé Pascal

(1623–1662), a French mathematician and physicist, in the 17th

century. The first hydraulic machine using water as the working

medium was built by English engineer Joseph Braman (1749–1814)

at the end of 18th century and had industrial applications thereafter. In

1905, oil replaced water as the power transmission medium, which

significantly increased the performance of hydraulics. In 1925, F.

Vickers proposed the pressure-balance type vane pump theory to

build a more stable foundation for hydraulics. Hydraulic systems are

still widely used today in industrial fields.

Restricted by their component properties, early hydraulic driving

systems could only control the movement of hydraulic cylinders

using an opening or closing control method. Hydraulic cylinder

pressure was regulated by a spring pressure regulator which was

adjusted manually, thus making it impossible to slightly adjust the

system structure. Therefore, achieving precision control was highly

limited and dynamic adjustment was inflexible.

However, due to the recent maturing of semi-conductor

development, various kinds of precision sensors and servo valves

have come into existence. Subsequently, hydraulic cylinder pressure

can be measured by a pressure sensor, and the pressure output of the

hydraulic cylinder can be controlled using a relief valve. Alternatively,

the force output is measured with a load cell, and the movement

direction of the hydraulic cylinder and the oil flow is controlled using

a servo valve. It is therefore no longer difficult to have precision

positioning and accurate output force control [1, 2, 5].

However, in a hydraulic system, hydraulic oil is compressible.

Friction can develop between the cylinder wall and the piston,

causing leakage problems [6]. In addition, the system characteristics

may also vary with oil temperature. The servo proportional valve also

has a dead-zone and hysteresis problems [7, 8]. The above properties

may consequently have a tremendous effect on the control

performance of the hydraulic servo system. It is therefore impossible

for a general linear controller to achieve effective control.

Therefore, many scholars have proposed using non-linear control

principles to achieve the control, of which a fuzzy controller [3, 4, 9]

is often used. The main feature of a fuzzy controller is that it does not

need to obtain an accurate system mode with no need to establish a

complex system equation. On the contrary, fuzzy controllers are

designed by using rules based on the control principles of experts or

http://dx.doi.org/10.6493/SmartSci.2014.233

Precision Force Control for an Electro-Hydraulic Press Machine

Hong-Ming Chen1,*, Guo-Wei Yang1 and Chong-Cyuan Liao1

1Department of Electronic Engineering, Chienkuo Technology University, Changhua City, Taiwan, ROC * Corresponding Author / E-mail: steven@ctu.edu.tw

KEYWORDS : Electro-hydraulic servo press system, Force control, Relief valve, Flow servo valve, Composite control

This thesis is primarily intended to design a PC-based control system to control the force of an electro-hydraulic servo press system for implementing precision force control. The main feature is to develop a composite control by using the relief valve and the flow servo valve. Using feedback from a force sensor, a fuzzy controller was designed with LabVIEW software as the system control core for achieving a precision force control for the hydraulic cylinder on its travel and output. The weakness of hydraulic systems is that hydraulic oil is compressible and prone to leaking, and its characteristics can vary with oil temperature, thus making it difficult for a general linear controller to achieve accurate control. Therefore, a fuzzy controller was designed with LabVIEW along with a NI-PCI_6221 interface card and a load cell to control the servo valve flow and the relief valve to control the pressure source. The testing results indicate that accurate force control output of an electro-hydraulic servo press system can be obtained.

Manuscript received: March 13, 2014 / Accepted: March 31, 2014

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experienced people. Its control effects are thus not inferior to the

performance of other advanced controllers.

In this study, force output was controlled with PC-based real-time

control. The control system consists of the controller designed with

the graphic control software LabVIEW which is developed by

National Instrument Corp. With feedback from the NI-PCI_6221

interface card and the force sensor, the fluid flow is controlled by the

servo valve while the pressure source is controlled by a relief valve,

thus completing precision force control of the electro-hydraulic servo

press system. A fuzzy controller is used in this system and was

designed with assistance of the Fuzzy toolkit in LabVIEW. Using a

real hydraulic servo press system platform for testing the control

principles, generally good control effects were observed with regard

single step, repeated pressure-load, sinusoid trace of force, and multi-

step force increase and decrease response, response speed and steady-

state error.

2. Electro-hydraulic servo force control system

The architecture of the electro-hydraulic servo press system is

shown in Fig. 1. The hydraulic system used in this study is a vertical-

type press. The hydraulic pump is driven by an AC motor, and the

pressure of the hydraulic cylinder is adjusted by a relief valve. The

moving direction of the hydraulic cylinder and the hydraulic flow

volume are controlled by the servo valve to achieve positioning of the

hydraulic cylinder and force control. The hydraulic servo system

consists of a servo valve, a relief valve, a hydraulic cylinder, an

optical ruler, a force sensor, a spring load, a PC and a data-acquisition

card. The hydraulic cylinder moves with a reciprocal stroke travel of

20 cm.

Fig. 1 Architecture of the electro-hydraulic servo press system

In the hydraulic servo system, hydraulic oil is supplied from the

pump driven by the motor to the hydraulic cylinder as a power source.

The control system used to design the fuzzy controller is PC-based as

it offers precision force control to the hydraulic cylinder. Its main

control method is through the hydraulic cylinder pressure, which is

measured by the pressure sensor, and the output force of the hydraulic

cylinder, which is measured by the force sensor. With the PCI-6621

interface card, the pressure and force signals are captured and sent to

the computer where the control level is calculated by LabVIEW as

subject to the control principles of the controller. The control level is

then output via the interface card to control the relief valve for initial

force control of the hydraulic cylinder pressure. At the same time, the

servo valve is controlled by the output of the interface card to fine-

tune the flow, thus achieving composite control and an accurate force

control effect.

3. Description of the electro-hydraulic force servo system

The hydraulic cylinder servo position system is shown in Fig. 2

[6-8]:

Fig. 2 Architecture of the hydraulic cylinder servo position system

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The hydraulic cylinder servo position system mainly consists of

the servo valve and the hydraulic cylinder. The main purpose of

control is to let the fluid flow rate of the hydraulic cylinder reach the

expected value LQ as soon as possible. The relationship between

the servo valve spool position vx and the load flow rate LQ at the

orifice can be determined from the orifice law [6] as follows:

LcvqL PkxkQ (1)

Where, qk is expressed in equation (2):

LvSdvq PxPwckk )sgn(1 (2)

and SP is the oil supply pressure, dC is the fluid emission

coefficient, w is the servo spool area gradient, vx is the servo spool

displacement, is the oil density, LP is the pressure variance

between two sides of the hydraulic cylinder, vk is the servo valve

gain, and ck is the gain constant of the pressure variance.

Since the moving speed of a normal hydraulic cylinder is within

the bandwidth of the servo valve, the servo valve can be assumed to

be under the zero-order system, the direct proportional relationship [7]

of the servo spool displacement vx and the control input u can be

expressed as follows in (3):

ukx av (3)

ka is the amplified gain of the servo valve and u is the control input of

the servo valve. In consideration of the internal leaking and

compressibility of oil in the hydraulic system, from the flow equation

[6] of the hydraulic cylinder the following equation is arrived at:

LtLe

tL PCP

VxAQ

4

(4)

LQ is the total oil flow volume to the hydraulic cylinder from the

orifice of the servo valve, A is the area of the effective cross-section

for the pressure of the hydraulic cylinder, tV is the effective volume

of the hydraulic system, e is the oil compressibility module, and

tC is the total oil leaking coefficient of the hydraulic cylinder. If we

rearrange equations (2) to (4), we can obtain the system mode as

follows:

4

)(44

)( 2

uV

KAK

tFV

KCxV

AtF

t

eaq

t

ect

t

e

(5)

From the equation (5) we see that the hydraulic cylinder force servo

system is a parameter-variable non-linear system. An accurate control

effect is impossible to achieve with a general opening or closing

control method. In the next chapter, a non-linear fuzzy controller will

be outlined specifically for this servo system in order to provide

precision force servo control.

4. Principles and design of a fuzzy controller

The basic structure of a fuzzy controller can be divided into four

parts [3, 4]: the fuzzification interface, the If-Then rule base, the

fuzzy inference engine and the defuzzification interface as shown in

Fig. 3. First, the input error and the error varying rate is fuzzificated

to allow the input value of the error or the error varying rate to fall

within the range set by the fuzzy inference. Therefore, at the input end,

the gain adjustment parameters K1 and K2 are added to allow the

output to fall within the control range. At the output end, a parameter

K3 is also added to adjust the output control. The membership

functions of the input error and the error variance for the fuzzy

controller are shown in Fig. 4 and Fig. 5 and the membership function

of the fuzzy output is shown in Fig. 6. In addition, the rule base

design is completed with the If-Then expressions of the

corresponding relationship between the system control requirements

and expert experience. For the fuzzy inference method, this paper

uses the common Min-Min-Max fuzzy inference of Mamdani. Lastly,

the fuzzy value derived from the fuzzy inference is converted into a

clear value for output through defuzzification to control the system.

There are many means of defuzzification, but this paper uses center of

gravity defuzzification [3, 4, 9] to determine the inference results. The

inference rule base of the fuzzy controller is shown in Fig. 7.

u

Fig. 3 Architecture of a fuzzy controller

Fig. 4 Error membership function of a fuzzy controller

Fig. 5 Error variance membership function of a fuzzy controller

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Fig. 6 Output membership function of a fuzzy controller

Fig. 7 Inference rule base of a fuzzy controller

5. Experimental results and discussion

Fig. 8 shows the real hydraulic servo press system used by this

study. This thesis is primarily intended to design a fuzzy controller

and apply it to the electro-hydraulic servo platform. First, the

LabVIEW software was used as the system control core to design the

human-machine interface control panel, as shown in Fig. 9, and the

fuzzy controller in order to achieve precision force control of the

hydraulic cylinder. The relationship between the input and output of

the relief valve is shown in Fig. 10, from which an obvious hysteresis

can be seen. This would make it difficult to achieve accurate force

control.

Fig. 8 Picture of the hydraulic press system

Fig. 9 Human-machine interface control panel

Fig. 10 Characteristic curve of the relief valve

To achieve precision force control, this study used a mixed force

output control structure to achieve precision control. This mixed force

output control structure is mainly uses the relief valve to control the

hydraulic pressure of the hydraulic cylinder. In the meantime, the

servo valve is also used to achieve an accurate flow. Finally, accurate

force output control can be achieved. Fig. 11 is the block diagram of

this force servo control system.

Fig. 11 Architecture of mixed force output control

To investigate the performance strengths and weaknesses of this

experimental platform as applied to various control structures,

performance was compared with a traditional PID controller as below.

In this study, parameters of the traditional PID controller are

determined by using the ZN rule. The values are then slightly

adjusted following a trial and error method. The parameter values are

as follows:

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Pressure relief valve: Kp=0.015 Ki=0.00237 Kd=0.00023

Servo valve: Kp=0.023 Ki=0.01115 Kd=0.00985

However, the membership function of this fuzzy controller is

designed specifically for the system's characteristics with

consideration of the minimum resolution of the sensor as fine-tuned

with the experimental data. The rule base was established according

to empirical rules. The parameters of K1, K2 and K3 are expressed as

follows:

Servo valve: K1=1.25 K2=2.75 K3=0.00425

This experiment compared the controller performance by single

step response, multi-step increase response, multi-step decrease

response, sinusoid response, and repeated force pressure-load

response of the force output. The red curve shown in each diagram is

the PID response curve, the green curve is the fuzzy response curve,

and the black curve is the setting amount.

First, the multi-step force increase response is shown in Fig. 12.

The initial force was set at 500 kgf with an increase of 100kgf for 10

seconds each until 1000 kgf was reached at which it stopped. The

multi-step increase error response is shown in Fig. 13, from which we

can see the error is about ±0.5 kgf. The multi-step force decrease

response curve is shown in Fig. 14. The initial force was set at 950

kgf with a decrease of 100kgf for 10 seconds each until 550 kgf was

reached at which it stopped. The multi-step decrease error response is

shown in Fig. 15, from which we see the error is also about ±0.5 kgf.

The force sinusoid trace response is shown in Fig. 16, and the

force sinusoid trace error response is shown in Fig. 17, from which

we see the trace error is about ±4 kgf. The force single step response

is shown in Fig. 18, and the force single step error response is shown

in Fig. 19. Its error is also about ±0.5 kgf.

In addition, the repeated force pressure-load response is shown in

Fig. 20, and the repeated force pressure-load error response is shown

in Fig. 21. The repeated pressure-load movement flow was set to the

origin position for the hydraulic cylinder when it started. When it

reached the set cycle, to prevent the hydraulic cylinder and the spring

load from excessive colliding, the hydraulic cylinder was set to

descend slowly. The control pressure did not increase until the

hydraulic cylinder touched the spring load. This is done cyclically.

Fig. 12 Multi-step increase response

Fig. 13 Multi-step increase error response

Fig. 14 Multi-step decrease response

Fig. 15 Multi-step decrease error response

Fig. 16 Force sinusoid trace response

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Fig. 17 Force sinusoid trace error response

Fig. 18 Single step response

Fig. 19 Single step error response

Fig. 20 Architecture of the fuzzy controller

Fig. 21 Repeated force pressure-load error response

The variance between each controller can be observed from the

series of response curve diagrams. This thesis compares the

maximum overshoot and the steady-state error by arranging the

curves as shown in Table 1 & 2. For the maximum overshoot, it is

apparent that the performance of the fuzzy controller is better than

that of the PID controller.

Table 1 Comparison of Maximum Overshoot (kgf)

Single step

response

Multi-step

increase

Multi-step

decrease

Sinusoid force

response

Repeated pressure-

load response

PID 12.25 11.8 5.5 9 12.8

Fuzzy 0.5 5.8 4.8 9.5 10.4

Table 2: Comparison of Steady-State Error (kgf)

Single step

response

Multi-step

increase

Multi-step

decrease

Sinusoid force

response

Repeated pressure-

load response

PID <0.5 <0.5 <5 <2 <0.5

Fuzzy <0.5 <0.5 <5 <3 <0.5

6. Conclusion

In the force control system, the most important is point that the

maximum overshoot must not be excessive. Otherwise, this could

cause damage to the object under pressure load. In consideration of

the press tester, an excessive overshoot could cause collapse of the

object under test. From the data in Tables 1 & 2, the fuzzy controller

can be seen to have a smaller result for the excessive overshoot. The

minimum resolution of the load cell used by this experiment is ±1 kgf.

On the whole, the precision force control effect was thus successfully

implemented. The results are briefly listed as follows:

(1) The design of a fuzzy controller has been completed and

applied to an electro-hydraulic servo force control system.

(2) Precision force servo control was achieved by using a mixed

control of the relief valve and the flow servo valve.

(3) Precision control effect was attained through the single step,

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multi-step, sinusoid trace and repeated pressure-load responses

of force.

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