Servo valve efficiency

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Hydraulic Servo Systems

Dynamic Properties and Control

by

Karl-Erik Rydberg

K-E Rydberg Feedbacks in Electro-Hydraulic Servo Systems 7

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Figure 12: A linear valve controlled position servo with velocity feedback

If the bandwidth of the valve is relatively high and threshold and saturation is neglected the velocity feedback will give the effect on the hydraulic resonance frequency and damping as shown in Figure 13.

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Figure 13: A linear position servo with velocity feedback included

From Figure 13 the new resonance frequency and damping ( hv and hv) caused by the velocity feedback can be evaluated as

vfv h hv vfv h

hv K K1

, , where the velocity loop gain is

p qi sav fv

vfv A

K K K

K 1 .

Designing the position control loop for the same amplitude margin as without velocity feedback gives the following relations:

Steady state loop gain without velocity feedback: _f

p qi sa

v K

A K K K

Steady state loop gain with velocity feedback: _f

vfv p

qi sav

vv K

K A K K K

A certain amplitude margin means that K_v _h _h. In this case _h _h _hv _hv, which +11

Karl-Erik Rydberg, Linköping University, Sweden 21

Closed loop stiffness for a position servo with velocity feedback

K s V

K s K

s K

s K K A K S

ce e

t h vfv

h h vfv vv

ce p vfv vv c

1 4

2 1

1 ₂

2 2

K s V A K

A K K K s K K s

K s X S F

ce e

t p ce

p qi sav f vfv h

vfv h h vfv p

L c

1 4 2 1

2 2 2

p qi sav fv

vfv A

K K K

K 1

f vfv p qi sav

vv K

K A K K

K 1

Steady state loop gain [1/s]

For the same amplitude margin, Kvmust have the same value in the system with and without velocity feedback.

Velocity feedback increases the steady state stiffness with the factor K_vfv.

feedback) elocity (without v

v

vv K

K

Karl-Erik Rydberg, Linköping University, Sweden 22

Closed loop stiffness for a position servo with velocity feedback

Without velocity feedback, K_v=20 1/s With velocity feedback, K_vv= 20 1/s, K_vfv= 9.0

hv= 387 rad/s

A_m= 6 dB

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K-E Rydberg Hydraulic Servo Systems – Dynamic Properties and Control

______________________________________________________________________

Linköping November 6, 2014 Revised October 27, 2016 Karl-Erik Rydberg Professor, PhD

Department of Management and Engineering Linköping University

SE-581 83 LINKÖPING ISBN: 978-91-7685-620-8

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Hydraulic Servo Systems – Dynamic Properties and Control

Table of Contents

Hydraulic Servo Systems - Dynamic Properties and Control ... 1

1 Introduction ... 5

1.1 What is a servo? ... 5

1.2 Technology comparisons ... 5

1.3 Capabilities of electro-hydraulic servos ... 7

1.4 Different electro-hydraulic concepts ... 7

1.5 Servo system efficiency ... 9

1.5.1 Servo valve efficiency ... 9

2 Configuration of electro-hydraulic servos ... 12

2.1 Position servo ... 13

2.2 Velocity and force servos ... 13

3 Servo valves and their characteristics ... 15

3.1 Number of lands and ports ... 15

3.2 Types of valve center ... 16

3.2.1 Valve sleeve ... 17

3.3 Examples of electro-hydraulic servo valves ... 18

3.3.1 Type of feedback ... 18

3.3.2 Number of stages ... 19

3.4 General steady state valve characteristics ... 24

3.4.1 Valve Coefficients ... 25

3.5 Critical center four-way valve ... 26

3.5.1 Practical null coefficients for a critical center valve ... 26

3.5.2 Leakage characteristics of a practical critical center four-way valve ... 27

3.5.3 Blocked line pressure sensitivity curve ... 27

3.5.4 Leakage flow curves ... 28

3.5.5 Real flow gain characteristics ... 29

3.6 Open center spool valve ... 30

3.7 Three-way spool valve analysis ... 32

3.8 Dynamic response of servo valves ... 34

4 Position servos with valve-controlled cylinders ... 36

4.1 Asymmetric cylinder ... 36

4.1.1 Example ... 38

Variation in resonance frequency for an asymmetric cylinder with line volumes ... 38

Parameters ... 38

4.2 Valve controlled symmetric cylinder ... 39

4.2.1 Servo system stability and bandwidth ... 42

4.4 Influence from flow forces on valve spools ... 45

4.5 Position servo with mechanical springs at connectors ... 47

4.5.1 Simulation of position servo with mechanical springs ... 48

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5.4 Pump controlled motor with variable displacement ... 58

5.5 Pump controlled symmetric cylinder ... 59

6 Hydraulic systems with complex load dynamics ... 61

6.1 Loads with one degree of freedom ... 62

6.2 Loads with two degrees of freedom ... 64

7 Feedbacks in Electro-Hydraulic Servo Systems ... 75

7.1 Linear valve controlled position servo ... 75

7.1.1 Influence of valve dynamics ... 76

7.1.2 Closed loop stiffness ... 77

7.2 Valve controlled position servo with load pressure feedback ... 78

7.3 Valve controlled angular position servo with acc. feedback ... 79

7.4 Velocity feedback in position control servos ... 81

7.5 Valve controlled velocity servo ... 83

7.6 Proportional valves with integrated position and pressure transducers .... 84

7.7 Electro-hydraulic servo actuators ... 84

7.8 Design examples ... 87

7.9 Summary of servo system design criterions ... 90

7.9.1 Control loop dynamics – possible improvements ... 90

8 Nonlinearities in Hydraulic Servo Systems ... 92

8.1 How to handle nonlinear properties in linear models? ... 92

8.2 Common Nonlinearities in Hydraulic Systems ... 93

8.2.1 Saturation and its effect on system performance ... 93

8.2.2 Dead-band ... 96

8.2.3 Threshold and Hysteresis ... 96

8.3.4 Nonlinear friction ... 97

9 Controller Design for Hydraulic Servo Systems ... 99

9.1 General structure of the controller ... 99

9.2 Feed forward gain for reduction of velocity error in position servos ... 100

9.3 PID Controller ... 101

9.3.1 Proportional gain ... 102

9.3.2 Integral gain ... 102

9.3.3 Derivative gain ... 103

9.3.4 Implementation and tuning of PID-controllers ... 103

9.4 A commercial digital controller ... 104

References ... 106

Appendix 1 ... 107

Design of a linear position servo ... 107

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1 Introduction

When closed-loop hydraulic control systems first began to appear in industry, the applications were generally those in which very high performance was required. While hydraulic servo systems are still heavily used in high-performance applications such as the machine-tool industry, they are beginning to gain wide acceptance in a variety of industries. Examples are material handling, mobile equipment, plastics, steel plants, mining, oil exploration and automotive testing.

Closed loop servo drive technology is increasingly becoming the norm in machine automation, where the operators are demanding greater precision, faster operation and simpler adjustment. There is also an expectation that the price of increasing the level of automation should be contained within acceptable limits.

1.1 What is a servo?

In its simplest form a servo or a servomechanism is a control system, which measures its own output and forces the output to quickly and accurately follow a command signal, se Figure 1-1. In this way, the effect of anomalies in the control device itself and in the load can be minimised as well as the influence of external disturbances. A servomechanism can be designed to control almost any physical quantities, e.g. motion, force, pressure, temperature, electrical voltage or current.

K-E Rydberg Hydraulic Servo Systems 1

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Hydraulic Servo Systems 2 Theory and Applications

1. Introduction

When closed-loop hydraulic control systems first began to appear in industry, the applications were generally those in which very high performance was required. While hydraulic servo systems are still heavily used in high-performance applications such as the machine-tool industry, they are beginning to gain wide acceptance in a variety of industries. Examples are material handling, mobile equipment, plastics, steel plants, mining, oil exploration and automotive testing.

Closed loop servo drive technology is increasingly becoming the norm in machine automation, where the operators are demanding greater precision, faster operation and simpler adjustment. There is also an expectation that the price of increasing the level of automation should be contained within acceptable limits.

What is a servo?

In its simplest form a servo or a servomechanism is a control system which measures its own output and forces the output to quickly and accurately follow a command signal, se Figure 1-1. In this way, the effect of anomalies in the control device itself and in the load can be minimised as well as the influence of external disturbances. A servomechanism can be designed to control almost any physical quantities, e.g. motion, force, pressure, temperature, electrical voltage or current.

Power source

Actuator Servo

electronics Mechanical

load Power

modulator

Feedback transducer

Motion Command

signal !

- +

Figure 1-1: Basic servomechanism

Technology comparisons

The potential for alternative technologies should be assessed in the light of the well- known capabilities of electro-pneumatic and electro-mechanical servos. High performance actuation system is characterised by wide bandwidth frequency response, low resolution and high stiffness. Additional requirements may include demanding duty

Figure 1-1: Basic servomechanism

1.2 Technology comparisons

The potential for alternative technologies should be assessed in the light of the well- known capabilities of electro-pneumatic and electro-mechanical servos. High performance actuation system is characterised by wide bandwidth frequency response, low resolution and high stiffness. Additional requirements may include demanding duty

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special interests in aerospace applications. The most important selection criteria can be summarised as follows:

• Customer performance

• Cost

• Size and weight

• Duty cycle

• Environment: vibration, shock, temperature, etc.

The performance available with electro-hydraulic servos encompasses every industrial and aerospace application. As indicated in Figure 1-2 electro-hydraulic servos will cover applications with higher performance then electro-mechanical and electro- pneumatic servos. This is easily explained because electro-hydraulic servo systems have been designed and developed to accomplish essentially every task that has appeared.

______________________________________________________________________

cycles and minimisation of size and weight. The last mentioned requirements are of special interests in aerospace applications. The most important selection criteria can be summarised as follows:

• Customer performance

• Cost

• Size and weight

• Duty cycle

• Environment: vibration, shock, temperature, etc.

The performance available with electro-hydraulic servos encompasses every industrial and aerospace application. As indicated in Figure 1-2 electro-hydraulic servos will cover applications with higher performance then electro-mechanical and electro- pneumatic servos. This is easily explained because electro-hydraulic servo systems have been designed and developed to accomplish essentially every task that has appeared.

0.1

Actuated load dynamics [Hz]

Actuation power [kW]

1 10 100

5 50

Electro-hydraulic actuation limit

Electro-pneumatic actuation limit

Electro-mechanical actuation limit

Figure 1-2: Typical performance characteristics for different types of servo actuators

The above figure indicates that applications in the lower range of power and dynamic response may also be satisfied with electro-pneumatic servos. However, the best choice is always determined by considerations, such as those selection criteria discussed above.

In most applications the aspect of cost is generally dominant.

Experience indicates that electro-mechanical or electro-pneumatic actuators tends to have lower cost than electro-hydraulic actuators in the low performance range. This cost difference rapidly dissipates for applications that require high power and/or high dynamic response.

In comparing costs, one must be careful to consider the total cost of entire servo- actuation system. The higher cost of an electro-hydraulic servo often results from the power conversion equipment needed to provide high pressure fluid with low contamination level. It is also clear that the relative cost of an alternative actuation system designed for a specific application will depend, primarily, on the actuation power level.

Figure 1-2: Typical performance characteristics for different types of servo actuators

The above figure indicates that applications in the lower range of power and dynamic response may also be satisfied with electro-pneumatic servos. However, the best choice is always determined by considerations, such as those selection criteria discussed above. In most applications the aspect of cost is generally dominant.

Experience indicates that electro-mechanical or electro-pneumatic actuator tends to have lower cost than electro-hydraulic actuators in the low performance range. This cost difference rapidly dissipates for applications that require high power and/or high dynamic response.

In comparing costs, one must be careful to consider the total cost of entire servo- actuation system. The higher cost of an electro-hydraulic servo often results from the power conversion equipment needed to provide high-pressure fluid with low

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contamination level. It is also clear that the relative cost of an alternative actuation system designed for a specific application will depend, primarily, on the actuation power level.

1.3 Capabilities of electro-hydraulic servos

When rapid and precise control of sizeable loads is required an electro-hydraulic servo is often the best approach to the problem. Generally speaking, the hydraulic servo actuator provides fast response, high force and short stroke characteristics. The main advantages of hydraulic components are.

• Easy and accurate control of work table position and velocity

• Good stiffness characteristics

• Zero backlash

• Rapid response to change in speed or direction

• Low rate of wear

There are several significant advantages of hydraulic servo drives over electric motor drives:

♦ Hydraulic drives have substantially higher power to weight ratios resulting in higher machine frame resonant frequencies for a given power level.

♦ Hydraulic actuators are stiffer than electric drives, resulting in higher loop gain capability, greater accuracy and better frequency response.

♦ Hydraulic servos give smoother performance at low speeds and have a wide speed range without special control circuits.

♦ Hydraulic systems are to a great extent self-cooling and can be operated in stall condition indefinitely without damage.

♦ Both hydraulic and electric drives are very reliable provided that maintenance is followed.

♦ Hydraulic servos are usually less expensive for system above several horsepower, especially if the hydraulic power supply is shared between several actuators.

1.4 Different electro-hydraulic concepts

In electro-hydraulic applications different concepts will be used in order to meet the actual requirements. One example of a system where the weight is of great importance is an Electro Hydraulic Actuator (EHA) to be used in aircraft applications. A typical EHA-concept is shown in Figure 1-3. This EHA consists of an electric motor, a speed controlled pump with low displacement, a cylinder and an accumulator used as a tank.

In a real application there is also a need for additional functions, such as by-pass

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as a consequence. Therefore, the losses and thereby the temperature of the fluid is of great importance in this application.

______________________________________________________________________

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Figure 1-3: Electro Hydraulic Actuator with electric motor control

Other more conventional concepts are a pump or a valve controlled actuator, as shown in Figure 1-4. The main difference between those systems is that the pump controlled system is one separate unit supplied by an electric wire to the motor (the same as for the system in Figure 1-3) and the valve controlled actuator is supplied by a constant pressure hydraulic line. In the last case the EHA is feed by a central hydraulic supply unit.

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Figure 1-4: Electro Hydraulic Actuators with pump and valve control respectively

Comparing the electric motor controlled EHA in Figure 1-3 and the pump controlled EHA in Figure 1-4, the overall efficiency will be better in the first case, where the pump shaft speed (np) is controlled. The efficiency curves for similar systems are shown in Figure 1-5. Maximum pump flow is the same in both cases and pressure drop over the directional valve is included as losses.

Looking at the variations in overall efficiency it is clear that speed control has a favour over displacement control, especially in the power range up to 50% of maximum power.

However, there are other problems to overcome in the pump speed control concept. For example, the amplitude of the flow pulsations from the pump must be very low at low shaft speeds in order to avoid problems with low frequency vibrations in the system.

This, require a special design of the pump. One suitable pump design is the inner gear concept. In such a pump both kinematic and compressibility dependent flow pulsations are extremely low compared to piston pumps.

Figure 1-3: Electro Hydraulic Actuator with electric motor control

Other more conventional concepts are a pump or a valve controlled actuator, as shown in Figure 1-4. The main difference between those systems is that the pump controlled system is one separate unit supplied by an electric wire to the motor (the same as for the system in Figure 1-3) and the valve controlled actuator is supplied by a constant pressure hydraulic line. In the last case the, EHA is feed by a central hydraulic supply unit.

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Figure 1-3: Electro Hydraulic Actuator with electric motor control

Other more conventional concepts are a pump or a valve controlled actuator, as shown in Figure 1-4. The main difference between those systems is that the pump controlled system is one separate unit supplied by an electric wire to the motor (the same as for the system in Figure 1-3) and the valve controlled actuator is supplied by a constant pressure hydraulic line. In the last case the EHA is feed by a central hydraulic supply unit.

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Figure 1-4: Electro Hydraulic Actuators with pump and valve control respectively

Looking at the variations in overall efficiency it is clear that speed control has a favour over displacement control, especially in the power range up to 50% of maximum power.

However, there are other problems to overcome in the pump speed control concept. For example, the amplitude of the flow pulsations from the pump must be very low at low shaft speeds in order to avoid problems with low frequency vibrations in the system.

This, require a special design of the pump. One suitable pump design is the inner gear concept. In such a pump both kinematic and compressibility dependent flow pulsations are extremely low compared to piston pumps.

Figure 1-4: Electro Hydraulic Actuators with pump and valve control respectively

Looking at the variations in overall efficiency it is clear that speed control has a favour over displacement control, especially in the power range up to 50% of maximum power. However, there are other problems to overcome in the pump speed control concept. For example, the amplitude of the flow pulsations from the pump must be very low at low shaft speeds in order to avoid problems with low frequency vibrations in the system. This, require a special design of the pump. One suitable pump design is the inner gear concept. In such a pump both kinematic and compressibility dependent flow pulsations are extremely low compared to piston pumps.

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Pump speed [rpm]

Overall efficiency [-]

0 1000 2000 3000

0.6 0.7 0.8 0.9 1.0

Pressure difference: 15 MPa Pump disp. setting: 1.0

0.5

0.6 0.7 0.8 0.9 1.0

Pump displacement setting [-]

0 0.2 0.4 0.6 0.8 1

Pressure difference: 15 MPa Pump shaft speed: 1500 rpm

Figure 1-5: Overall efficiencies with pump speed control and displacement setting control

2. Configuration of an electro-hydraulic servo

The basic elements of an electro-hydraulic servo is shown in Figure 2-1. The output of the servo is measured with a transducer device to convert it to an electric signal. This feedback signal is compared with the command signal. The resulting error signal is then amplified by the regulator and the electric power amplifier and then used as an input control signal to the servo valve. The servo valve controls the fluid flow to the actuator in proportion to the drive current from the amplifier. The actuator then forces the load to move. Thus, a change in the command signal generates an error signal, which causes the load to move in an attempt to zero the error signal. If the amplifier gain is high, the output will vary rapidly and accurately following the command signal.

Actuator Servo ampl.

and regulator

Mechanical load Servo

valve

Feedback transducer

Output

Command signal

-

+ ! ^signal^Error !

External disturbances

+ -

Figure 2-1: Components in an electro-hydraulic servomechanism

External disturbances (forces or torque) can cause the load to move without any changes in the command signal. In order to offset the disturbance input an actuator output is needed in the opposite direction (see Figure 2-1). To provide this opposing output a finite error signal is required. The magnitude of the required error signal is minimised if the amplifier gain is high. Ideally, the amplifier gain would be set high enough that the accuracy of the servo becomes dependent only upon the accuracy of the transducer

Figure 1-5: Overall efficiencies with pump speed control and displacement setting control

1.5 Servo system efficiency

The overall efficiency of a servo system depends upon the system configuration, which has been demonstrated in Chapter 1.4. However, the criteria for selection of system concept to a specific application belongs to the system power level, system bandwidth, control accuracy, number of axis to control and the installation conditions.

In general, it’s a well-known fact that pump control is much more energy efficient than valve control. A speed or displacement controlled pump has much lower response than a servo valve at the same nominal power. Especially at high power (> 50 kW) pump control will drastically reduce the bandwidth of the system compared to the dynamic performance of a servo valve. Therefore, for servo systems with requirements on high response and accurate control, valve controlled actuators are the best choice. Also, if several axes have to be controlled simultaneously, valve control is less costly instead of using one pump for each axis.

1.5.1 Servo valve efficiency

The use of servo valves is not energy efficient, because they introduce quite heavy throttling losses into the system. This can be illustrated by looking at valve specification. The nominal flow capacity of a servo valve is specified at a total valve pressure drop of 70 bar. Assuming a supply pressure of 210 bar gives that only the valve losses represents 33% of the input power at nominal flow conditions and only 67% remains for load actuations.

The most simple way to supply a servo valve is to use a fixed displacement pump,

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Servo valve efficiency

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p q

!

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Variable pump

q_L = load flow

p_L = load pressure q_s = supply flow

P_s = supply pressure P_s = constant

Karl-Erik Rydberg, IEI/Linköping University 8

Servo valve efficiency

Fixed displacement pump

( s L)

v q

L C w x p p

q = " " !

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Load flow Supply flow (max valve flow)

Max valve efficiency acc. to pL:

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sv p p

p d d

3 2

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sv [-]

0.385

Figure 1-6: Servo valve with supply unit and corresponding valve efficiency

The servo valve efficiency is defined as load power over supply power, ηsv=q_L⋅ pL

q_s⋅ ps

(1-1) The flow equation for a four port symmetric and zero-lapped valve is expressed as,

q_L = Cq⋅ w ⋅ xv0

1

δ( p_s− pL) (1-2) The constant pump flow will have the same value as max valve flow, which gives,

q_s = Cq⋅ w ⋅ xv0

1

δ p_s (1-3)

The above equations make it possible to express the valve efficiency as a function of the load pressure pL and ps as a parameter,

ηsv= p_s− p_L⋅ p_L p_s ⋅ ps

(1-4) The load pressure that gives max efficiency is found from the efficiency/pressure derivative as,

d(ηsv)

d( p_L)= 0 ⇒ pL=2

3p_s (1-5)

From Figure 1-6 it can be noticed that max valve efficiency is 38,5%, which can’t be acceptable for a modern servo system.

In today’s servo system the supply unit is commonly a variable displacement pump equipped with a constant pressure controller, see Figure 1-7. The pump line is also supplied by an hydraulic accumulator. In a servo system it is important to keep the supply pressure constant, because variations in ps has an impact system response. The accumulator helps the pump to control its pressure when the servo valve are opened or closed very fast.

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Servo valve efficiency

s s

L sv L

p q

!

= !

"

Fixed pump

Variable pump

q_L = load flow

p_L = load pressure q_s = supply flow

P_s = supply pressure P_s = constant

Figure 1-6: Servo valve supplied by a variable pressure controlled pump and accumulator

The use of a variable displacement pump has the advantage that the pump flow always is adjusted to fit the load flow, qs = qL. According to equation (1-1), the servo valve efficiency can be expressed as,

ηsv= p_L

p_s (1-6)

Nominal load flow gives a valve efficiency of η^sv = 0,67.

In practice, the valve efficiency will be lower than the above figures, because of the fact that the valve leakage flow has been neglected. For single stage servo valve (zero- lapped) the leakage flow loss is about 2 % of nominal flow and for a two or three stage valve up to 5 %. Under-lapped valves can have much higher losses.

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2 Configuration of electro-hydraulic servos

The basic elements of an electro-hydraulic servo are shown in Figure 2-1. The output of the servo is measured with a transducer device to convert it to an electric signal. This feedback signal is compared with the command signal. The resulting error signal is then amplified by the regulator and the electric power amplifier and then used as an input control signal to the servo valve. The servo valve controls the fluid flow to the actuator in proportion to the drive current from the amplifier. The actuator then forces the load to move. Thus, a change in the command signal generates an error signal, which causes the load to move in an attempt to zero the error signal. If the amplifier gain is high, the output will vary rapidly and accurately following the command signal.

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0.5

Pump speed [rpm]

0 1000 2000 3000

0.6 0.7 0.8 0.9 1.0

Pressure difference: 15 MPa Pump disp. setting: 1.0

0.5

0.6 0.7 0.8 0.9 1.0

Pump displacement setting [-]

0 0.2 0.4 0.6 0.8 1

Pressure difference: 15 MPa Pump shaft speed: 1500 rpm

Figure 1-5: Overall efficiencies with pump speed control and displacement setting control

2. Configuration of an electro-hydraulic servo

The basic elements of an electro-hydraulic servo is shown in Figure 2-1. The output of the servo is measured with a transducer device to convert it to an electric signal. This feedback signal is compared with the command signal. The resulting error signal is then amplified by the regulator and the electric power amplifier and then used as an input control signal to the servo valve. The servo valve controls the fluid flow to the actuator in proportion to the drive current from the amplifier. The actuator then forces the load to move. Thus, a change in the command signal generates an error signal, which causes the load to move in an attempt to zero the error signal. If the amplifier gain is high, the output will vary rapidly and accurately following the command signal.

Actuator Servo ampl.

and regulator

Mechanical load Servo

valve

Feedback transducer

Output Command

signal

-

+ ! ^signal^Error !

External disturbances

+ -

Figure 2-1: Components in an electro-hydraulic servomechanism

External disturbances (forces or torque) can cause the load to move without any changes in the command signal. In order to offset the disturbance input an actuator output is needed in the opposite direction (see Figure 2-1). To provide this opposing output a finite error signal is required. The magnitude of the required error signal is minimised if the amplifier gain is high. Ideally, the amplifier gain would be set high enough that the accuracy of the servo becomes dependent only upon the accuracy of the transducer itself. However, since the control loop gain is proportional to the amplifier gain, this

Figure 2-1: Components in an electro-hydraulic servomechanism

External disturbances (forces or torque) can cause the load to move without any changes in the command signal. In order to offset the disturbance input an actuator output is needed in the opposite direction (see Figure 2-1). To provide this opposing output a finite error signal is required. The magnitude of the required error signal is minimised if the amplifier gain is high. Ideally, the amplifier gain would be set high enough that the accuracy of the servo becomes dependent only upon the accuracy of the transducer itself. However, since the control loop gain is proportional to the amplifier gain, this gain is limited by stability considerations. In some applications, stability may be critical enough that the desired performance is not possible to reach.

The three common types of electro-hydraulic servos are:

• Position servo (linear or angular)

• Velocity or speed servo (linear or angular)

• Force or torque servo

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2.1 Position servo

Probably the most basic closed-loop control system is a position servo. A schematic diagram of a complete position servo is shown in Figure 2-2.

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gain is limited by stability considerations. In some applications, stability may be critical enough that the desired performance is not possible to reach.

The three common types of electro-hydraulic servos are:

• Position servo (linear or angular)

• Velocity or speed servo (linear or angular)

• Force or torque servo Position servo

Probably the most basic closed-loop control system is a position servo. A schematic diagram of a complete position servo is shown in Figure 2-2.

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Figure 2-2: Symbol circuit of a position servo

In Figure 2-2 the actuator or load position is measured by a position transducer, which gives an electric signal (uf) in voltage as an output. The servo amplifier compares the command signal (uc) in voltage with the feedback signal (uf). Then, the resulting error signal are gained the with the factor Ksa. The output current signal (i) from the amplifier will control the servo valve.

Velocity and force servos

Another common types of closed loop control systems are velocity (speed) and force (torque) servos. The configuration of these systems are identical to the position servo depicted in Figure 2-2, expect that the transducer measures velocity or force instead of position and that the controller may have different characteristics. Figure 2-3 shows both a speed and a force servo. It is notable that the same type of servo valve can be used in all of these applications. As indicated in Figure 2-3, velocity or speed servos are more commonly used to control the shaft speed of an hydraulic motor than to control linear velocity.

In the velocity servo the servo amplifier is of integrating type, as shown in Figure 2-3.

Compared to a position servo the velocity servo has no integration between servo valve displacement and the output velocity. Therefore, the integration in a velocity servo is generally provided electronically in the amplifier. The integration is desirable to minimise static errors and to maintain stability.

Figure 2-2: Symbol circuit of a position servo

In Figure 2-2 the actuator or load position is measured by a position transducer, which gives an electric signal (uf) in voltage as an output. The servo amplifier compares the command signal (uc) in voltage with the feedback signal (uf). Then, the resulting error signal is gained with the factor Ksa. The output current signal (i) from the amplifier will control the servo valve.

2.2 Velocity and force servos

Another common types of closed loop control systems are velocity (speed) and force (torque) servos. The configuration of these systems are identical to the position servo depicted in Figure 2-2, expect that the transducer measures velocity or force instead of position and that the controller may have different characteristics. Figure 2-3 shows both a speed and a force servo. It is notable that the same type of servo valve can be used in all of these applications. As indicated in Figure 2-3, velocity or speed servos are more commonly used to control the shaft speed of a hydraulic motor than to control linear velocity.

In the velocity servo the servo amplifier is of integrating type, as shown in Figure 2-3.

Compared to a position servo the velocity servo has no integration between servo valve displacement and the output velocity. Therefore, the integration in a velocity servo is generally provided electronically in the amplifier. The integration is desirable to minimise static errors and to maintain stability.

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Figure 2-3: Symbol circuit of a speed and a force servo

In a real force servo the transducer measures the output force and this signal is fed back to the amplifier. A more simpler way to implement a force servo is to use the load pressure in the actuator as a feedback signal. This is quite close to a true force servo except from the friction force in the actuator.

3. Servo valves and their characteristics

The heart of the hydraulic servo system is the servo valve and it is essential that its characteristics be thoroughly understood. A servo valve is a component which work as an interface between an electrical (or mechanical) input signal and the hydraulic power represented by the product of flow and pressure. Depending of the application there are different types of servo valves to use.

3.1 Number of lands and ports

The most widely used valve is the sliding valve employing spool type construction.

Typical spool valve configurations are shown in Figure 3-1. As explained in the figure, spool valves can be classified by the numbers of ways the flow can enter and leave the valve and the number of lands. Because all valves require a supply, a return and at least one line to the load, valves are either of three-port or four-port type.

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Figure 3-1: Typical confuguration of the main stage of hydraulic servo valves Figure 2-3: Symbol circuit of a speed and a force servo

In a real force servo the transducer measures the output force and this signal is fed back to the amplifier. A more simple way to implement a force servo is to use the load pressure in the actuator as a feedback signal. This is quite close to a true force servo except from the friction force in the actuator.