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High Speed Solenoid Valves in Pneumatic Servo Applications

Željko Šitum, Tihomir Žilić and Mario Essert Faculty of Mechanical Engineering and Naval Architecture

Department of Robotics and Automation of Production Systems, Zagreb, Croatia

Abstract— Pneumatic driving systems are mainly used in industrial applications where the moving parts are usually fixed by the mechanical stops. For flexible and precise positioning tasks of pneumatic drives relatively expensive proportional valves have been implemented. In order to develop cheaper pneumatic servo systems, the employment of low-cost on/off solenoid valves have received considerable attention. But, the traditional technologies used in on/off solenoid valves manifest different problems caused by the electric and thermal effects, inertial forces of their mechanical parts and friction phenomenon. However, it seems that innovative technology used in fast switching solenoid valves gives better possibilities in control of pneumatic systems due to their negligible internal friction and modular architecture. In this paper a control method based on pulse-width-modulation algorithm for position control of a pneumatic actuator with high-speed solenoid valve is considered and experimentally verified.

I. INTRODUCTION Technological improvements and innovations in

pneumatic components and digital signal processing have made possible some new modalities in traditional pneumatic systems applications. Over the years, pneumatic actuators are extensively used in industrial automation only in the open-loop control mode i.e. for ‘pick-and-place’ positioning problems [1], with simple on/off solenoid valves to control their motion. This sequence control technique is widely used in pneumatic driving systems for relatively simple tasks. Namely, in manufacturing processes there exist a great number of pneumatic systems where the process operation is based on the use of such simple valves of the on/off type, which are operated electronically or even in some cases adjusted manually. For less demanding applications this form of control is fairly satisfactory and their employment will be continued in the future. Increasing requirements for precise positioning when the pneumatic systems are used for robotic manipulators and mechatronics applications make demands on more effectively control techniques and closed-loop control strategies. Accurate position control of pneumatic driving systems is generally a difficult problem because of the fundamental problems associated with the air compressibility and significant friction effects of moving parts. During the past two decades a notable amount of research has been realized to development of position-control systems for pneumatic drives. In most of them relatively expensive proportional directional control valves are used [2]–[6]. Proportional solenoid valves place a certain position between full open or closed in terms of

an electric signal from control device. In order to develop an inexpensive pneumatic actuator system a considerable research has been addressed to implement inexpensive on/off solenoid valves rather then proportional or servo valves. Binary-position solenoid valves put their open or closed position in accordance with an electric on/off control signal. The use of pulse-width modulation (PWM) technique in pneumatic systems makes possible the use of inexpensive, fast switching on/off valves in closed-loop control applications, which are implemented to digital systems by using real-time programming. Such systems can provide actuation characteristics at a significantly lower cost compared to the electromechanical actuation systems. However, precise control is difficult to achieve because of significant delay time of the solenoid valves and their discrete on/off characteristic [7]. Also, it is not easy to derive and to use an analytical dynamic model of pneumatic system, which uses a PWM control signal for switching solenoid valves. From the viewpoint of engineering practice, the standard solenoid valves realized by traditional technologies have several imperfections due to the inertial forces of their moving parts and thermal effects of electromagnet caused by the electric current.

In this paper instead of traditional solenoid valves we have been implemented a high-speed solenoid valve, which uses some new construction conceptions [8]. The mass reduction of the moving parts and the using of material with energetic high efficiency have resulted with a negligible internal friction during opening and closing the valve. This innovative solution enables fast valve switching, which is crucial in the implementation of PWM based control method. The construction of the valves is based on the modular architecture, allowing the assembly of several outlets in a single body. In this study the implementation of a pulse-width modulated, computer controlled, closed-loop electro-pneumatic drive, which consists of a rodless cylinder and fast switching on/off valve is investigated and experimentally verified.

II. DESCRIPTION OF THE EXPERIMENTAL DEVICES In order to study different control methodologies for

pneumatic servodrives the experimental system with a cylindrical actuator and different control components has been made. The laboratory equipment is schematically shown in Fig.1, while photograph is shown in Fig. 2. The actuator is a double acting pneumatic rodless cylinder (SMC CDY1S15H-500) with stroke length 500 mm and diameter 15 mm. The position of the piston is measured by the horizontal linear potentiometer (Festo MLO-POT-500-TLF), which is attached to the actuator. The linear

Proceedings of the 15th Mediterranean Conference onControl & Automation, July 27 - 29, 2007, Athens - Greece

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motion of the piston is controlled with a high-speed on/off valve (Matrix valve 758 series H-X758-8-E-2-C-3-24), where two valve’s outlets are connected to each cylinder's chamber. Three pressure transducers (SMC ISE4-01-26) are added to measure the pressures in each cylinder’s chamber and also the pressure of the air supply. The control software is coded in "C" language and the feedback control algorithms are implemented on a Pentium based PC with PCL-812PG acquisition card. All signals from the process are reading to a microcomputer via a 12-bit A/D converter. The calculated control signals from the microcomputer are sending via digital outputs and 8-bit ULN 2803 darlington driver to the utilized valve.

Figure 1. Schematic diagram of the control system

1–Linear potentiometer 2–Pneumatic rodless cylinder, 3–Pressure sensor, 4–High-speed valve, 5–Filter-regulator unit, 6–Darlington driver, 7–Control computer with data-acquisition card, 8–Proportional valve, 9–On/off solenoid valves, 10–Proportional pressure valves

Figure 2. The photo of the laboratory equipment

Then the air mass flow rate through the valve can be regulated and the position of the cylinder can be controlled.

The experimental setup also includes proportional directional control valve (Festo MPYE-5 1/8 HF-010B), then two proportional pressure regulator valves (SMC VY1A00-M5), and two on/off solenoid valves (SMC EVT307-5D0-01F). In this paper the control of pneumatic drive using these valves is not considered, although we have done an extensive research with them [9]–[13].

A. Fast switching valve technology The solenoid valves are basic elements of many

pneumatic systems in industry, and they will keep their important role also in the near future. But, due to their discontinuous on/off nature the accuracy of system falls down when they are used in precise position or force control tasks. However, technological improvements of solenoid valves reduce the difference between the overall characteristics of solenoid on/off valves and proportional directional control valves, so that on/off valves can be a suitable solution in many operations, which have traditionally been operated by proportional valves.

The innovative solution in high-speed on/off valve with O-ring and shutter has resulted with multiple reducing the mass of the moving elements and allows the valve to run in a speed-up mode. When the valve is energized, the coil attracts the steel shutter and opens the way from inlet to outlet and the valve in ‘on’ state. When the current is cut from the coil, the shutter is released and the O-ring returns the shutter and blocks the air passing through the port. The valve is then in ‘off’ state. The investigated model of the valve manufactured by Matrix S.p.A. of Italy was a 758 series, 3-way 2-state (3/2) type with 8 outlets, Fig. 3. By using such valve with multiple outputs the number of active valves for a certain technical project realization can be minimized, which reduces the cost and improves reliability of the system.

III. PNEUMATIC SERVODRIVE CONTROL SYSTEM The method chosen for controlling the passing flow rate

through the valve was PWM control method as investigated by a number of researchers [14]–[19], among others. Also, a number of various configurations of pneumatic circuit design in the research have been used. The most common experimental systems use two 3-way 2-state (3/2) solenoid valves or one 5/3 solenoid valve, but in some cases also a rotary control valve [20]–[21] or several 2-position solenoid valves [22]–[23] have been used.

8 outletsO-ring

ShutterSpring

2

2

Coil

1

Figure 3. High-speed on/off valve with eight outlets


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By using PWM control technique it is possible to obtain an almost proportional flow rate with on/off valves. In order to obtain a smoother control or a ‘continuous like’ signal a higher PWM frequency is preferable, i.e. period of the carrier wave should be as small as possible. In distinction from DC motor control drives, where PWM control technique was originally developed and in which process, practical frequencies can be over 1000 Hz, in pneumatic systems the utilized frequencies are significantly reduced. The real valves have finite opening and closing times, which puts a limit on the smallest modulation period. The dead-time of the used valve is important in the design of PWM control algorithm. Namely, the valve failed to open if the duration time when the valve is energized is shorter then the dead-time of the valve.

When on/off solenoid valves are used to position control of the pneumatic drive, the control signal should be transmitted from the microcomputer into individual pulsing of each valve. The desired PWM signal can be realized by comparing the continuous control signal and a high-frequency carrier wave [15], as is illustrated in Fig. 4. The carrier wave is usually a high-frequency tooth-wave with the period T . The frequency and amplitude of the carrier wave must change faster then those of the continuous signal.

For the case when the rise and fall times of the pulses are negligible, then the mathematical description of the PWM signal can be given by the following relation:

<≥

=)()(,0)()(,

)(tVtVtVtVU

tUdc

dcpPWM . (1)

with [ ] TVTjttV pd /)1()( −−= , for TjtTj <≤− )1(

nj ...,2,1= , where j is j -th modulation period. If we suppose that the solenoid valve is an ideal relay, then the valve will be in the state "on" when the control PWM signal has the value Up, and in the state "off" when the PWM signal is set to zero. However, in a real case the valve can be presented like an ideal relay with dead time [24], Fig. 5.

Figure 4. Principle of the PWM signal realization

U

mT

maxy

maxy−

vy∆y

Figure 5. The real valve presented as an ideal relay with dead time

Thus, in a real case if the period of PWM signal pjT is

shorter then the valve's dead time mT , i.e. mpj TT < , then there is not air mass flow through the valve. And also, if the period of PWM signal pjT is greater then the difference between period T and the time for valve switching )( sm TT + , i.e. )( smpj TTTT +−> , then the valve stays open till next cycle. This reasoning may be written as follows:

+−≥

+−<<

≤

=

)(,

)(,)(,0

)(

max

maxmax

smp

smpm

mp

TTTTy

TTTTTU

kTUy

TT

kTy (2)

The ratio of the ‘on’ state pjT to that of the j -th modulation period T is defined as the duty ratio (or pulse width ratio) τ .

0 10 20 30 40 50 60 70 80 90 1000

1

2

3

4

5

6

p (

Pa/

1e5

)

24 V20 V15 V13 V12 V

0 2 4 6 8 10 12 14 16 18 200

0.2

0.4

0.6

0.8

1

p (

Pa

/1e

5)

0 10 20 30 40 50 60 70 80 90 1000

1

t (ms)

'on' state

'off' statecont

rol c

omm

and

24 V20 V15 V13 V12 V

Figure 6. The reaction of the valve for different voltage supply


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0 20 40 60 80 100 120 140 160 180 2000

1

2

3

4

5

6p

(P

a/1e

5)

pApB

0 20 40 60 80 100 120 140 160 180 2000

20

40

60

80

100

x (m

m)

0 20 40 60 80 100 120 140 160 180 200

-1

0

1

t (ms)

out1-ON, out2-OFF

out1-OFF, out2-ON

out1-ON, out2-OFF

out1-OFF, out2-ON

cont

rol c

omm

and

Figure 7. The reaction of the valve for alternatively control signal

The careful attention in a realization of the control algorithm should be given to issues such as frequency of carrier wave and the open/close delay time of the valve.

From the experimentally obtained results shown in the Fig. 6, it can be seen that in the case when the coil is energized and the port is opened, then after approximately 3 milliseconds the pressure in the cylinder chamber (or precisely on the pressure sensor) started to grow. The manufacturer claims that the port is opened after the coil is charged in time less than 1 milliseconds. However, because of relatively long connection lines between the actuator and the valve, the pressure change in the chambers has an additional delay. Under normal operating conditions in industry, equipment design must ensure the required voltage level within reasonably close tolerances (here 24 VDC). However, in the case of smaller values of the power supply, the pressure response time will be significantly higher, and for the voltage supply below 12 V the valve reaction fail to appear. For practical applications of solenoid valves a crucial point is the time necessary for rising or falling of the pressures in the cylinder chambers after the valve is energized or deenergized. Due to the air compressibility and friction effects small pressure differences in cylinder chambers will not be able to achieve the actuator motion.

It can be seen from Fig. 7 that the cylinder will begin to move when the actuating forces, as a consequence of the pressure difference, overcome the reaction forces of inertia and friction effects. In the case of fast valve switching, due to a significant delay time for charging and discharging process of the cylinder chamber, the differential pressure is building up insufficiently fast according to the pulse width ratio, and can not establish a required force to move the piston. As a circumstance, the actuator failing to respond and the motion will be unrealized. The pressure responses in the cylinder chambers for the control signal relevant to the two valve

0 10 20 30 40 50 60 70 80 90 1000

2

4

p (

Pa

/1e

5)

0 10 20 30 40 50 60 70 80 90 1000

2

4

p (

Pa/

1e5

)

0 10 20 30 40 50 60 70 80 90 1000

2

4

t (ms)

p (

Pa

/1e

5) f = 50 Hz

f = 25 Hz

f = 10 Hz

Figure 8. Pressure output of the cylinder chamber for different frequency of PWM signal

outputs and the cylinder piston displacement x are shown in Fig. 7.

In order to choose appropriate parameter values for generating PWM signal, the frequency of the workable carrier wave should be experimentally identified. In Fig. 8 the pressure output of the cylinder chamber for different frequency of PWM signal is shown. Based upon recommendation from the literature [24], that the period of the carrier wave should be at least 10 times greater than the valve’s dead time it follows that the frequency of the PWM signal should be less than 33 Hz. However, from Fig. 8 it can be seen that the frequency of the PWM signal which is greater than 25 Hz is not appropriate, because of the delay time for opening and closing the valve it is unable to obtain the sufficient pressure difference in the cylinder chambers for the actuator motion. In order to achieve a satisfactory performance of the control system, one must find a suitable switching time of the valve and pulse width ratio. If the period of the carrier wave is too large then the system output tent to be stepwise, and if the period of the carrier wave is too small then the system output is too slow and even the motion of the actuator can fail to appear.

IV. EXPERIMENTAL RESULTS When binary-position solenoid valves are used for

pneumatic actuator positioning tasks, it is well known that the simple on/off control method (bang-bang control) gives an unstable output of the process. Due to time delay in the valve response in the actuator output there is undamped oscillation about desired position with a period and amplitude which depend of the valve switching, Fig. 9.

By using a PC the pulse-width modulated control signal has been generated in a real-time programming process and directly sent to the digital outputs, thus removing the D/A converter card. The control algorithm has been realized with software by comparing a generated periodic toot-wave as the carrier wave and a continuous signal of the control error. The carrier wave frequency of 17 Hz has been applied. The sample time was 1 ms.

The experimental measured response of the pneumatic cylinder for square-wave reference signal is shown in Fig.


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10. The figure shows the well-damped response of the pneumatic actuator on the control signal. It can be seen that the control method based on PWM control algorithm provides a relatively good positional accuracy with fast output response. The absolute value of steady-state control error is less than 1 mm for the rightward motion of the cylinder, and some larger values are obtained for leftward motion, probably due to the larger control volume of the chamber as a consequence of the cylinder construction. The ITSE criterion defined as ∫

Tdtte

02 , with the time

window T=10 s, is used for the comparison of three different control components (high-speed valve, on/off valves, proportional valve) and the performance index is shown in Fig. 11. It can be seen that the utilization of high-speed valve using PWM control method yields the small value of performance index. But, it should be noticed that the proportional valve is used with simple PD controller. When proportional valve is used with more sophisticated control algorithm it can be obtained better control performance [10].

Also, the tests for tracking control were carried out for three different frequencies of sinusoidal reference signal, Fig. 12. At lower frequencies of the sinusoidal input, the nonlinear frictional effects incorporated in the system dynamics dominate in the features of the system output, and at higher frequencies the main influence for the system output have the PWM switching effects.

V. CONCLUSION In the paper, the PWM control technique for position

control of a pneumatic servodrive with high-speed solenoid valve has been applied. The control method has been designed with software by a transformation of the on/off pulse duration according to continual signal.

The study has shown that the fast switching valves with the control method based on PWM algorithm can be successfully used in accurate position control of pneumatic servodrives. For generating PWM signal, the pulse frequency should be carefully selected. When the high-speed on/off solenoid valve is used to the pneumatic system control, in order to establish a sufficient pressure

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

200

400

x (m

m)

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

0

1

outle

t A

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

0

1

t (s)

outle

t B

ReferenceActual

Figure 9. Experimental results of pneumatic actuator position control

for step input using on/off control method

0 1 2 3 4 5 6 7 8 9 100

100

200

300

400

500

x (m

m)

High-speed valve (Matrix H-X758-8-E-2-C-3-24)

0 1 2 3 4 5 6 7 8 9 10-10

0

10

t (s)

erro

r (m

m)

ReferenceActual

a)

0 1 2 3 4 5 6 7 8 9 100

100

200

300

400

500

x (m

m)

On/off valves (SMC EVT307-5D0-01F)

0 1 2 3 4 5 6 7 8 9 10-10

0

10

t (s)

erro

r (m

m)

ReferenceActual

b)

0 1 2 3 4 5 6 7 8 9 100

100

200

300

400

500

x (m

m)

Proportional valve (Festo MPYE-5 1/8 HF-010B)

0 1 2 3 4 5 6 7 8 9 10-10

0

10

t (s)

erro

r (m

m)

ReferenceActual

c)

Figure 10. Experimental results of pneumatic actuator position control for step inputs using: a) High-speed valve (PWM control method), b)

On/off valves (PWM control method), c) Proportional valve (PD-controller)


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Figure 11. Comparison of system performance using ITSE criterion

0 1 2 3 4 5 6 7 8 9 100

200

400

0.16 Hz x (

mm

) ReferenceActual

0 1 2 3 4 5 6 7 8 9 100

200

400

0.32 Hz x (

mm

)

0 1 2 3 4 5 6 7 8 9 100

200

400

1.6 Hz

t (s)

x (

mm

)

Figure 12. Experimental results of pneumatic actuator position control for sinusoidal input tracking using PWM control method

difference for the actuator motion the workable frequency of the carrier wave in this study was 10-25 Hz. As a result, it has been demonstrated that the use of fast switching valves is effective in response speed and positional accuracy, and the control system overall performances are completely comparable to the results accomplished by using proportional valves. The innovative solutions in Matrix valves technology and the construction conception of the valves based on modular architecture with a higher number of outlets in a single body allowing them to be used in pneumatic systems instead of several conventional on/off solenoid valves, and also for less demanding industrial servo applications instead of using costly proportional valves. Through use of inexpensive high speed solenoid valves it is obtainable cost saving over a pneumatic system solution using proportional or servo valves. The application of high speed solenoid valves in real industrial environment set requirements for implementing the control algorithm on a Programmable Logic Controller (PLC), because PLC’s are preferred by industrial operating staff over control computer, which has been used in the laboratory set up. This way, an inexpensive multifunctional pneumatic position control system can be realized, which has a reasonable acceptance rate for industrial applications.

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a Proportional Pneumatic Positioning System”, 1. Int. Fluidtechniches Kolloquium, Aachen, pp. 365-378, 1998.

[2] S. Liu, J.E. Bobrow, “An Analysis of a Pneumatic Servo System and Its Application to a Computer-Controlled Robot”, ASME J. Dyn. Syst., Meas., Control, vol.110, pp. 228-235, Sept. 1988.

[3] J. Pu, R.H. Weston, P.R. Moore, “Digital Motion Control and Profile Planning for Pneumatic Servos”, ASME J. Dyn. Syst., Meas., Control, vol. 114, Dec. 1992.

[4] E. Richard, S. Scavarda, “Comparison Between Linear and Nonlinear Control of an Electropneumatic Servodrive”, ASME J. Dyn. Syst., Meas., Control, vol. 118, June 1996.

[5] B.W. Surgenor, N.D. Vaughan, “Continuous Sliding Mode Control of a Pneumatic Actuator”, ASME J. Dyn. Syst., Meas., Control, vol. 119, pp. 578-584., Sept. 1997.

[6] J. E. Bobrow, B. W. McDonell, “Modeling, Identification, and Control of a Pneumatically Actuated, Force Controllable Robot”, IEEE Trans. on Robotics and Automation, vol. 14, No. 5, Oct. 1998.

[7] R.B. van Varseveld, G.M. Bone, “Accurate Position Control of a Pneumatic Actuator Using On/Off Solenoid Valves”, IEEE/ASME Transactions on Mechatronics, vol.2, No.3, September 1997.

[8] Matrix S.p.A., Pneumatic Division – General Catalogue. [9] Ž. Šitum, “Control of Pneumatic Servosystem Using Fuzzy

Controller”, PhD dissertation, (in Croatian), Univ. of Zagreb, 2001.

[10] Ž. Šitum, D. Pavković, B. Novaković, “Servo Pneumatic Position Control Using Fuzzy PID Gain Scheduling”, ASME J. Dyn. Syst., Meas., Control, vol. 126, No.2, pp. 376-387, June 2004.

[11] Ž. Šitum, J. Petrić, “Modeling and Control of Servopneumatic Drive”, Strojarstvo, vol. 43 (1-3), pp 29-39, Jan.- June 2001.

[12] Ž. Šitum, M. Essert, “Modeling, Simulation and Control of a Pneumatic Servo System with On/off Solenoid Valves”, 5th EUROSIM Congr. on Model. and Simul., ESIEE Paris, Sept., 2004.

[13] Ž. Šitum, M. Crneković, “Control of a Pneumatic Actuator Using Proportional Pressure Regulators”, 8th Int. Scient. Conf. on Prod. Eng. CIM 2002, Brijuni, Croatia, pp. II-035-II-046, 2002.

[14] J. Y. Lai, C. H. Menq, R. Singh, “Accurate Position Control of a Pneumatic Actuator”, ASME J. Dyn. Syst., Meas., Control, vol. 112, pp 734-739, Dec. 1990.

[15] N. Ye, S. Scavarda, M. Betemps, A. Jutard, “Models of a Pneumatic PWM Solenoid Valve for Engineering Applications”, ASME J. Dyn. Syst., Meas., Control, vol. 114, pp. 680-688, Dec. 1992.

[16] R. B. Varseveld, G. M. Bone, “Accurate Position Control of a Pneumatic Actuator Using On/Off Solenoid Valves”, IEEE/ASME Trans. on Mechatronics, Vol. 2, No.3, pp. 195-204, Sept. 1997.

[17] M. C. Shih, C. G. Hwang, “Fuzzy PWM Control of the Positions of a Pneumatic Robot Cylinder Using High Speed Solenoid Valve”, JSME Int. Journal, Series C, Vol. 40, No. 3, pp. 469-476, 1997.

[18] M. C. Shih, M. A. Ma, “Position control of a pneumatic cylinder using fuzzy PWM control method”, Mechatronics, Vol. 8, pp. 241-253, 1998.

[19] Y. Qi, B. Surgenor, “Pulse-Width Modulation Control of a Pneumatic Positioning System”, Proceedings of IMECE’03, ASME Int. Mech. Eng. Congress & Exposition, Washington, 2003.

[20] C. Kunt, R. Singh, “A Linear Time Varying Model for On-Off Valve Controlled Pneumatic Actuators”, ASME J. Dyn. Syst., Meas., Control, vol. 112, pp. 740-747, Dec. 1990.

[21] T. Royston, R. Singh, “Development of a Pulse-Width Modulated Pneumatic Rotary Valve for Actuator Position Control”, ASME J. Dyn. Syst., Meas., Control, vol. 115, pp. 495-505, Sept. 1993.

[22] G. Belforte, S. Mauro, G. Mattiazzo, “A method for increasing the dynamic performance of pneumatic servosystems with digital valves”, Mechatronics, Vol. 14, pp. 1105-1120, 2004.

[23] K. Ahn, S. Yokota, “Intelligent switching control of pneumatic actuator using on/off solenoid valves”, Mechatronics, vol. 15, pp. 683-702, 2005.

[24] Y. Lü, “Elektropneumatischer Positionierantrieb mit schnellen Schaltventilen”, PhD dissertation, RWTH Aachen, 1992.


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