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POSITION CONTROL OF A WATER HYDRAULIC CYLINDER

Kari T. KOSKINEN, Esa MAKINEN, Matti J. VILENIUS and Tapio VIRVALO

Tampere University of Technology

Institute of Hydraulics and Automation (IHA)

Korkeakoulunkatu 2, P.O.Box 589

FIN-33101 Tampere, Finland

ABSTRACT

From control point of view there are some serious problems in water hydraulic cylinder drives. Low viscosity of water

causes low damping and bad lubrication. Friction is quite high and its behaviour is not very clear. Bulk modulus of

water is high and it could cause some problems with short strokes and at the ends of cylinder. The biggest problem is

however a lack of good valves. On the market there are some commercial water hydraulic servo and proportional

valves, but the specification for them are not very high, especially for proportional valves. In this study experimental

tests of the water hydraulic cylinder drive with different valves are presented. The characteristics of different water

hydraulic cylinder drives are discussed.

KEY WORDS

Fluid Power, Water Hydraulics, Servo Control, Control Systems

NOMENCLATURE

A: Area

Q: Flow

F: Force

KP, KV, KA: State controller gains

PS: Supply pressure

PT: Tank pressure

U: Voltage

P: Pressure

m: Mass

s: Valve position

y: Cylinder position

ΔP: Pressure difference

INTRODUCTION

The first water hydraulic applications are known

already from the end of 18th century. After that over

100 years all hydraulic systems used water as a

pressure medium. In the beginning of 19th century oilhydraulics became into the picture when oil-resistant

seal materials were developed. In recent years water

hydraulic applications have increased and the trend

seems to be upwards. One reason for that are the

environmental aspects, which have become very

important in all over the world. Of course water as a

pressure medium offers many other benefits compared

Fluid Power. Third JHPS International Symposium (C) 1996 JHPS . ISBN4-931070-03-5

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to oil like easy maintenance, good availability, lowcost, non-flammable etc.[1]. One other kick-off for thedevelopment of water hydraulics has been the recentdevelopment of material technology. Many newmaterials have been developed, which can be usedlonger time with water without corrosion and erosion.

Water hydraulics has been used traditionally forexample in coal mines, steel mills and aluminiummills. The reason for the use of water hydraulics hasbeen the risk of fire or explosion and also reducing ofoperation costs in high fluid volume applications. Allthese water hydraulic applications has been mainly on-off technology. So actual proportional control of forceor speed has not been used. However, in newapplication areas such as food processing, pulp and

paper industry and manipulators, the need forcontrolling and positioning of an actuator is obvious.The basic element needed then can be for example a

position controlled cylinder.

The positioning of a water hydraulic cylinder can bedone with several different methods. The method to beselect depends naturally on the application and itsrequirements. Sometimes the required accuracy andspeed can be achieved with simple on-off -control, butin some other application servo control is hardlyenough. Even more difficult the problem is coming,when we consider the small variety of water hydraulicvalves available. In following some commercial waterhydraulic valves suitable for cylinder position controlare discussed and also the characteristics of cylinder

position control with different valves are presented.

WATER HYDRAULIC CONTROL VALVES

On-Off -valvesOn-off -valves are the simplest control valves, whichcan be used in cylinder position control. There are afew different on-off valves suitable for tap wateravailable in the market. Traditionally ball seat valveshave been used with smaller flows and piloted seatvalves in larger flow systems. Spool valves has notbeen used for tap water until recently, when ceramicspool valve was introduced.

One new on-off valve for tap water use is presented inFigure 1 [2]. The valve is electrically activated valve,which includes built-in pressure peak dampening. Themaximum pressure is 140 bar and the maximum flowis either 301/min or 601/min. Valve opening time withmaximum pressure is 70ms and closing time is about350 ms. On-off -valves can be used for positioning of awater hydraulic cylinder with sophisticated controlsystem.

Figure 1. Electrically operated on-off -valve (Danfoss)

Flow control valvesThe easiest way to control speed of a cylinder is to use

flow control valves. Pressure compensated flow controlvalves for tap water are not many in the market, but inFigure 2 one example is presented [2].

The speed of a hydraulic cylinder can be controlled

Figure 2. Manual operated flow control valve

(Danfoss)

easily with pressure compensated flow control valve.Especially in applications where the speed is same inevery working cycle, the right flow can be adjustedbeforehand for both directions. The direction controlcan be handled with separate arrangements, such as on-off -valves.

Proportional valvesBall seat valves (Figure 3) are quite commonly used as

proportional tap water valves. Ball seat has theadvantage of small leakage and reliable operation. Themanufacturing is also relatively easy, because the partscan be designed very simple and modern materials canbe used. The balls for example can be made of stainlesssteel or industrial ceramics. Ball seat valves areavailable both 2/2-way and 3/2-way -versions in themarket. So in position servo system the 4/3-functioncan be established with 2 pieces of 3/2-valves or 4

pieces of 2/2-valves.

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Figure 3. Proportional ball seat valve

(Hauhinco)

Ceramic spool valve (Figure 4) is the newestdevelopment in the range of water hydraulic

proportional valves. There are not many spool valvesavailable that can be used with tap water, althoughspool valves have many advantages compared to othervalve types. When we are considering the properties ofwater, the main problems in spool valves are high

Figure 4. Proportional ceramic spool valve (Hauhinco)

leakage, high flow forces and cavitation and erosioncaused by the high flow velocity. The spool and thehousing sleeve is made out of industrial ceramics andthe outer housing is made of stainless steel. To achievehigh pressure levels, low leakage rates and longlifetime the manufacturing tolerances have to be very

fine. The manufacturing technology of ceramicmaterials also sets different demands for designscompared to steel valves.

Servo valvesThe servo valves for especially designed for tap watercannot be found in the market at the moment. Somemanufacturers have stainless steel versions of theirnormal oil hydraulic servo valves. These characteristicsof these valves are guaranteed for HFA-fluids, but notnecessary for tap water. Of course these valvesoperates properly with tap water, but the lifetime is a

questionmark, because industrial reference cases do notyet exist. In Figure 5 is presented Ultra Hydraulics4658 servo valve [3]. According to manufacturer thestainless steel version of the valve has beensuccessfully tested with water, water glycol and HFA-

fluids. In TUT/IHA 4658 servo valve performance is

tested in position servo system, but the durability test

will be carried out in the future. The operation of the

valve until now has been correct.

Figure 5. Stainless steel servo valve (Ultra)

Hysteresis of the valve is < 3% and threshold is < 0,5

%.Null leakage of the valve at 140 bar supply pressure

is 1,6 1/min. The opening and closing times from 0%

to 100% at 210 bar are about 8ms. These values has

been measured with hydraulic oil. The characteristics

with tap water are obviously slightly different, but

these values are not yet available. This have to be taken

account when designing a water hydraulic servo

system.

Moog Sea Water Servo Valve (Figure 6) is the onlyservo valve, which is found to be originally designedfor to be used with water-in this case sea water [4,5].The valve is a prototype valve, which is developedfrom standard Moog 26 servo valve. The valve hassuccessfully tested with sea water, but it has not been

put into the production yet.

DEFLECTOR JET (DJ) SERVOVALVE

Figure 6. Sea water servo valve (Moog)

Other servo valves are also developed all the time. Oneexample is Ebara-servo valve, which is usinghydrostatic bearing in support of spool [6].

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POSITION CONTROL OF A CYLINDER WITH

DIFFERENT CONTROL VALVES

In all following measurements the hydraulic power is

generated by the water hydraulic power pack and thefluid used is tap water. The power pack includes

pressure relief valve, flow control valve and on/off-valve for pressureless side flow. The components of

the power pack are from Danfoss NESSIE -component

program. The maximum flow of the power pack is 371/min and the maximum pressure is 140 bar. The

cylinder dimensions are 32/22-1000 and the

manufacturer of the hydraulic cylinder is Finnish

company Kone-Ketola Oy. The cylinder is

manufactured of stainless steel and it is specially

designed for to be used with tap water. The load of the

cylinder can easily be varied. The system also includes

the pressure filter before the servo valve and the

accumulator for preventing supply pressure changes

during the movement.

Using on-off and flow control valves

The principal diagram of the studied system is depictedin Figure 7.

Figure 7. Position control with on-off valves.

The working principle of the system is following: Insteady state position 2-way on/off-valves are closed. Inoutward movement valves pdl and rd2 are opened andvalves pd2 and rd 1 are closed. In this case velocity istuned with flow control valve rf2. In inward movementvalves pd2 and rd 1 are opened and valves pdl and rd2are closed. In this case velocity is tuned with flowcontrol valve rfl . The type of the flow control valvesare NESSIETM VOH3OPM and direction control on-offvalves are NESSIETM VDH30E2/2. These on-offvalves leaves open with high output pressures (higherthan the supply pressure). So, it would also benecessary to use check valve in the supply line of thevalve. In these tests the check-valve was replaced witha manual operated valve.

Step responses were measured with different strokes.

Two velocities 50 mm/s and 150 mm/s, and three

masses, 25, 90 and 320 kg were used. Velocity and

pressure diagrams in a typical step response from 0.4to 0.6 m are presented in figure 8 and 9.

Figure 8. Typical velocity step response in on-off-

position control.

Figure 9. Typical pressure step response in on-off-

position control.

At the beginning of the responses there are a couple ofvery strong oscillations. At the end of the movementoscillation is clearly smaller. With the smaller massand lower velocity also oscillation is smaller. Thevelocity seems to effect quite directly to the positionrepeatability. The variation of the mass can been seenmore in the oscillation. With the lower velocity (50mm/s) repeatability were about 0.5 mm and withhigher velocity (150 mm/s) 1.5 mm. These results are

preliminary and they are based on short term tests.

Using proportional valves

Proportional valves can be applied several ways to

cylinder position control. The simplest method is

presented in Figure 10.

A differential cylinder is controlled by one 3/2-way

proportional ceramic spool valve. The connection iswell known differential connection. Using 4/3-way

proportional valve could give better characteristics, butsuch proportional valve is available for tap water yet.

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Figure 10. Cylinder position control using proportional

valve.

The control card used is digital control card, whichincludes two independent programmable PID-controllers [7]. The controllers can be connected alsoin cascade. The card includes also a measurementfacility, which have been used for measuring pressures

PL and PR. Position of the valve spool and position ofcylinder have been used as feedback signals for thecontrollers.

When manually tuned PD-controllers are used both invalve control and in cylinder control and the steps are

given between 400 mm and 600 mm, the responsespresented in Figures 11 and 12 can be obtained.

Figure 11. Cylinder and valve position step responses.

The speed of the cylinder is quite low, because the

valve size used is small. The accuracy is about 1 mm

and the stability of the system is good. The valve

movements are quite fast and the actual control time,

when the cylinder reaches the target value, is about 250

ms. From Fig.12 we can see, that the supply pressure

was about 65 bar and it keeps its level during the

movement. The pressure pR is vibrating, because the

water volume is quite small and the valve movements

Position control with ceramic spool valve

Figure 12. Cylinder chamber pressure step responses.

relatively fast. However, the pressure peaks are staying

in reasonable level. Small dither signal was used in

ceramic spool valve to avoid static friction.

Using servo valves 181In Figure 13 the test system installation of the waterhydraulic position servo system is presented. Thedifferential cylinder is controlled by symmetrical servovalve and the position of the piston is measured with

pulse transducer. The supply pressure ps and cylinderchamber pressures pi, and PR are also measured.

Figure 13. Cylinder position control using servo valve.

The digital control of the system is established with

PC-computer. The control program is C-language

program, which is processing the measurement data

and forming the control out put signals via DT2831-

control card including 12 bits DAC. The servo valve

used is stainless steel version of Ultra Hydraulics 4658

servo valve. The nominal flow of the valve is 19 limin

with Ģp= 70 bar.

Behaviour of the water hydraulic position servo have

been studied with using P-controller and state

controller. The controller is realised in a PC and

velocity and acceleration are calculated from the

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position signal with differentiation. For example pole

placement method can use to find appropriate valuesfor the gains KP, KV and KA, but in this case values

the both controllers has been tuned experimentally.

Figure 14. Simulated and measured step responses

with P-control, mass= 220kg.

Figure 15. Simulated and measured step responses

with state control, mass= 220 kg.

Step responses from 550 mm to 600 mm with P-

control are shown in Fig. 14 and with state control in

Fig. 15. With non-linear simulation model simulated

responses are also presented in the figures. The open

loop gain K in P-controller was 5 1 /s and the

measured steady state error was 1 mm. The open loop

gain K in state controller was 20 1/s and the measuredsteady state error was 0.27 mm. It means, that

hysteresis value for the servo valve given by

manufacturer is also appropriate for water system.

CONCLUSIONS

Because water hydraulics is quite new technology area,

the applications of tap water based positioning systems

are still quite unusual. Just recently appropriate

components for position control systems have arrived

in to the market. Obviously this will increase the

willingness for accomplishing some positioning

applications with tap water technology.

Three different methods for positioning a waterhydraulic cylinder were briefly discussed based on thelaboratory measurements. Positioning with four 2-wayonloff -valves was also studied. The system workedwell and the results are reasonable. However, somedetails still wait for explanations and improvements.At least something has to do for velocity and pressure

peaks at the beginning of the movements. The secondmethod was based on proportional ceramic spool valvecontrolled system. Ceramic spool valve has goodlifetime properties and quite good valve characteristicscan be achieved with using digital controller. Simple

positioning system is possible, if the accuracy within 1mm is reasonable. The dynamic characteristics of thesystem can be adjusted according the application.Servo valve based system gives naturally the fastestresponses and also with using modern controller asstate controller better accuracy can also be achieved.However, the servo valve used is not originallydesigned for tap water use and therefore the lifetimecould be shorter than with onloff -valves and ceramicspool valve.

REFERENCES

1. Koskinen K.T., Vilenius M.J., Water as a Pressure Medium in Fluid Power Systems. IFAC Workshop on Trends inHydraulic and Pneumatic Components and Systems Nov. 8-9, 1994, Chicago, IL, USA, 13 p.

2. Danfoss, NESSIE component brochures.3. Ultra Servoline, Quality Servo Valves, Brochure, Ultra Group 1993.4. Moog Model 26-102 Servo Valve Brochure, Moog Aircraft Controls Division.5. Moog Brochure, Electrohydraulic Servovalves for Aquaeous Operating Fluids.6. Urata, E., Miyakawa, S., Yamashina, C., Hydrostatic Support of Spool for Water Hydraulic Servovalves, The

Fourth Scandinavian International Conference on Fluid Power, Tampere, Finland, September 27-29, 1995.7. Hytar Pro 105, Operation manual, Hytar Ltd, 1995.8. Koskinen K.T., Vilenius M.J., Virvalo T., Makinen E., Water as a Pressure Medium in Position Servo Systems,

The Fourth Scandinavian International Conference on Fluid Power, Tampere, Finland, September 27-29, 1995.

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