evaluation of velocity control concepts involving counter balance vavles in mobile cranes

8/9/2019 Evaluation of Velocity Control Concepts Involving Counter Balance Vavles in Mobile Cranes

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EVALUATION OF VELOCITY CONTROL CONCEPTS INVOLVING

COUNTER BALANCE VAVLES IN MOBILE CRANES

Torben Ole AndersenInstitute of Energy Technology

Aalborg University, 9220 Aalborg , [email protected]

Michael Rygaard HansenInstitute of Mechanical Engineering

Aalborg University, 9220 Aalborg , [email protected]

ABSTRACT

This contribution reports about some simulation andexperimental studies carried out on flow controlconcepts used in mobile loader cranes equipped withcounter balance valves. First it is shown that aconventional flow control system based on meter-in, is

potentially unstable. Then three concepts based onmeter-out is described. It is found that none of theconcepts can fulfill all the requirements asked for bycrane manufactors, and the performance will to somedegree depend on the specific application.

KEYWORDS: Velocity control Balance valveMobile crane

NOMENCLATURE

tM : total mass of piston and load referred to piston

e : effective bulk modulus

P : pressure

Q : flow

V : volume

qK : overcenter valve flow gain

qpK : overcenter valve flow-pressure coefficient

PQ : displacement flow of piston

Subscripts:

r: piston side f: piston rod side

Other used symbols are shown in Fig. 1

INTRODUCTION

Counter balance valves are used to provide smoothcontrol, preferably load independent, when loweringloads, to give protection in the event of a hydraulic hosefailure and, in most circuits, to provide overloadprotection for the actuator.A check valve allows freee flow into the actuator, thenholds and locks the load against movement. A pilotassisted relief valve section will give controlled

movement when pilot pressure is applied. The reliefvalve is nomally set to open at a pressure at least 1.3times the maximum load induced pressure but thepressure required to open the valve and allow movementdepends on the pilot ratio of the valve.For optimisation of load control and energy usage, achoice of pilot ratios is available [4].

The development within mobile fluid power systems isin general directed towards increased functionality,improved response and controllability. For increasedcontrollability the directional valve is typically a spooltype valve with pressure compensation, and in loadercranes the cylinder is often equipped with a counterbalance valve (CBV). Both of these valves handle awide variety of functions. However, the combination isa well known source of oscillatory behavior or eveninstability [5], [6]. Legislations and safety reasonsthough make them a necessary element in many loadercranes, and put pressure on the requirement for velocitycontrol.

This paper describes experience with the design andoperation of different velocity control concepts insystems involving counter balance valves.

BASIC SCHEME

Some general proporties of the flow control schemesunder study can be found by considering theconventional system seen in Fig. 1.

Capacitance(Cr)

Pilot ratio (PR) =

Ao

C

P


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sCsH

f

f

1)( = ;

sCsH

r

r

1)( = ;

sM

AsG

t

rM

2

)( = (1)

The basic equations, describing the dynamics of thesystem in Fig. 1, are for purpose of reducing theequations, conveniently represented in a block diagram,as shown in Fig. 2.

qo

+

K

+

fQ

PRKqo

Hf(s)

Pr

CRQ r+ +

HHHK+ qp (s)r

M

CR

+PfG (s)

QP

Fig. 2 Block diagram of conventional system

Closing the inner loops gives the open loop gain

function )(sGo .

)1/2/(

/1)(

323

22

++

+=

sss

sKsG

nn

to

(2)

Where

2

)1(

CRVK

KKCRPRAKK

fo

eqpooqe ++=

ro

eqpooqe

tVK

KKCRPRAK

++=

)1(2

t

e

r

Pn

MV

A

=3 ;r

te

Po

qpooq

V

M

AK

KKAK

+=

2

1

Where ooqqo KAKK /= , eff VC /= , err VC /= .

From the above mathematical description, stability andother performance characteristics can be computed.

The characterististic equation for the system describedin Fig. 2 can be written as

012

23

3 =+++ oasasasa (3)

Where

tfr MCCa =3 ; { }qpqoro KPRCRKAa ++= )1(2

)( 221 rfr CCRCAa += ; )(2 qpqotf KKMCa +=

Using the Routh-Hurwitz criterion, it is necessary andsufficient that the coefficient be positive and

312aaaa

o>. See also [1] and [2].

Therefore, for a stable system, we require that

+>

qpqo

qo

r

f

KK

K

CR

PR

V

V(4)

In a crane we often have high inertia loads combinedwith static loads. This means that the pressuresensitivity of the overcenter valve will be high and theterm in the brackets in Eq. 4. will tend to 1. A lowerpilot ratio will increase stability, but from this simplestability result it is obvious that the stability margin ofthe overall system will be very small as the hydrauliccylinder is a full stroke component. The enclosedvolumes fV and rV on the meter-in and the meter-out

side, respectively influences the gain and the dampingin the system. Hence, a big fV and a small rV leads to

increased stability (Fig. 3), and vice versa (Fig. 4).

0 1 2 3 40

50

100

150

200

250

Time [sec]

Pressure[Bar]

Pr

Pf

Fig. 3 Pressure response with step in fQ

0 1 2 3 40

50

100

150

200

250

300

350

Time [sec]

Pressure[Bar]

Pf

Pr

Fig. 4 Pressure response with step in fQ

A commen way to try to achieve a more smooth andstable valve opening and load lowering is by addingdifferent pilot features, i.e. different combinations of


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damping throttles, bypass lines and non-return valves inthe pilot line to the counter balance valve.

EXTENDED SCHEMES

Clearly, the conventionel scheme is not suited to flowcontrol. However, it is the lack of pressure control in anormal meter-in flow control system that causes theproblem.

Fig. 5 Two ways of achieving pressure control

This suggest that one should look at meter-outconcepts. A counter balance valve is a pilot operatedvalve, and ideally a conventional pressure compensateddirectional valve can not control pressure though it isnatural to look at meter-out systems. The basic idea is tocontrol pressure in the meter-in side and flow in themeter-out side. In the control schemes presented in thenext sections the pressure in the meter-in side iscontrolled by two methods, as shown in Fig. 5. The leftdrawing symbolizes a pressure compensated directionalvalve with a sequence valve placed downstream.

Thereby is it possible to control the pressure in front ofthe sequence valve as function of the flow. In the figureto the right there is an adjustable pressure drop acrossthe main orifice. Letting the adjustable spring forcedepend on the lever motion and keeping the main orificesufficiently open, the valve will behave like a pressurecontrol valve. In the three schemes presented next theconcept figures is shown with the flow control-sequencevalve combination.

Scheme AIn this scheme, Fig. 6., the meter-out control is achievedby letting the CBV act as a pressure reducing valve.

This reduced pressure compensates the meter-out orificepassage created by the proportional valveWhen using a sequence valve the opening characteristicmust be matched to the characteristic of the CBV, insuch a way that the outlet flow is higher than the inletflow. The meter-in and meter-out side is then decoupledand the system operates stable. Using either pressurecontrol or a sequence valve the pressure characteristicshould be independent of the spool movement, and heldconstant. In case of hose break the lowering speed isincreased. If the load changes direction, the conceptwith a sequence valve will change to meter-in flowcontrol.

LS

T P

Fig. 6 Scheme A

Using pressure control some lever movement must beused to create a higher pressure, and the velocity will beload dependent.

0 1 2 3 40

20

40

60

80

100

Time [sec]

Pressure[B

ar]/Velocity[mm/s] Pr,p

Pr,s

Vp

Vs

Fig. 7 Simulation results with step in fQ

In Fig. 7. some simulation results are shown, where astpe has been applied to the spool in the directionalvalve. Subscript p stands for pressure control andsubscript s stands for sequence valve. The upper

curves is the pressure in the piston side of the cylinderand the lower curves is the piston velocity. From thefigure it is clear that the system is rather undamped.

Scheme BIn this scheme a pressure reducing valve is placedbefore the outlet orifice of the directional valve, thusacting as a meter-out presure compensator (Fig. 8).The pressure characteristic in the inlet side is made sothat the CBV always is fully opened.There is no need to match the area characteristics of thepressure controlling elements and the CBV.


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As with scheme A, the concept with a sequence valvewill change to meter-in flow control and with pressurecontrol the scheme will be load dependent if the loadgoes over center.

T

LS

P

Fig. 8 Scheme B

If the compensator is placed near the CBV away fromthe proportional valve there might be some dependancyon oil temperature due to the restriction in the returnline between the CBV and the proportional valve.Simulation results are shown in Fig. 9.

0 1 2 3 40

50

100

150

Time [sec]

Pressure[Bar]/Velocity[mm/s]

Pr,p

Pr,s

Vp

Vs


An advantage of the system is that it works with mostcounter balance valves.

Scheme CThe contradistinction between this scheme and the othertwo is that pressure compensation is done over theorifice of the CBV and not over the return orifice of theproportional valve (Fig. 10).

The velocity control is related to the postion of the spoolin the CBV, and thereby to the pressure in the inlet side.

PT

LS

Fig. 10 Scheme C

Hence some adjustment is necessary. The speed isunchanged in case of hose break. Simulation result isshown in Fig. 11.

0 1 2 3 40

20

40

60

80100

120

140

Time [sec]

Pressure[Bar]/Velocity

[mm/s]

Pr,p

Pr,s

Vp

Vs


EvaluationAlthough all three schemes can be made stable, schemeC only have the advantage of unchanged speed in caseof hose break. Adjustment can be difficult as the systemholds many parameters that may obstruct theadjustment. Also the outlet compensator must be sealedto the spring chamber in order to prevent leakage fromcylinder to tank.In scheme A and B with a sequence valve, thecharacteristic of the inlet restriction must match thecharacteristic of the CBV, so that the opening of theCBV corresponds to the inlet flow. Further the outletcompensator must be adjusted to a setting which breaks


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the conventional control loop, though the flow out of thecylinder must slightly exceed the flow into the cylinder.In scheme A the pilot pressure must be held constant,and in scheme B the CBV should just be kept open.

Based on the above discussion scheme A and B werechosen for experimental investigation. Scheme A for itssimplicity and relatively stable behaviour, and scheme Bwith pressure control for its robustness and stabilityproperties.

EXPERIMENTAL RESULTS

The simulation and measurements were carried out withan HMF 680 mobile crane shown in Fig. 12. Only the1st boom cylinder was used in the study.All simulations and measurements were made with thecrane extended 8 meter and with a payload of 500 kg,

the input being a step in required velocity.

1E11 - P 4W - 50S 34-6

Danfoss Fluid Power

LS

T P

Fig. 12 HMF 680 mobile loader crane

By changing the spools in the proportional valve it ispossible to have load-independent flow control orpressure control. In Fig. 13 is shown some experimentalresults with scheme A and B .

0 1 2 3 40

50

100

150

200

Time [sec]

Pressure[Bar]

Scheme B, Pressure valve

Scheme A, Pressure valve

Scheme A, Sequence valve

Fig. 13 Experimental results with step in fQ

In the figure we see the same tendency as in thesimulation study. Scheme A being prone to loadoscillations and the stable behaviour of scheme B withpressure control.

CONCLUSIONS

In this study the main goal was to compare threedifferent flow control schemes for mobile cranesequipped with counter balance valves. They are allbased on meter-out control, utilizing pressure control inthe meter-in side. The pressure control is achievedeither by letting the meter-in side cavitate or bycontroling the pressure by the lever on the proportionalvalve. They all show a stable behaviour. Scheme A andB were chosen for an experimental study thatcorresponded well with the simulation results.

Despite their relative advantages and disadvantages it isvery difficult to compare the schemes. This is becausean important part of the function is the operator's"feeling" of the control of the machine.Most of todays hydraulic systems have a standardsingle-spool configuration and provide limitedfunctionality while the controlling land affects bothsides of the actuator. The scheme B and C are morecomplex and require use of pressure compensators forcontrolling meter-in and meter-out seperately. No doubtthat the a new generation of valves will appear that willutilize electronic control with individual valve softwarefor flow and pressure requirements but due to their

relative sophistication, we believe they will cohabitswith more traditional schemes as the one presented here.

REFERENCES

[1] Lisowski E., Stecki J. S., Stability of a HydraulicCounterbalancing System of a Hydraulic Winch. TheSixth Scandinavian International Conference on FluidPower, Vol. 2, 1999, 921-933.[2] Persson T., Palmberg J-O., The Dynamic propertiesof Over-Center Valves in Mobile Cranes. 9th AFKConference for Hydraulics and Pneumatics, Vol. 2,1990, 233-251.[3] Zhe B., Stability of Load Holding Circuits withCounterbalance Valves. Eighth Bath International FluidPower Workshop, 1995, 60-73.[4] Zhe B., Lasthalteventile in Patronenbauweise.lhydraulik und Pneumatik, 42, nr. 3, 1998, 158-166.(in German).[5] Overdiek G., Hydraulikhubwerke mit senkbrems-Sperrventilen als Regelkreis. lhydraulik undPneumatik, 26, nr. 3, 1982, 139-143. (in German).[6] Zoebl H., Senkbremsventil und Ventil-kombinationen zur Steuerung der Bewegungs-geschwindigheiten. lhydraulik und Pneumatik, 42, nr.

11-12, 1998, 751-756. (in German).

evaluation of velocity control concepts involving counter balance vavles in mobile cranes

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