221RE 00 207/10.00

LS Controls in Public Utility Vehicles and Tool Carriers

LS Controls in Public Utility Vehiclesand Tool Carriers

A. WolfRexroth HydraulicsLohrPhone +49 (0) 93 52 18-38 98Fax +49 (0) 93 52 18-21 59E-mail andreas.wolf@rexroth.de

Mechanical engineers employ hydraulic systems totransmit the drive power generated by diesel en-gines to the tools attached to public utility vehi-cles. Such a hydraulic system consists of a hydro-static drive in closed circuits, and is often found inhigh-performance travel drives or rotating drives,or it may be valve-controlled in an open-circuitconfiguration.

The table below shows a list of machines withopen-circuit service hydraulics featuring eitherthrottle control or load-sensing control:

Trend: The more sophisticated the function,the more precise the repeat accuracy ofa movement, and the more functionshave to be controlled in parallel by onepump, the greater the need for LS con-trols.

The table below indicates the most important dis-tinctions between throttle and LS controls:

Table 1: Service hydraulics in public utility vehicles - some typical fields of application

Open circuit,throttle control

Sweeping machinesRefuse collection trucks, rear loadersTruck-mounted cranes up to 25 mt, manualTow-away vehiclesHook/skip loadersScissor-type lifting platformsAirport vehiclesFork lift trucksSewer flushing vehiclesBulk handling vehiclesStabilizers and outriggers of all types

Open circuit, LS control(closed centre), flow control

Snow ploughs/winter service vehiclesRefuse collection trucks, front and side loadersTruck-mounted cranes over 20 mt,radio-remote controlledMotor turntable laddersSlope mowers, mulch spreadersSki slope crawler vehiclesLarge lifting platforms

Table 2: Throttle and flow control characteristics

Throttle control OC

Throttling of a pump circulationport and simultaneous opening ofa actuator connection via a vari-able metering orifice (spool). Assoon as pump pressure exceedsactuator pressure, oil is pumpedto the actuator. Precise controldepends strongly on load andoperator.

Load sensing control LS

Flow to actuators proportional tometering orifice aperture. Load-in-dependence ensured by sectionalpressure compensator as long asthe amount delivered by the pumpmeets actuator demand. Ifundersupply occurs, the speed ofthe actuator(s) with the highestload pressure will be reduced.

LUDV control CC

Flow to actuators proportional tometering orifice aperture. Load-in-dependence ensured by sectionalpressure compensator as long asthe amount delivered by the pumpmeets actuator demand. Ifundersupply occurs, the speed ofall actuators will be reduced inparallel and in proportion to theirmetering orifice aperture.

222 RE 00 207/10.00

LS Controls in Public Utility Vehicles and Tool Carriers

High-precision control of individual movements isrequired in particular if movements are automated;this includes positioning accuracy as well as speedcontrol of parallel movements. The example givenbelow, an LS control in refuse collection trucks withoverhead loading facility operated by one person,illustrates this point:

The PLC is fed the command value for the liftingcycle from a reference position at which sensorsrun a safety check after the fork has been insertedin the container pockets under joystick control.

Now, up to three parallel movements follow, whichmust not interfere with one another, to ensure thatthe container moves parallel to the ground until itis above the driver’s cab. Only then will its contentsbe dumped into the - now open - container body,without any refuse dropping on the driver’s cab.

Since pump pressure ranges between 50 and100 bar or between 80 and 200 bar when liftingan empty or full container, an electro-proportionalLS control is the only way to meet the requirementfor repeatable and load-independent speed con-trol.

To this end, an A10VO100DFR1 pump and anM4-15 control valve are used. While two sectionscontrol the lifting movement of the container, thethird section controls the further compacting of therefuse under variable drive speed and constantcompactor speed while the truck is moving.

Since a considerably lower amount of volumetricflow is required to open/close the flap and to con-trol other auxiliary functions, a 5SP-08 controlvalve featuring a 2-way inlet pressure compensa-tor is used. As long as the solenoids are in theirneutral position, the integrated pilot-controlledcheck valves keep the cylinders in their holdingposition without any oil leakage.

Load holding valvesLeakage-free shut-off load lowering valves that aredirectly mounted on the cylinder for safety reasons(to preclude any danger from pipe rupture) holdthe lifting frame, with or without a refuse container.

Parallel fork movement overa wide swing angle of thelifting frame to prevent thepremature emptying of therefuse container. The flap ofthe container body openssimultaneously.

Picking up and lowering acontainer with an overhead-loading refuse collectiontruck

LS control block diagram for truck-mounted cranes

223RE 00 207/10.00

LS Controls in Public Utility Vehicles and Tool Carriers

The relief of the spring chamber of the load low-ering valves must be ensured if the return pressurefrom the valve is transmitted via narrow pipes, adirectional valve control land and return filter ar-rangement.

The machine does not need an oil cooler because,during lowering operations, the period of time withhigh throttling losses is very short compared to theload cycle when only the load pressure is gener-ated by the pump.

Load loweringControl pressures required forlowering loads

Note: The equations below have been simplifiedand do not take into account pressure at connec-tion 2.

Note: In valves not having a fourth connection forspring chamber relief, the pressure at connection2 equates to the opening pressure on the basis ofa factor of one plus the control ratio. E.g., for a 3:1load lowering valve, a pressure of 15 bar at con-nection 2 will entail a cutting-in pressure increaseby 60 bar.

L = load pressureP = control pressure required for

openingP(r) = control ratio (e.g. 3:1 = 3)S = valve timingC(r) = cylinder area = (spool Ø_)

spool Ø_ - rod Ø_)

1) Load lowering valves on the cap side of thecylinder with load on the piston side

P = S - LP(r) + 1/C(r)

2) Load lowering valves on the rod side of the cyl-inder with pulling load

P = S - LP(r) + C(r)

3) Load lowering valves on a motor or a double-rod cylinder with pulling load

P = S - LP(r) + C(r)

The machine does not need an oil cooler because,during lowering operations, the period of time withhigh throttling losses is very short compared to theload cycle when only the load pressure is gener-ated by the pump.

Example application: truck-mounted crane control

Truck-mounted crane controls are another typicalLS control application. Again, if radio-remote con-trolled, a change in speed of individual, usuallysuperimposed movements is required that is pro-portional to the excursion of the command unitactuator (e.g. joystick).

However, heed must be paid to the safety regula-tions stipulated in crane guideline prEN 1501 or

other relevant crane regulations. For instance, allcrane valves must feature redundant emergencycut-off systems, and the entire crane control sys-tem must feature a load moment limiter to pre-vent the crane from overturning when extendingthe jib with an attached load. These safety func-tions are best integrated into the main controlvalve of a crane to avoid additional pipework.

Emergency OFFIn fixed displacement pump operations, the up-stream selector valve switches from the ‚stabilizer‘or ‚crane control‘ position to neutral with spring-actuated centering. The resulting circulation pres-sure is too low to even serve as auxiliary energyand is unable to bring downstream directionalvalve spools, which are also spring-centred, intotheir operating position.

In variable displacement pump operations, theactuator connections of the upstream emergency-off directional valve will be blocked. In addition tothis, the electrically-powered directional valves willbe de-energized, so unloading the LS signallingcascade back to the tank. Consequently, the spoolsof the pressure compensators will shut as well sothat any accidental movement of the control isdoubly prevented.

Load moment limiter (LMB)Modern crane controls feature small load momentcomputers that calculate the current load momenton the basis of the pressure in the lifting cylinder,for instance. (In truck cranes with several telescop-ing sections or a level-luffing jib extension, thesituation is more complex due to the mastdeadweight and would exceed the remit of thispaper.)

Whenever the admissible load moment is ex-ceeded, all movements increasing the load-mo-ment must be disabled. One means to achieve thisend is to route each LS actuator-connection sig-nal individually to the tank to render the springforce too weak for shifting the pressure compen-sator spool against the pressure exerted by thepump.

If the LS unloading valves isolate both LS connec-tions of an inactive valve at the same time, opera-tion is impossible even if an override manual le-ver is used. Alternatively, the emergency-OFF valvemay go into neutral whenever the admissible loadmoment limit is infringed, so cutting off pump flow.All pilot valves will be isolated at the same time,

5M4-12 directional valve with LS limiting valves

224 RE 00 207/10.00

LS Controls in Public Utility Vehicles and Tool Carriers

Public utility vehicle tool carrier with attached devices

LS valve SP-08: monoblock inlet with two sections

causing all directional valve spools to go into neu-tral as well. In this case, even manual (emergency)operation will not trigger any movement becausethere is no pump pressure left, and since load low-ering or pilot-controlled check valves will immo-bilise all the actuators.

Catching manual leversUntil now, directional valve spools with fixed-con-nection manual levers have been available on themarket which are either covered or unscrewed toprevent inadvertent use. However, they do increasevalve hysteresis due to the requisite external sealof the directional valve spool. Catching manuallevers, in contrast, do not follow the spool eitherin the neutral position or during electrohydraulicoperation. During mechanical operation, however,the control spool engagement mechanism is au-tomatically actuated, enabling the manual lever tobe used for flow control.

Tool carrier controlIn public utility vehicles, tool carriers are used fora wide variety of purposes as is reflected by thelist of attachable devices. For this reason, hydrau-lic control technology must cover a most diverserange of functions.

The A10VO45DFR1 variable displacement pumpwith a capacity of up to 90 l/min at 280 bar formsthe core of such a system. A valve control blockbelonging to the SP-08 series transmits the hydrau-lic power via a monodesign inlet element to theindividual actuators such as tipper, water pump,mounted plough, or sweeper drive.

Two section pressure compensators with variableorifices for the main actuators and one sectionpressure compensator for the downstream direc-tional valve sections have been installed to makethe individual actuators load-independent. Pres-sure relief valves provide electroproportional con-trol of the directional valve spools in the monobloc.To prevent any control start-up problems in win-ter, a flushing valve has been installed in themonobloc for continual flushing of the controlblock at approx. 20 bar and 5 l/min. As soon asthe pump receives an LS signal from an actuator,the flushing valve closes to prevent any unneces-sary heating and to make the full pump capacityavailable for the drive.

LS valve control block SP08, monodesign inlet el-ement with 2 flange-mounted directional valvesections

The same monodesign inlet element is used inagricultural machines and winter service vehicles.

SummaryLS valves are increasingly used in public utilityvehicles with exacting demands for the precisecontrol and repeat accuracy of individual move-ments. Compared to throttle valve control, thesevalves are advantageous because they are load-independent when several actuators have to beoperated simultaneously. With control blocks of thetypes SP-08 (up to 30 l/min. per section), M4-12(up to 80 l/min. per section) and M4-15 (up to 130l/min. per section), Rexroth offers a balanced range

of on/off and proportional valves offering typicalfeatures such as emergency OFF control, latch/delatch manual levers, LS pressure limiting or shut-off valves, and integrated electronic systems thatare suitable for CAN bus control. With matchingmicro-controllers and a complete range of pumpsas well, Rexroth is able to offer machine buildersthe full spectrum of drive and control technologyfrom a single source.