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Drive System and Boom Controlfor Aerial Work Platforms

Drive System and Boom Controlfor Aerial Work Platforms

Jrn PetersenGlenn SchaalRexroth HydraulicsBethlehem/USAPhone +1 610-694-83 00Fax +1 610-694-84 67

Aerial work platforms (AWPs) have proven them-selves to be an indispensable tool on constructionsites as well as in building maintenance on aworld-wide basis. First developed in the U.S.,AWPs have experienced a gigantic boom over thelast 10 years and are now widely used in Europe.Rexroth has made great strides to support andparticipate in this exciting market place. On a moregeneral basis Rexroth views the AWPs to be anexcellent example of the increasing degree of tech-nology in mobile equipment. This technology takesplace on two levels: In reviewing specific machines, for example an

excavator, there is an obvious trend towardshigh tech. Today a sophistiated LUDV systemwith an electronic microprocessor control wouldbe used in place of a simple 6/3-way throttle

control with hydraulic joystick pilot controllersused a mere 10 years ago. As demonstrated bythe many presentations Rexroth has contributedto this conference, we have always been on theforefront of this technology development.

At the same time more and more technologyhas been implemented into the ConstructionIndustry in general. Tasks that would have re-quired a simple ladder or scaffording 10 years

ago, can be accomplished by scissor lifts today.Therefore, we see a trend for hydraulic controlsystems and components suitable for increas-ingly simple machinery. Due to the tremendousquantities of machines required for these sim-ple tasks, there are large market opportunitiesopening up. Rexroth embraces these challengesas substantiated by the following AWPs article.

RT scissors lifts

Slab scissors lift

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Drive System and Boom Controlfor Aerial Work Platforms

AWPs can generally be categorized in 4 groups:1. Scissor lifts, battery powered2. Scissor lifts, engine powered3. Boom lifts, battery powered4. Boom lifts, engine powered

Scissors lifts have less complex hydraulic systemsdue to their comparatively simple design. Normallythere are only three functions needed for eachmachine: Propel Lift Steer

Small, electrically-driven machines are used prima-rily indoors. These drive systems utilize low-speedorbit motors, while lift and steer operations arecylinder functions. All these functions are control-led by an integrated manifold. 4WE6 directional-control valves in combination with screw-in car-tridge valves can be a very compact package thatis cost-competitive and a reliable solution. TheCompact Hydraulic Program, which was officiallylaunched by Rexroth earlier this year, enablesRexroth to be the competent supplier of choice forcomplete manifold systems which are engineered,designed, machined, preassembled, tested, andshipped to the OEM for their production line as-sembly.

For these battery-driven machines it is very impor-tant to reduce pressure drops to save energy. Evenmore important is to utilize the optimum systemdesign to prevent power draw out of the battery,which is then transfered into heat in control valves.Therefore, intelligent energy transformation is re-quired, and for that reason load-sensing systemsare found in many of these machines. In addition,there is a trend toward additional proportionalcontrols for these small machines, especially forpropel and lift functions. To accomplish this in apower-saving fashion the gear pump is driven witha variable speed DC motor. All directional-controlvalves can then work as on/off valves, which sim-plifies the electronic controls as well as the valving.Adjustment of the flow rate as well as its distri-bution have to be controlled electronically by amicrocontroller. This may appear to be an expen-sive controller, however, the large quantities in-volved drive development costs down, so that avery sound technical and economical solution canbe achieved. Internal gear pump PGF

Articulatedboom lift

Compact hydraulics control manifold Directional control valve 4 WE6 (on/off)

With increasing machine weight, and thereforelarger prime movers, the drive systems areequipped with a closed-loop transmission circuit.

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Drive System and Boom Controlfor Aerial Work Platforms

Advantages of this type system are: Better controllability of the propel function. Better energy utilization due to the direct

pump control of the drive, without meteringlosses caused by control valves.

Elimination of the counterbalance valves,which results in additional power savings.

Simple horsepower limitation without majorinfluences to the other actuators.

The easiest way to control the drive speed for thetransmission is with an electro-proportional pump.Horsepower control can then be easily and reliablyaccomplished by an analog controller card withfeedback capability. The EDA card was specificallydesigned for this need and has proven beneficialin many applications. Depending on the machinedesign, the EDA card can work in conjunction withan impulse speed pickup from the engine or anamperage draw feedback from an electric motorto limit pump stroke and maintain horsepower asa constant. This is an off-the-shelve product whichcan be used for any type of feedback control.

Although less precise, an even simpler and less-costly solution is a newly-developed pump controlfor the A10VG, called ED, or non-feedback control.This control eliminates the internal feedback sig-nal. Lets look at a normal pump control to ex-plain this effect. With an axial piston pump con-trol there is the usual undesired effect of the in-ternal forces of the rotary group trying to destrokethe pump as load pressure increases. This effect isdetrimental to the principle of a displacement con-trol. In a standard EP control, for example, thiseffect is eliminated by an internal feedback fromthe control cylinder of the pump to the controlvalve spool. The internal forces are continually try-ing to automatically destroke the pump back to asmaller displacement angle. With the new, simpli-fied control these forces are now utilized to achievea horsepower limiting control. The control lets therotary group stroke the pump back with higherpressures, which performs essentially as a horse-power limiting function to decrease the vehiclespeed with increasing load pressure. Otherwise, thecontrol resembles a normal EP with 2 solenoidsfor forward and reverse and can be controlled witha standard amplifier card. Note that calibrationof the pump and rotary group must be performedby a qualified service technician to avoid malfunc-tions.

Axial piston motor A10VM / A10FM

Transmission pump A10VG

Horsepower controller EDA

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Drive System and Boom Controlfor Aerial Work Platforms

Wheel drives for transmission circuits, as comparedto the smaller gerotor wheel motors, can be donein two different ways. For medium sized machines,usually small rough terrain machines, radial pistonwheel motors of the MCR type are providing aneconomical solution. The compact package in onlyone component saves in logistics as well as assem-bly costs. Their pressure rating is suited towards themedium duty transmission pumps A10VG whichare usually used for these drive circuits.

Wheel drive gear box GFT7

Microcontroller MDC

Wheel motor MCR with brake

For even bigger machines, usually boom lifts, plan-etary wheel drives in conjunction with axial pis-ton motors are used. Rexroths L+S division pro-vides this type of wheel drives with their GFT line.Smaller drives have also been designed as a sin-gle component, comparable to the MCR motors.

For bigger sized drives the gear box and the hy-draulic motor have to be split into separate com-ponents for design reasons. With increasing wheelloads and torque requirements this solution be-comes more viable in many applications from 40feet up to 130 feet working height (12 m to 52 m).

Electric articulated Boom Lift

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Drive System and Boom Controlfor Aerial Work Platforms

Cartridge valves

Since AWPs are provided in most cases with a 4-wheel drive propel system the transmission circuitalso has to include a flow divider manifold. Thatmanifold is an integral part of the drive function-ality and has to be considered during the designstage of the machine at least as much as the othercomponents of the circuit. Rexroth is putting a bigemphasis on this product with its latest ventureinto Compact hydraulics. The cooperation withSUN Hydraulics is a big part of this program. Weat Rexroth feel that by utilizing SUNs flow divid-ers the best performance for the system can beachieved. Especially the high pressures used intransmisson circuits make the SUN product theproduct of choice in these demanding circuits.

When compared to scissor lifts, boom lifts rely ona larger number of actuators for lift function. Inaddition to the three above mentioned functionson scissor lifts, there are several different functionsto be provided. This requires a much larger con-trol manifold, which usually entails all boom con-trol functions as well as steering. In most cases theflow requirements can be met with 4WE6 direc-tional-control valves. Only the telescopic cylinderfor boom extend necessitates flows that require alarger size 4WE10 directional valve. Cartridge valvetechnology is utilized to provide pressure, flow andother control functions. The control systems arenormally load-sensing systems, which provide bet-ter controllability to the operator. Again, in battery-driven machines this technology provides energyefficiency and longer recharge intervals.

With these bigger machines the load pressures areelevated as compared to scissor lifts. Therefore,internal gear pumps become an attractive solution.Rexroths G2 Series pumps fall into this category.Their unique design makes their overall efficiency,and in particular their volumetric effeciency, supe-rior to any other gear pump. Significantly reducednoise levels are another feature of these pumps.The higher cost, however, justify their usage onlyin larger electrical boom lifts with sufficiently high-load pressures. In engine-powered machines theeconomics do not justify the technical gain theyprovide.

Program - Compact HydraulicContents of the catalogues

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Drive System and Boom Controlfor Aerial Work Platforms

Another very important feature with increasingmachine size is proportional controls. On m a -chines utilizing manifolds with load-sensing sys-tems, this can be accomplished with proportionaldirectional valves 4WRAB6. These DO3 type spoolvalves offer increased controllability, hysteresis, andrepeatability in comparison to any proportionalcartridge valves for directional 4-ported valves.Further, they provide a true operation in all 4 loadquadrants. This means in the valve, and thereforethe operator, can accelerate both positive andnegative loads proportionally. The same is true fordeceleration. This provides significantly improvedcontrollability of the machine functions, which hasbeen well received by operators who are 60 feetabove ground. The functions normally providedwith proportional features are swing and boom lift.

Rexroth 4WRAB6 is a cost-effective valve thatmakes these proportional controls possible. Itsdesign utilizes as many components of the on/offvalve 4WE6 as possible to provide commonality ofparts and drive costs down. The control of the valve,as well as a multitude of other proportional de-vices, can be accomplished with a MDSD drivercard, which is also a cost- effective solution. Addi-tionally, a large variety of analog and digital con-troller cards are available from Rexroth, includingcomplete plug and play microprocessor controllersthat provide analog outputs for all drive and boomcontrol functions on a boom lift hydraulic system.For communication between basket and chassiscontrols, a CAN-Bus interfacing is utilized. Whatshould be noted here is that Rexroth is a capableand accessible source to answer system designneeds for this application.

Obviously, the boom control manifold is also amajor target for the new Rexroth Compact Hy-draulics Program. The solenoid cartridges, whichare an integral part of this program, will help toprovide reliable manifold solutions to the AWPindustry. The main target of this program, how-ever, is for Rexroth to be the best-suited supplierfor hydraulic system solutions to the AWP indus-try. The control manifolds are the heart of themachine design and the key component to provideeasy and smooth controllability of the boom func-tions. These manifold systems, now available inEurope and in the U.S., complete the system ca-pabilities that Rexroth has to offer to the AWPindustry.

Proportional control valve 4WRAB6

Boom control manifold

Controller card MDSD

Index