millennium introduction

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One of the realities of fluid power inthe future is that more of it will becontrolled with electronics. There aretwo broad methods of control in hy-draulic fluid power: valve control andpump control. In this context, the valveand the pump are the elements that con-nect the controlling electronics to thehydraulic system.

Factors too numerous to mentionmake electronics the control medium ofchoice, but one that must be reckonedwith is the rush to wire the worldwith communication buses. More will

be written about buses in subsequent is-sues. For now, however, let it sufficethat the ability for inanimate devices tofreely communicate with one another isso compelling, and the advantages areso enormous, that the bus juggernautcannot be stopped.

Two-way communication via bus al-lows centralized data-gatherers to in-terrogate the status of the system liter-ally from anywhere in the world. Eachinstrument or device on the networkcan broadcast its presence and essentialcharacteristics to any other instrument,

controller, or data-gatherer. From anengineering and system design point ofview, the enormous amount of infor-mation available will make initial star-tups of systems and even whole facto-ries go much faster. And the samedevices that are integral to the controlsystem will broadcast diagnostic datathat will either tell explicitly or be usedto derive the general health of both de-vices and components.

The fluid power industry is just be-ginning to embrace this exciting newbus technology. The purpose of the dis-

cussion to follow is to review the chal-lenges confronting our mature industry

in order to link up to the network. Inpursuit of that aim, let us consider apump architecture that is bus-ready and in the process, expose at least someof the more important problems andvexing issues.

The aim here is not to provide all theanswers, but rather, to start a dialog byasking some of the right questions atleast from my perspective. Readers areencouraged to contribute, too, becausethere are many difficult decisions thatmust be made. Many of you are goingthrough that process right now. But fear

not, you are not alone. Our standards-writing bodies will have a big role toplay, too, because the issue of whichbus will emerge as the ultimate winnerin the bus wars is by no means settled.This makes our job all the more diffi-cult. CAN may be all but settled in themobile equipment arena, but in the in-dustrial arena, the battle continues torage, and big fortunes are at stake.

Pump control vs. valve controlThe use of valves to control hydrau-

lic actuators is inherently inefficient,

but pump control of actuators is veryefficient. In spite of this reality, thereare ample reasons why valve control ispreferred. Primarily, valves can re-spond much faster than pumps can.Therefore, the most demanding appli-cations those requiring both rapidresponse and accuracy in controllingposition, speed, pressure, force, or anycombination of these must usevalves. Pumps do not have sufficientbandwidth to accomplish these tasks.

This statement is not meant to implythat pumps cannot be used as a means

of control; they are well-suited formany applications. Furthermore, I hope

that someday pumps will be developedthat will indeed produce real band-widths of 10 Hz or more. The changestaking place in industry, and those that

will take place, reflect the need for thevery best pumps that can be designedand built. In the text that follows, Illtry to describe the pump of the futurethat is needed to meet industrial de-mands of this, the new millennium.

Advantage of pump controlThe main advantage of using a pump

as the control element is that pumps canregenerate power, whereas valves can-not. For example, during the accelera-tion of a load mass or inertia, the pumpdisplacement is increased, motivating

the actuator to run at a higher speed.This transfers energy from the pump,through the actuator, and into the load.Upon deceleration, pump displacementis reduced, and the actuator pressureundergoes sign reversal. Essentially,the actuator then becomes a pump, tak-ing energy from the load. The pressurereversal causes the pump to become amotor, overdriving the prime mover.With an electric motor as the primemover, it becomes a generator and putspower back onto the electrical grid tobe used by other electrical consumers.

Thus, power is not consumed duringdeceleration, but, rather, is regenerated,and put back into the electrical powerbus. This technology has been highlydeveloped by those dedicated to electri-cal actuation. It will be exploited bythem against fluid power until pumpcontrol is made more responsive andcost effective in more applications.

Now, contrast this with valve con-trol. Upon acceleration, the valve

14 AUGUST 2000 /HYDRAULICS & PN EUM ATICS

By JACK L. JOHNSON, P. E.M otion Control

Introducing the millennium pump

nium pump, take a detailed look atthe features and operation of themillennium pump, and address is-sues surrounding sensors used formonitoring pressure, speed,torque, temperature, and displace-ment in the millennium pump.

This series of columns on motioncontrol begins with an introductionto the theoretical hydraulic pumpof the future the millenniumpump. Subsequent installmentswill cover serial bus technologyand how it relates to the millen-

Whats coming up

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J ack Johnson is an electrohy-draulic specialist, fluid powerengineering consultant, and presi-dent of IDAS Engineering, EastTroy, Wis. Contact him at 262/642-7021, fax 262/642-7025, [email protected], or visit his website at www.idaseng.is4.com.


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Now, back to the pump. The conven-

tional pressure-compensated pumpuses internal pressure-sensing pistons,springs, and spools. The energy outputof the pump itself effects a reduction indisplacement as pressure rises abovethe compensators cracking pressure.Hydromechanical logic is used to es-tablish the rate of displacement changeand its sensitivity to the output-pres-sure change. This has led to the use ofintricate spools, poppets, sleeves, andother hydromechanical pieces that areexpensive and difficult to develop. Theresulting machines represent well-de-

veloped technology and provide userswith a variety of reliable pressure-com-pensated pumps with an even greatervariety of performance characteristics.

Pumps of the future will use a uni-versal prototype as the kernel, and thelogic to control them will be increas-ingly relegated to the computer orsome kind of so-called intelligent con-troller. There are several reasons whythis can be expected:

Requirements for the small, intri-cate parts that are used in the hydrome-chanical sensing and controlling will be

all but eliminated, replaced by thesmall parts for the displacement varia-tors for the kernel pump.

The degree of control will be im-proved.

Specific performance characteris-tics will be easily modifiable merely byentering a different parameter, say loopgain, as a digital input parameter.

Initial design will be expedited, be-cause it is far easier to tune an electro-hydraulic controller than it is to pro-duce spools, poppets, and springs inalmost endless combinations to achieve

a specific response. Anyone who hastuned an electronic controller may dis-agree that it is easier; however, the pro-cess must be put into context. In thecase of conventional hydromechani-cally regulated pumps, all the tuning isdone by sizing the spools and sleevesand poppets and springs during productdevelopment in the laboratory which can take weeks, if not months.

In the case of the electronic pump,the tuning is done at application time,

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Control

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and even novices can fumble their wayto acceptable tuning in a few hours ormaybe days. And the important fact isthat the electronic pump will be tunedto application conditions, not labora-tory conditions. In the end, the net tun-ing time will be reduced for the elec-tronic pump.

Electronic compensation methodsare available that will allow refinementof the pump responses, especially whenconsidering the tendency of some hy-dromechanical pumps to go unstableunder certain loading conditions.

Variations on integral control willmake it possible to provide an electron-ically controlled, pressure-compen-sated pump with essentially perfectsteady-state pressure control. That is,the output pressure will not have tochange with changing load flow. Thedeadhead pressure will be the same as

the average running pressure.More sensors will be necessary in

the system of the 21st century, but theiradded cost will be more than offset bythe advantages derived from them. Thegreatest advantage will be that all thehydromechanical parts for a pressure-compensated pump will be the same asthose for, say, a load-sensing pump.The only difference between these twomachines will be in the control soft-ware. In fact, the control program foreach machine will be loaded into thecontrol computer, and the operator will

select the pump of choice based on theapplication circumstances at that in-stant. Once the software has been de-veloped, the cost to reproduce it will bealmost trivial. Compare this to the costof producing thousands of identicalprecision-manufactured parts for hun-dreds of identical pumps.

A manufacturers ability to standard-ize on all the mechanical parts will re-sult in substantial price reductions inproducts.

Now, anyone who has suffered thepains of developing software knows

that this can accrue a significant ex-pense. Software development is a tech-nical specialty, and the quality of thecontrol program will determine the ulti-mate degree of success of the millen-nium pump in the application. Debug-ging can be particularly frustrating tothe uninitiated, because it is usuallyonly the programmer who can under-stand how easy it is to write in the bugs,how difficult they can be to locate, and

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how challenging the corrections can be.

This is especially true when a commit-tee is specifying the final performanceof the machine, but only one program-mer is writing the code.

Most control-software programmersdo not have the luxury of redundancythat designers do. A checker typically re-views each and every dimension and linedrawn by the original designer beforemechanical drawings are finalized. Pro-grammers often work alone. And be-cause most lack lack intimate quantita-tive fluid power skills, they are inclinedto blame the hardware when a system be-

haves unacceptably. Of course, the hy-draulic engineers are prone to blame thesoftware. The real problem, most often,is on the system level.

Simulation and real-time controlLearning simulation and mathemati-

cal modeling is an excellent prepara-tion for developing programs for real-time control. Real-time control refersto the process where the computer pro-gram is written to execute with suffi-cient speed so that its calculations andlogic always stay ahead of the machine

it is controlling. It involves, amongother things, the use of techniques thatnot only result in very efficient (fast ex-ecuting) code, but also synchronize theprogram with the controlled machine.Such is the purpose of wait loops,polling of input devices, and softwareand hardware interrupts.

In the industrial seminars I conduct,control programmers often attend whoneed engineering details of servo andproportionally controlled systems. Moreoften than not, they are electrical engi-neers with programming experience,

which provides them with a good back-ground for modeling of hydraulic sys-tems. Unfortunately, there are enoughdifferences between electric and hy-draulic circuits to make the quantitativechoices difficult for one who is notspecifically schooled in the art of digi-tally controlled hydraulic machinery. Itshould almost be a prerequisite that any-one responsible for writing real-timecontrol software to have developed acompetence in simulation.


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