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8 Hydraulics & Pneumatics s September 2002

Analog hydraulic valves bothservo and proportional

have served industry well for sev-eral decades. Of course, for veryprecise applications, system design-ers had to learn to make allowancesfor their limitations. Among them:

susceptibility to electromagneticinterference and transmissionlosses over long lengths of cable.Care had to be taken to ensure in-tegrity of low-power analog signals. only modest diagnostic and mon-

itoring capabilities. Typically, ana-log valves provide only an all ornothing digital fault signal. Withthis limited information, trackingdown a root-cause problem can bea time-and-money-con-suming task. inflexibility. Functional-ity is set at the factory.Analog valves need cen-tralized control-system ar-chitecture with a power-ful central controller,

which must interface witheach independent deviceon the machine.

Today, electronic tech-nology and miniaturiza-tion can bypass these limi-tations of analog valves. AtMoogs Industrial ControlsDiv., we believe that thetime for digital valves has ar-rived. Our experience with this newproduct platform suggests that thefuture will focus on configurable,

outer-loop motion and pressureclosed-loop controls directly atthe valve.

Digital valves integral micro-processors can close control loopsand provide extensive device-mon-itoring, fault-detection, and com-munication capabilities. They offerthe many benefits of open and de-centralized system architecture

from reduced installation andwiring costs to greater functional-ity, increased flexibility, and sim-plified valve setup.

Today, sectors of the hydraulicsindustry are driving towards open-architecture components. Open ar-chitecture calls for such develop-ments as standardized cables andconnectors, clearly defined data-exchange mechanisms, and pub-lished device profiles. Open archi-tecture interfaces protect machine

builders investments in the devel-opment of digital device communi-cation software.

At the protocol layer, Moogs ini-tial Digital Interface Valve (DIV)

conforms

to CANopen. Thisstandard clearly defines the

data-exchange mechanism so thatthe inter-operability between con-forming devices is guaranteed. At

the application layer, the DIV com-plies with the Proportional ValveDevice Profile (first proposed bythe VDMA organization of Ger-many). This standard specifies thefunctionality of all devices of thesame family. Detailed knowledgeof the fieldbus is unnecessary witha human/machine interface inplace, so system designers can fo-

cus on the application.The DIV fits well into both cen-

tralized and decentralized systems.One benefit of decentralized archi-tecture is that serial wiring to eachcomponent replaces parallel wiring.Serial wiring reduces installationand connection costs. It also en-hances flexibility because systemscan be changed or extended easily.

Tasks of subsystems are assigned tolocal devices, reducing the com-plexity and required processing

power of the main controller.The DIVs embedded digital mi-

croprocessor is the foundation forenhanced functionality and flexibil-ity (as compared with its analog

counterparts). Configurablevalve functions enableusers to define the valvesdynamic behavior andadapt its characteristics toparticular application re-quirements. Field-program-mable functions include

four-quadrant demandramp conditioning, deadband compensation, de-mand limits, direction de-pendent gains, and access

to valve tuning parameters. De-tailed diagnostic status information,available over the digital bus, im-proves safety and reliability andsimplifies maintenance. This infor-mation includes programmable con-trol deviation monitoring, vendorname, model numbers and the like.

Elimination of external analog de-mand conditioning boards and sim-plified P/Q valve setup save cost.

Daniel Halloran, head of the Con-trols and Electronics Section atMoogs Industrial Controls Div.,provided this discussion. ContactMoog at 800 / 272-6664 or visitwww.moog.com/industrial.

Ideas&Applications ByRichard T. Schneider

Moogs Digital Interface Valve is

based on integral 32-bit, floating-

point microprocessor that closes

all control loops. Extensive device

monitoring and fault detection are

built in. Valve supports CANbus

interface, one analog input,

and one analogoutput.

Consider the advantages of digital hydraulic valves