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  • Topic: Pulse-width modulation, Square wave, Profibus
  • Pages : 17 (2851 words )
  • Download(s) : 126
  • Published : November 27, 2012
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Assume,.for.example,.that.the.encoder.generates.600.pulses. per.revolution,.and.it.takes.000.motor.revolutions.to.move. the.platform.from.one.position.to.another,.moving.the.platform. from.position..to.position.6.(5.positions).takes.5000.motor. revolutions.or.30,000.encoder.pulses..In.most.practical. applications,.the.frequency.of.these.pulses.is.too.high.for.them. to.be.counted.with.inputs.that.are.not.associated.with.a.highspeed.counter.. Interrupts. S7-200.PLCs.incorporate.instructions.for.use.with.interrupts.. Interrupts.are.used.to.initiate.a.specific,.short.PLC.program. segment,.called.an.interrupt.routine,.when.an.internal.or. external.event.occurs..After.the.interrupt.routine.has.been. executed,.control.is.returned.to.the.main.program. Three.types.of.interrupts.are.supported.by.S7-200.PLCs,. communication port interrupts,.I/O interrupts,.and.timebased interrupts..Communication.port.interrupts.are.used. to.control.a.communication.port.operated.in.Freeport.mode.. I/O.interrupts.are.used.to.respond.quickly.to.high-speed.I/O. transitions,.such.as.those.associated.with.high-speed.counters. or.pulse.train.outputs..Time-based.interrupts.allow.the.user. program.to.execute.an.interrupt.routine.on.a.cyclic.basis. Each.of.these.types.of.interrupts.has.an.associated.priority. that.determines.which.interrupt.is.processed.first.in.the.event. that.two.or.more.interrupts.are.requested.at.the.same.time.. Communication.port.interrupts.have.the.highest.priority.and. time-based.interrupts.have.the.lowest.priority. Pulse Training Output (PTO). S7-200.PLCs.have.two.PTO/PWM generators.that.create.either. a.high-speed.pulse.train.or.a.pulse.width.modulated.waveform.. One.generator.is.assigned.to.output.point.Q0.0.and.the.other.to. output.point.Q0...When.a.generator.is.activated,.it.controls.its. respective.output. Pulse Train Output (PTO).is.used.to.provide.a.series.of.pulses. to.an.output.device,.such.as.a.stepper.motor.driver..The.PTO. provides.a.square.wave.output.for.a.specified.number.of.pulses. and.a.specified.cycle.time..The.number.of.pulses.can.be.from.. to.4,294,967 ,295.pulses..The.Pulse.Train.Output.has.a.50%.duty. cycle..This.means.the.pulse.is.off.for.the.same.amount.of.time. that.it.is.on.

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The.number.of.pulses.and.the.cycle.time.can.be.changed.with. an.interrupt..In.the.accompanying.example,.each.pulse.is.initially. on.for.500.ms.and.off.for.500.ms..After.four.pulses,.an.interrupt. occurs.which.changes.the.cycle.time.to.2.seconds,..second.on. and..second.off. . 1 sec 1 sec

500 ms

Interrupt Occurs

Pulse Width Modulation. (PWM)

The.Pulse Width Modulation (PWM) function.provides.a. fixed.cycle.time.with.a.variable.duty.cycle..When.the.pulse. width.is.equal.to.the.cycle.time,.the.duty.cycle.is.00%.and.the. output.is.turned.on.continuously..In.the.following.example,.the. output.initially.has.a.0%.duty.cycle.(on.0%,.off.90%)..After.an. interrupt,.the.output.switches.to.a.50%.duty.cycle.(on.50%,.off. 50%). On Off On Off

10% Duty Cycle 50% Duty Cycle Interrupt Occurs

The.PWM.function.can.be.used.to.provide.a.programmable. or.adjustable.control.of.machine.timing..This.allows.machine. operation.to.be.varied.to.compensate.for.product.variations.or. mechanical.wear. And Much More. The.instructions.listed.in.this.section.are.only.examples.of. the.types.of.instructions.available.for.S7-200.PLCs..The.full. instruction.set.includes.a.much.broader.range.of.capabilities.. Refer.to.the.S7-200.System.Manual.for.additional.information.

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Specialized Expansion Modules

In.addition.to.the.expansion.modules.previously.discussed. that.provide.additional.discrete.or.analog.I/O,.several.expansion. modules.are.available.to.provide.communication.interfaces.or. specialized.I/O.functions. EM 241 Modem Module One.of.these.modules.is.the EM 241 Modem module..This. module.supports.communication.between.a.computer.with. STEP.7.Micro/WIN.and.an.S7-200.PLC.

SF/DIAG

EM 241 MODEM

241-1AA22-0XA0

S7-200 PLC...
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