Eaton HFX Series Installation And Operation Manual

EATON HFX Product Family Installation and Operation Manual E-ELCL-II002-E2 July 20193518.0 OUTPUT CIRCUIT (PASSIVE REPRESENTATION)18.1 Overcurrent ShutdownThe individual outputs of the unit are fully protectedand will switch off if overloaded. When diagnosticsare enabled and an overcurrent event occurs,the output will switch and remain off until anotheroff/on command is received. The overcurrentshutdown is dependent upon the specific output.2 A rated outputs have a slow blow level ofapproximately 2.1 A and a fast blow level ofapproximately 2.4 A. 4 A rated outputs havea slow blow level of approximately 4.1 A and a fastblow level of approximately 4.4 A. All outputs areadditionally protected via an internal solid state relay.The relay can be active monitored for current andcontrolled in the user program.19.0 Pulse Width Modulation (PWM)19.1 What is PWM?PWM stands for Pulse Width Modulation. It isa method that can be used to efficiently drivesolenoid valves. Typically the output device isswitched at a fast rate (60 Hz – 2 kHz).In the past solenoid valves were driven usinga transistor or op amp in linear mode. This workedwell to control the valve as a directly proportionalvoltage signal could be easily controlled usingfeedback. The negative of this technique is thatit generates a great deal of heat, is inefficient, andrequires a larger enclosure since the output deviceis operated in between cutoff and saturation and isbehaving like a variable resistor constantly having todissipate the power not used by the load as heat.PWM uses the output device digitally and thereforethe device is either on or off. Using this technique,the output device supplies a series of pulses of thesame voltage level to the load. Since transistors arevery efficient when either on or off, much less heat isdissipated. Varying the duty cycle (on time/switchingperiod) the output power is modulated effectivelyemulating an analog signal, especially at the higherswitching frequencies through a solenoid valve coilsinductance.Current feedback can be used to more effectivelycontrol the valve. With a known current flowingthrough the valve coil, the valve spool position can beprecisely determined. The added advantage of thismethod is that it is independent of temperature.19.1.1 DitherDither is a small modulation of the PWM signal. Thisis intended to compensate for stiction and hysteresisby continually changing the PWM signal slightly andthus keeping the valve spool in constant motion.This can dramatically improve the valve performanceby improving response to small signal changes.The following oscilloscope images illustrate both thePWM voltage and current waveforms. Also illustratedare the effects of varying PWM duty cycle and ofadding dither.19.1.2.1 PWMDepiction of 2 kHz PWM. Lower waveform isvoltage, upper is current through a typical inductance.19.1.2.2 PWMDepiction of 2 kHz PWM waveforms (note fastertime scale factor of 10). Lower waveform is voltage,upper waveform is current through a typicalinductance.HFX Product Family