EATON
HFX Product Family Installation and Operation Manual E-ELCL-II002-E2 July 2019
35
18.0
OUTPUT CIRCUIT (PASSIVE REPRESENTATION)
18.1
Overcurrent Shutdown
The individual outputs of the unit are fully protected
and will switch off if overloaded. When diagnostics
are enabled and an overcurrent event occurs,
the output will switch and remain off until another
off/on command is received. The overcurrent
shutdown is dependent upon the specific output.
2 A rated outputs have a slow blow level of
approximately 2.1 A and a fast blow level of
approximately 2.4 A. 4 A rated outputs have
a slow blow level of approximately 4.1 A and a fast
blow level of approximately 4.4 A. All outputs are
additionally protected via an internal solid state relay.
The relay can be active monitored for current and
controlled in the user program.
19.0
Pulse Width Modulation (PWM)
19.1
What is PWM?
PWM stands for Pulse Width Modulation. It is
a method that can be used to efficiently drive
solenoid valves. Typically the output device is
switched at a fast rate (60 Hz – 2 kHz).
In the past solenoid valves were driven using
a transistor or op amp in linear mode. This worked
well to control the valve as a directly proportional
voltage signal could be easily controlled using
feedback. The negative of this technique is that
it generates a great deal of heat, is inefficient, and
requires a larger enclosure since the output device
is operated in between cutoff and saturation and is
behaving like a variable resistor constantly having to
dissipate the power not used by the load as heat.
PWM uses the output device digitally and therefore
the device is either on or off. Using this technique,
the output device supplies a series of pulses of the
same voltage level to the load. Since transistors are
very efficient when either on or off, much less heat is
dissipated. Varying the duty cycle (on time/switching
period) the output power is modulated effectively
emulating an analog signal, especially at the higher
switching frequencies through a solenoid valve coils
inductance.
Current feedback can be used to more effectively
control the valve. With a known current flowing
through the valve coil, the valve spool position can be
precisely determined. The added advantage of this
method is that it is independent of temperature.
19.1.1 Dither
Dither is a small modulation of the PWM signal. This
is intended to compensate for stiction and hysteresis
by continually changing the PWM signal slightly and
thus keeping the valve spool in constant motion.
This can dramatically improve the valve performance
by improving response to small signal changes.
The following oscilloscope images illustrate both the
PWM voltage and current waveforms. Also illustrated
are the effects of varying PWM duty cycle and of
adding dither.
19.1.2.1 PWM
Depiction of 2 kHz PWM. Lower waveform is
voltage, upper is current through a typical inductance.
19.1.2.2 PWM
Depiction of 2 kHz PWM waveforms (note faster
time scale factor of 10). Lower waveform is voltage,
upper waveform is current through a typical
inductance.
HFX Product Family
