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The graph was plotted from tests using four ferrous blocks set equidistantly on a 406 mm
(16 inch) diameter circle on a non-ferrous disc.
The physical shape of the ferrous target generally becomes important at low velocities or
large gaps. At these points, tests indicate that a cubic shape gives the best results due to the
sudden change it causes in the magnetic field.
An increase in block size beyond 50 x 50 x 25 mm (2 x 2 x 1 inch) is generally not as effective
as minimizing the gap, except at very low velocities.
Milltronics manufactures probes to suit a wide variety of environments: low temperature,
high temperature, corrosive, and Class I, II, and III applications.
Pre-Amplifier (IMA and RMA)
The pre-amplifier accepts the voltage pulses generated by the probe and converts them into
noise-immune current pulses. Current levels are 12 mA low and 45 mA high. The preamplifier
comes internally mounted in the probe, or in an enclosure for remote mounting.
Internally mounted pre-amplifiers are called IMAs. Remote mounted pre-amplifiers are called
RMAs.
MFA 4p Operation
The MFA 4p provides a short circuit protected, +24 V DC unregulated supply to the preamp.
In the event that the interconnecting wiring is shorted, output current from the MFA 4p is
automatically limited and the on-board alarm relays are de-energized to indicate failure.
The output current pulses from the pre-amp are super-imposed onto the DC current supply.
These are monitored by Probe LED 1, which is illuminated at the rate of the incoming pulses
and is useful for positioning the probe.
The rate at which the pulses are received by the MFA 4p is compared to a setpoint reference
signal from the time base generator.
Although two pulses within range are required to energize the relays, as long as the
frequency of the incoming pulses exceeds the setpoint frequency (or is less than that of the
setpoint in the case of overspeed detection), the MFA 4p keeps the alarm relays energized.
The reference generator is frequency adjustable by the pulses per minute (ppm) switch and
potentiometer.
Milltronics MFA 4p
Operating Instructions, 12/2020, A5E33988839-AC
Technical Reference
A.1 Principles of Operation
33
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