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Motor Commutator Noise Filter without Phase Margin Degradation

IP.com Disclosure Number: IPCOM000110587D
Original Publication Date: 1992-Dec-01
Included in the Prior Art Database: 2005-Mar-25
Document File: 3 page(s) / 116K

Publishing Venue

IBM

Related People

Malmberg, JE: AUTHOR [+2]

Abstract

Disclosed is a non-linear filter which discriminates against high frequency impulse noise while introducing negligible phase shift at lower signal frequencies. The specific application for which the filter was developed involved the suppression of brush noise in a motor servo loop, however, the technique is general and can be applied in any instance where the signals must be separated from impulse noise. The filter uses the high dV/dt of the impulse noise to forward bias a diode which then clamps the peak of the impulse. The desired signals, having lower frequencies, do not generate the necessary dV/dt to forward bias the diode and are consequently not affected by the filter.

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Motor Commutator Noise Filter without Phase Margin Degradation

       Disclosed is a non-linear filter which discriminates
against high frequency impulse noise while introducing negligible
phase shift at lower signal frequencies.  The specific application
for which the filter was developed involved the suppression of brush
noise in a motor servo loop, however, the technique is general and
can be applied in any instance where the signals must be separated
from impulse noise.  The filter uses the high dV/dt of the impulse
noise to forward bias a diode which then clamps the peak of the
impulse.  The desired signals, having lower frequencies, do not
generate the necessary dV/dt to forward bias the diode and are
consequently not affected by the filter.

      The motors used on the 3490 tape drives are brush-type DC
motors.  Noise generated by the commutation process had a detrimental
effect on the system.  Attempts to use capacitors to bypass the noise
or to limit the noise peaks with Metal Oxide Varistor (MOV) devices
were contraproductive.  Capacitors or MOV devices large enough to
suppress the noise introduced so much capacitance into the circuit
that the loop stability of the motor current driver circuit was
unacceptably degraded.

      The problem in essence was to filter out pulses having fast
rise times and large peak over-voltages without affecting the lower
frequency components.  The use of higher order filters was
unacceptable because the current levels involved were several Amps
and the cost of inductors would have been prohibitive.

      The filter shown in Fig. 1 is the design implemented.

      The response of the filter to low amplitude, low frequency
signals is not affected by the diodes and is
                         Z=R (1+RCs)
                              (1+(R+RL)Cs)
where RL is the load resistance.  At low frequencies, s approaches
zero and the impedance is purely resistive (no phase shift).  Thus,
at low frequencies, the filter circuit appears as a resistor since
the dV/dt associated with low frequencies is not sufficient to
develop enough current through R to cause the diodes to conduct.  If
a noise pulse with rapid rise time (high dV/dt) and large amplitude
is applied, the current in the resistor R rises to the point where
the voltage across R exceeds the forward conduction threshold of one
or the other of the diodes and the dynamic resistance of the circuit
becomes essentially that of the capacitor C connected to ground in
parallel with the load resistance RL .
                         Z= ____RL
         ...