DEPARTMENT OF
ELECTRONICS AND MECHANICAL
INSTRUMENTATION AND MEASUREMENTS:
DC MOTOR CHARACTERIZATION
INTRODUCTION:
Measuring speed is one of the most common applications of using electronic sensors to record
mechanical events. Non-contact optical, magnetic, and capacitive sensors are often utilized
because of their versatility, high reliability and low cost. These sensors do not measure speed
directly, but rather act as simple counters to record the number of revolutions of a cyclic
process, or the number of discrete items on a moving conveyor or similar process. Once the
count has been obtained, a comparison to a known time interval can be used with a scaling
factor to provide standardized units of speed or number of revolutions per unit time. In its most
basic form, the number of counts per unit of time provided by the sensor and acquisition device
result in a measured frequency for the application.
The two major components to be used in this lab in addition to the NI-ELVIS system are a small
permanent magnet, direct-current (DC) motor and a packaged photo interrupter. The photo
interrupter consists of an infrared light emitting diode (IR LED) and a phototransistor detector.
The emitter and detector are separated by a 3mm gap, and objects passing between this gap
provide an output that can be used for counting purposes.
As seen in a previous lab, if the output signal form the phototransistor creates a periodic signal,
the FFT algorithm can be used to determine the fundamental frequency.
Based on the equation,
T
f1
where
f
= frequency, and
T
= period,
the period can be calculated if the frequency is known. Alternatively, if the period is measured,
the frequency can be calculated.
Power to the DC motor will be provided from the +12V variable power supply terminal of the
ELVIS workstation. To reduce programming complexity, the variable power supply will be used
in the MANUAL mode which allows the user to adjust a ¾ turn potentiometer on the front of the
ELVIS to vary the output from 0 to 12V potential. Be sure the voltage is set at “0” before
hooking up the motor leads.
Information on the Panasonic CNZ-1021 photo interrupter can be found on the digi-key.com
website, and the motor (P/N 206949) can be found on the robotstore.com site.
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