Product specifications

Instrumentation Laboratory Exercise 47
Power up the motor and read the sense resistor voltage, and the trace
from the foto- sensor. Display the foto- sensor trace in a Strip Chart
graph (found in the Display menu), and convert the sense voltage to
motor current by dividing by the resistance. Spinning the motor at
5 V with no load (without rubbing the spinning disk) you should get
like the following:
We now need to analyze the foto- sensor wave form to determine the
speed of the motor. The speed of the motor is related to the period of
the pulses (t) by the following:
Where n is the number of pulses per revolution (eight in this case),
t is the period from one pulse to the next (in seconds) and Speed is
in Hz (cycles per second).
To calculate the period we need to loop through the data and
determine when the signal crosses a certain threshold value. The
threshold is a voltage that the signal crosses once each cycle. We
define our upper threshold (UT) as being half way between the
maximum value of the wave form and the average value of the wave
form. Placing the UT in the upper half of the signal reduces the
sensitivity to noise on the lower half of the wave form.
Speed 1 nt×()⁄=