A stroboscope & frequency counter design
To measure the speed of rotation of a motor, or any other part of teh robot,
it is useful to have a stroboscope. This allows a non-contact method to
measure the rotational speed, simply by drawing a chalk mark on the item
to be measured. The area is then lit by a flashing light, whose frequency
is adjusted upwards until the mark appears stationary. The rotational speed
is then equal to the frequency of the flashing, which can be read off the
display of the stroboscope.
The design here uses a voltage to frequency converter driving a high-brightness
LED to generate the flashing light, and a single-chip frequency counter
IC from Intersil to drive a 4-digit 7-segment display to display the frequency.
The frequency goven is in Hz, to convert this into RPM, use:
rpm = frequency * 60
2. A strobe circuit
2.1. Circuit diagram
2.2. Circuit description
The AD654 is used to generate the strobe frequency. This converts a voltage
(from teh potentiometer) to a logic level frequency. The transistor then
drives the high-brightness LED. The circuit values for this part is directly
from the AD654 datasheet. The trimmer resistor is to set the maximum range
of the frequency. We need a maximum of around 20,000rpm, which is 333Hz.
The capacitor value sets this range, which is fine tuned with the trimmer.
3. The frequency counter / tachometer section
3.1. Frequency counter on a meter
Some DMMs also have a frequency counter mode on them. If you need to buy
a DMM, get one with a frequency counter. Note however that some of them
have a very restrictive frequency range, sometimes only starting at several
kHz. If you also need to measure high DC currents, the clamp
meter available from Maplin is very good.
3.2. Tachometer module
Maplin also sell a tachometer section whoch can be used to measure the
frequency of the flashing LED section of the stroboscope. Using this you
can do away with the ICM7224 section. Maplin order code VE01B.