Build Your Own Combat Robot

Determining the Motor Constants
Chapter 4: Motor Selection and Performance 67
Grant Imahara and Deadblow (continued)
With only an hour left and a 20-minute drive to get back to the competition,
Grant still wasn’t overly concerned. “But then we hit Sunday evening traffic back
into San Francisco. We were going to be late. Forty-five minutes later, I ran into
Fort Mason with the new hammer in hand. And we threw it into the robot.” As the
announcer called Team Deadblow to line up for the fight, they were still screwing
the armor back onto the robot. “If you look carefully,” Grant says, “you can see that
my normally put-together look had become severely disheveled. I was out of breath
and about to pass out and the match hadn’t even started yet! I had a ‘go for broke’
attitude for that match, and the adrenaline was pumping. Deadblow went in and
pummeled Pressure Drop with a record number of hits. By the end, I could barely
feel my hands because they were tingling so much.”
To use the equations, the motor constants, K v , K t , I 0 , and R must be known. The
best way to determine the motor constants is to obtain them directly from the motor
manufacturer. But since some of us get our motors from surplus stores or pull
them out of some other motorized contraption, these constants are usually unknown.
Fortunately, this is not a showstopper, because these values can be easily
measured through a few experiments.
You’ll need a voltmeter and a tachometer before you start. To determine the
motor speed constant, K v , run the motor at a constant speed of a few thousand
RPMs. Measure the voltage and the motor speed, and record these values. Repeat
the test with the motor running a different speed, and record the second values.
The motor speed constant is determined by dividing the measured difference in the
motor speeds and the difference between the two measured voltages:
All permanent magnet DC motors have this physical property, wherein the
product of the motor speed constant and the motor torque constant is 1352. With
this knowledge, the motor torque constant can be calculated by dividing the motor
speed constant by 1352. The units for this constant is (RPM / Volts) × (oz.-in. / amps).
Equation 13 shows this relationship.
The next step is to measure the internal resistance. This cannot be done using
only an ohmmeter—it must be calculated. Clamp the motor and output shaft so
that they will not spin. (Remember that large motors can generate a lot of torque
and draw a lot of current, so you need to make sure your clamps will be strong
4.12
4.13