Page 83 - Build Your Own Combat Robot

Build Your Own Combat Robot
64
that by looking at Equation 4, the voltage must also exceed the no-load current
multiplied by the internal resistance for the motor to start turning.
4.5
Some motors advertise their no-load speed and not their no-load current. If the
motor’s specifications list the internal resistance of the motor, the no-load current
can be determined from equation 4.
With these equations, as well as the gear ratio, wheel size, and coefficient of
friction between wheels and floor, you can determine how fast the robot will move
and how much pushing force the robot will have. (How you actually determine
this will be explained in Chapter 6.) If you want the robot to go faster, you can ei-
ther run the motors at a higher voltage or choose a lower gear reduction in the
drive system.
Equation 5 is an important equation to know and understand, because it will
have a direct effect on the type and size of the batteries that you will need. By rear-
ranging this equation, the current draw requirements from your batteries can be
determined. Equation 6 shows this new relationship.

4.6

For any given torque or pushing force, the battery current requirements can be
calculated. For worst-case situations, stalling the motors will draw the maximum
current from the batteries. Equation 7 shows how to calculate the stall current,
where I is the stall current in amps. The batteries should be sized to be able to de-
stall
liver this amount of current. Batteries that deliver less current will still work, but
you won’t get the full performance potential of the motors. Some builders pur-
posely undersize the battery to limit the current and help the motors and electron-
ics survive, and others do this simply because they have run out of weight
allowance. For some motors, the stall current can be several hundreds of amps.

4.7

Another set of relationships that needs to be considered is the overall power being
supplied by the batteries and generated by the motor. The input power, P ,tothe
in
motor is shown in equation 8. Note that it is highly dependent on the current draw
from the motor. The output power, P , is shown in mechanical form in equation 9
out
and in electrical from in equation 10. Motor efficiency is shown in equation 11.
The standard unit of power is watts.
4.8

4.9
4.10