Build Your Own Combat Robot

FIGURE 8-5
Motor with three
capacitors to
reduce radio
frequency
interference.
Chapter 8: Remotely Controlling Your Robot 171
running. This noise can be picked up by the radio system and can jam or interfere
with the normal control signal. If your robot’s weapons unexpectedly actuate by
themselves when you drive it, or if your robot twitches back and forth by itself
when you trigger the weapon, you may be experiencing radio interference from
your motors that is altering your radio control.
To combat this interference, start by neutralizing it at the source. You cannot
do anything about the arcing at the terminals, but you can divert most of the noise
before it leaves the motor. Small ceramic capacitors can be attached to filter the
noise from the brushes (see Figure 8-5). Capacitors have a low impedance to high
frequencies and can short-circuit the noise before it even leaves a motor’s case.
You should use non-polarized ceramic capacitors in the range of .01 to .1 µF, with
a voltage rating of at least twice your motor’s running voltage. If possible, use
three capacitors—one from each brush terminal to the motor case, and one across
each of the two motor terminals. The capacitors should be connected as close to
the actual brushes as possible, ideally inside the motor case itself, and they should
be mounted carefully and secure to avoid the chance of shorting out the motor if
one comes loose.
What noise that does manage to escape from the motor will radiate from the motor
power wires like a broadcast signal from an antenna. You can minimize this by
twisting the motor wires together (leave the insulation on the wires); the noise emitted
by the motor leads will be significantly reduced. Placing these twisted wires
within a braided shield grounded to the robot’s structure also helps. You can also
reduce the transference of noise from the power system to the radio by placing your
receiver as far as possible from the motors and their wires. Placing the receiver in a
shielded metal container will also help reduce the noise interference.
note Do not run the lines from your radio receiver to the servos and speed controllers
near or parallel to the motor power lines, if you can help it. As current goes through a wire,
a circular magnetic field is generated. If a wire is running parallel to this wire, and it is inside
the magnetic field, the field can induce a current flow in the adjacent wire. The physics behind
this is why motors and transformers work in the first place. Twisting the servo leads and power
leads also helps minimize their tendency to pick up electrical noise from the motor system.