Build Your Own Combat Robot

180 Build Your Own Combat Robot
FIGURE 8-7
A commercially
available onechannel
failsafe
unit. (courtesy of
Futaba)
Some AM and FM radios have the unfortunate habit of transmitting a few garbled
servo pulses when they are switched on or shut off. Known as chirp, this behavior
can cause the robot to twitch or fire its weapon when the radio is switched
on or off. When using this kind of radio, the operator should adopt a policy of
never switching the radio on or off when the robot is powered up; instead, the radio
transmitter should be switched on before the robot is turned on, and it should
stay on until after the robot is powered down.
Many of these problems can be solved with a failsafe board (Figure 8-7). Several
manufacturers of radio control equipment sell modules, such as Futaba’s
FP-FSU1 Fail Safe Unit, which is connected between the radio receiver and the R/C
servo or electronic speed controller. The failsafe board monitors a signal from the
radio receiver; and in the event of a lost or badly garbled signal, the board generates
a servo signal output that commands the servo to move to a preset level, or it
shuts off the attached electronic speed controller. Some failsafe boards will even
store enough power to center a servo in the event of battery failure.
Radio systems with computerized receivers, such as the PCM-type receivers, are
smart enough to recognize when the radio signal has been lost and take appropriate
action. Depending on the controller type and parameter settings, the shutdown
behavior might be to return all outputs to a preset level or to keep all outputs at
whatever level they were in when radio contact was lost. The latter is the default
behavior on many model airplane and helicopter radios because it will keep the
plane or helicopter in stable flight until radio control is regained. But this is not the
behavior you want in a combat robot radio; it will cause your robot to keep moving
on radio contact loss. This behavior is usually programmable. For a combat robot,
the failsafe units should be programmed to shut down all motors, apply brakes to
spinning weapons, and move servos to a safe position.