Page 162 - Build Your Own Combat Robot

Chapter 7:
Controlling Your Motors
the heat from it more quickly, either by providing airflow with cooling fans or by 143
attaching the FET to a large heat sink, or both.
The current capacity of an FET switching system can also be increased by wiring
multiple FETs together in parallel. Unlike relays, FETs can be switched on and off
in microseconds, so there is little possibility of one FET switching on before the
others and having to carry the entire current load by itself. FETs also automati-
cally load-share—because the resistance of an FET increases with temperature,
any FET that is carrying more current than the others will heat up and increase its
resistance, which will decrease its current share. Most high-powered commercial
electronic speed controllers use banks of multiple FETs wired in parallel to handle
high currents.
Bi-directional and variable-speed control of a motor can be accomplished with
a single bank of PWM-control FETs and a relay H-bridge for direction switching,
or with four banks of FETs arranged in an H-bridge. A purely solid-state control
with no relays is preferable but electronically more difficult to implement. Building
a reliable electronic controller is a surprisingly difficult task that often takes longer
to get to work than it did to put the rest of the robot together. The design and con-
struction of a radio controlled electronic speed controller is an involved project
that could warrant an entire book of its own.

Commercial Electronic Speed Controllers
Fortunately, several commercial off-the-shelf speed controller solutions are
readily available for the combat robot builder. Several companies make
FET-based motor controllers designed to interface directly to hobby R/C gear;
and many brands of commercial motor drivers and servo amps, with some engi-
neering work, can be adapted to run in combat robots. Building a motor control-
ler from scratch will usually end up costing you more money and more time than
buying an off-the-shelf model, so there is little reason for a robot builder to use
anything other than a pre-made motor control system.

Hobby Electronic Speed Controllers

Hobby ESCs were originally designed to control model race cars and boats. Early
R/C cars often had gas-powered engines, but refinements in electric motors and
the use of nickel-cadmium rechargeable batteries saw a switchover to electric
drive cars. The first systems used a standard R/C servo to turn a rheostat (a
high-power version of a potentiometer) in series with the drive motor to control
the speed of a race car. This system had a bad feature, in that the rheostat literally
“burned away” excess power in all settings except for full speed. Needless to say,
this did not help the racing life of the batteries.
There had to be a better way to conserve battery life and allow better control of
the motors. The result was the hobby electronic speed controller. All of the major
R/C system manufacturers are now producing various styles and capacities of