Build Your Own Combat Robot

Chapter 8: Remotely Controlling Your Robot 167
AM, FM, PCM, and Radio Interference
While all the R/C sets use the same electrical signals for communicating with the servos
and motor speed controllers, they differ in how they deliver that information
from the radio transmitter to the radio receiver. Most R/C sets use a single radio frequency
to transmit the control information from the transmitter to the receiver. To
deliver information to drive multiple servo channels, the servo pulse information is
transmitted serially, one pulse following another on the radio signal.
The transmission of control information between the transmitter and the receiver
is usually sent as radio waves in one of two different ways: AM or FM.
Amplitude Modulation
In an AM radio system, the strength of the transmitted radio signal is varied to encode
the control information. This means that the radio signal is being switched
between high and low power output levels to encode the pulse data stream. AM
radio transmission is inexpensive and easy to implement electrically, but it is
highly susceptible to radio interference.
The AM transmitter sends each channel’s servo position as an analog pulse
with a width that varies from 1 to 2 milliseconds. All the pulses are transmitted as
a continuously “on” radio frequency (RF) carrier, with each channel’s beginning
and ending marked by an “off” for 0.35 millisecond. All the channels are sent sequentially
with the .35-millisecond end mark between each channel serving as the
beginning mark of the next channel. A special framing pulse designates the beginning
of the channel series by resetting the receiver. The receiver uses the marks to
determine which servo to control based on the proper 1- to 2-millisecond command
pulse. Any radio interference could be interpreted as a marker and cause the
servos to go to a wrong position or to sit and “jitter” erratically.
Using AM, any electrical noise from electric motors, fluorescent lights, or gasoline
engines, for example, can cause unwanted movement of the robot because the
electrical noise can be added to the original AM transmitting signal. Because AM
receivers interpret the intensity of the incoming radio signal as specific information,
they have trouble distinguishing electrical noise from the actual transmitted
signals. This results in the receiver sending false signals to the motor controllers
and servos. Because AM radios may cause uncontrolled movement in combat robots,
most competitions prohibit the use of AM radios entirely.
Frequency Modulation
A more robust and reliable method for transmitting control signals is to use frequency
modulation (FM). In an FM radio system, the amplitude of the signal is
held constant, and the transmitted information is encoded by varying the frequency
of the transmitted carrier signal. The FM receiver locks onto the constant