Page 187 - Build Your Own Combat Robot

Build Your Own Combat Robot
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transmitted signal and is much less likely than AM to be distracted by random
electrical noise from the environment. This does not say that FM systems are immune
to radio interference, though, because all radios are subject to radio interference.
However, FM radio signals are far less susceptible to radio interference than AM
radio signals.

Pulse Code Modulation
To further improve the reliability of FM radios, a more advanced system of
signal transmission known as Pulse Code Modulation, or PCM, can be used. A
PCM radio signal uses an FM radio transmission similar to an ordinary FM ra-
dio set, but the servo commands are transmitted as a digital data stream rather
than time-coded pulses.
A PCM receiver contains a microcontroller to develop and interpret the pulse
code for servo control. PCM systems form the servo commands using a set of algo-
rithms and precise code timing. PCM allows accurate signal reception, even when
severe radio frequency interference (RFI) or other noise is present.
The process begins in the transmitter by converting each joystick, switch, trim
knob, and button position into a 10-bit digital word, plus the extra bits to enable
the receiver to verify the word. The PCM radio system compacts this data repre-
senting 1,024 servo positions per channel into the FCC-specified radio band-
width, while maintaining responsive real-time control. The PCM data is
transmitted synchronously; each bit has a particular position in time, within a
frame. The frame continuously repeats. A crystal-controlled clock in the receiver
locks onto the transmitted signal to maintain synchronization with the data, bit by
bit. Thus, the receiver can process data immediately after interference instead of
waiting for a framing pulse.
Received data is evaluated channel by channel. When the microcontroller de-
tects an error, previously stored valid channel data is used. If an error persists,
failsafe servo operations previously specified by the operator are initiated until ac-
curate commands are again received. The microcontroller converts the proper
data into pulse widths to command the servos, and you no longer have servo “jit-
ters.” Some receivers can be programmed to shut down if they receive bad data, or
they can be programmed to output specific commands so that the robot enters a
controlled and safe state. Because the actual data signal and a data checksum sig-
nal are sent at the same time and compared together at the receiver, it is nearly im-
possible for a robot to move out of control accidentally because of radio
interference.