While all the R/C sets use the same electrical signals for communicating with the ser-vos 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 fre-quency 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 re-ceiver 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 en-code 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 se-quentially with the .35-millisecond end mark between each channel serving as the beginning mark of the next channel. A special framing pulse designates the begin-ning 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 com-mand 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 gaso-line 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 informa-tion, 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 ro-bots, most competitions prohibit the use of AM radios entirely.
Frequency Modulation
A more robust and reliable method for transmitting control signals is to use fre-quency modulation (FM). In an FM radio system, the amplitude of the signal is held constant, and the transmitted information is encoded by varying the fre-quency of the transmitted carrier signal. The FM receiver locks onto the constant
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 datachecksum 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.
The other advantage of PCM radios is that they grant you the ability to customize the control interface. Because the signals are being digitized and encoded, it is easy for the internal computer to perform custom mixing and scaling operations on the data before transmitting it. Known ascomputer radios,these units have a liquid crystal display (LCD) screen and a miniature keypad that can be used to write cus-tom programs for the controller interface. Typical settings include cuscus-tom gain, and center and end points on individual controls, as well as custom mixing of two channels to generate left and right motor drive signals from a single joystick for driving skid-steer robots.
When choosing a radio system, you may want to consider more than just the robot you are currently using. While the rest of a robot may be scrapped, recycled, or even completely destroyed in combat, your R/C system can be reused on robot after robot. If you intend to participate in robotic combat competition year after year, it makes sense to spend a little more on your R/C system at the start, rather than buy-ing a low-end radio and then havbuy-ing to pay more on a better radio down the road.
If you buy a PCM radio with at least seven channels, you will probably never have to buy another radio for as long as you are competing. Most veteran combat robot builders will recommend that if you use a traditional R/C system, you should use a PCM radio with your robot. It will save you a lot of headaches when testing and competing with your robot, since you will know that erratic motion is not due to radio interference.
Tables 8-1 and 8-2 contain short lists of the available R/C systems. The column under “Band, MHz” lists the frequency bands these systems can use. If two different
Manufacturer Model Channels Band, MHz PCM Available
Futaba 3PDF 3 27 and 75 No
3PJS 3 27 and 75 Yes
Airtronics CX2P 2 27 and 75 No
M8 3 27 and 75 No
Hitec Lynx 2 2 27 and 75 No
Lynx 3 3 27 and 75 No
TABLE 8-1 Pistol-Grip–Style Radio Control Systems ■
frequencies are listed, a system can be obtained to operate under either frequency, not both frequencies. The “Channels” column shows the number of servo chan-nels the R/C system can control at once, and the “PCM Available” column lists whether the system uses PCM error-correction controls.