Model aircraft radios are designed to control airplanes at ranges over thousands of feet; yet in the arena, robots less than 50 feet away from their controllers can go wildly out of control or fail to move at all. The difference between the two environments is in the ambient radio interference and the antenna placement. Installing a radio that was designed to be run inside a balsa wood or plastic airplane with only small servos and a single glow-plug engine, and making it run inside a metal-cased com-bat robot with large noisy electric or gasoline motors, is more difficult than you might think.
The first challenge to overcome is radio interference, most of which will come from inside the robot itself. As a brush-type DC motor turns, the sliding contact of the brushes over the commutator segments is constantly making and breaking cir-cuits and reversing the flow of current in the motor’s armature winding segments.
This constant arcing creates high-frequency electrical noise whenever the motor is
Manufacturer Model Channels Band, MHz PCM Available
Futaba 4VF 4 72 and 75 No
6VH 6 72 No
6XAS 6 50 and 72 No
6XAPS 6 72 Yes
8UAPS 8 50 and 72 Yes
9ZAS 9 50 and 72 Yes
Airtronics VG400 4 72 No
VG600 6 72 and 75 No
RD6000 6 72 Yes
Hitec Ranger 3 3 27 and 75 No
Laser 4 4 72 No
Laser 6 6 72 No
Eclipse 7 7 72 Yes
TABLE 8-2 Stick-Style Radio Control Systems ■
running. This noise can be picked up by the radio system and can jam or interfere with the normal control signal. If your robot’s weapons unexpectedly actuate by themselves when you drive it, or if your robot twitches back and forth by itself when you trigger the weapon, you may be experiencing radio interference from your motors that is altering your radio control.
To combat this interference, start by neutralizing it at the source. You cannot do anything about the arcing at the terminals, but you can divert most of the noise before it leaves the motor. Small ceramic capacitors can be attached to filter the noise from the brushes (see Figure 8-5). Capacitors have a low impedance to high frequencies and can short-circuit the noise before it even leaves a motor’s case.
You should use non-polarized ceramic capacitors in the range of .01 to .1µF, with a voltage rating of at least twice your motor’s running voltage. If possible, use three capacitors—one from each brush terminal to the motor case, and one across each of the two motor terminals. The capacitors should be connected as close to the actual brushes as possible, ideally inside the motor case itself, and they should be mounted carefully and secure to avoid the chance of shorting out the motor if one comes loose.
What noise that does manage to escape from the motor will radiate from the mo-tor power wires like a broadcast signal from an antenna. You can minimize this by twisting the motor wires together (leave the insulation on the wires); the noise emit-ted by the motor leads will be significantly reduced. Placing these twisemit-ted wires within a braided shield grounded to the robot’s structure also helps. You can also reduce the transference of noise from the power system to the radio by placing your receiver as far as possible from the motors and their wires. Placing the receiver in a shielded metal container will also help reduce the noise interference.
n o t e Do not run the lines from your radio receiver to the servos and speed controllers near or parallel to the motor power lines, if you can help it. As current goes through a wire, a circular magnetic field is generated. If a wire is running parallel to this wire, and it is inside the magnetic field, the field can induce a current flow in the adjacent wire. The physics behind this is why motors and transformers work in the first place. Twisting the servo leads and power leads also helps minimize their tendency to pick up electrical noise from the motor system.
FIGURE 8-5 Motor with three
capacitors to reduce radio frequency interference.
Of course, minimizing the transmission of noise from one system to another does no good if your radio control and power circuits are not electrically isolated.
No common ground or shared power source should exist between your radio and your drive motor power. Electronic speed controllers (ESCs) that make a direct electrical connection between the servo signal line and the motor battery, or those that tap power off the drive batteries to feed to the radio (known as abattery eliminator circuit, or BEC), should not be used. Electrical isolation through opto-isolators or relays should be mandatory. A separate battery should be used to power the radio. If a power converter is used to provide power to the radio from the motor batteries, it should be a type with full electrical isolation, such as the Team Delta’s R/CE85-24.
n o t e If speed controllers with BEC must be used, the power pin connecting the ESC to the receiver can be removed from the connector and insulated to prevent an electrical connection.
A separate battery should then be used to power the receiver.
Gasoline engines can be a huge source of electrical noise—particularly the small, RPM, two-stroke motors used in chainsaws and lawn trimmers. The high-voltage pulses generated by the ignition system can play massive havoc with a nearby R/C system. To prevent noise from the engine from getting into the radio circuitry, place the radio control system in a metal box, test the servo leads for in-terference, and keep the distance between the radio receiver and the engine’s elec-trical system as far as possible in the robot. The elecelec-trical noise that is radiated from the motor can be minimized by using resistor-type spark plugs and replacing the ignition wire with a shielded line. Resisting this sort of electrical noise is where PCM radios really prove themselves to be worth the extra money. The error-checked digital transmission system is much better at rejecting extraneous noise than simpler non-PCM setups.
Radio to Radio Interference
Radio interference commonly occurs when two radios transmit on the same fre-quency. In such a case, your robot will have a difficult time distinguishing between the two signals. The robot can stop responding, or it might respond to whichever radio has the strongest output power, or it might do some combination of the two.
This can be a dangerous situation, because the robot can suddenly start to move or trigger weapons when it shouldn’t. You should always carry various frequency crystals with you, and make sure that you are the only robot driver transmitting at a particular frequency. As noted, this is ensured at some events by the transmitter impound.
Some people build their own R/C systems that transmit under the 300-MHz, 900-MHz, 1.2-GHz, and 2.4-GHz frequency bands. Many companies sell products designed to transmit data or control signals that can be used to control a robot.
Some of these systems offer more control flexibility than traditional R/C systems.
The drawback of using these frequencies is that other ground-use systems also transmit at the same frequencies. For example, cordless phones transmit at the 900-MHz and 2.4-GHz frequencies. A cordless phone near your robot could cause radio interference with your robot. Because of this, it is recommended that you use only radio transmitting equipment that has built-in error correction methods that can filter out unwanted information, such as the IFI Robotics system.