Science Fair Project Encyclopedia
Radio control is the use of radio signals to remotely control another device. The term is used almost universally to refer to the control of model cars, boats and airplanes from a user-held control box.
Radio control was originally developed during World War II, primarily by the Germans who used it in a number of missile projects. Their main effort was the development of radio controlled missiles and glide bombs for use against shipping, a target that is otherwise both difficult and dangerous to attack. However by the end of the war the Luftwaffe was having similar problems attacking allied bombers, and developed a number of radio controlled anti-aircraft missiles, none of which saw service.
The effectiveness of their systems was greatly reduced by British efforts to jam their radio signals. After initial overwhelming successes, the British launched a number of commando raids to collect the missile radio sets. Jammers were then installed on British ships, and the weapons basically "stopped working". The German development teams then turned to wire guidance once they realized what was going on, but these systems were not ready for deployment until the war had already moved to France.
Both the British and US also developed radio control systems for similar tasks, in order to avoid the huge anti-aircraft batteries set up around German targets. However none of these systems proved usable in practice, and the one major US effort, Project Aphrodite, proved to be far more dangerous to its users than to the target.
Radio control systems of this era were generally mechanical in nature. A small radio receiver was placed in the missile, the signal from the controller (transmitter) was "played" into a small speaker. In front of the speaker were a number of small metal "fingers" with different resonant frequencies, each one tuned to vibrate when a particular tone was played in the speaker (a so called reed relay). The vibration would push on electrical contacts connected to the actuators of the control surfaces of the missile. The controller's radiotransmitter would play the different frequencies in response to the movements of a control stick. These were typical on/off signals.
These systems were widely used until the 1960s, when the increasing use of solid state systems greatly simplified radio control. The mechanical resonant systems using reed relays were replaced by similar electronic ones, and the continual miniaturization of electronics allowed more signals, referred to as control channels, to be packed into the same package. While early control systems might have two or three channels using Amplitude modulation, modern systems include 20 or more using Frequency modulation.
Typical radio control systems for Radio controlled models employ pulse width modulation (PWM) or pulse position modulation (PPM), and actuate the various control surfaces using servos. These R/C systems made 'proportional' control possible, where the position of the control surface in the model is proportional to the position of the control stick on the transmitter.
In the type of system most commonly used today PWM is used, where transmitter controls change the width (duration) of the pulse for that channel between 920 Ás) and 2120 Ás, 1520 Ás being the centre (neutral) position. The pulse is repeated in a frame of between 14 and 20 milliseconds in length. Off-the-shelf servos respond directly to pulse trains of this type using integrated decoder circuits, and in response they actuate a rotating arm or lever on the top of the servo. An electric motor and reduction gearbox is used to drive the output arm and a variable resistor or "potentiometer". The latter produces a voltage proportional to the output position which is then compared with the position commanded by the input pulse and the motor is driven until a match is obtained. The pulse trains representing the whole set of channels is easily decoded into separate channels at the receiver using very simple circuits such as a johnson counter . The relative simplicity of this system allows receivers to be small and light, and has been widely used since the early 1970s.
More recently, high-end hobby systems using "Digital Proportional" features have come on the market that provide a computerized digital bit-stream signal to the receiving device, instead of analog type pulse modulation. Advantages include error checking capabilities of the data stream (good for signal integrity checking) and fail-safe options including motor (if the model has a motor) throttle down and similar automatic actions based on signal loss.
Remote control military applications are typically not radio control in the direct sense, but take the form of instructions sent to a completely autonomic, computerized automatic pilot. Instead of a "turn left" signal that is applied until the aircraft is flying in the right direction, the system sends a single instruction that says "fly to this point".
Today radio control is used almost entirely by hobbyists.
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