Radio control
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This remote control airplane is carrying a scale model of X-33 and is taking part in NASA research. |
Radio control is the use of
radio signals to remotely control another device. The term is used frequently to refer to the control of model cars, boats, airplanes, and helicopters from a user-held control box (radio.) Industrial, military and scientific research all make use of radio-controlled vehicles as well.
The possibility of radio remote control was appreciated almost as soon as the first demonstrations of radio itself; the credit for the first to suggest radio control of aircraft may belong to
Patrick Young Alexander as early as 1888.
Nikola Tesla patented a radio-control scheme as early as 1899. In 1917
Archibald Low as head of the
RFC Experimental Works, was the first person to use radio control successfully on an aircraft. In the 1920s, various radio-controlled ships were used for naval artillery target practice.
Radio control was further 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 the Luftwaffe 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
servomechanisms. 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
µs and 2120 µs, 1520 µs being the center (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 component such as a resistor "
potentiometer" or tuning capacitor. The variable capacitor or resistor produces an error signal 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
bit 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, directly operating flight control surfaces and propulsion power settings, but instead take the form of instructions sent to a completely
autonomous, 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".
The most outstanding example of remote radio control of a vehicle are the
Mars Exploration Rovers such as
Sojourner.
Today radio control is used in industry for such devices as overhead
cranes and switchyard
locomotives. Radio-controlled
teleoperators are used for such purposes as inspections, and special vehicles for disarming of
bombs. Some remotely-controlled devices are loosely called
robots, but are more properly categorized as teleoperators since they do not operate autonomously, but only under control of a human operator.
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RC Groups - A popular discussion forum and information website about radio controlled vehicles.
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offroad-CULT- very popular German RC-Site dedicated to RC offroad vehicles
