Radio control
From Wikipedia, the free encyclopedia
Radio control (sometimes abbreviated R/C) is the use of radio signals to remotely control a device. The term is used frequently to refer to the control of model cars, boats, airplanes, and helicopters from a hand-held radio transmitter. Industrial, military, and scientific research organizations make use of radio-controlled vehicles as well.
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[edit] History
In 1898 at an exhibition at Madison Square Garden Nikola Tesla demonstrated a small boat which could apparently obey commands from the audience but was in fact controlled by Tesla interpreting the verbal requests and sending appropriate frequencies to tuned circuits in the boat. He was granted a US patent on this invention on November 8, 1898.[1] In 1904, Bat, a Windermere steam launch, was controlled using experimental radio control by its inventor, Jack Kitchen. In 1909 the French inventor Gabet demonstrated what he called his "Torpille Radio-Automatique", a radio controlled torpedo[2] 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. The Soviet Red Army used remotely controlled teletanks during 1930s in the Winter War against Finland and fielded at least two teletank battalions at the beginning of the Great Patriotic War. A teletank is controlled by radio from a control tank at a distance of 500–1,500 meters, the two constituting a telemechanical group. There were also remotely controlled cutters and experimental remotely controlled planes in the Red Army. In the 1930s Britain developed the radio controlled Queen Bee, a remotely controlled unmanned Tiger Moth aircraft for a fleet's gunnery firing practice. The Queen Bee was superseded by the similarly named Queen Wasp, a later, purpose built, target aircraft of higher performance.
[edit] Military applications in the Second World War
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 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.
The German Kriegsmarine operated FL-Boote (ferngelenkte Sprengboote) which were radio controlled motor boats filled with explosives to attack enemy shipping from 1944.
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 electromechanical in nature, using small metal "fingers" or "reeds" with different resonant frequencies each of which would operate one of a number of different relays when a particular frequency was received. The relays would in turn then activate various actuators acting on the control surfaces of the missile. The controller's radio transmitter would transmit the different frequencies in response to the movements of a control stick; these were typically on/off signals.
These systems were widely used until the 1960s, when the increasing use of solid state systems greatly simplified radio control. The electromechanical systems using reed relays were replaced by similar electronic ones, and the continued 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.
[edit] Radio-controlled models
The first general use of radio control systems in models started in the early 1950s with single-channel self-built equipment; commercial equipment came later. Initially remote control systems used escapement (often rubber driven) mechanical actuation in the model. Commercial sets often used ground standing transmitters, long whip antennas with separate ground poles and single electron-tube (valve) receivers. The first kits had dual tubes for more selectivity. Such early systems were invariably super_regenerative circuits, which meant that the use of two controllers in close proximity would interfere with each other. The requirement for heavy batteries to drive tubes also meant that model boat systems were more successful.
The advent of transistors greatly reduced the battery requirements, since the current requirements at low voltage were greatly reduced and the high voltage battery was eliminated. Low cost systems employed a superregenerative transistor receiver sensitive to a specific audio tone modulation, the latter greatly reducing interference from 27 mHz Citizens' band radio communications on nearby frequencies. Use of an output transistor further increased reliability by eliminating the sensitive output relay, a device subject to both motor-induced vibration and stray dust contamination.
In both tube and early transistor sets the model's control surfaces were usually operated by an electromagnetic escapement controlling the stored energy in a rubber-band loop, allowing simple rudder control (right, left, and neutral) and sometimes other functions such as motor speed.[1]
By the early 1960s transistors had replaced the tube and electric motors driving control surfaces were more common. The first low cost "proportional" systems did not use servos, but rather by driving a bidirectional motor with a proportional pulse train. This system, commonly known as "Galloping Ghost", was driven with a pulse train that caused the rudder to "wag" though a small angle (not affecting flight owing to small excursions and high speed), with the average position determined by the proportions of the pulse train. Single-channel gave way to multi channel (at significantly higher cost) with various audio tones driving electromagnets affecting tuned resonant reeds for channel selection.
The use of Superheterodyne receivers with high frequency stability via crystals had much better selectivity and sophisticated equipment became more readily available at affordable cost. The constantly diminishing equipment weight was crucial to ever increasing modelling applications. Superhetrodyane circuits became more common, enabling several transmitters to operate closely together and enabling further rejection of interference from adjacent Citizen's Band voice radio bands.
Multi-channel developments were of particular use to aircraft, which really needed a minimum of three control dimensions, (yaw, pitch and motor speed) as opposed to boats, which can get away with two or one. Radio control 'channels' were originally outputs from a reed array, in other words, a simple on-off switch. To provide a usable control signal a control surface needs to be moved in two directions, so at least two 'channels' would be needed, unless a complex mechanical link could be made to provide two-directional movement from a single switch. Several of these complex links were marketed during the 1960s, including the Graupner Kinematic and the Galloping Ghost.
As the electronics revolution took off, single-signal channel circuit design became redundant, and instead radios provided coded signal streams which a servomechanism could interpret. Each of these streams replaced two of the original 'channels', and, confusingly, the signal streams began to be called 'channels'. So an old 6-channel transmitter which could drive the rudder, elevator and throttle of an aircraft was replaced with a new 3-channel transmitter doing the same job. Controlling all the primary controls of a powered aircraft (rudder, elevator, ailerons and throttle) was known as 'full-house' control. A glider could be 'full-house' with only three channels.
Soon a competitive market place emerged, bringing rapid development. By the 1970s the trend for full-house proportional radio control was fully established. Typical radio control systems for radio-controlled models employ pulse width modulation (PWM), pulse position modulation (PPM) and more recently spread spectrum technology, 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.
PWM is most commonly used in today's equipment, 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 10 and 30 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 Pulse-Code Modulation (PCM) 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. However, those systems that use pulse code modulation generally induce more lag due to lesser frames sent per second as bandwidth is needed for error checking bits. It should also be noted that PCM devices can only detect errors and thus hold the last verified position or go into failsafe mode. They can not correct transmission errors.
In the early 21st century, 2.4 gigahertz tramsissions have become increasingly utilised in high-end control of model vehicles and aircraft. This range of frequencies has many advantages from the perspective of radio-controlled applications. Because the 2.4 gigahertz wavelengths are so small (around 10 centimetres), the antennas on the receivers do not need to exceed 3-5 cm. Electrostatic noise, for example from r/c batteries, is not 'seen' by 2.4 gigahertz receivers due to its frequency (which tends to be around 10 to 150 MHz). The transmitter antenna only needs to be around 10-20cm long, and receiver power usage is much lower; batteries can therefore last longer. In addition, no crystals or frequency selection is required as the latter is performed automatically by the transmitter. However, the short wavelengths do not diffract as easily as the longer wavelengths of PCM/PPM, so 'line of sight' is required between the transmitting antenna and the receiver. Also, should the receiver lose power, even for a few milliseconds, or get 'swamped' by 2.4 GHz interference, it can take a few seconds for the receiver-which, in the case of 2.4ghz, is almost invariably a digital device-to 'reboot'.
[edit] Modern military and aerospace applications
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".
Some of the most outstanding examples of remote radio control of a vehicle are the Mars Exploration Rovers such as Sojourner.
[edit] Industrial control
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.
[edit] See also
- Air engine
- Radio-controlled model
- Radio-controlled car
- Radio-controlled boat
- Radio-controlled airplane
- Radio-controlled helicopter
- Teletank
[edit] Notes
- ^ Fort Smith Flightmasters: Radio Control Systems (Part 2) with illustrations of an early installation and descriptions of various escapements.
