Radio-controlled model


Radio-controlled model

A radio-controlled model (or RC model) is a model that is steerable with the use of radio control. All types of vehicles imaginable have had RC systems installed in them, including cars, boats, planes, and even helicopters and scale railway locomotives.

History

Radio control has been around since the late 1800s with Nikola Tesla having demonstrated a remote control boat in 1893. The World War II era saw increased development in radio control technology. The Luftwaffe used controllable winged bombs for targeting Allied ships. During the 1950s pioneering work was done by enthusiastic amateurs to create valve based control units. Originally simple 'on-off' systems these evolved to use complex systems of relays to control speed and direction. The information was encoded by varying the signals mark/space ratio (pulse proportional). Rapidly commercial versions of these systems became available. The tuned reed system brought new sophistication, using metal reed switches to resonate with the transmitted signal and operate one of a number of different relays. In the 1960s the availability of transistor based equipment led to the rapid development of fully proportional servo-based systems, again driven largely by amateurs but resulting in commercial products. In the 1980s, integrated circuits made the electronics cheap, small and light enough for multi-channel fully proportional control to become widely available.

In the 1990s miniaturised equipment became widely available, allowing radio control of the smallest models, and by the 2000s radio control was commonplace even for the control of inexpensive toys. At the same time the ingenuity of modellers has been sustained and the achievements of amateur modelers using the latest technology has extended to such subjects as gas-turbine powered aircraft, aerobatic helicopters and submarines, to name but a few examples.

Before the days of radio control many models would use simple burning fuses or clockwork mechanisms to control flight or sailing times. Sometimes clockwork controllers would also control and vary direction or behaviour. Other methods included tethering to a central point (popular for cars and hydroplanes), round the pole control for electric model aircraft and control line (USA: u-control) for internal combustion powered aircraft!

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. [ [http://www.fortsmithflightmasters.com/fp/fp040701.html Fort Smith Flightmasters: Radio Control Systems (Part 2)] with illustrations of an early installation and descriptions of various escapements.]

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.

Crystal oscillator superheterodyne receivers with better selectivity and stability made control equipment more capable and at lower 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 2.4 GHz, is almost invariably a digital device-to 'reboot'.

Design

RC electronics have three essential elements. The transmitter is the controller. Transmitters have control sticks, triggers, switches, and dials at the user's finger tips. The receiver is mounted in the model. It receives and processes the signal from the transmitter, translating it into signals that are sent to the servos. The number of servos in a model determines the number of channels the radio must provide.

Typically the transmitter multiplexes all the channels into a single pulse-position modulation radio signal, and the receiver demultiplexes and translates it to the special kind of pulse-width modulation used by standard RC servos.

In recent years, electronic speed controllers (ESCs) have been developed to replace the old variable resistors, which were extremely inefficient. They are entirely electronic, so they do not require any moving parts or servos.

In the 1980s, a Japanese electronics company, Futaba, introduced wheeled steering for RC cars. It has been widely accepted along with a control for throttle. It's often configured for right hand users, so the transmitter looks like a gun with a wheel attached on its right side. Pulling the trigger would accelerate the car forward, while pushing it would either stop the car or cause it to go into reverse. There are some models that comes in left-handed versions too.

Mass production

There are thousands of RC vehicles available. Most are toys suitable for children. What separates toy grade RC from hobby grade RC is the modular characteristic of the standard RC equipment. RC toys generally have simplified circuits, often with the receiver and servos incorporated into one circuit. It's almost impossible to take that particular toy circuit and transplant it into other RCs.

Hobby grade RC

Hobby grade RC systems have modular designs. Many cars, boats, and aircraft can accept equipment from different manufacturers, so it is possible to take RC equipment from a car and install it into a boat, for example.

However, moving the receiver component between aircraft and surface vehicles is illegal in most countries as radio frequency laws allocate separate bands for air and surface models. This is done for safety reasons.

Most manufacturers now offer "frequency modules" (known as crystals) that simply plug into the back of their transmitters, allowing one to change frequencies, and even bands, at will. Some of these modules are capable of "synthesizing" many different channels within their assigned band.

Hobby grade models can be fine tuned, unlike most toy grade models. For example, cars often allow toe-in, camber and caster angle adjustments, just like their real-life counterparts. All modern "computer" radios allow each function to be adjusted over several parameters for ease in setup and adjustment of the model. Many of these transmitters are capable of "mixing" several functions at once, which is required for some models.

Types

Aircraft

Radio-controlled aircraft (also called RC aircraft) are small aircraft that can be controlled remotely. There are many different types, ranging from small park flyers to large jets and mid-sized aerobatic models.The aircraft use many different methods of propulsion, ranging from brushed or brushless electric motors, to internal combustion engines, to the most expensive gas turbines. The fastest aircraft, with gas turbines, can reach speeds of up to convert|250|mi/h|km/h|abbr=on. Newer jets can achieve above convert|300|mi/h|km/h|abbr=on in a short distance.

Cars

A radio-controlled car is a powered model car driven from a distance. Both gas and electric cars exist, designed to be run both on and off-road. "Gas" cars traditionally use petrol (gasoline) though the majority now use nitromethanol, mixture of methanol and nitromethane, to get their power. Building, driving, and modifying radio-controlled car kits is a hobby enjoyed by enthusiasts of all ages.

Helicopters

Radio-controlled helicopters, although often grouped with RC aircraft, are unique because of the differences in construction, aerodynamics and flight training. Several designs of RC helicopters exist, some with limited maneuverability (and thus easier to learn to fly), and those with more maneuverability (and thus harder to learn to fly).

Boats

Radio-controlled boats are model boats controlled remotely with radio control equipment. The main types of RC boat are: Scale models (12"(30 cm) - 144"(365 cm) in size), the sailing boat and the power boat. The latter is the more popular amongst toy grade models. Radio controlled models were used for the children's television program "Theodore Tugboat".

Robotics

The majority of robots used in shows such as battlebots and Robot Wars are remotely controlled, relying on most of the same electronics as other radio-controlled vehicles.

Power

Internal combustion

Internal combustion engines for remote control models have typically been two stroke engines that run on specially blended fuel. Engine sizes are typically given in cm³ or cubic inches, ranging from tiny engines like these .02 in³ to huge 1.60 in³ or larger. For even larger sizes, many modelers turn to four stroke or gasoline engines (see below.) Glow plug engines have an ignition device that possesses a platinum wire coil in the glow plug, that catalytically glows in the presence of the methanol in glow engine fuel, providing the combustion source.

Since 1976, practical "glow" ignition four stroke model engines have been available on the market, ranging in size from 3.5 cm³ upwards to 35 cm³ in single cylinder designs. Various twin and multi-cylinder glow ignition four stroke model engines are also available, echoing the appearance of full sized radial, inline and opposed cylinder aircraft powerplants. The multi-cylinder models can become enormous, such as the Saito five cylinder radial. They tend to be quieter in operation than two stroke engines, using smaller mufflers, and also use less fuel.

Glow engines tend to produce large amounts of oily mess due to the oil in the fuel. They are also much louder than electric motors.

Another alternative is the gasoline engine. While glow engines run on special and expensive hobby fuel, gasoline runs on the same fuel that powers cars,lawnmowers, weed wackers etc. These typically run on a two-stroke cycle, but are radically different from glow two-stroke engines. They are typically much, much larger, like the 80 cm³ Zenoah. These engines can develop several horsepower, incredible for something that can be held in the palm of the hand.

Electrical

Electric power is often the chosen form of power for aircraft, cars and boats. Electric power in aircraft in particular has become popular recently, mainly due to the popularity of park flyers and the development of technologies like brushless motors and lithium polymer batteries. These allow electric motors to produce much more power rivaling that of fuel-powered engines. It is also relatively simple to increase the torque of an electric motor at the expense of speed, while it is much less common to do so with a fuel engine, perhaps due to its roughness. This permits a more efficient larger-diameter propeller to be used which provides more thrust at lower airspeeds. (eg an electric glider climbing steeply to a good thermalling altitude.)

In cars, trucks and boats, glow and gas engines are still used even though electric power has been the most common form of power for a while. The following picture shows a typical brushless DC motor and speed controller used with these radio controlled vehicles. As you can see, the speed controller is almost as large as the motor itself.

External links

* [http://electronics.howstuffworks.com/rc-toy1.htm How Radio Controlled Toys Work]
* [http://www.heliguy.com Remote Controlled Helicopter Specialist]


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