Radio-controlled model

源自：http://en.wikipedia.org/wiki/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

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Crystal oscillatorsuperheterodyne 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. Superheterodyne 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 be controlled 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 theGraupner Kinematic Orbit, Bramco, and Kraft simultaneous reed sets.

Doug Spreng is credited with developing the first "digital" pulse width feedack servo and along with Don Mathis developed and sold the first digital proportional radio called the "Digicon" followed by Bonner's Digimite, and Hoovers F&M Digital 5.

With the electronics revolution, single-signal channel circuit design became redundant and instead, radios provided coded signal streams which aservomechanism 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 on/off 6-channel transmitter which could drive the rudder, elevator and throttle of an aircraft was replaced with a newproportional 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 marketplace 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 employpulse width modulation (PWM), pulse position modulation (PPM) and more recently spread spectrum technology, and actuate the various control surfaces using servomechanisms. These systems made 'proportional control' possible, where the position of the control surface in the model isproportional to the position of the control stick on the transmitter.

PWM is most commonly used in radio control equipment today, where transmitter controls change the width (duration) of the pulse for that channel between 920µs and 2120 µs, 1520 µs being the center (neutral) position. The pulse is repeated in a frame of between 10 and 30milliseconds 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. Anelectric motor and reductiongearbox 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 aJohnson counter. The relative simplicity of this system allows receivers to be small and light, and has been widely used since the early 1970s. Usually a single-chip4017 decade counter is used to decode the transmitted multiplexed PPM signal to the individual "PWM" signals sent to eachRC servo.^[2][3][4]

More recently, high-end hobby systems using Pulse-Code Modulation (PCM) features have come on the market that provide acomputerizeddigitalbit-stream signal to the receiving device instead of analog type pulse modulation. Advantages includebit error checking capabilities of the data stream (good for signal integrity checking) andfail-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 asbandwidth is needed for error checking bits. PCM devices can only detect errors and thus hold the last verified position or go intofailsafe mode. They cannot correct transmission errors.

In the early 21st century, 2.4 gigahertz (GHz) transmissions have become increasingly utilised in high-end control of model vehicles and aircraft. This range of frequencies has many advantages. Because the 2.4 GHz wavelengths are so small (around 10 centimetres), the antennas on the receivers do not need to exceed 3 to 5 cm. Electromagnetic noise, for example from electric motors, is not 'seen' by 2.4 GHz receivers due to the noise's frequency (which tends to be around 10 to 150 MHz). The transmitter antenna only needs to be 10 to 20 cm 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 re-sync.

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. Thereceiver 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 ofchannels the radio must provide.

Typically the transmitter multiplexes all the channels into a single pulse-position modulation radio signal. The receiver demodulates and demultiplexes the signal and translates it to the special kind ofpulse-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, copied wheeled steering for RC cars. It was originally developed by Orbit for a transmitter specially designed for Associated cars It has been widely accepted along with atrigger control forthrottle. Often configured for right hand users, the transmitter looks like a pistol 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 intoreverse. Some models are available in left-handed versions.

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 separatebands 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 allowtoe-in,camber andcaster 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.

Many of the most popular hobby grade radios were first developed, and mass produced in Southern California by Orbit, Bonner, Kraft, Babcock, Deans, Larson, RS, S&O, and Milcott. Later, Japanese companies like Futaba, Sanwa and JR took over the market.

Power

Internal combustion

Internal combustion engines for remote control models have typically beentwo 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 theSaito 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 whackers 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 likebrushless motors andlithium polymer batteries. These allow electric motors to produce much more power rivaling that of fuel-powered engines. It is also relatively simple to increase thetorque 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. (e.g. an electric glider climbing steeply to a good thermalling altitude.)

In aircraft, 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 motor and speed controller used with radio controlled cars. As you can see, due to the integrated heat sink, the speed controller is almost as large as the motor itself. Due to size and weight limitations, heat sinks are not common in RC aircraftelectronic speed controller (ESCs), therefore the ESC is almost always smaller than the motor.