Radio remote control

Transmitter of a programmable radio remote control

The radio remote control (engl. Radio control or RC ) is based on the use of radio signals for control of a technical apparatus from a distance. The term is mostly used to describe the control of model cars , boats , airplanes or helicopters with a control device operated by the pilot. Only certain frequency bands (without a license) are released for the public . In order to be able to operate several models in a competition, released frequency bands (27 MHz, 35 MHz, 40 MHz, 433 MHz, 2.4 GHz) are divided by a channel grid . The sending and receiving channels are specified by a quartz oscillator . Modern transmitting and receiving crystals (pair of crystals) of each channel have a frequency difference equal to the intermediate frequency of the receiver. The latest microprocessor-operated receivers manage without channel crystals thanks to the use of a PLL .

history

With the invention of radio technology , the foundations for wireless transmission of control signals were prepared: Nikola Tesla demonstrated a radio remote-controlled model of a ship in New York as early as 1898 and had this radio remote control design patented . The first radio remote controls were used professionally in the 1920s to control target ships for target practice in the navy. In the second half of the 1930s, the first successful attempts at remote control of model aircraft were carried out, particularly in England and Germany (including the 1938 Rhön competition ). As early as 1936, the first radio remote control built by Ernst Namokel was successfully tested on the Wasserkuppe with the BF 52 glider model . A replica of the K tube receiver with an anode voltage of 12 volts and a heating voltage of the tubes of 3 volts was made available by Ernst Namokel, who is over 80 years old, for the exhibition in the German Glider Museum with model flight on the Wasserkuppe. This flight can be seen as another milestone in the history of model flying. The replica of the BF 52 model is also in the exhibition.

Radio remote control in World War II

During the Second World War, radio remote controls were used for a number of flying objects. The aim was, in particular, to develop radio-controlled anti-ship missiles for use against ship formations, which otherwise could only be attacked with difficulty and at very high risk. Towards the end of the war, the Luftwaffe faced similar questions when attacking bomber formations, and numerous remote-controlled missiles were developed (electropneumatic systems), but they were no longer used.

Only for ballistic V2 - missile remote controls were 20 flights experimentally using the first radar frequencies tested. But even here it was only possible to transmit 1 bit (1 channel on / off), a desired proportional control was reserved for later development stages.

Radio control systems have also been developed in Great Britain and the USA to reduce the risk to crews when deploying against strongly defended targets. However, none of these systems were found to be useful in practice. One device, Project Aphrodite , turned out to be more dangerous to its users than to the target.

Radio control systems of this era were generally electromechanical in nature. For example, a radio was built into a missile, the signal transmitted by the control panel was demodulated and fed to a small loudspeaker. A few small metal tongues with different resonance frequencies were attached in front of the loudspeaker, but these made enormous demands on the frequency stability of the transmitter-side tone generators. The control impulses for the oars were finally triggered by their vibrations.

The idea of tone modulation and tongue relay switching stages was taken up again in the early post-war era in the A2 model remote controls - however, it was not until the invention of the transistor and the highly sensitive resonance tone circuit switching stage with feedback that the hoped-for breakthrough in operational reliability was achieved.

First model radio controls

As early as 1898, the physicist Nikola Tesla presented a radio-controlled electric model boat that he had developed at the world exhibition in New York. The first model radio remote controls were available in the second half of the 1930s, at that time they were still usually self-built with tube transmitters and receivers. From the beginning of the 1950s, the first mass-produced tube remote controls were available in Germany - technically very simple devices with carrier keying ; the high frequency of the radio transmitter was only switched on or off. As a result, only a single control command (single-channel transmitter) could be transmitted, with the model control usually taking place via a command sequence: e.g. E.g. tapping once (= switching on the transmission signal) means left, second tap again neutral position, the next tap right rudder etc., or (more often) first time transmitter on = right rudder, transmitter off = neutral, next time transmitter on = left rudder and so on further. By evaluating the signal duration with the help of complicated techniques (the “kinematics” rowing machine was an example of this), additional functions such as engine control were also possible. The "kinematics" from Graupner ran with long pulses right - neutral (transmitter off) - left - neutral and with short pulses motor forwards - stop - backwards - stop and so on.

A development of early remote control technology that was important for the further development of radio control, but not very widespread, was the highly innovative electromechanical single-channel proportional system from Webra , which was also known as "flutter control": the pulse duty factor could be controlled with the help of a mechanically operated pulse generator Change the HF pulse / pause on the transmitter side. The evaluation principle in the receiver servo was based on the mechanical balance of a spring return force and a counterforce by an electric motor controlled by the pulse duty factor on the transmitter side.

The introduction of tone modulation, i.e. H. a simple amplitude modulation of the transmission frequency with low audio frequencies, but above all the increasing transistorization of the remote control electronics in the second half of the 1950s, revolutionized the possibilities of the model control: from the functionally reliable single-channel systems to the top class of the ten-channel system, which with several tone generators even one Simultaneous control of up to three rudder functions allowed, the range of industrially manufactured remote control systems extended. The better remote control transmitters now received correct control sticks instead of simple tip buttons despite the still existing on / off restriction, and with the help of multi-channel technology a more realistic and reliable model control was possible.

The tone- modulated single-channel system sold by Graupner with Bellaphon B transmitter and Ultraton receiver .

From the end of the 1950s, the tube systems, which were particularly heavy because of their batteries, were gradually replaced by lighter hybrid devices and later complete transistor systems - due to the low cut-off frequencies of the germanium transistors of the time, the high-frequency stages of the transmitting and receiving circuits were still equipped with tubes during a transition period, For low-frequency circuit functions (tone generators, DC voltage converters for generating anode voltage, switching amplifiers, LF amplifiers, etc.), current and space-saving transistor technology was becoming increasingly established. Examples of well-known hybrid devices were the widespread transmitters from the southern German companies Graupner and Metz (Graupner Bellaphon A and B, Metz Mecatron) and the Graupner “Mikroton” single-channel receiver.

The unstoppable advances in semiconductor technology have made fully electronically controlled proportional systems possible since around 1965, in which each rudder precisely follows the deflection of the stick movement on the transmitter, with which aircraft models in particular can be controlled precisely and safely. Since their appearance, the proportional systems have been refined to include computer systems, but the basic functionality has not changed since then.

Current development

The use of microelectronics soon made it possible to transmit very complex control signals. While early control systems initially only worked with unmodulated Trägertastung, which were later replaced by the reliable tonmodulierten radio control systems include advanced systems ten or more digital -proportionale command channels. These RC systems enable proportional control - the control variable in the controlled vehicle, such as the position of a rudder, is always proportional to the position of the control stick on the transmitter.

With the introduction of digital technology, the functionality of current remote controls has again increased significantly. Channels can be freely exchanged and their characteristics changed or mixed with other functions. Furthermore, servos can be programmed, for example to adjust the direction of rotation, installation angle and rudder positions after installation, which in analog systems is only possible by intervening in the transmitter electronics (function modules).
With pulse code modulation (PCM), the susceptibility to interference is also reduced, since the data stream can be checked digitally for errors. If there are reception problems, e.g. B. in flight, the receiver can set a defined helm position (fail safe, hold). Furthermore, the signal transmission can be more responsive, since those channel values that have changed are preferably transmitted.

Newer technologies (PCM, Spread Spectrum ) from digital and computer technology will further improve transmission security in the future and enable additional functions (e.g. telemetry). The broader use of spread spectrum technologies are currently still being prevented by regulatory restrictions imposed by the authorities, so that this technology is currently limited to the ISM band .

A significant further development concerns the miniaturization of the systems. While a remote control receiver from 1955 in tube technology with the associated rowing machine and required batteries still weighed around 300 g and could only control a single function, today a receiver system in commercial technology with four proportional functions using a LiPo battery of 2.6 g 5 g can be realized. The receiver weighs less than 1 g and the rudder elements each about 0.35 g. In the case of a higher load requirement for larger models, of course, correspondingly stronger and heavier components are required.

Modern remote control technology

The position of the levers or regulators of the control function is picked up internally on the transmitter using a potentiometer or switch, converted into an electrical control signal and modulated to the HF transmission frequency . Depending on the type of control signal, a distinction is made between pulse pause modulation (PPM) and pulse code modulation (PCM). The latter two are referred to as digital - proportional transmission, since the control signal is a digital signal which in turn encodes the position of the control lever in a directly proportional manner.

A distinction is made between AM and FM transmissions and, more recently, spread spectrum in the HF modulation method .

The RF energy picked up by the antenna is amplified and demodulated in the receiver, thereby recovering the control signal. The receivers are mostly designed as superheterodyne receivers (super), often with double frequency conversion (double super). The regenerated control signal is then decoded in order to separate the individual control functions and fed to the appropriate command switching devices on separate electrical outputs.

Mechanical steering gear ( servos ) or electrical controllers , actuators or switches can be used as command activation devices.

Servos convert the value of the control function into a proportional mechanical movement. A potentiometer measures the actual value, which is compared with the target value specified by the transmitter. The motor is now started up until the target value is reached. Due to the constant readjustment, the position is held even under load.

Electrical actuators / regulators convert the value of the control function into a proportional electrical signal for a consumer (usually an electric motor ). For powerful electric motors, the supply voltage is usually pulse-width modulated by means of an electronic circuit breaker ( transistor ) . Controllers differ through a built-in control loop. For this purpose, the electrical signal of the consumer is readjusted by control electronics in such a way that a parameter of the consumer (e.g. speed of the electric motor) is proportional to the control function regardless of environmental influences (e.g. change in supply voltage, mechanical load).

Signal structures

Remote control with analog signals

High frequency keying 13.56 and 27.12 MHz

With the first tube remote controls, the control signal was transmitted simply by switching the high frequency on and off (A1 mode). The high-frequency part of the remote control receiver initially worked exclusively on the principle of the pendulum receiver , with which highly sensitive receiving stages could be implemented at low cost: without a high-frequency signal, the circuit principle, also known as pendulum audion , generates a strong noise voltage; if the high frequency is received, the pendulum noise disappears. The noise voltage of the pendulum receiver is amplified and forms the basis for controlling an electromechanical relay.

With special rowing machines ('self-neutralizing') a model could already be remotely controlled with a fixed control sequence: straight ahead / to the right / straight ahead / to the left / straight ahead / etc.

Some complex rowing machine mechanics also succeeded in evaluating the duration of the switching impulses sent (long / short) and thus additional functions such as B. to influence the drive control of the model.

Single-channel proportional control

As early as the early sixties of the last century, experiments were carried out with simple single-channel proportional controls (Webra-Picco system), in which the time ratio of the HF switch-on time and the subsequent transmission pause (the duty cycle ) via a mechanical control device ( Control stick) could be varied. A control signal was derived from this pulse duty factor, which was converted into appropriate rudder deflections in the receiving-side steering gear. Due to the inertia and the low repetition rate of the pulse generator, the fluttering movements of the connected rudders, which are typical for these systems, resulted, which led to the name "flutter control".

Sound modulation of the carrier frequencies 27.12 and 40.68 MHz

The further development of sound modulation (A2 operation), the basic principle of which was once used for military purposes, brought great progress in early remote control technology: the carrier frequency of the remote control transmitter was modulated with the audio frequency voltage of a low-frequency oscillator and converted into switching signals in the receiver using special circuit technology. The widespread pendulum audio in the entrance section of the A2 remote control receiver was able to maintain itself for a long period of time, despite its undeniable disadvantages (broadband interference radiation, poor selectivity), but it was increasingly displaced by the powerful and selective superheterodyne principle , which, thanks to its high selectivity, enables the simultaneous operation of several Models in the same frequency band made possible.

Similar to the analog multi-frequency dialing method of the telephone, the control functions in the tone modulation are coded by different tone signal frequencies modulated onto the high frequency, the number of tone frequencies available corresponding to the number of remote control channels.
The individual audio signal frequencies are assigned selective on / off switching stages in the remote control receiver, which are responsible for controlling the electromechanical steering gear - proportional control is not possible with the system. With the development of the extremely powerful and reliable transistor audio circuit switching stage with feedback by the Munich manufacturer of remote control devices ( Graupner -Bellaphon) Hans Schumacher, the audio-modulated remote control achieved its final market success.

At that time, three-channel control was the standard in remote control operation: two modulation channels, together with a self-neutralizing rowing machine, provided left-right control, while the third modulation channel enabled motor control to be implemented with the help of special servo types.

Analog AM / FM proportional control 27 MHz - 40 MHz

In the systems (27; 35 or 40 MHz) used today in ship, aircraft and car model construction, proportional control based on pulse width modulation is standard. This is usually abbreviated as PWM (Pulse Width Modulation). Today the impulse mostly varies in the range of 1.5 ms ± 0.5 ms (system multiplex: 1.6 ms ± 0.5 ms). In the model to be controlled, for example, 1.0 ms means left, 1.5 ms means neutral and 2.0 ms means right or vice versa. Only one pulse width (one pulse) is assigned to each transmission channel, the pulses of all channels are transmitted cyclically one after the other via PPM .

Pulse diagram

In the transmitter each pulse is generated by a monostable multivibrator ; the potentiometer of the transmitter stick represents an RC timing element with a fixed capacitor. Each servo in the receiver also has a monostable multivibrator, whereby the time-determining potentiometer is mounted on the axis of rotation of the servo.

The difference between the pulse coming from the transmitter and the pulse supplied by the servo potentiometer is positive or negative and corrects the direction of rotation of the servo motor in the opposite direction. The servo only comes to rest when the transmit and servo pulses are of the same length and the pulse difference between the two is zero. The pulse definition varies slightly depending on the manufacturer (Multiplex / JR Propo / Futaba systems).

The impulses of several channels (functions) follow one another immediately. However, it is not the servo pulses themselves that are used for transmission. The end of one pulse also means the beginning of the next. Depending on the manufacturer, only this transition edge is sent as a 0.2 ms to 0.5 ms wide impulse and decoded in the receiving decoder.

The resulting pulse telegram is repeated cyclically. Depending on the number of channels of the transmitter and manufacturer (2 to 12 channels) within 15 to 24 ms. This creates a somewhat longer pause between two consecutive pulse groups, which is used by the receiver for synchronization. Since each channel is retransmitted within 15 to 24 ms, the model reacts very quickly, so that depending on the latency of the respective system, the controller can have the feeling of an abrupt or only slightly delayed reaction.

Advantage of the technique is about 5 kHz Extremely low RF - bandwidth , whether as a transmission method AM or FM is used.

This ingeniously simple and powerful technology was developed in the sixties. The first receivers worked with amplitude modulation (AM), with the transmitter being partially or completely blanked. Problems with the fast readjustment of the gain of the IF amplifier favor frequency modulation (FM), which was introduced into remote control technology from around 1970. The frequency modulation manages with a frequency deviation of approx. 4 kHz, enough to meet a frequency grid of 20 kHz. ${\ displaystyle \ Delta {f} = f_ {1} -f_ {2}}$

Remote control with digital signals

PCM

In the field of competition, pulse code modulation (PCM) has been gaining ground in large model aircraft since 2000 . Here all control information is transmitted as numerical values in a digital data stream. Additional check bits allow error correction and increase transmission security. The advantages are increased immunity to interference (fail save does not function reliably with digitally proportional receivers) as well as a lower zero point drift of the servos and higher precision of the deflections. The disadvantage is that the bandwidth requirement is several times higher , since significantly more pulses have to be transmitted during a frame, which results in a higher signal frequency. As a way out, FM-PCM transmitters use data compression techniques to reduce the amount of data during a frame. Depending on the manufacturer, z. B. not changed servo channels omitted or sent with a lower repetition rate. Depending on the system, this can lead to increased latency times. In higher frequency bands, PCM systems can use higher bandwidths in order to be able to be as fast as digitally proportional systems. For example, the 2.4 GHz frequency band is used with a channel grid of 1024 kHz.

PCM in the ISM band

With the introduction of PCM systems there were occasional latency problems. This is due to the dilemma of potentially higher bandwidth requirements. Various attempts have often failed due to the availability of suitable frequency bands or inexpensive technologies. With the release of frequencies in the 2.4 GHz ISM band and the availability of inexpensive components through the development of WLAN in the PC area, applications for remote control purposes were to be expected. Due to high-quality modern modulation methods, significant technical advances in transmission security are possible here.

There are now different transmission systems used by all well-known manufacturers for remote control, such as DSSS and FHSS , and mixed forms, which differ mainly in the transmission power permitted in Europe (DSSS 10 mW / MHz, FHSS 100 mW) after the legal and the technical situation was a bit confusing.

However, smaller wavelengths lead to a new problem, that of reflexive extinction, see Fresnel zone . A z. B. on the ground reflected wave the receiver in phase opposition to the direct wave, it comes to signal cancellation (dead points). This is remedied by dynamic frequency band changes ( spread spectrum ) or diversity receivers. Here two independent antennas supply two independent receivers, of which the higher effective value is fed to the pulse decoder. A double receiver with offset antennas is already integrated in modern receivers.

Components

Components of a radio remote control: servo, battery, receiver.

A radio remote control is conceptually and usually also structured in the components transmitter, receiver and servos or actuators:

Channel

Common transmitters have two control sticks that can be moved to the right / left and up / down. A control function (e.g. elevator) is also referred to as a “channel” or “function”, so four functions result from the two control sticks. Additional channels are implemented using rotary and slide controls or switches, if necessary.

In terms of handling, a distinction is made between hand-held transmitters, in which the thumbs usually rest on the control sticks, and desk-top transmitters, which are worn on a belt, and in which the sticks are operated with the fingers when the heel of the hand is on top. A special “pistol” design is sometimes used to control car and ship models, with a rotary ring for the rudder and a lever for the gas / motor.

The position of the lever or controller is internally picked up by a potentiometer , coded and modulated to the transmission frequency in the HF stage .

In Europe, the frequency bands 27 MHz ( short wave ) and 35 MHz ( VHF ), 40 MHz ( VHF ), 433 MHz ( UHF ) and 2.4 GHz (short microwaves ) are permitted for remote controls, with the exact frequency being determined by crystals on the transmitter becomes. On modern transmitters and receivers, the frequencies can be set as desired using PLL technology. With the new transmission methods, such as Bluetooth or Spread Spectrum, users no longer need to worry about a channel. Here the frequencies are set dynamically or assigned via a unique transmitter and receiver ID. In some countries, frequencies in the 41, 72 and 75 MHz bands are also approved for remote control of models. In Germany, the 35 MHz band has also been free of charge and registration for model aircraft since 2003.

In each frequency band, only well-defined frequency channels are available that are so far apart that adjacent channel interference is avoided, which would be particularly fatal in the case of aircraft models. In the 27 MHz band in particular, there must also be other interference, e.g. B. can be calculated by the CB radio , which is why the 35 MHz band is preferred, especially for model aircraft.

Programmable so-called “computer transmitters”, mostly with an LC display , enable the storage of servo parameters and mixes of channels, which are mainly used for helicopters and flight models . In most cases, parameter sets for a number of models can be saved and changed quickly. Additional functions of high-quality transmitters are selectable modulation methods ( PCM , PPM ), transmission channels that can be selected by software, exchangeable RF modules for the different bands or functions for scanning free channels.

receiver

The receiver should combine light weight and reliable reception; The antenna, power supply and one or more servos or other control devices are connected to it. The receivers are mostly designed as superheterodyne receivers (super), often with double frequency conversion (double super). The receiver needs a receiver crystal that matches the transmitter frequency (transmitter crystal), but the frequency of which differs from the transmitter frequency by the amount of the first intermediate frequency (depending on the receiver type) because it sets the frequency of the reference oscillator.

Servos and actuators

Servos are generally classified according to weight (from 4 g, standard 40 g) and torque (e.g. 25 Ncm); Further important parameters are the positioning time (e.g. 0.15 seconds for 60 degrees) and gear design (plastic / metal, possibly ball bearings). Bell armature motors and digital controls are also used in more complex servos .

There are special servos for special applications:

Sail winch: Servo to operate cables, usually with a very long travel range;
Positioning servo: Servo with long travel or high positioning angle, mostly slow
Switching servo: Servo with fixed or adjustable end positions, not proportional;
Linear servo: Instead of an output shaft that rotates, there is a lever that is moved. In contrast to a normal servo, the travel is proportional to the lever position, not the sine of the travel.

In addition to servos, electrical actuators or regulators are also used to convert control signals on the receiver side , which, for example, control the speed remotely in models with electric motors. In addition to voltage regulators, speed controllers are also used, e.g. B. for helicopter models, where the speed of the engine is kept constant even with changing mechanical loads. Special controllers for brushless motors regulate the electrical rotating field directly according to the control specification ( three-phase current controller ).

In models with several motors, actuators can also be used for direction control. Examples are B. Tracked vehicles with separately driven chain drives, two or more engine model airplanes or coaxial helicopters . In these cases, mechanically complex servos and controls can be dispensed with.

Other uses

Today the radio remote control is also used in the industry for the control of z. B. overhead cranes as well as shunting and small locomotives are used. Radio controlled robots are z. B. used for defusing bombs.

Individual evidence

Web links

literature

Günter Miel: Electronic model remote control. Military Publishing House of the GDR, Berlin, 1976 (compendium)
Lothar Hennicke: RC model aircraft and RC model flight. VEB Transpress Verlag for Transport, Berlin 1976 (simple introduction with pulse protocol and circuits)
Gerald Kainberger: The big book of model flying. VTH Verlag, Baden-Baden 2010, ISBN 978-3-88180-793-7 .
Manfred-Dieter Kotting: Modern remote controls for RC model aircraft: receivers, servos, accessories. 2.4 GHz and 35/40 MHz. VTH Verlag, Baden-Baden 2000, ISBN 3-88180-780-2 .

[1] Project Aphrodite

[2] Article on the admissibility of 2.4 GHz systems on RC-Network from February 2009

[3] Page about the applicability of the EN300328V1.7.1 standard to remote controls