Radio communication

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The radio communication system of an RC car allows the driver to control the throttle and steering of the car from a distance. It is composed of a radio transmitter (often referred to as TX) and a radio receiver (often referred to as RX). The transmitter is operated by the driver while the receiver is fixed onto the car.

Vintage RC radio transmitter

History

The first ever radio controled vehicle was built and demonstrated in 1898 by inventor, electrical engineer and mechanical engineer Nikola Tesla. During World War II, development in radio control technology increased. The German Luftwaffe used controllable winged bombs for targeting Allied ships. During the 1930s the Good brothers Bill and Walt pioneered vacuum tube based control units for R/C hobby use. Following World War 2, many other R/C designs emerged and some were sold commercially sudh as the Berkeley’s Super Aerotrol.

In the 1960s the availability of transistor-based equipment led to the rapid development of fully proportional servo-based “digital proportional” systems, achieved initially with discrete components, again driven largely by amateurs but resulting in commercial products. In the 1970s, integrated circuits made the electronics small, light and cheap enough for the 1960s-established multi-channel digital proportional systems to become much more 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.

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.

Overview of an RC car Radio Communication system

The entire Radio Communication system of a modern RC car consists of a radio transmitter and a radio receiver. The transmitter is meant to be handheld and operated by the driver, the receiver is meant to be attached to the chassis of the car. The transmitter is powreed by everyday batteries, rechargeable batteries or a battery pack. The receiver is powered by the battery by way of the Electronic Speed Controler (ESC).

Overview of RC radio transmitter and receiver

Overview of the main components

The understanding of the components of the Radio Communication system of an RC car is paramount t the optimal operation of the vehicle:

  • Transmitter: a handheld electronic device which holds the sticks, wheels and buttons to command an RC vehicle. It is powered by batteries and needs to be paired with a receiver inside the RC model to be operational. Standard transmitters may only have a throttle trigger, a steering wheel and a couple trim buttons while high-end modern transmitters are micro-computers capable of handling anti-braking systems, limiting the amplitude of motion of the car servo (EPA), adjusting tensions in both throttle trigger and sterring wheels and much more.

  • Receiver: a small electronic device often with an antenna which when paired with a transmitter, receives and transfers the commands from the driver onto the servo and Electronic Speed Controler (ESC). The receiver is fixed onto the car chassis and connected to and powered by the ESC. It can be connected to other devices such as multiple servos, LED lights, fans and more.

Radio communication system operation and tuning

The operation of "pairing" a transmitter and a receiver in order for them to communicate exlusively with each other safe from interference is called binding. Entry-level receivers will require a binding cable to be connected to a specific Bind port on the receiver and powered up while the transmitter has been turned on in 'Binding Mode'. Once binding is achieved, the binding cable will need to be removed and both transmitter and receiver turned OFF and ON to be fully operational. High-end transmitters and receivers are capable of automatic binding.

Types of radio transmitters

RC Pistol Radio vs Stick Radio
A pistol grip RC radio next to a stick radio

There are primarily two types of radios available on the market: the pistol grip radio (AKA wheel radio), and the stick radio. Pistol grip radios are mostly for surface vehicles such as RC cars, buggies, and trucks. Stick radios, on the other hand, cater to all models including RC planes, cars, boats, and drones.

The pistol grip is the preferred choice when it comes to surface vehicles as the steering mechanism closely resembles that of a real car and handling is made much easier. The "pistol" pedal also gives users better throttle control and responses which are crucial for executing small and precise maneuvers on a racetrack.

Radio Frequency And Protocols

Most modern RC radios work on the 2.4GHz (Gigahertz) frequency. The frequency can be viewed as the medium or channel by which the radio signals are sent. In older RC radio comunnication systems, most RC signals were sent via the 27, 35, and 40 MHz frequency, which suffered from bandwidth (number of simultaneous users) and interference problems. In the 2.4GHz spectrum, bandwidth, interference, and range are greatly enhanced, allowing modern hobbyists to enjoy flying or racing with minimal technical deficiencies.

RC protocols can be viewed as the operating system a Tx/Rx is working on. The most widely used protocols include DSM (DSMC/ DSM2), ACCST, AFHDS (AFHDS2), SBUS, IBUS, FASST, PWM, PPM, and CPPM. The main constraint for an RC driver is to use receivers which share the protocol(s) of the radio transmitter.

Configuration of an RC transmitter

Passed the binding process of a transmitter and receiver, modern radio systems offer extensive configuration capabilities. Since all commands to he car come from the transmitter, the proper configuration is paramount to an optimal driving experience.

Channels

RC receiver close up of connectors and channels numbers
RC receiver, channels and connectors

Channels (CH) refer to the number of controllable functions on the RC model. A 4CH transmitter will allow the driver to control four functions of the model while a 6CH transmitter will allow the driver to control up to six functions. Throttling is one function, steering is another so generally speaking, most RC cars only require two channels. Extra channels can be used to activate a handbrake function, turn on and off LED lights and more. Each channel corresponds to a numbered connector on the receiver while the binding connector will be labaled 'B' or shared with a channel.

With most RC receivers, it is standard for the steering servo to be connected to Channel 1 and the Electronic Speed Controler (ESC) to Channel 2.

RC radio transmitter adjustments

RC radio transmitters feature adjustment controls that alter the signals sent to the receiver and that ultimately control the car. They allow the driver to remotely alter such settings as the steering dead point and how sensitive the steering or throttle controls are.

Trim

Trim adjustment compensates for a servo and a car that would steer to one side (left or right) while the driver's transmitter wheel is pointing straight forward. The fact for the car steering rather than going straight can be caused by a servo horn that was mounted while the servo was not in a neutral position of by the misalignment of one or several of the parts of the steering system or other factors such as the suspension, the wheels, the tires etc.

Trim adjustments allow for the driver to correct the deviation by trimming incrementaly in the opposite direction of the unwanted curving of the car.

Trims are also relevant for throttle, adjusting the neutral position to ensure the car doesn't accelerate or reverse by itself. They are especially useful for refining inputs on transmitters that offer a physical ratio adjustment (50:50 to 30:70) in the throttle.

Dual Rate

Dual rate (also called rate adjustment) will adjust the 'throw' of the input from the transmitter. The entire travel of the steering/throttle inputs will be in use but scaled down. If a driver sets the dual rate all the way down and pulls the throttle trigger to 100%, the transmitter will not convey a 100% throttle command but a scaled down command to whatever percentage has been set by the Dual Rate. Throttle dual rate adjustment can be used to cap power when driving on particularly loose surfaces or for a beginner to start driving on a powerful RC car.

The process is identical for steering where winding the wheel all the way over to one side will only give a fraction of the steering angle it normally would.

Such adjustment is useful when driving a car at higher speeds (without any gyro assist or traction control) where extreme steering-angle changes would result in loss of control.

Reverse switches

Used to reverse the signal sent from the transmitter to the receiver. This setting is necesasry when a car is moving backward when the driver is pushing forward or when a car is turning left when the driver is turning right.

Although the reversing steering is straightfoward, when it comes to power, the reason for the car operating in reverse should be investigating since some combination of motors and ESC, when in reverse, will provide full driving power going backward and limited power going forward. In this case, the cause for the reverse operation would be found in the wiring of the motor to the ESC.

End-point Adjustment (EPA)

End-Point-Adjustment (EPA) allows the driver to set the servo "throw", or how far the servo moves, when input is detected. It may be used to prevent the servo from pushing the wheels to much to the left or right when the mechanical configuration of the steering system does not match the servo's amplitude, causing parts in the steering system to bend and prematurely aging the servo. The opposite can be said of a servo that does not make use of the full amplitude of the steering system cpabilities, not turning the wheels enough to the left and right.

By default, servo throw is at 100%. EPA can limit the servo's movement by a certain amount (in percentage of the total amount). End points can be adjusted individually for left and right, as well as forward and back.

Some transmitters do not offer manual end point adjustment but are often capable of limited configuration by entering a programming mode. These typically have the driver holding the throttle/steering at the desired point before pressing a button on the receiver and setting the next end point etc.

Exponential Adjustment

High-end controllers offer an exponential adjustment allowing the driver to modify the behaviour of the transmitter inputs. It can be sued to soften the initial steering input without sacrificing the full steering lock, making the car more stable when inputting slight steering adjustments at high speed.

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