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7/31/2019 RC Transmitter

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RC Transmitter, Receiver and Servos

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Added byGary Mortimer, last edited byBerkelyon Sep 16, 2012 (view change)

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Content Content

Definitions

Introduction

Transmitters and receivers

Modulation

Pulse Position Modulation (PPM)

PCM

Spread Spectrum

Number of channels

Reserved Frequencies

Mode 1 or Mode 2?

Servos

Connecting control surfaces.

Futaba serial connector

All those hard to understand terms

Sources

Definitions

Where the term 'RC system' is used on this page, it refers to a radio controlled vehicle such as a car or an airplane.

RPM = Revolutions per minute. This is a term commonly used to describe the speed of rotary objects (e.g.

propellers on an aeroplane or wheels on a car)

Introduction

This page describes the basic operation of model radio controlled (RC) systems. Visual aids, such as videos, will be

used where possible.

For RC control, you will need:

-A transmitter, which takes the form of a handset. This, unsurprisingly, 'transmits' commands to your RC system. If

you've ever had an RC toy, this is the part you hold in your hand. :)

-A receiver. This will usually be a small plastic box with one or more wires attached. It receives the commands from

the transmitter and sends them to the output devices.
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-One or more output devices such as an ESC/motor combo orservos(see below). These plug into the receiver and

are usually used to spin wheels or move control surfaces such as a steering mechanism.

-A battery to power the RC system. The batteries most commonly used today areLithium Polymerbatteries. They

are very similar to the battery in a cellphone-- although using a cellphone battery to power an RC system is NOT

recommended! More on this later.

Transmitters and receivers

As described above, this is a kind of handset. It usually has an aerial sticking out the top end and two gimbals

('joysticks') for control of your RC system. These are usually free to move both up/down and left/right, like the

joysticks on a playstation controller.

There may be other knobs and switches which can be used for added functionality of your RC system. For example,

a button on your transmitter may be set to switch an RC car's headlights on and off.

The receiver has several connectors, usually at least four, on one side. These are for RC outputs to electric motor

controllers and servos.

There are many different brands of transmitters and receivers. Hitec, Futaba, JR, Multiplex, and Spektrum are

among the most common, but increasingly the market is being penetrated by low-cost generic transmitters.

Generally speaking, the two golden rules when deciding which transmitter and receiver to use are:

1) Look for a transmitter with a short plastic aerial rather than a long metal one. These transmitters operate on very

high frequencies known as Spread Spectrum, described below. They are much more reliable than other transmitters

and less likely to allow interference to cause unintended operation from your RC system. You don't want

interference to crash your brand new RC plane!

2) It is best to ensure that your receiver is the same brand as your transmitter. If you don't, you are more likely to get

problems with operation of your RC system that could lead to a crash. For spread-spectrum radio systems, the

transmitter/receiver combination *MUST* speak the same protocol-- for instance DSM2 or SBUS-- or they simply

won't work at all!

OK, so far we've dealt with the transmitter controlling the RC system and the receiver which ouputs the transmitted

commands. But how do they talk to each other?

First, we must realize that the transmitter is sending radio waves to the receiver at all times whilst the RC system is

operating. (Radio control.)

By changing, or modulating the wave somehow, it is possible to put messages into these waves. For example,

"make the motor spin at a certain speed", or "rotate the servo arm 20 degrees".

Modulation

Let's have a look at exactly how this modulation works.
http://www.google.com/search?um=1&hl=en&client=safari&rls=en&biw=1265&bih=1217&site=search&tbm=isch&sa=1&q=rc+servo&aq=f&aqi=g10&aql=&oq=http://www.google.com/search?um=1&hl=en&client=safari&rls=en&biw=1265&bih=1217&site=search&tbm=isch&sa=1&q=rc+servo&aq=f&aqi=g10&aql=&oq=http://www.google.com/search?um=1&hl=en&client=safari&rls=en&biw=1265&bih=1217&site=search&tbm=isch&sa=1&q=rc+servo&aq=f&aqi=g10&aql=&oq=http://en.wikipedia.org/wiki/Lithium-ion_polymer_batteryhttp://en.wikipedia.org/wiki/Lithium-ion_polymer_batteryhttp://en.wikipedia.org/wiki/Lithium-ion_polymer_batteryhttp://en.wikipedia.org/wiki/Lithium-ion_polymer_batteryhttp://www.google.com/search?um=1&hl=en&client=safari&rls=en&biw=1265&bih=1217&site=search&tbm=isch&sa=1&q=rc+servo&aq=f&aqi=g10&aql=&oq=


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The 'basic wave' sent to the receiver, with no movement of the transmitter controls, looks like this:

____l'''''''''''l___l''''''''''''l___l''''''''''''l___l''''''''''''l___

It's a bunch of pulses, rather like a loudspeaker going 'beep'...'beep'...'beep'... or a light flashing on and off

constantly.

This signal says to the receiver,

"Do absolutely nothing!"

As soon as the wave changes in some specific way, the meaning of the wave changes.

We will look three ways to change, or modulate, the signal: Pulse position modulation, Pulse Code modulation and

Spread Spectrum.

Pulse Position Modulation (PPM)

This is the simplest way to change the signal. The distance between the pulses is changed:

____l'''''''''''l___l''''''''''''l___l''''''''''''l___l''''''''''''l___ "Do nothing"

____l'''''''''''l_l''''''''''''l_l''''''''''''l_l''''''''''''l___ "Go faster"

____l'''''''''''l_____l''''''''''''l_____l''''''''''''l_____l''''''''''''l___ "Go slower"

PPM receivers are usually pretty cheap, and it's often used in low-cost models which you wouldn't worry about

if they happened to go wrong.

So why use anything more complicated?

Well, it's essentially down to precision and safety.

Firstly, although a PPM receiver understands that rapid pulsing means 'speed up' and slow pulsing means 'slow

down', it doesn't actually know by how much it should speed up/slow down. Let's say you want to speed up a

motor (to make a car go faster, for example), but only by a little bit. You increase the throttle on your

transmitter and, as we've seen, the transmitter then sends closely spaced pulses to the receiver. The motor may

speed up by, say, 400 RPM or 800.

The problem is that the signals mean NOTHING to the receiver; it's just blindly following what it sees the

transmitter telling it to do.

Not very precise! If only we could encode a message in the pulsing that said "Increase throttle by 20%" rather

than just "err...a little bit faster, please".

And what if our pride and joy (our RC system, of course) happens to lose signal from the transmitter? With

PPM, there is no way to implement a failsafe command to tell it to switch itself off, or at least anything


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dangerous like a propeller (believe me, they can hurt!) in the event of signal loss. Once again, if we're worried

about anything going wrong, we must somehow encode a command to this effect.

And so we move from PPM to PCM, which addresses these issues.

PCM

Pulse Code Modulation. A scheme in which the commanded position for each servo is transmitted as a

digitally encoded number. Manufacturers use their own proprietary system to encode this number with various

levels of precision (i.e. variable number of bits per servo position). JR use Z-PCM (9 bits, 512 different values:

0...511) then S-PCM (10 bits, 1024 values: 0...1023). Futaba use PCM-1024 and G3 PCM (11 bits, 2048

values: 0...2047). With PCM not all positions are broadcasted at one time (each frame) to save time. The odd

numbered positions are sent as absolute in one frame, with the even sent only as differences from their

previous values. The next frame the opposite is done. PCM includes achecksumat the end of the frame to

check the signal's validity. Hence, if there is interference and the signal arrives distorted at the Receiver,

utilizing the checksum it is able to know if it is the original. In case it is not, a feature called Fail-Safe is

implemented to set servo positions to a predefined position, or to hold them at the last valid position.Pulse-

position modulation. A scheme in which the commanded position for each servo is transmitted as theduty-cycleof the transmitted pulses 1 per servo position.

Spread Spectrum

Systems such as FHSS (Frequency-hopping spread spectrum) used byFutabaemploy frequency hopping on the

2.4 GHz band instead of the various frequencies in the lower MHz ranges. The advantage is that radios are no longer

using a fixed frequency during flight but a multitude of frequencies.

Systems such as Spectrum and JR use the DSM2 DSSS (Direct-sequence spread spectrum) method, where they

transmit on a pair of fixed channels chosen when the radio and receiver are turned on. Any subsequent systems

would avoid using these channels and continue searching for another unused pair of channels.

With either method many radios can be transmitting at once without interfering with each other. The Futaba systems

change frequency approximately every two milliseconds, so even if two transmitters are using the same channel they

are not doing so for long. The pilot will not notice any abnormal behavior of the model in the 1/500th of a second

that they are interfering. This gives one the advantage of turning on a transmitter without regard to channels

currently in use by other pilot's radios.

One downside to 2.4 GHz is that precautions must be taken during installation since certain materials such as carbon

fiber can mask the signal. In some cases, "satellite" receivers with secondary antennas need to be used to maintain

better line-of-sight with the transmitter radio. Another drawback is that a 2.4 GHz standard has yet to evolve so that

receivers and transmitters can be mixed regardless of their respective manufacturer

Number of channels

The number ofchannelsa plane requires is normally determined by the number of mechanicalservosthat have been

installed (with a few exceptions such as the aileron servos, where two servos can operate via a single Y harness

(with one of the two servos rotating in the opposite direction)). On smaller models, usually one servo per control

surface (or set of surfaces in the case of ailerons or a split elevator surface) is sufficient.
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Ailerons- controlsroll.

Elevator- controlspitch(up and down).

Throttleor, if electric, motor speed.

Rudder(or Vertical Stabilizer)- controlsyaw(left and right).

Retracts - controls retractable landing gear.

Flaps - Increase lift, but also increase drag. Using Flaps, an aircraft can fly slower before stalling. Flaps are often

used to steepen the landing approach angle and let the plane land at a slower touchdown speed (as well as letting the

aircraft lift off at a slower takeoff speed). In both cases, flaps enable using a shorter runway than would otherwise be

required.

Auxiliary Switches - can control anything such as Spoilers, Flaperons, Elevons, Lights, Cameras, etc.

Three channels (controlling rudder, elevator and throttle) are common on trainer aircraft. Four channel aircraft add

aileron control. For UAS flight a minimum of six channels are normally required

For more information on the available receiver types & connections, please refer to theReceiver & Transmitter

Optionspage.

Reserved Frequencies

Many countries reserve specific frequency bands (ranges) for radio control use. Due to the longer range and

potentially worse consequences ofradio interference, model aircraft have exclusive use of their ownfrequency

allocationin some countries.

USA and Canada reserved frequency bands

72 MHz: aircraft only (France also uses US/Canada channels 21 through 35).

75 MHz: surface vehicles.

50 & 53 MHz:, on the6-meter bandfor all vehicles, with the operator holding a validamateur radio(FCC in the

USA) license.

27 MHz: general use, toys.

2.400-2.485 GHz: Spread Spectrum band for general use (amateur radio license holders have 2.39-2.45 GHz

licensed for their general use in the USA) and usually usingfrequency-hopping spread spectrumRF technology to

maximize the number of available frequencies on this band, especially at organized events in North America.

US frequency chart available at [2|http://www.modelaircraft.org/events/frequencies.aspx], Canadian frequency chart

available at [3|http://www.maac.ca/freq_chart.php]

European reserved frequency bands

35 MHz: aircraft only.

40 MHz: surface vehicles or aircrafts.

27 MHz: general use, toys, citizens band radio.

2.4 GHz spread spectrum: surface vehicles.

Within the 35 MHz range, there are designated A and B bands. Some European countries allow use only in the A

band, whereas others allow use in both bands.

Singapore reserved frequency bands
http://en.wikipedia.org/wiki/Aileronhttp://en.wikipedia.org/wiki/Aileronhttp://en.wikipedia.org/wiki/Rollhttp://en.wikipedia.org/wiki/Rollhttp://en.wikipedia.org/wiki/Rollhttp://en.wikipedia.org/wiki/Elevator_%28aircraft%29http://en.wikipedia.org/wiki/Elevator_%28aircraft%29http://en.wikipedia.org/wiki/Pitch_%28flight%29http://en.wikipedia.org/wiki/Pitch_%28flight%29http://en.wikipedia.org/wiki/Pitch_%28flight%29http://en.wikipedia.org/wiki/Throttlehttp://en.wikipedia.org/wiki/Throttlehttp://en.wikipedia.org/wiki/Rudder#Aircraft_ruddershttp://en.wikipedia.org/wiki/Rudder#Aircraft_ruddershttp://en.wikipedia.org/wiki/Yaw_anglehttp://en.wikipedia.org/wiki/Yaw_anglehttp://en.wikipedia.org/wiki/Yaw_anglehttp://wiki.openpilot.org/pages/viewpage.action?pageId=19300506http://wiki.openpilot.org/pages/viewpage.action?pageId=19300506http://wiki.openpilot.org/pages/viewpage.action?pageId=19300506http://wiki.openpilot.org/pages/viewpage.action?pageId=19300506http://en.wikipedia.org/wiki/Electromagnetic_interferencehttp://en.wikipedia.org/wiki/Electromagnetic_interferencehttp://en.wikipedia.org/wiki/Electromagnetic_interferencehttp://en.wikipedia.org/wiki/Frequency_allocationhttp://en.wikipedia.org/wiki/Frequency_allocationhttp://en.wikipedia.org/wiki/Frequency_allocationhttp://en.wikipedia.org/wiki/Frequency_allocationhttp://en.wikipedia.org/wiki/6-meter_bandhttp://en.wikipedia.org/wiki/6-meter_bandhttp://en.wikipedia.org/wiki/6-meter_bandhttp://en.wikipedia.org/wiki/Amateur_radiohttp://en.wikipedia.org/wiki/Amateur_radiohttp://en.wikipedia.org/wiki/Amateur_radiohttp://en.wikipedia.org/wiki/Frequency-hopping_spread_spectrumhttp://en.wikipedia.org/wiki/Frequency-hopping_spread_spectrumhttp://en.wikipedia.org/wiki/Frequency-hopping_spread_spectrumhttp://wiki.openpilot.org/display/Doc/RC+Transmitter,+Receiver+and+Servoshttp://wiki.openpilot.org/display/Doc/RC+Transmitter,+Receiver+and+Servoshttp://wiki.openpilot.org/display/Doc/RC+Transmitter,+Receiver+and+Servoshttp://wiki.openpilot.org/display/Doc/RC+Transmitter,+Receiver+and+Servoshttp://wiki.openpilot.org/display/Doc/RC+Transmitter,+Receiver+and+Servoshttp://wiki.openpilot.org/display/Doc/RC+Transmitter,+Receiver+and+Servoshttp://wiki.openpilot.org/display/Doc/RC+Transmitter,+Receiver+and+Servoshttp://wiki.openpilot.org/display/Doc/RC+Transmitter,+Receiver+and+Servoshttp://en.wikipedia.org/wiki/Frequency-hopping_spread_spectrumhttp://en.wikipedia.org/wiki/Amateur_radiohttp://en.wikipedia.org/wiki/6-meter_bandhttp://en.wikipedia.org/wiki/Frequency_allocationhttp://en.wikipedia.org/wiki/Frequency_allocationhttp://en.wikipedia.org/wiki/Electromagnetic_interferencehttp://wiki.openpilot.org/pages/viewpage.action?pageId=19300506http://wiki.openpilot.org/pages/viewpage.action?pageId=19300506http://en.wikipedia.org/wiki/Yaw_anglehttp://en.wikipedia.org/wiki/Rudder#Aircraft_ruddershttp://en.wikipedia.org/wiki/Throttlehttp://en.wikipedia.org/wiki/Pitch_%28flight%29http://en.wikipedia.org/wiki/Elevator_%28aircraft%29http://en.wikipedia.org/wiki/Rollhttp://en.wikipedia.org/wiki/Aileron


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29 MHz: aircraft only

Australian reserved frequency bands

36 MHz: aircraft and water-craft (odd channels for aircraft only)

29 MHz: general use

27 MHz: light electric aircraft, general use 2.400-2.485 GHz: Spread Spectrum band for general use (ACMA references available

at [4|http://www.acma.gov.au/])

New Zealand reserved frequency bands



27 MHz: general use

29 MHz: general use

36 MHz: general use

72 MHz: general use (US 72 MHz "even-numbered" channels 12 through 56, at 40 kHz spacing)

2.4 GHz is permitted under NZMAA and MED/RSM regulations, provided equipment bears a C-Tick compliancelabel

Mode 1 or Mode 2?

The way in which the sticks in an RC transmitter send information to an airframe have formed different evolutionary

paths in different parts of the world, hence mode 1 and mode 2.

Servos

This section will really be of interest to aircraft owners as multicopters don't use servos.
http://wiki.openpilot.org/display/Doc/RC+Transmitter,+Receiver+and+Servoshttp://wiki.openpilot.org/display/Doc/RC+Transmitter,+Receiver+and+Servoshttp://wiki.openpilot.org/display/Doc/RC+Transmitter,+Receiver+and+Servoshttp://wiki.openpilot.org/display/Doc/RC+Transmitter,+Receiver+and+Servos


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RC servos are hobbyist remote control devices servos typically employed inradio-controlled models, where they

are used to provideactuationfor various mechanical systems such as the steering of a car, the control surfaces on a

plane, or the rudder of a boat.

RC servos are composed of an electric motor mechanically linked to a potentiometer.Pulse-width

modulation(PWM) signals sent to the servo are translated into position commands by electronics inside the servo.

When the servo is commanded to rotate, the motor is powered until the potentiometer reaches the value

corresponding to the commanded position.

Due to their affordability, reliability, and simplicity of control by microprocessors, RC servos are often used in

small-scaleroboticsapplications.

The servo is usually controlled by three wires: ground, power, and control. The servo will move based on the pulses

sent over the control wire, which set the angle of the actuator arm. The servo expects a pulse every 20 ms in order to

gain correct information about the angle. The width of the servo pulse dictates the range of the servo's angular

motion.

A servo pulse of 1.5 ms width will set the servo to its "neutral" position, or 90. For example a servo pulse of 1.25

ms could set the servo to 0 and a pulse of 1.75 ms could set the servo to 180. The physical limits and timings of

the servo hardware varies between brands and models, but a general servo's angular motion will travel somewhere in

the range of 180 - 210 and the neutral position is almost always at 1.5 ms.

RC servos are usually powered from eitherNiCdorNiMHpacks common to most RC devices. More recently these

systems are powered bylithium-ion polymer battery(LiPo) packs orLiFePO4packs. Voltage ratings vary from
http://en.wikipedia.org/wiki/Radio-controlled_modelhttp://en.wikipedia.org/wiki/Radio-controlled_modelhttp://en.wikipedia.org/wiki/Radio-controlled_modelhttp://en.wikipedia.org/wiki/Actuationhttp://en.wikipedia.org/wiki/Actuationhttp://en.wikipedia.org/wiki/Actuationhttp://en.wikipedia.org/wiki/Pulse-width_modulationhttp://en.wikipedia.org/wiki/Pulse-width_modulationhttp://en.wikipedia.org/wiki/Pulse-width_modulationhttp://en.wikipedia.org/wiki/Pulse-width_modulationhttp://en.wikipedia.org/wiki/Roboticshttp://en.wikipedia.org/wiki/Roboticshttp://en.wikipedia.org/wiki/Roboticshttp://en.wikipedia.org/wiki/Nickel-cadmium_batteryhttp://en.wikipedia.org/wiki/Nickel-cadmium_batteryhttp://en.wikipedia.org/wiki/Nickel-cadmium_batteryhttp://en.wikipedia.org/wiki/Nickel-metal_hydride_batteryhttp://en.wikipedia.org/wiki/Nickel-metal_hydride_batteryhttp://en.wikipedia.org/wiki/Nickel-metal_hydride_batteryhttp://en.wikipedia.org/wiki/Lithium-ion_polymer_batteryhttp://en.wikipedia.org/wiki/Lithium-ion_polymer_batteryhttp://en.wikipedia.org/wiki/Lithium-ion_polymer_batteryhttp://en.wikipedia.org/wiki/LiFePO4http://en.wikipedia.org/wiki/LiFePO4http://en.wikipedia.org/wiki/LiFePO4http://en.wikipedia.org/wiki/LiFePO4http://en.wikipedia.org/wiki/Lithium-ion_polymer_batteryhttp://en.wikipedia.org/wiki/Nickel-metal_hydride_batteryhttp://en.wikipedia.org/wiki/Nickel-cadmium_batteryhttp://en.wikipedia.org/wiki/Roboticshttp://en.wikipedia.org/wiki/Pulse-width_modulationhttp://en.wikipedia.org/wiki/Pulse-width_modulationhttp://en.wikipedia.org/wiki/Actuationhttp://en.wikipedia.org/wiki/Radio-controlled_model


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product to product, but most servos are operated at 4.8 V or 6 VDCfrom a 4 or 5 cell NiCd or NiMH battery, or

aregulatedLiPo pack

http://wiki.openpilot.org/display/Doc/RC+Transmitter,+Receiver+and+Servos
http://en.wikipedia.org/wiki/Direct_currenthttp://en.wikipedia.org/wiki/Direct_currenthttp://en.wikipedia.org/wiki/Direct_currenthttp://en.wikipedia.org/wiki/Voltage_regulatorhttp://en.wikipedia.org/wiki/Voltage_regulatorhttp://en.wikipedia.org/wiki/Voltage_regulatorhttp://wiki.openpilot.org/display/Doc/RC+Transmitter,+Receiver+and+Servoshttp://wiki.openpilot.org/display/Doc/RC+Transmitter,+Receiver+and+Servoshttp://wiki.openpilot.org/display/Doc/RC+Transmitter,+Receiver+and+Servoshttp://en.wikipedia.org/wiki/Voltage_regulatorhttp://en.wikipedia.org/wiki/Direct_current

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