Propeller Powered Skateboard

Radio Control is the basis for a lot of fun projects. Here I briefly overview a system, show how a servo works, and give a demo.

Transmitter

The transmitter is often the most expensive part of an RC system. A transmitter, as with most RC gear, will survive through many projects, so it is wise to shop for a transmitter with future projects in mind.

Control Style

Pistol

The pistol grip is primarily used for controlling RC cars and trucks as the wheel simulates a steering wheel, the trigger being used as the throttle.

Stick

The stick interface is the most common, and is used for practically everything including RC cars, helicopters, planes, boats, and general use. The sticks are spring loaded to return to the center position with the exception of the left stick which has no spring for the vertical direction. The ability to hold the vertical position is commonly used to hold a throttle position making the left hand at least momentarily available for switching other controls. The exception to this is in special "helicopter" radios in which all sticks return to center.

Radio Transmission Technique

There are many types of transmission techniques available on the market today. Each newer generation improves on the last, so when selecting which one you want it is really a trade-off between the new features and price. I'll start with the oldest and describe the improvements through the generations.

AM

Amplitude Modulation (AM) systems would transmit the data as changes in amplitude on a constant transmission frequency wave. The simplicity of the electronics made this method inexpensive. Noise directly effects the output signal causing a noisy output. AM is cheap, but the costs of FM transmitters have come down so much AM is not longer in the game.

FM (PPM)

Just like car radio transitioned from AM to FM, so did hobby RC gear. FM (Frequency Modulation) changes the frequency of the waveform as opposed to the amplitude. Most random radio noise is in pops in amplitude as opposed to consistent frequencies. Since FM is insensitive to pops in amplitude while that's all AM has got, FM can handle radio noise much better than AM can.

PPM is a way to encode the data before sending it over the air, (Pulse Position Modulation). This is how the hobby radio implementation works. The transmitter sends a series of highs and lows in a specific sequence which the receiver then decodes. The output is binary, high or low. The sequence starts with a long high followed by a low, which signals the start of the sequence. The position of channel 1 on the transmitter is indicated by the length of the next high, which is again followed by a low. The length of the following high corresponds to the position of channel 2 on the transmitter. And so on, and so on, followed by a long high which is the start of a new transmission. This sequence is repeated regularly, typically at 50Hz.

The receiver's job is to split the sequence up into isolated individual pulses on different channels. Each channel receives one of the pulses. The length of each pulse ranges from about 1ms to 2ms. The receiver knows when to change outputs when it sees a low. If the low was caused by noise, the rest of the channels would receive bad data due to an off by one error. This type of error is avoided in the next technology...

PCM

Referred to as PCM (Pulse Code Modulation), this technique is still uses FM at it's core but sheds the FM name. Whereas PPM created a pulse of a certain length specified by the position of the controls, PCM creates a digital representation of the position of the controls (8-bit, or 16-bit, etc) and sends the position data as a stream of 1s and 0s. For example, as opposed to a neutral stick being represented by a 1.5ms high pulse, it may be represented in 8-bit as a high pulse followed by seven low pulses (1,0,0,0,0,0,0,0 [binary]), the pulse length being determined by a clock.

One advantage of this is the ability to do quick error checking with a CRC check. The error checking allows for noise to be detected and for bad data to be halted before going out of the receiver.

The noise screen practically eliminates interference, yet we find a new problem: when the model gets out of range, the servos hold the last good transmission, possibly until a spectacular crash. To reduce this problem, some receivers have the ability to keep "fail-safe" outputs in memory. When the receiver has not received a noise free transmission for a certain amount of time it starts outputting the "fail-safe" values. These values may, for example, turn the throttle down or put a glider into level flight which could help avoid the spectacular crash.

2.4GHz

Now comes the latest generation of wireless transmitters, using a technology much similar to WiFi computer networks. Unfortunately, there are currently many implementations of transmitters/receivers using the 2.4GHz spectrum, making generalizing difficult. The previous technologies used a crystal to determine the transmission frequency, allowing for interference from someone using the same crystal (of which there are a limited number), and requiring additional crystals to change frequencies. This new generation of transmitters get around this problem in different ways. Some scan for clear frequencies then latch on to a clear one, others use spread spectrum frequency hopping, to continuously evade noise by continuously changing channels. Other advantages of this new generation include the reduction of size in the transmitter antenna, smaller receiver size, and better battery life. Of course, these new features will hit you in the pocketbook.

Receiver

The receiver's job is to split the incoming signal into however many channels you require. Although transmitters and receivers of different transmission technologies can't talk to each other, the output of all current receivers is the same: a pulse of a certain length that corresponds to the position of one of the controls on the transmitter. It is then left up to the servo, motor controller or other accessory to decode that pulse into a position, speed, etc.

It is most common to buy a receiver in a set with a transmitter. That way you know they are compatible and get a better deal on the purchase.

Servos, Motor Controllers, and More - Oh My!

Servo

A servo is a motor that has position feedback, allowing the servo to rotate the motor to an angle, then hold that specified angle of rotation. Check out the video for more on how servos work.

Motor Controller

Motor controllers control the output (speed or torque) of a motor connected to it given various types of input. Many motor controllers accept the servo signal output from an RC receiver, allowing drop-in functionality with an RC system. With electric motor powered airplanes becoming more popular due to advancing lightweight and powerful battery technology more motor controllers are hitting the market and they are coming down in price. Since electrical power loss goes as I^2*R, twice as much current produces twice as much heat. In order to prevent part loss due to high heat, motor controllers are usually rated in terms of current (here is a link to an example motor controller, rated for 2 channels outputting 25 Amps each). Many motor controllers have onboard heat sensors and will shut down the motor controller if too high of a temperature is reached.

High electrical loads and spikes that come with motors and motor controllers may disrupt the signal going into the receiver causing potentially disastrous interference. Error checking radios (PCM or later) may help a great deal in these situations.

Other

One cool part I have used is a relay that is triggered by the servo signal called the: Battle Switch

(Wikipedia Article For More Info )

Demo

Construction

RC Gear