Radio Control Systems - Principles
control (RC) is currently a must-have component for RW. All existing contenders
use a RW system of some kind. It is also one of the areas that is least
understood from a technical view. Since for many purposes RC is a plug-and-play
technology, it is not necessary to have a grasp of the engineering.
There are numerous frequency bands for RC systems. These vary from country to country, so included here are the UK bands. Roboteers from other nations, mail RWE with details for your country.
There are 3 main bands used for hobbys - 27, 35 and 40Mhz. Generally 27Mhz tends to be used for cheaper models and RC cars. It is acceptable for antweight and lightweight robots only due to its susceptibility to interference from CB radio, taxis and other nearby sources.
35Mhz is the most common frequency and is used exclusively for model aeroplanes. It may not be used for any robot control.
40Mhz is a general purpose band for hobbies and the most common for RW. It is also used for boats, hovercraft and some helicopter use.
These 3 frequencies are made up of a number of bands - seperate, more specific frequencies. There are also a handful of other frequencies which have a few available bands. These are 75, 418 and 458. In the UK, the house robots use 458Mhz but as there are only about 10 channels available for public use, this is not generally open for contestants unless agreed with the organisers first. 75Mhz is a recently allocated band designed for general hobby use (Note: in the USA, this is the main band for robots instead of 40Mhz) and 418 is normally used for telemetry and has only one band free for hobby purposes.
Whatever frequency is used, the principle for RC is the same. The transmitter produces a pulse which is between 1 and 2 milliseconds (ms) in length. A 1.5ms pulse represents the neutral position for the control stick. 50 times a second, the transmitter (Tx) produces a pulse train (1 pulse from each channel plus a syncronization 'pulse') and the receiver (Rx) can decode and send the pulses to the correct channel. The sync 'pulse' is actually a gap with a minimum length of 4ms.
If the channel is connected to a servo, the servo will start a timer when it receives the pulse. The circuit will compare the length of the incoming pulse with the internal pulse and try to adjust the servo motor in the correct direction to match the two. The servo contains a feedback pot which is mechanically connected to the motor to provide the 'positioning' information.
This simple pulse information can be used in fully electronic circuits to alter switching, voltage or other parameters as required. The pulse train system is the same regardless of whether the radio is AM, FM.