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martes, 30 de abril de 2013

Build a Radio control car


Build a Radio control car Design and build radio-control system works just like the ones  used in commercially available RC cars. 
Radio Control
There are many applications for a radio-control (RC) system; for instance, an RC system might be used to control i model cars or planes, toys, and even household devices. In this article, we'll describe the theory, construction, and interfacing of a RC transmitter/receiver system to a model car. The RC: system that we'll present here is based r on a pair of companion IC's specifically designed for simple remote- control applications. One chip- along with its external support components-performs the needed en- coder/transmitter functions. The second chip along with its support components provides receiver/decoder functions. The receiver (which operates from nominal 6-volt DC supply) is a crystal controlled superheat design that can be easily interfaced to small DC motors or directly connected to standard pulse-type hobby servo motors. The receiver-to-DC motor interface we'll, discuss provides on-off and direction-of-rotation control functions.
The receiver system can work with four channels, two digital and two analog. The two analog channels can be independently controlled to per- form separate functions. In our circuit only one of the analog outputs is used, although both channels are supported by hardware. The extra channel can be used to perform some other remote function if desired. The two digital output channels are used for simple on-off and direction (forward/reverse) control.
Our system uses two separate variable-width 1- to 2-ms pulses for its analog channels and 1 to 4 fixed-width 1- ms pulses for the digital channels.
Generally Speaking. A block diagram of the RC system is shown in Fig 1. The basic system is comprised of a pair of special-purpose chips: the LM1871 RC encoder/transmi'lter and its companion, the LM1872 RC receiver/decoder. A third chip, the LM18293 4-channel push-pull driver (which is controlled by the LM1872) is used to control the drive motor,
The LM1871 (see 'Fig. 1A) contains 4.6-volt regulator, a frame timer, a pulse timer, an encoder, and chan- nel-add logic circuitry, The LM1871 generates an encoded pulse-width encoded waveform that is amplified, amplitude modulated, and fed to an antenna for transmission.
The receiver (see Fig. 1B) is a single- conversion superheterodyne circuit that is built around an LM1872 RC receiver/decoder, The LM1872 contains a local oscillator, voltage regulator, a mixer, IF circuitry with AGC, a detector, sync circuitry, and logic circuitry (which is used to decode the demodulated signal), It has a total of six out- put pins-two analog (pins 11 and 12,
CH1 and CH2, respectively) and four digital (CHA at pins 7 and 8, and CHB at pins 9 and 10).
The receiver (which operates from a 5- to 6-volt supply) has a single tuned network in its front end. The net work feeds a mixer, where the signal is mixed with the output of the crystal- controlled local oscillator (La). (The crystal is cut for 455 kHz below or above the desired radio frequency.) Since there is only one tuned circuit used for RF pre-selection, the re- ceiver's image rejection is about 6 to 10 dB, but image interference is not a problem.
The mixer output is fed to the IF stage, whose output is kept constant by the chip's AGC circuit. The output of the IF is then used to drive the detector/decoder-logic circuitry. The receiver's analog outputs (pulse-width signals emitted from pins 11 and 12) can be used to directly drive a standard pulse servo.
However, in our application, the A and B digital outputs of the LM1872 are fed to an additional IC-an LM18293 four-channel push-pull driver (setup in an H-bridge configuration), which is, in turn, used to control the drive motor. One of the LM1872's two digital outputs (CHB) is fed directly to the enable input of the LM18293 to provide on/off control of the motor. The other digital output (CHA) is fed directly to one leg of the H-bridge circuit. A simple inverter, connected to the CHA output, inverts that signal, which is, in turn, used to control the drive motor. One of the LM1872's two digital outputs (CHB) is fed directly to the enable input of the LM18293 to provide on/off control of the motor. The other digital output (CHA) is fed directly to one leg of the H-bridge circuit. A simple inverter, connected to the CHA output, inverts that signal, which is then fed to the other leg of the H-bridge driver. In that way, the output from CHA is used to rotate the motor in either direction (backward or forward), depending on the logic level output by the receiver chip. The LM18293 can handle loads of up to 1 amp without overloading.
Encoding. The lM1871's encoding scheme is best understood with the aid of the control waveforms shown in Fig 2, As that illustration shows, the encoded signal (a stream of pulses) is output during a 20-ms period called the frame time, denoted Tm' At the be- ginning of each frame, there is a period (the modulation time, denoted T,  during which the output signal goes to zero for a fixed duration 500 µS. (0.5 ms), After T m' the signal goes high for a variable duration called the channel time (Tc,,)' Note that Tm (the width of a recovered channel pulse) is the sum of Tm and T ch In the first analog channel, T ch has a pulse width of 0,5 to 1.5 ms, making Tm equal to between 1 and 2 ms,
0.5(Tm) + 0.5-1.5 (Tcc)-which appears at the CH1 output.

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