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Specification

Controller:

The Controller will produce 2 analog signals, one for each motor, and a digital abort signal.

The Encoder:

The Encoder receives the 2 analog signals and converts them into 3-bit digital signals which are fed into the Flex chip. The abort signal from the controller is passed to the Flex chip.

The Flex Chip:

The Flex chip receives 2 3-bit signals, one for each motor. Each of these signals drives a state machine which will produce pulses of varying width every 20 ms. These pulses will be fed to the two servo motors of the robot. The abort signal will disable the production of the pulses for as long as it is held high.

Block Diagram

Circuit Description

Controller:

The controller is composed of 2 potentiometers, as well as the abort button. The potentiometers produce analog signals (for left and right wheels respectively), which are passed to the encoder. Each potentiometer is connected between power and ground (which come from the encoder) and will produce an analog signal between 0 and 5 V. Each potentiometer has 3 pins: 5V power, the ground and the analog output signal. The abort signal (active high) is also passed to the controller, for relaying to the Flex chip. The abort signal is controlled by a normally off button between power and the output pins. The abort output is in the high impedance state when the button is not pressed.

Encoder:

The encoder receives the abort signal from the controller. This signal is connected to ground through a tie down resistor and to the abort input on the Flex chip. The tie down resistor pulls the signal being passed to the Flex chip low when the abort signal received is in the high impedance state. When the abort signal received is high, it is simply relayed to the Flex chip.

The encoder receives the analog signals from the controller and encodes them into 8 bit digital signals, of which the 3 most significant bits are relayed to the Flex chip. An ADC0804 chip is used to encode each of the analog signals. The power pin of each of the potentiometers is connected between the power and the Vcc pin on the respective ADC chip. The ground pin of each potentiometer is connected between ground and the V- input on the ADC chip. the output pin of each potentiometer is connected to the V+ input of the respective ADC. The Vref/2 of each of the ADC chips is connected to a voltage reference diode (lm336 2.5), supplying 2.5 V. The same lm336 chip supplies both ADCs. The CLK R pin of each of the ADCs is connected to ground through a resistor and a capacitor, while the CLK In pin is connected between the resistor and capacitor to run the ADCs in the free running mode. When each of the ADCs is initiated, INTR and WR inputs must be tied low momentarily and then the WR input must either go high or float. The RD input must be tied low for the ADc to return the digital result. The INTR and WR inputs are tied together on each chip. A normally off button connecting the INTR and WR inputs to ground was used for this purpose (i.e. the button was pressed when the circuit was first powered on).

Flex:

The Flex chip receives 2 3-bit inputs (6 inputs altogether) and the abort signal. The 3-bit signals are sampled by the PLD and drive the state machine. The state machine (see VHDL code) generates 2 sets of pulses (1 for each servo) which occur every 20 ms. The width of these pulses (again set by VHDL code depending on the digital signal received) control the speed the servos rotate at.

Schematic Design

Controller: