FORTS Car

EE 1159

Senior Design Project Midterm (work in progress)

Objective

This objective for the project is a conversion of a radio-controlled model car to computer control. The car will be controlled via  a remote control interfaced to a RS-232 serial port in turn connected to "Intel box" PC host computer running an interface under Windows. The protocol used to communicate with the vehicle will be platform neutral to allow porting to other operating systems.

Overview

The radio-controlled car chosen for conversion is a RadioShack BLACK WOLF II Off-Road Racing car. This car was chosen because it was the least expensive car available with proportional steering and throtle. The transmitter uses two potentiometers, one to conctrol steering and the other for throttle. Each potentiometer is replaced with a parallel combination of two Xicor X9312wp 10KOhm EEPOTs to form an equivalent 5KOhm potentiometer. The digital potentiometers are initialized and controlled by a Parallax BASIC Stamp II running a PBASIC program which  communicates to a host computer by a RS-232 connection.

Progress

The BASIC Stamp II was purchashed soon after the car was chosen and purchased. While researching available and apropiate potentiometer replacements, a test circuit built around an eight bit serial to parallel converter was designed and constructed to explore writing programs.

The appropiate digital potentiometers were located but availability was in question. While attempting to obtain purchasing information, an alternative plan to control analog potentiomters with stepper motors was begun. A stepper motor controller was designed and constructed with software for demonstration.

Digital potentiometers were located and obtained. A test circuit was constructed and initial driver software developed by using an analog volt meter to display the potentiometers' position. The analog potentioneters in the transmitter were replaced and the operation of the transmitter with the digital potentiometers tested. It was found that the operation of the car's controlles occured only over a small range of the digital potentiometers movement. Resistors were added to both the high and low sides of the digital potentiometer so that control occured over the digital potentiometer'sentire range. A non-linerity of throtle speed was discovered when the steering was moved from left to right. This will be corrected in software by a lookup table.

Constant Throtle Speed vs Steering Position Graph

Sensors are being developed. I have begun by building an audio frequency sonar unit. It can measure the distance from the speaker to the microphone up to about 5 feet. However, it will not measure a distance by bouncing sound off objects. I plan to use true ultra-sonic transducers for the final unit.

The car is a modified remote controlled toy car. The car's system consists of two intelligent subsystems: one that controls movement and one that gathers data about the car environment.

Car Organization Diagram

The control system is a modified version of remote controller that was original to the toy. The steering and throttle controls were removed and replaced with electronic equivalents. An embedded micro controller enables two way communications to a PC via RS-232.

Schematic of serial port to digitally controlled potentiometers interface

The car carries an onboard system to collect data about the car environment. At the heart of the system is a micro controller that coordinates communications between a PC and sensors. The PC communicates to this micro controller via radio modem.

Micro Controller Schematic
Micro Controller Photo

The sensor controllers reside on I/O cards, which connect to the micro controller by way of a proprietary bus. This bus allows the connection of up to 16 I/O cards. Each I/O card may further be addressed to access up to 16 devices per card.

The 16550D UART I/O card handles serial communication between the micro controller and the radio modem. It provides a PC standard RS-232 interface, with16 bytes of buffering for data being transmitted and 16 bytes for data received. A single bit indicating received data available lowers communication overhead. Indicator LEDs provide a means of visible feedback of car operations by showing the status of critical UART signals.

UART Photo

The Bumper Switch I/O card allows the status of 8 switches to read as a single byte.

Bumper Switch Schematic
Bumper Switch Photo

The Sonar I/O card contains all the circuitry to run up to 16 sonar heads. The micro controller addresses one of the sonar heads, toggles a single bit to begin measurement and then reads a byte that contains the sonar reading.

Sonar Schematic
Sonar Photo


Fault tOlerant Real-Time Systems FORTS Projects

forts@cs.pitt.edu