74*245 Motor Driver

H-Bridge Driver

Simple PWM Gen.

Handy Method Measuring RPM

Measuring RPM via Photo reflector

Introduction to Robotic

DC, Stepper,and Servo Motor

Microcontroller Tutorial

Computer Interface
Tutorial
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Rollie: A Two-Wheeled Robot,

William D. Sherman

Abstract

The idea of building a robot with two wheels was tested. Requirements for this project included the use
of readily available and inexpensive materials. The use of standard size hobby servos to move the robot,
required weight to be minimized and distributed in order for the robot to maintain balance. The design included the use of sensors to detect obstacles and people. The possible development and potential problems in detecting a remote beacon or IR command transmission was fully explored.

1 Why Two Wheels?

Why build a robot (fig. 1) with just two wheels? The use of two wheels enables the robot to execute turns centered upon it axis. Such turning allows accurate scanning and maneuvering without using a large ground area. Tight turning is not possible with a fixed four-wheel design. The use of two wheels with an idler wheel located in the front or back of the robot to maintain balance was considered but found undesirable. The use of two wheels allows the robot’s electronics to be protected inside the space between the wheels. Additionally, the height of the wheels gives the robot good ground clearance over obstacles. If the robot comes up against a wall it simply flips over, with the robots interior completely protected. Interrupting rotation while rolling causes a gentle rocking action of its interior, allowing scanning of the area ahead.

Figure 1 Front view

2 Wheel Design

A simple wheel design, easily duplicated by others, was desired. Use of a CD-ROM disc was just the right size, occasionally found in one’s mailbox as part of the junk mail everyone receives. The use of a single disc lacked the dimensional stability and strength required for a wheel. Using two discs with a ½ inch foam disc sandwiched between solved this problem. The foam disc (fig.2) is made ½ inch larger in diameter than the CDs, giving a ¼ inch edge for traction with the floor.

To hold this disc-foam-disc sandwich together, four ½ inch spacers were used to separate the CDs (fig. 6) while one side was attached to a round servo horn. Existing holes in the servo horn were opened in size to allow the use of 4-40 machine screws to hold everything in place. For increased traction, the edge could be scalloped, though this was not tried.

Figure 2 Wheel

3 Circuitry

3.1 Controller
A 16F84 PIC microcontroller from Microchip was selected for the robot’s controller. The 16F84 is inexpensive, easily obtained, uses very little power and can be programmed “in-circuit” with the serial port of a computer. In-circuit programming allows programming of the PIC microcontroller without removing the device from the robot. Only four lines brought out to the PC serial port along with a 12v power source are all that is required to program the PIC. Programs were written and compiled in Basic
using Micro Engineering Lab’s PIC Basic Pro©, then downloaded using IC-Prog©, a freeware program found on the Internet.
The 16F84 flash memory is limited to 1024 words, while other PIC microcontrollers with more memory, faster speed and increased I/O lines can be used, the 16F84 running at 20 Mhz was adequate for this design. A small 1-7/8” by 2-3/4'’ perf board from Radio Shack (274-150) contains the electronic circuit of the controller and is mounted in the center of the robot’s base. Connection of the sensors to the circuit was accomplished by mounting female headers to the board. This allowed easy plug-in connections. The headers provide power, common, and input/output to the PIC’s port. Mounted on each end of the base are the servos that provide rotation of the wheels.

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