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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Obstacle-Avoiding Robot.

by Pololu

1. Introduction

One of the biggest challenges in building your own robot is controlling its motors. You can find new or surplus motors and gearboxes in many places, and low-cost microcontrollers and books on how to use them abound. However, microcontrollers cannot directly drive DC motors, leaving robotics beginners with the possibly overwhelming challenge of building their own motor controller. This task is even more complicated if the motors require bidirectional operation and speed control.

This project demonstrates how easy it is to make a simple robot controller using the micro dual serial motor controller with a Microchip PIC16F628 microcontroller. We then use the circuit with the Pololu robot chassis to create a small, obstacle-avoiding robot that can serve as a starting point for more advanced projects. The low-voltage operation of the motor controller allows a small, 3.6 V cordless telephone battery pack to power the entire robot. Since the motor controller only requires two of the PIC's 13 I/O lines, there is plenty of opportunity for expansion.

2. Materials and Tools

Here are the essential parts you will need if you want to build a similar robot. These items are available either from Pololu or from most electronic component distributors.

• Motor controller
• Robot chassis plate
• Parts to build the robot chassis, which come with our chassis combination kits:
  -Tamiya dual gearbox (Tamiya #70097)
  -Tamiya ball caster (Tamiya #70144)
  -Tamiya truck tires (Tamiya #70101)
• PIC16F627 or PIC16F628 microcontroller in a DIP (dual in-line package) from Microchip. The 16F62X microcontrollers are the only 18-pin PICs that have a built in UART (universal asynchronous receiver and transmitter), which makes transmitting data serially (to the motor controller) very simple. The code presented in this project should be portable to any other PIC with a hardware UART; with the other PICs, you would have to write your own serial routines (which isn't that bad since you only need to transmit, and the motor controller should handle any baud rate you come up with).
• Clock source for the PIC. We used a 4 MHz ceramic resonator with built-in capacitors; any crystal, resonator, or oscillator in the 1-20 MHz range should be fine.
• 18-pin DIP socket for the PIC. You may also want a socket for your motor controller; a crude way of obtaining a 9-pin SIP (single in-line package) socket is to cut an 18-pin DIP socket in half.
• Two long-lever, snap-action switches for use as bumpers switches.
• 3.6 V, 650 mAh cordless telephone battery pack (or three AA size NiCd or NiMH batteries in a battery holder). Cordless phone batteries are available in many consmer electronics stores (e.g. Radio Shack, Best Buy) and discount stores (e.g. Wal-Mart) for around $10, making them a great power source for small robots.
• General-purpose prototyping PC board (or proto board) with space for two 18-pin DIP sockets, the ceramic resonator, and whatever other electronics you might want to fit. Such boards are available from most electronics component stores, including Radio Shack (e.g. part number 276-150A). To avoid soldering, this project could also be done using a small wireless breadboard, such as the one used in project 2.
• Hook-up wire and solder for making all of your connections.
• Double-sided foam tape provides a quick way of temporarily mounting items such as the battery pack. Alternatively, you could use standard mounting hardware or cable ties for fixing your components to the chassis.

3. Hardware Construction

Begin by assembling the robot chassis. You should mount the battery pack on the rear of the robot, above the ball caster, to balance the weight of the motors. Double-sided foam tape is a convenient method of attaching the battery pack; it can also be secured with cable-ties by using the rectangular holes on both sides of the ball caster. If you are using a battery holder, you can easily drill mounting holes through the holder or the chassis if existing holes do not line up.

When soldering to the motor leads, be careful not to damage them. Soldering a small capacitor across the motor leads can improve the performance of the motor controller and lower interference with other electronics on your robot. We used a 0.1 uF ceramic capactitor.

The picture to the right shows a resistor in series with the capacitor. In general, such a resistor limits the current wasted by the PWM (pulse width modulation) in charging and discharging the capacitor. However, the relatively low, 600 Hz PWM frequency of the motor controller makes this resistor unnecessary; we saw no added benefit when we added the resistor.

Because the motor leads are fragile, it is important to provide strain relief for the wires you connect to the motor. For our example, we hot-glued the leads to the side of the gearbox, as shown in the picture. Securing the wires this way will allow you to manipulate the other end of the wires without worrying about breaking off the motor leads. Note that the glued wires prevent removal of the motors from the gearbox. Hot glue has the advantage of not being entirely permanent; if necessary, it's not too difficult to free the wires.

We have kept the motor capacitor exposed for the purposes of these pictures, but it's a good idea to protect them as much as possible, especially since they are low to the ground and on the front end of the robot.

To keep this project as simple as possible, we limit our sensors to two snap-action swtiches for front collision detection. Of course, you can add more sophisticated sensors for more interesting behavior.

As you can see in the picture to the below, there isn't much to the electronics. We soldered the circuit on a small perforated board, but you can also use a solderless breadboard. The small pushbutton switch on the top right and the resistor below it make up an optional reset circuit. The only other components are the PIC, the resonator (lower right), and the motor controller. No additional resistors are required for the bump switches because we use the PIC's internal pull-up resistors on port B.

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