Constructing a Differential Drive Mobile Robot from

Simple Components

Introduction
One of the most difficult problems associated with construction mobile robots is starting. This document is designed to give you a quick introduction into the theory behind a differential drive mobile robot followed by a description of how such a robot can be built from commonly available components.
Differential Drive Theory

One meaning of "differential" is simply "proceeding at different rates".  Keeping this in mind, a differential drive is one where the drive--or movement--components of the vehicle move at different rates. For example, the diagram below illustrates a very simple differential drive vehicle.

Figure 1 Simple Differential Drive

Each wheel is connected to its own motor. A glide or unpowered castor wheel provides balance at one end of the vehicle. If both motors spin at the same rate and all components are identical then the vehicle will move in a straight line. If one wheel spins faster than the other does, the vehicle will have a tendency to turn in the direction opposite to the fast wheel. If each wheel spins in the opposite direction of the other, the vehicle will turn in place.

This form of steering is commonly found on tracked vehicles such as tractors and tanks. The drive has the advantage of simplicity and a very tight turning radius.

Making a Differential Drive Vehicle

A simple differential drive can be constructed in various ways. One cost-effective means is to use components that are readily available as junk or at a local retailer. To build sucha vehicle you will need several components as illustrated in the photo below.

Figure 2 Vehicle Components

These components are,
1.  An old radio or wire controlled car
2.  A carpet protector for a chair leg
3.  Several small screws
4.  Two 1" brackets
5.  Several narrow wire ties
6.  A hobby AA size batter holder
7.  A bamboo barbecue skewer
8.  Some Hook-and-loop tape (Velcro)
9.  A sample piece of counter-top arborite or Formica

The Drive

Remove the drives from the toy car. These cars tend to have very low voltage motors (3-9 volts DC) driving an arrangement of gears that supply power to the wheels. You can usually remove this arrangement by undoing appropriate screws. The goal is to remove the motor and gears intact. You will need two of these motor and gear arrangements. If a single car has only one, it will be necessary to use two similar cars. The best results are achieved when the drives are similar or identical on either side of your vehicle.

The Vehicle Body
Drilling holes in the Formica with a small drill in front of where the drives will be located will provide anchor points for the wire ties that will hold the motors in place. The diagram above shows a bottom view of the vehicle. Note how the wire ties hold the motors in place. The barbecue skewer is broken and jammed under the ties to firm them up and keep the motors from moving. Also shown in the picture is the glide made from
the carpet protector.

The photo above shows the top view of the vehicle. As on the bottom, a skewer is forced

under the wire ties. The black piece in the middle of the vehicle is one part of the hook

and loop tape. The other part is placed on the bottom of the battery holder. One bracket is

held in place with the bolt that holds the glide in place under the vehicle.

Providing Balance

A side view of the vehicle is provided below. Note how the glide balances the drive

wheels. For best performance, most of the vehicle's weight should ride over the drive

wheels. This can be accomplished by placing the battery holder over or near the rear

wheels.

Additional Vehicle Components

In the diagram below, note how the battery holder is placed on the hook and loop tape.

To allow for the mounting of additional components (possibly mounted on additional

Formica samples), a second bracket is attached to the first in the manner shown. The

remaining vehicle components will be fixed to this second bracket.

Final note

The vehicle should now be ready for final wiring to some form of controller. This is not

shown but can include something as simple as a joystick or something as complex as a

microcontroller or computer.  It is wise to determine how much current the motors draw

from the batteries before attempting to hook them to any electronic controller but, apart

from this caveat, the vehicle should be ready to go.