you are planning on building a robot to roam
around on the Moon or Mars, the acceleration
value will be different. This means your robot’s
weight differs depending on what planet it is
roaming around on. Let’s take a robot that weights
25 lbs here on Earth. Remember, pounds are the
English unit of weight, not mass. Each of the
engineering equations we will use later has specific
units. It is important to watch out for this. Nothing
would be more embarrassing than sending a robot
to Mars because weight was used when you
should have used mass. Figure 1 is a simple free
body diagram of a simple robot with two front
drive wheels and a rear caster. This demonstrates
the various forces that will act on the robot. We
will start by examining the effect of gravity on our
robot. What is force?
increase the current. For example, a 12 volt DC motor can
supply the same power as a six volt DC motor, but at 1/2
the current. This is important because most components
are limited by the amount of current they can carry. If your
robot will be extremely heavy, you may even want to look
at 24 volt DC or even 90 volt DC motors. One of the trade
offs for the higher voltage is safety. It is hard to shock
yourself at 12 or 24 volts, but 90 volts can cause shock and
possible injury. Another key property of DC motors is that
the speed is controlled by changing the voltage. When
sizing a DC motor, we will use the rated voltage of the
motor. This is the maximum voltage the motor is designed
to handle. There are several different types of DC motors to
select from. In most cases, I use brushed DC gearhead
motors. Gearhead motors have a gearbox installed as part
of the motor. This gearhead is a gearbox attached to the
output shaft of the motor. A few other types of DC motors
include brushless and stepper motors.
Gearboxes are sometimes called reducers because the
output shaft of the gearbox will be less than the output
shaft of the motors. The reduction in speed results in an
increase in torque. This is a good place to start because
most DC motors have output shaft rpms (revolutions per
minute) of several thousand and very little torque. Using a
gearbox will reduce the shaft speed and increase the
When selecting DC motors, you must understand some
of the basic physics that will affect your robot. Some of
these physics concepts you should understand are force,
weight, mass, torque, acceleration, and velocity, and the
relationships between them.
What is Weight?
Most of us have talked about weight all of our lives.
We know that every object has a weight. But how is weight
defined in the world of physics? Weight is actually defined
as the force due to the acceleration of gravity on a body.
On Earth, we normally use 9. 8 m/(s^ 2) or 32 ft/(s^ 2). If
Force = Mass x Acceleration
Most commonly written as:
Force = ma
On the free body diagram, the key forces are:
Weight = mass x acceleration = mass x gravity = mg
This is the force pulling the robot down toward the
center of the Earth due to gravity.
On a free body diagram, the force of weight is broken
down into its two components.
Force Pulling Robot Down Incline = fg = mgsinθ
The force is pulling your robot back down the incline
and must be overcome by your drive motors. The greater
the angle of incline, the greater this force will be. The
incline a robot is trying to climb makes a significant
difference in the torque required from your drive motors.
Force Pulling Robot to Incline = fn = mgcosθ
This force is holding your robot onto the incline. This
force is required along with friction to allow your robot’s
drive wheels to push the robot forward up the incline. We
will ignore friction, because the force of friction is what the
drive wheels need to push to move the robot forward.
On our free body diagram, torque is the force at the
edge of the drive wheels pushing our robot up the incline.
But What is Torque?
Torque is the measurement of the force applied to
rotate a body around a particular axis. In a mobile robot,
the body will be the wheel and the axis will be a motor
Tips For Selecting DC Motors
34 SERVO 01.2010