This chapter introduces several types of motors commonly used in robotic and related
are inexpensive, small, and powerful motors that are widely used. Gear-train reductions
are typically needed to reduce the speed and increase the torque output of the motor.
also called actuators, do not rotate continuously, but turn in fixed increments, and
resist a change in their fixed positions. They require special driving circuits to apply
the correct sequence of currents to their multiple coils. They are commonly used in
robotics, particular in mechanisms that perform linear positioning, such as floppy and
hard disk drive head motors and X-Y tables.
are used for angular positioning, such as in radio control airplanes to control the
position of wing flaps, or in RC cars to turn the wheels. The output shaft of a servo does
not rotate freely as do the shafts of DC motors, but rather is made to seek a particular
angular position under electronic control. In effect, a servo motor is a combination of a
DC motor, a shaft position sensor, and a feedback circuit. A servo motor also usually
includes a built-in gear-train and is capable of delivering high torques directly. No
servo motors are included in the 1999 ELEC 201 kit.
DC motors are widely used in robotics because of their small size and high energy
output. They are excellent for powering the drive wheels of a mobile robot as well as
powering other mechanical assemblies.
Several characteristics are important in selecting a DC motor. The first two are its
input ratings that specify the electrical characteristics of the motor.
- Operating Voltage.
- If batteries are the source of power for the motor, low operating voltages are desirable
because fewer cells are needed to obtain the specified voltage. However, the electronics
to drive motors are typically more efficient at higher voltages. Typical DC motors may
operate on as few as 1.5 Volts or up to 100 Volts or more. Roboticists often use
motors that operate on 6, 12, or 24 volts because most robots are battery powered, and
batteries are typically available with these values.
- Operating Current.
- The ideal motor would produce a great deal of power while requiring a minimum of
current. However, the current rating (in conjunction with the voltage rating) is usually a
good indication of the power output capacity of a motor. The power input (current times
voltage) is a good indicator of the mechanical power output. Also, a given motor draws
more current as it delivers more output torque. Thus current ratings are often given when
the motor is stalled. At this point it is drawing the maximum amount of current and
applying maximum torque. A low voltage (e.g., 12 Volt or less) DC motor may draw from
100 mA to several amperes at stall, depending on its design.
The next three ratings describe the motor's output characteristics:
- Usually this is specified as the speed in rotations per minute (RPM)
of the motor when it is unloaded, or running freely, at its specified
operating voltage. Typical DC motors run at speeds from one to twenty
thousand RPM. Motor speed can be measured easily by mounting a disk
or LEGO pulley wheel with one hole on the motor, and using a slotted
optical switch and oscilloscope to measure the time between the switch
- The torque of a motor is the rotary force produced on its output shaft.
When a motor is stalled it is producing the maximum amount of torque
that it can produce. Hence the torque rating is usually taken when the
motor has stalled and is called the stall torque. The motor
torque is measured in ounce-inches (in the English system) or Newton-meters
(metric). The torque of small electric motors is often given in milli-Newton-meters
(mN-m) or 1/1000 of a N-m. A rating of one ounce-inch means that the
motor is exerting a tangential force of one ounce at a radius of one
inch from the center of its shaft. Torque ratings may vary from less
than one ounce-inch to several dozen ounce-inches for large motors.
- The power of a motor is the product of its speed and torque. The power
output is greatest at about half way between the unloaded speed (maximum
speed, no torque) and the stalled state (maximum torque, no speed).
The output power in watts is about (torque) x (rpm) / 9.57.