If you want your project to have some form of
movements, linking a motor to some mechanism is one effective way of achieving
the purpose. Whether it is to rotate a platform, or to swing a door, or to
manipulate a lever, a motor can be the power source to operate the mechanism.
However, you have to select the correct type of motor to match the job.
Using the wrong motor would result in under-performance and in the worse case,
the project ending in a complete failure. The following is an introduction to
the different types of motor and their applications.
1.
DC Motor
This is the
most common type of small battery operated motor that can be used for a variety
of projects. The main advantage is their small size and they do not require
complicated control. It can be connected directly to the rated battery voltage
for simple functions. Straight from the box, a typical DC motor spins in excess
of 15,000 rpm (revolution per minute), which is way too fast for most
applications. To be useful, it must be geared down to the appropriate speed. Another
reason for gearing down a DC Motor is to increase the tuning torque
(turning power) which is required to do useful work.
Applications:
Because of their small sizes and easy control, DC motors
find many uses in modern appliances. They can be found cooling power supplies
and CPUs
in PCs and operating CD ROM drives. Printers use a few of them in each
unit to feed paper to the print head. As a result, they can be found easily in
used equipment and modified to for use in a project. However, it is important
to note that DC motors are not very precise in their operations. Their speed
can vary quite a bit even under the same operating condition. This is
especially the case when they are under load.
Motor Selection When selecting a DC motor, ask yourself
the following questions:
What is the load it is expected to bear? This is the
turning torque of the motor and is measured in kg.cm. If the expected load is higher
than the motor’s rating, it would heat up very quickly when in use and will
soon breakdown. This is a very common cause of motor failure in projects.
Always cater for some extra allowance in your selecting the power rating. For
example, if your project requires 3kg.cm force, then choose a unit that gives 6kg.cm
force. This would prevent motor failure in the event of the mechanism becoming jammed,
which is quite common in the development stages.
What speed is the motor expected to turn at? Motor
speed is measured in rpm. In applications where the speed is an important
factor, it would have to be reduced by a series of gears. Some motors are sold
with gear box with user-selectable ratios. These allow the user to select the
gear ratio by interchanging the positions of the gears to arrive at a suitable
speed for the output shaft. Further speed refinement can be done with an
electronic speed controller.
DC
Motor Control:
Some kind of control circuit is required if you need
the motor to do more than just spinning. An example is when a motor is used to
slide a tray open and close. The motor will need to turn in one direction to open
the tray and stop. This is followed by turning in the opposite direction to
close the tray. The control circuit would have to accept a signal to turn the
motor in one direction and reverses it when a second signal is received to
close the tray. The
example cited above is relatively easy.
It can be accomplished by means of some switches
connected together. For more complicated movements, an intelligent controller
would be required. These may include movements that return the output shaft to
the original position or one that requires several stops in one revolution.
2.
AC Motor
AC motors
are complicated electrical equipment. They are mentioned here just so that you
know that there is such a type of motor. Unlike DC motors, they cannot be
powered by batteries and requires special connections to enable them to work
properly. The most commonly available AC motor in small size is the synchronous
motor. It spins at a constant rpm and are found in electric clocks (not
battery-operated quartz).
Applications
These days, AC motors are found mostly in heavy
industries like machine tools and equipment that require continuous operation
like the compressor in refrigerators. Invariably, these tend to be large.
Occasionally they may be used in places where speed is not an important factor.
A new generation of AC (synchronous) motor has found its way into hobby uses.
They provide very high speed power for electric model airplanes.
Because of mass production, they have become quite
economical although the complicated electronic controllers are still expensive.
AC motors are not useful for most projects unless there is a specific requirement.
3.
Servo motor
These are,
strictly speaking not designed as motors like in the above categories. Servos
were first used as escapement control to move valves
and levers
in mechanical systems. Ocean-going oil tankers make extensive use of servos to
transfer crude oil to and from their bulk tanks. They have a built-in DC motor
with reduction gears to increase their operating torque. An electronic circuit controls
the movement of the motor precisely. All these are housed in a single casing
making servo very compact and easy to use. For application in small projects,
hobby servo is an ideal source of power.(photo - servos, inside).
Applications
Hobby servos can be used as they are or with
modification. Ordinarily, they are the power houses that push and pull the
elevators and ailerons of model airplanes. In recent years, they are used in
robotic controls. Modified servos were first used as wheel motors because of
their compact size, high power and easy control. A pair of continuous turning
servos can easily power a 5 kg mobile robot. There are now purpose-made units
for robotic applications. These have higher power output and very precise displacement
control. Many find applications in moving limbs and sensors. Hobby servos can
be linked to levers or cranks to effect useful movements in a project.(photo –
door mech, mobile robot, brat).
Servo
Selection
As in DC motor, servos come with different power
rating. They range from 1.5 to12 kg.cm, varying also in physical size and
battery consumption. Servos are more tolerant to overload because of the
built-in reduction gears and sometimes current-limiting capability. The end
result of using an underpowered servo is a stalled movement, usually without
serious damage to the servo. It is still advisable to use a higher power servo
motor for any application to cater for friction and mechanical losses due to
sloppy linkages. Servo Control Servo can only be controlled by a servo
controller designed for the purpose.
The use of other types of controller can
result in a damaged servo. There is a three-wire cable attached to all hobby
servos. This cable must be connected to the controller in the right direction.
The standard unmodified servo can move its output disc within limits, usually
100 degrees. The disc can be set to move between different angles and external
linkages can be attached to the servo output to activate other mechanisms.
Modified servos have their internal mechanical stops removed so that the motor
can spin without restriction. In this case, the controller only controls the
rotational speed. This is usually within a very limited range.
4.
Stepper Motor
Of all the
motors considered so far, the stepper motor offers the greatest precision in
term of displacement. However, they are inherently slow and bulky (and heavy)
for the same power rating. A series of magnetic windings are placed around the
casing (stator) at regular interval. Permanent magnets are attached on the rotor
(the central spinning part) with no external electrical contact. The rotor
therefore is freewheeling. Electric current is passed through the stator
windings sequentially to cause the rotor to spin or to displace a specific
angle and hold its position. A stepper motor with an appropriate controller can
be expensive. You may want to consider the cost effective before deciding on using
one.(photo – stepper, small and big).
Applications:
The stepper motor is the motor of choice in
controlling the position of print heads in printers and laser sensor in CD ROM
drives. These applications require very precise positioning of mechanical parts
within a system. Except in the special areas of robotics, stepper motors are
not very useful for general project because of their low torque and complicated
control set-up.(photo – CD ROM, printer)
Stepper
Motor Control
Special electronic controllers are used to control
the movement of steppers. Depending on the motor used, the unit angle of
displacement, speed and total displacement can be controlled. Steppers have the
unique equality of holding a rotor position without damaged.
