User guide

7
Motor speed will determine the encoder frequency that
is needed. A high CPR encoder will need a high frequency
rating in order to maintain the same RPM.
The motor RPM is increasingly important in providing high
dynamic and fast response to various applications, such as
high speed pick and place machines. An encoder needs
to have the frequency response to keep up with the
motor RPM.
The mathematical relationship between encoder CPR
count frequency and motor RPM is dened by the
following formula:
f = RPM x CPR/60
Where:
CPR = Encoder counts Per Revolution
RPM = Motor Speed (Revolution Per Minute)
f = Encoder Count Frequency (hertz)
3.6 Motor Diameter
The diameter of the motor will determine the type of
encoder that can be used. A small diameter motor will
require a similar size encoder so that the overall diameter
is within the same t and form.
For miniature motors, the challenge is to have a high
resolution encoder which can t the same small diameter,
and still perform at the same level.
Motor sizes will inuence the force/torque that the motor
can deliver and the space it requires.
4.0 Electrical Motor System
Load
Motor
Amp
PID
Closed-Loop
Open-Loop
0
0 1 2 3 4 5 6 7
Time (Seconds)
8 9 10 11 12 13 14 15
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Encoder
Feedback
Host
Decoder/
Counter
An electric motor converts electrical energy into
mechanical energy. The reverse task of converting
mechanical energy into electrical energy is accomplished
by a generator or dynamo. Traction motors used on
locomotives often perform both tasks if the locomotive
is equipped with dynamic brakes.
Electric motors are found in household appliances such
as fans, exhaust fans, fridges, washing machines, pool
pumps and fan-forced ovens. They are also widely used
in industrial automation and applications such as robotic
arms, conveyor and automated assembly lines, and pick
and place machines.
Most electric motors work by electromagnetism. Motors
based on other electromechanical phenomena, such as
electrostatic forces and the piezoelectric eect, also exist.
The fundamental principle upon which electromagnetic
motors are based is that there is a mechanical force on
any current-carrying wire contained within a magnetic
eld. The force is described by the Lorentz force law and is
perpendicular to both the wire and the magnetic eld.
Most magnetic motors are rotary, but linear motors also
exist. In a rotary motor, the rotating part (usually on
the inside) is called the rotor, and the stationary part is
called the stator. The rotor rotates because the wires and
magnetic eld are arranged so that a torque is developed
about the rotor’s axis. The motor contains electromagnets
that are wound on a frame. Though this frame is often
called the armature, the term is often erroneously applied.
Correctly, the armature is that part of the motor across
which the input voltage is supplied. Depending upon the
design of the machine, either the rotor or the stator can
serve as the armature.
4.1 Open and Closed Loop System
An open system does not have a feedback system to
the controller to change its input. A closed loop system
provides a feedback loop to the controller where the
output is constantly monitored and the input varies as
the output changes.
4.2 Types of Electrical Motors
There are many types of electrical motors in the market
today. These motors can be categorized into six types:
•BrushlessDirectCurrent(BLDC)Motors
•DirectCurrent(DC)Motors
•UniversalMotors
•AlternatingCurrent(AC)Motors
•StepperMotors
•Piezomotors
For the purpose of this design guide we focus our attention
on the BLDC motor system. The other types of motors will
be included into this design guide in the near future.
4.2.1 Brushless DC Motors
Avago Technologies oers a broad range of products that
provide a complete system solution for the brushless DC
motor. The diagram below shows a simple block diagram
of the brushless DC motor system.