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Stepper Motors 1
Stepper Motors 1
Stepper Motors
Source :
Control of Stepping Motors, a Tutorial
http://www.cs.uiowa.edu/~jones/step/
Stepper Motors 2
Stepper Motors
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Contents
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Working principle
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Characteristics
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Types
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Control
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Applications
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Stepper Motors 3
DC Motors
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Familiar with the
hobby motors
or
free-spinning DC
motors
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Difficult to position accurately
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Even if the timing is just right for starting and stopping
the motor, the armature
does not stop
immediately
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Have very
gradual
acceleration and deceleration curves stabilization is slow
Stepper Motors 4
DC Motors
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DC Motors
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A line carrying a current I and placed in a
magnetic field B tends to move by the force
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F = I * B
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Terminal Voltage, U = E
bemf
+ I * r
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Back-emf, E
bemf
= k
bemf
* n
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Torque, T = k
T
* I
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I: armature current
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r: armature resistance
"
n: rotation speed
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DC Motor Parts
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DC Motor : Parts
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Motor Source of torque and power
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A gear box To amply torque at reduced speeds
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An encoder To measure the rotation angle of the motor
axis
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DC Motor - Encoders
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The optical incremental Encoder
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Emitter
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Photo-detector
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Wheels
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Stepper Motors 7
Stepper Motors
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An electromechanical device
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converts electrical pulses into discrete mechanical
movements
"
the shaft or spindle of a stepper motor rotates in discrete
steps increments when electrical command pulses are
applied to it a proper sequence
Stepper Motors 8
Stepper Motors
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The motors rotation has several direct
relationships to the applied pulses
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The sequence of applied pulses is directly
related to the direction of motor shaft
rotation
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The speed of the motor shaft rotation is
directly related to the frequency of the
input pulses
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The length of rotation is directly related
to the number of input pulses applied
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Stepper Motors 9
Stepper Motors
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Advantages
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The rotation angle of the motor is proportional to the input pulse.
"
The motor has full torque at stand­still (if the windings are energized)
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Precise positioning and repeatability of movement since good stepper
motors have an accuracy of 3 - 5% of a step and this error is non
cumulative from one step to the next.
"
Excellent response to starting/stopping/reversing.
"
Very reliable since there are no contact brushes in the motor.
Therefore the life of the motor is simply dependant on the life of the
bearing.
"
The motors response to digital input pulses provides open­loop control,
making the motor simpler and less costly to control.
Stepper Motors 10
Stepper Motors
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Advantages (continued)
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It is possible to achieve very low speed synchronous rotation with a
load that is directly coupled to the shaft.
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A wide range of rotational speeds can be realized as the speed is
proportional to the frequency of the input pulses.
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Less cost.
"
Degree of rotation is a function of their construction and therefore
consistent.
"
No feedback is necessary for positional or speed control.
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Any errors present are non-cumulative.
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Disadvantages
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Resonance can occur if not properly controlled.
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Not easy to operate at extremely high speeds.
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The Floppy Drive
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Floppy Drives
The floppy drive uses a thin circular ceramic disk for data
storage. The disk is coated with magnetic particles and is
flexible (hence the term
floppy
).
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The disk rotates at 360rpm. A read/write head makes physical
contact with the disk surface. Data is recorded as a series of
tracks subdivided into sectors.
!
The read/write head is positioned using a moveable arm
attached to a
stepper motor
. The stepper motor moves a small
amount for each pulse applied to it. A mechanical switch
detects when the read/write head is in the outermost position
(track 00).
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Open Loop Operation
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One of the significant advantages of a stepper motor is its
ability to be accurately controlled in an open loop system.
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Open loop means no feedback information about position is
needed.
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Open loop type of control eliminates the need for expensive
sensing and feedback devices such as optical encoders.
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The position is known by knowing track of the input step
pulses.
Input - step pulses
Output - rotor position
No feedback
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Stepper Motors 13
Stepper Motors
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Stepper motors have been developed extensively since
the 1920s.
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Often recognizable from their
clicking
action.
"
In clocks the second hand clicks from position to
position, rather than sweeping continuously.
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Can be viewed as electric motors without commutators
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All of the commutation must be handled externally by the
motor controller, and typically, the motors and controllers are
designed so that the motor may be held in any fixed position as
well as being rotated one way or the other.
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Stepper Motors
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Typically, all windings in the motor are part of the
stator
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The rotor is either a permanent magnet or, in the
case of variable reluctance motors, a toothed
block of some magnetically soft material
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Stepper Motors
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For some applications, there is a choice between using
servomotors and stepping motors.
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Both types of motors offer similar opportunities for precise
positioning, but they differ in a number of ways
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Servomotors require analog feedback control systems of some
type
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Typically, this involves a potentiometer to provide feedback
about the rotor position, and some mix of circuitry to drive a
current through the motor inversely proportional to the
difference between the desired position and the current
position.
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Stepper Motors
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Choice between stepping motors and servomotors
"
depends on the application
"
the repeatability of positioning done with a stepping motor depends on
the geometry of the motor rotor, while the repeatability of positioning
done with a servomotor generally depends on the stability of the
potentiometer and other analog components in the feedback circuit
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Stepping motors can be used in simple open-loop control systems
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Stepper motors are generally adequate for systems that operate at low
accelerations with static loads, but closed loop control may be essential
for high accelerations, particularly if they involve variable loads.
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If a stepping motor in an open-loop control system is overtorqued, all
knowledge of rotor position is lost and the system must be reinitialized;
servomotors are not subject to this problem.
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Stepper Motors 17
Types of Stepper Motors
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Three varieties
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Permanent magnet stepper motor
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Variable reluctance stepper motor
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Hybrid stepper motor
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The permanent magnet type and the hybrid type stepper
motors are the more commonly used
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Lacking a label on the motor, you can generally tell the first
two apart by feel when no power is applied Permanent magnet motors tend to "cog" as you twist the rotor
with your fingers variable reluctance motors almost spin freely (although they
may cog slightly because of residual magnetization in the rotor)
Stepper Motors 18
Types of Stepper Motors
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Three varieties
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Permanent magnet stepper motor
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Variable reluctance stepper motor
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Hybrid stepper motor
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The first two varieties can also be distinguished with an
ohmmeter Variable reluctance motors usually have three (sometimes
four) windings, with a common return permanent magnet motors usually have two independent
windings, with or without center taps. Center-tapped
windings are used in uni-polar permanent magnet motors
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Types of Stepper Motors
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Variable reluctance stepper motor
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Have been around for a long time
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Easiest to understand from a structural point of view
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Consists of a soft iron multi-toothed rotor and a wound
stator
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When the stator windings are energized with DC current
the poles become magnetized
"
Rotation occurs when the rotor teeth are attracted to
the energized stator poles
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Types of Stepper Motors
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Permanent magnet stepper motor
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Low cost
"
Low resolution Typical step angles of 7.5
°
to 15
°
(48 24 steps per revolution)
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The rotor has no teeth as with the reluctance type
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The rotor is magnetized with alternating north and south
poles situated in a straight line parallel to the rotor shaft The magnetized rotor poles provide an increased magnetic flux
intensity and due to this the PM motor exhibits improved torque
characteristics compared with the variable reluctance type
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Stepper Motors 21
Types of Stepper Motors
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Hybrid type
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Combines the best features of the of both PM and VR type
stepper motors
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More expensive than the PM stepper motor
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Better performance with respect to: Step resolution, Torque, Speed
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Typical step angles 3.6
°
to 0.9
°
(100 400 steps per revolution)
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The motor is multi-toothed like the VR motor