UNIT1 SEM Stepper motors ym.pptx

UNIT I: STEPPER MOTOR
Constructional features – Types – Operating principles
Wednesday, October 26,
2022
Y.Mastanamma, EEE Department 1

 A stepper motor is an incremental motion machine, i.e.
its rotation is not continuous as in conventional
machines, but in steps.
 The motor rotates through a fixed angular step in
response to each input current pulse received by its
controller.
 A stepper motor is a “pulse-driven” motor that changes
the angular position of the rotor in “steps”.
 They can be controlled directly by computers,
microprocessors, and programmable controllers.
Stepper Motor
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Stepper Motor
 The unique feature of a stepper motor is that its shaft
rotates in a series of discrete angular intervals or steps,
one step being taken each time a command pulse is
received.
 When a definite number of pulses are supplied, the shaft
moves through a definite known angle. This fact makes
the motor well suited for open-loop position control.
 Stepper motors develop torques ranging from 1μN-m
(e.g. in a wrist watch motor whose diameter is 3mm)
upto 40 N-m (e.g. in motors used in machine tools whose
diameter is 15mm).
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Stepper Motor
 The power output ranges from 1 W to a maximum of
2500 W.
 The only moving part in a stepper motor is its rotor,
which has no windings, commutators or brushes.
 Typical types of stepper motors can rotate 1.8°, 2°,
2.5°, 5°, 7.5°, and 15° per input electrical pulse.
 The step angles are as small as 0.72° or as large as 90°.
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Stepper Motor
 There are several features common to all stepper motors
that make them ideally suited for these types of
applications. They are as under:
1. High accuracy: Operate under open loop.
2. Reliability: Stepper motors are brushless.
3. Load independent: Stepper motors rotate at a set
speed under different load, provided the rated
torque is maintained.
4. Holding torque: For each and every step, the motor
holds its position without brakes.
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Step Angle, β
 The angle through which the motor shaft rotates for
each command pulse is called the step angle, β.
 Smaller the step angle, greater the number of steps per
revolution and higher the resolution or accuracy of
positioning obtained.
 The value of step angle (β) can be expressed either in
terms of the rotor (Nr) and stator (Ns) poles (teeth) or in
terms of the number of stator phases (m) and the
number of rotor teeth.

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β =
(𝑵𝒔 − 𝑵𝒓)
𝑵𝒔 . 𝑵𝒓
x 360°
or
β=
360°
𝒎 . 𝑵𝒓
=
360°
𝒏𝒐.𝒐𝒇 𝒔𝒕𝒂𝒕𝒐𝒓 𝒑𝒉𝒂𝒔𝒆𝒔 × 𝒏𝒐.𝒐𝒇 𝒓𝒐𝒕𝒐𝒓 𝒕𝒆𝒆𝒕𝒉
where,
β = the step angle (per input pulse)
NS = no. of stator poles or teeth
Nr = no. of rotor poles or teeth
m = no. of stator phases
Step Angle, β

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Resolution
 It is given by the no. of steps needed to complete one
revolution of the rotor shaft.
 When the resolution is higher, the accuracy of
positioning of objects by the motor is greater.
∴ Resolution =
(No. of steps)
(revolution)
=
360°
β
where,

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Total rotor angle, θ
 It is total angle travelled by the rotor for a given step
angle.
 A stepping motor has the ability to operate at very high
stepping rates (up to 20,000 steps per second in some
motors) and they remain fully in synchronism with the
command pulses.
∴ θ = β × No. of steps
where,

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Slewing
 When the pulse rate is high, the shaft rotation seems
continuous.
 Operation at high speeds is called Slewing.
 At higher pulse rate, the motor operates with a howling
sound, having fundamental frequency (fp) equal to
stepping rate.
 When the stepping rate is increased too quickly, then
the machine stops.

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Shaft speed, n
 It is also called as Pulse frequency resolution.
n =
β × 𝑓𝑝
360°
in rps
where,
n = shaft speed / pulse frequency resolution
fp = stepping frequency or fundamental frequency
= No. of pulses per second

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By energizing one or more of the stator phases, a magnetic field is
generated by the current flowing in the coil and the rotor aligns with
this field. By supplying different phases in sequence, the rotor can be
rotated by a specific amount to reach the desired final position.
Figure shows a representation of the working principle. At the beginning, coil A is
energized and the rotor is aligned with the magnetic field it produces. When coil B
is energized, the rotor rotates clockwise by 60° to align with the new magnetic field.
The same happens when coil C is energized. In the pictures, the colors of the stator
teeth indicate the direction of the magnetic field generated by the stator winding.
Stepper Motor Principle of Operation

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Stepper Motor (Constructional
features)
2
1
S
N
1
2
Outside Casing
Stator
Rotor
Internal components of a Stepper Motor

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features)
2 2
1
N
S
1
S
N
Stator
Rotor
Cross Section

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features)
2 2
1
1
S
N
S
N
N
N
S S
1
a b
Winding number 1
2
a b
Winding number 2
One
step
6 pole rotor

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Stepper Motor (Operating Principles)
The top electromagnet (1) is turned on,
attracting the nearest teeth of a gear-shaped
iron rotor. With the teeth aligned to
electromagnet 1, they will be slightly offset
from electromagnet 2
The top electromagnet (1) is turned off,
and the right electromagnet (2) is
energized, pulling the nearest teeth
slightly to the right. This results in a
rotation of 3.6° in this example.

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Stepper Motor (Operating Principles)
The bottom electromagnet (3) is
energized; another 3.6° rotation
occurs.
The left electromagnet (4) is enabled, rotating
again by 3.6°. When the top electromagnet (1)
is again enabled, the teeth in the sprocket will
have rotated by one tooth position; since there
are 25 teeth, it will take 100 steps to make a
full rotation in this example.

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Types of Stepper Motors
 Although various types of stepping motor have been
developed, they all fall into three basic categories:
1. Variable Reluctance Stepper Motor (VRM)
2. Permanent Magnet Stepper Motor (PMSM)
3. Hybrid Stepper Motor (HSM)
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Permanent Magnet Stepper. PM steppers have rotors that are
constructed with permanent magnets, which interact with the
electromagnets of the stator to create rotation and torque.
PM steppers usually have comparatively low power requirements
and can produce more torque per unit of input power.
Variable Reluctance Stepper. VR stepper rotors are not built with
permanent magnets. Rather, they are constructed with plain iron
and resemble a gear, with protrusions or “teeth” around the
circumference of the rotor. The teeth lead to VR steppers that have
a very high degree of angular resolution; however, this accuracy
usually comes at the expense of torque.
Hybrid Stepper. HS rotors use the best features of both PM and VR
steppers. The rotor in an HS motor has a permanent magnet core,
while the circumference is built from plain iron and has teeth. A
hybrid stepper motor, therefore, has both high angular
resolution and high torque.

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The operating modes
Full Step Mode. For each 360° rotation of the motor shaft, the rotor proceeds
through 200 distinct steps, each exactly 1.8°. During full step operation, one phase
on the stator is always energized. This provides maximum torque, but angular
resolution is limited by the number of teeth on the rotor.
Half Step Mode. For each 360° rotation of the motor shaft, the rotor proceeds
through 400 distinct steps, each exactly 0.9°. During half step operation, there is
an alternation between having one or two phases on the stator energized. This
provides twice the level of angular resolution for increased positioning accuracy
but comes at the expense of torque.
Micro Step Mode. For each 360° rotation of the motor shaft, the rotor proceeds
through 51,200 distinct steps, each exactly 0.007°. During micro-step operation,
phases on the stator can be either energized, de-energized or partially energized.
This mode is used in applications where highly accurate positioning is needed,
although torque rating can be reduced by as much as 30%.

VARIABLE RELUCTANCE
STEPPER MOTOR
Constructional features – Principle of operation – Variable
reluctance motor – Single and multi stack configurations –
Torque equations – Modes of excitation–Applications.
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Variable Reluctance Stepper Motors
– Constructional Features
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Toothed Rotor and Toothed Stator
Reluctance of the magnetic circuit
formed by the rotor and stator teeth
varies with the angular position of
the rotor
Here, energize coils A and A’ (Phase
A)
Rotor “steps” to align rotor teeth 1
and 4 with stator teeth 1 and 5
– Principle of Operation
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Energize coils B and B’
(Phase B)
Rotor steps “forward”
Rotor teeth 3 and 6 align with
Stator teeth 1 and 5
Let Ns = # of teeth on the stator
Nr = # of teeth on the rotor
β = Step Angle in space
degrees
360
s r
s r
N N
N N


  

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Energize Phase C
Rotor steps forward another 15°
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Energize Phase D
Rotor steps forward another 15°
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Repeat the sequence
Energize Phase A
Rotor steps forward again
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Switching Circuit for the
stepper motor
Close switches in order 1, 2, 3,
and 4 to turn the rotor “clockwise”
Close switches in reverse order -
4, 3, 2, and 1 to change rotation to
the opposite (counter-clockwise)
direction
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– Modes of Excitation

. Single-Coil Excitation - Each successive coil is energized in turn.
Step Coil 4 Coil 3 Coil 2 Coil 1
a.1 on off off off
a.2 off on off off
a.3 off off on off
a.4 off off off on
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. Single-Coil Excitation - Each successive coil is energized in turn.
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Variable Reluctance – Single Coil
Excitation

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Two-Coil Excitation - Each successive pair of adjacent coils is energised in turn.
Step Coil 4 Coil 3 Coil 2 Coil 1
b.1 on on off off
b.2 off on on off
b.3 off off on on
b.4 on off off on
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Two-Coil Excitation - Each successive pair of adjacent coils is energised in turn.
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Variable Reluctance – Two Coil Excitation

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Variable Reluctance – Half Step Excitation

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VR Stepper Motor – Single Stack

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VR Stepper Motor – Multi Stack

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Pulse Sequence – VRSM Single
Stack

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Pulse Sequence – VRSM Multi Stack

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VR Stepper Motor – Torque Equation
When,
i(t) = Current per phase
(t) = Angular displacement
T = Total Torque produced by the motor
L() = Inductance per phase
 = Angular displacement made by the rotor
We = Energy stored
= (½ L().{i(t)}2)
Pe = Power due to energy stored
= dWe/dt or (Pm + P)
Pm = Mechanical power
P = Power available
= (Pm – Pe) or (.T)

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Paper feeder on printers
Rotor
Stator coils
Stepping Motor to
move read-write head
Practical Applications of Stepper
Motors

Control circuits for stepping motor-open loop controller for a
2-phase stepping motor
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 The control and drive circuits form an important part of the
overall stepping motor drive.
 A large variety of control circuits are possible depending on
whether the motor is operating in open loop or closed loop, and
whether it is a 2-Phase or 3-Phase motor.
 In open loop control, the rotor position is not sensed to find out
whether the motor has actually executed a step or not for the
given command.
 Nevertheless, open-loop control is quite satisfactory for many
applications, provided the motor is able to develop enough
torque to position the load against external and frictional
torques and the time interval between the input pulses is
sufficiently large.
Control circuits for stepping motor

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 If these conditions are met with, the motor does not fail to step
(except for unforeseen mechanical jams), and when once it
executes the steps, the positional error even for a large number
of steps is limited only to a fraction of the last step, and that
too only if there is some load torque.
 In closed-loop control, the rotor position is actually sensed by
suitable position transducers like photosensors or Hall
sensors, and the command for the next step is given only if the
motor has executed the previous command.
 Here, the pulse input rate is not constant but increases
automatically as the motor picks up speed.
 At the same time, the closed-loop operation prevents the motor
from losing a step.

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 From physical considerations, it should be obvious that even in
open loop control the input pulse rate can be gradually
increased as the motor picks up speed.
 However, it is difficult to adjust the pulse rate manually as there
is always the danger of losing synchronisation, resulting in
total failure.
 It is therefore safe to apply the minimum stepping rate
(corresponding to the one at the starting) throughout the
operation.
 In closed-loop control this disadvantage is overcome by
automatic adjustment of the pulse input rate.
 However, the final speed attained in closed-loop control may be
so high that after the application of last pulse the motor may
not come to rest within that step but overshoot the target.

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 Therefore, the equality between the number of pulses sent and
the number of steps made is lost, in closed loop control.
 It is therefore necessary to brake the motor just before it
reaches the final position, and to reverse the direction in case
of overshoot.
 These can be achieved by properly sequencing the
energisation of the phase windings.
 The operation of closed-loop stepping motor approaches that
of a conventional velocity servo except that the input and
output signals are in digital form.

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Open loop controller for a 2-phase stepping
motor
J K Q
0 0 Qn (Previous State Output)
0 1 0
1 0 1
1 1 Qn
’ (Toggle State)
Truth Table of Jack-Kilby Flip Flop

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motor

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Closed loop controller for a 2-phase stepping
motor

UNIT1 SEM Stepper motors ym.pptx

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