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Stepper Motor
 A stepper motor is an electromagnetic incremental
actuator which converts digital pulse inputs to analog
output shaft motion;
 The shaft of the stepper motor rotates in equal
Stepper Motors increments in response to a train of input pulses.
 When properly controlled, the output steps of a stepping
motor are always equal in number to the number of input
pulses.

Advantages of Stepper Motor

 A stepper motor is inherently a discrete motion device,
therefore it is more compatible with modern digital control
techniques;
 It is more easily adaptable for interfacing with other digital Applications
components;
 The positional error in a stepper motor is non-cumulative;
 It is possible to achieve accurate position and speed
control with a step motor in an open loop system;
 Thus, avoiding the ordinary instability problems and
elimination of feedback transducers;

Instrumentation (low torque applications) Instrumentation (low torque applications)

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Quartz watches Synchronized clocks

Camera shutter operation

Synchronized clocks

Dot matrix and line printers Floppy disc drives

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Digital x-y plotter

Autonomous Underwater Vehicle

Unmanned Aerial Vehicle Jobs that are dangerous for humans

Decontaminating Robot
Cleaning the main circulating pump housing in the nuclear power plant

Welding Robot Computer peripherals and office equipments

Repetitive jobs that are boring, stressful, or labor-intensive for humans

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Numerical control of machine tools Electro medical Equipment

Fax Machine X ray Machines

Constant Flow Hydraulic Pumps

Stepper Motor Basics
S

N S N S
rotor

Stator: made out of coils of wire Current switch in winding

Electromagnet called “winding” ==>Magnetic force
Rotor: magnet rotates on ==>hold the rotor in a position
bearings inside the stator
• Direct control of rotor position (no sensing needed)
• May oscillate around a desired orientation (resonance at low speeds)
• Low resolution

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HOW STEPPER MOTORS WORK

Stepper motors behave differently than standard DC

motors.
They cannot run freely by themselves.
Stepper motors do as their name suggests -- they "step" a
little bit at a time.
They have several windings which need to be energized in
the correct sequence before the motor's shaft will rotate.
Reversing the order of the sequence will cause the motor to
rotate the other way.

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Increased Resolution Half stepping More teeth on rotor or stator

S

N S
torque

N S

N
angle
N
 The smallest step of angular rotation a stepper motor
Half stepping
can make is called its resolution.
 Unlike the example, which had 90 degrees per step
resolution, real motors employ a series of mini-poles
on the stator and rotor to increase resolution.

 Stepper motors have a resolution of 1.8 degrees per step.

How to Control
 Because the rotor is fixed by magnetism in the stationary 4 Lead Wire Configuration

condition, the stationary torque is large. It allows one to

Step Table
make a precise stop at some angle and hold it there Step Red Blue Yellow White A+
Red

0 + - + - 4 lead
 It can better hold its position on a ramp.
1 - + + - motor
A-
 Speed control can be achieved by digitally cycling through 2 - + - + Blue

3 + - - +
the phases at a desired speed of rotation.
4 + - + -
Yellow White
 A microcontroller are used in practice to reverse the
B+ B-
Clockwise Facing Mounting End
current after each step, which changes the poles of the
corresponding electromagnets. Each step, like the second hand of a clock => tick, tick

Increase the frequency of the steps => continuous motion

Differences between Servo & Stepper Motors

Classification of Stepper Motor

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Permanent Magnet Stepper Motor

 It has permanent magnet on the rotor and also known as
can-stack motor;
 Due to its low manufacturing cost, it is employed in paper
feed motor of printers or head drive motor of a floppy disc
drive.

Cutaway View of 2 ph PM Stepper Motor Variable Reluctance Stepper Motor

It has no permanent magnet either on the rotor or the
stator;
The rotor carries no windings and has salient pole
construction;
Rotor and stator both are made of soft iron stampings.
The stator also has salient poles and carry stator windings.
The number of stator poles is an even multiple of the
number of phases for which the stator windings are wound.
The number of phases must be at least three for
bidirectional control of stepper motor.
It can have single stack or multi stack construction;
The torque developed by the motor shall be more in multi
stack motor.

Variable Reluctance (VR) Stepper Motor VR Multi Stack Stepper Motor

6/4 pole 12/8 pole

Single Stack Cross Section Parallel to Shaft

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VR Multi Stack Stepper Motor VR Multi Stack Stepper Motor

Cross Section Perpendicular to Shaft

Hybrid Stepper Motor Hybrid Stepper Motor

 It has permanent magnet mounted on the rotor;

 It provides détente torque with windings de-energized;
 Less tendency to resonate;
 Higher holding torque capability;
 Better damping due to the presence of rotor magnet;
 High stepping rate capability;
 High efficiency at lower speeds and lower stepping rates.
 It suffers from high inertia and weight due to presence of
rotor magnets;
 Performance is affected by change in magnet strength.

Cross Section Parallel to Shaft

Operation of Hybrid Stepper Motor

Hybrid Stepper Motor  As long as the stator winding A and B are energised in a
particular manner, the rotor stays put in the corresponding
position;
 The rotor will move further only when the pattern of
energization of the stator windings is changed;
 Figures a – h in next slide represent programmed
sequence;
 Figures a,c,e,g represent single phase energisation,
however, b,d,f,g represent two phase energisation;
 These figures constitute 8 step sequence in which the
rotor moves 45º per step;
 Here one or two phases are alternatively energized,
Cross Section Perpendicular to Shaft therefore this sequence is known as hybrid or mixed or 1-
2 sequence.

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Hybrid Stepper Motor operation 1.8º Rotation Hybrid Stepper Motor

 It carries a cylindrical
permanent magnet
sandwiched between two
rotor discs;
 The stator and rotor both are
made of soft iron stampings;
 Each rotor disc has 50 teeth
and the stator has 8 poles
with 5 teeth per pole;
 There are 40 poles on the
stator which can be in perfect
alignment or misalignment
depending on the switching.

1.8º Hybrid Stepper Motor 1.8º Hybrid Bifilar Stepper Motor

Static Characteristics

Characteristics of Stepper Motors

Torque Angle Curve

Torque Current Curve Hybrid motor torque and detente torque

profile

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Dynamic Characteristics Dynamic Characteristics

 Pull in curve: corresponds to start-stop or single  Start-stop Mode: rotor comes to rest after one step;
stepping mode;  Slewing Mode: rotor still moves in response to previous
pulse when next pulse arrives; Therefore motor can overrun
 Pull out curve: corresponds to slewing mode;
by several steps before stopping.

 Mid Frequency Resonance :

Pull in curve of stepper
motor suddenly dips very
low in particular range of
Start Stop Mode stepping rates;
 This phenomenon is a
manifestation of instability
of motor operation.

Torque speed characteristic Slewing Mode

Some Definitions Some Definitions

 Step angle (θs) : It is the angle through which an unloaded stepper
 Holding Torque (TH) : It is the maximum load torque which the
motor rotates for every step of the energization sequence; i.e. θs =
energized stepper motor can withstand without slipping from
360º/Z
equilibrium position;
 Steps/revolution (Z) : For hybrid motor : Z=Nr.kws,
 Detente Torque (TD) : It is the maximum load torque which the un-
Kws = 4 for 4 step sequence (i.e. 1 ph on or 2 ph on)
energized stepper motor can withstand without slipping;
Kws = 8 for 8 step (i.e. 1-2 or hybrid) sequence
Détente torque is due to residual magnetism and is therefore present
of energization of stator winding;
in PM or hybrid stepper motor only;
For single stack VR stepper motor :
 Torque Constant (Kt) : It is the initial slope of the torque-current curve
Ns = Nr ± p where p is no. of teeth per phase (minimum 2)
of the stepper motor, and also known as torque sensitivity;
Z = p.m(m-1) for Ns > Nr; p.m(m+1) for Ns < Nr where Nr and Ns
 Pull-in Torque (TPI) : It is the maximum torque that the stepper motor
are number of teeth in rotor and stator respectively;
can develop in the start-stop mode at a given stepping rate Fs
m is number of phases (minimum 3) (steps/sec), without losing synchronism;

Some Definitions

 Pull-out Torque (TPO) : It is the maximum torque that the stepper motor
can develop at a given stepping rate Fs (steps/sec), in the slewing
mode, without losing synchronism;
 Pull-in Rate (FPI) : It is the maximum stepping rate at which the stepper
motor will start or stop, without losing synchronism, against a given


load torque T.
Pull-out Rate (FPO) : It is the maximum stepping rate at which the
Analysis of Stepper Motors
stepper motor will slew, without missing steps, against a load torque T.
 Response Range : It is the range of stepping rate at which the stepper
motor can start or stop, without losing synchronism, at a given torque
T. Response range spans stepping rates Fs ≤ FPI;

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Static Torque Production Static Torque Production

 Considering a 4 phase motor with two-phases-on
 For a motor with p rotor teeth and a peak static torque TPK
excitation which gives approximately sinusoidal
at a rotor displacement θ from the step position, the torque
torque/position characteristic as shown;
developed by the motor is approximately T = - TPK sin pθ;
 The first step command changes excitation to phases B &
 When a load torque TL is applied the rotor is displaced
C and the static torque at the position θe then exceeds the
from the demanded position by the angle θe, at which the
load torque, so the motor accelerates in positive direction;
load and motor torques are equal, i.e. TL = T = - TPK sin
pθe;  Assuming that the motor has
 The static position error is θe = {sin-1 (- TL /TPK)}/ p; moved to θ1 with phase BC
 Therefore, static position error can be reduced either by excited, the average torque
increasing the peak static torque or by increasing the produced is:
number of rotor teeth.
 A motor with high stiffness develops a large torque for a 1 1 TPK
TM   TPK sin  p   2  d  sin p1  sin p e 
small displacement from equilibrium. 1   e e p 1  e 

Static Torque Production

The equation of motion for the system inertia (J) is
TM  TL  Jd 2 / dt 2 which, after solving, gives
  TM  TL  t 2 / J  e ;
After one period of escitation t p the rotor is at position 1 ,
therefore Converter Topologies for Stepper
1  TM  TL  t / J   e or t p   J (1   e ) /(TM  TL ) 
2
p
1/ 2
Motor Drives
So the starting rate for the four phase motor is
approximately :
Starting rate = 1/t p   (TM  TL ) / J (1  e ) 
1/ 2

3 phase Uni-polar Drive Circuit Bipolar Drive Circuit (one phase)

+Vs

Forcing Resistance R

Free wheeling Resistance Rf

1 2 3

Phase 1 Base
control
Drive
Signal

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Uni-polar Drive Circuit for one phase of Bifilar Uni-polar Circuit Bi-level Drive (one phase)
Wound Stepper Motor
Phase Current
+Vs
Rf R Rf
T2
VH

• D2
1
• D1
1
-
Phase 1 + Phase 1
Base Base control
control VL
Drive Drive Signal
Signal
T1

Uni-polar Chopper Drive Circuit

(one phase)

T2
D2

Micro stepping Control of Stepper

1
VH D1 Motors
T1

Rc Vc

Micro Stepping Control Micro Stepping Control

 Micro stepping control
 In micro stepping control, the stator magnetic field is made to rotate
enables the stepper motor through a small angle θ << 90º in response to an input pulse;
to move through a tiny  This is achieved by modulating currents through windings B2 and A1 in
micro step of size ∆θ << θs such a way that IB2 = IR Cos θ, while IA1 = IR sin θ;
 The resulting stator magnetic field will be at an angle θº (elect.) w.r.t. the
in response to input pulse;
positive real axis;
 This overcomes the limited  The phasor diagram and the sequence table for micro stepping control
resolution and mid is given in next slides.
frequency resonance  The torque developed is same as developed under one-phase-on
sequence, as the resultant current remains IR;
problems;
 However, if the torque required is as under two-phase-on sequence
 The pull out curve of micro then the magnitudes of the currents should be such that IA2 + IB2 = 2.
stepping control is shown

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Micro Stepping Principle Circuit for Micro Stepping Control

+

Current Current
Controller Controller
A B

IA IB
Power
supply

Phase Phase
A B

S1 S2 S3 S4

Advantages of Micro Stepping References

 Improvement in resolution by the factor MSR (micro 1. A.C. Leenhouts, “The art and practice of step motor control,” Intertec
stepping ratio) i.e. MSR = θs /∆θ in as much as the communications Inc., 1987.
2. B.C. Kuo, “theory and applications of step motors,” west publishing
smallest angle through which the motor rotates per input
co., 1974.
pulse is : ∆θ = (1/MSR). θs usual values of MSR are 5, 3. P.P.Acarnley, “Stepping Motors : a guide to modern theory and
10, 125 and powers of 2 up to 128; practice,” Peter Peregrinus Ltd., 1982.
4. V.V. Athani, “ Stepper Motors: fundamentals, applications and design”,
 Rapid motion at a micro stepping rate MFs = MSR.Fs New age international (P) ltd. publishers, New Delhi, 1997.
where Fs is full stepping rate;
 DC motor like smooth performance;
 Elimination of mid frequency resonance.

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