hệ thống điều khiển động cơ Servo

Chapter 3

Motors
& Servo Drives
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3.1 Stepper Motors
3.1.1 Introduction
Stepper motors convert electrical energy into discrete
mechanical rotation. Stepping motors have the following
advantages and disadvantages
 Advantages:
 Full torque when rotation is stopped. This aids in

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maintaining the current position.
 Precise open-loop positioning and repetition. High quality
stepping motors have three to five percent precision
within a single step.
 Quick starts, stop, and reverse capability.
 High reliability because there is no brush or physical
contact required for commutation.
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3.1.1 Introduction
 Disadvantages:

 Inherent resonance can cause noise, jerky rotation, and at

extreme levels, loss of position.
 It is possible to lose position control in some situations,
because no feedback is natively provided.
 Power consumption does not decrease to zero, even if load
is absent or motor is in stop mode.
 Stepping motors have low-power density and lower
maximum speed compared to brushed and brushless DC
motors. Typical loaded maximum operating speeds for
stepper motors are around 1000 RPM.
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3.1.2 Classification
 Types of Stepping motors:

 Variable reluctance motors
 Permanent magnet motors
 Hybrid motors

 Variable Reluctance (VR) Motors
VR stepping motors have three to five
windings and a common terminal
connection, creating several phases on
the stator. The rotor is toothed and made
of metal, but is not permanently
magnetized.

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4 teeth and 3 independent windings (six phases), creating 30 degree steps.
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VR Stepper Motors
Operation: The rotation of a VR motor is produced by
energizing individual windings.
When a winding is energized, current flows and magnetic
poles are created, which attracts the metal teeth of the rotor.
The rotor moves one step to align the offset teeth to the
energized winding. When the phases are turned on
sequentially, the rotor rotates continuously.

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12 steps per revolution


PM Stepper Motors
 Permanent Magnet (PM) Motors

 A PM stepping motor consists of a
stator with windings and a rotor with
permanent magnet poles. Alternate
rotor poles have rectilinear forms
parallel to the motor axis.

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 Stepping motors with magnetized
rotors provide greater flux and torque
3 rotor pole pairs and than motors with variable reluctance.
2 independent stator
windings, creating 30  PM motors are subjected to
influence from the back-EMF of
degree steps.
the rotor, which limits the
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maximum speed.


PM Motors
Operation: Rotation of a PM stepping motor is produced by
energizing individual windings in a positive or negative
direction.
When a winding is energized, a north and south pole are
created, depending on the polarity of the current flowing.
These generated poles attract the permanent poles of the rotor.
The rotor moves one step to align the offset permanent poles to
the corresponding energized windings. When the phases are
turned on sequentially the rotor is continuously rotated.

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12 steps per
revolution


PM Motors
Another alternative to rotate a permanent magnet rotor is to
energize both windings in each step. The vector torque
generated by each of the coils is additive; this doubles the
current flowing in the motor, and increases the torque.

12 steps per
revolution

Typical PM motors have more poles to create smaller steps.
To make significantly smaller steps down to one degree or
even lower.
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Hybrid Stepper Motors
 Hybrid Motors

 Hybrid stepping motors combine a
permanent magnet and a rotor with
metal teeth to provide features of the
VR and PM motors.
 Hybrid motors are expensive, but

they use smaller steps, then have
greater torque, and have greater
maximum speeds.

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 Rotation of a hybrid stepping motor is produced with the
same control method as a PM motor, by energizing
individual windings in a positive or negative direction.
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3.1.3 Motors Connection and Wiring
 Identify the motor leads

The color code of the wires coming out of the motor are
not standard; however, using a multimeter/ohmmeter, it
is easy to identify the winding ends and center tap.
 4 leads: the motor is a bipolar motor. If
the resistance measured across two
terminals is finite, then those are ends
of a coil. If the multimeter shows an
open circuit then the terminals are of
different windings.
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3.1.3 Motors Connection and Wiring
The color code of the wires coming out of the motor are
not standard; however, using a multimeter/ohmmeter, it is

easy to identify the winding ends and center tap.
 4 leads: the motor is a bipolar motor. If the
resistance measured across two terminals
is finite, then those are ends of a coil. If
the multimeter shows an open circuit then
the terminals are of different windings.

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 5/6 leads: the resistance across
one terminal and other terminals
will be almost equal (5 leads) or
double (6 leads).


3.1.3 Motors Connection and Wiring
 8 leads: it is similar to 4 leads case. However, 8 wire motors
have two coils per phase. The coils can be run in series,
parallel or half coil mode.

In all the above cases, once the terminals are identified, it is
important to know the sequence in which the windings should
be energized. This is done by energizing the terminals one
after the other, by rated voltage.
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3.1.4 Torque and Speed
 Torque

Torque is a critical consideration when choosing a stepping
motor. Stepper motors have different types of rated torque.

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 Holding torque: The torque required to rotate the motor‟s
shaft while the windings are energized
 Pull-in torque: The torque against which a motor can
accelerate from a standing start without missing any
steps, when driven at a constant stepping rate.
 Pull-out torque: The load a motor can move when at
operating speed.
 Detent torque: The torque required to rotate the motor‟s
shaft while the windings are not energized.
Stepping motor manufacturers will specify several or all of
these torques in their data sheets for their motors.


3.1.4 Torque and Speed
 Speed

The speed of a stepper motor depends on the rate at which you
turn on and off the coils, and is termed the step-rate.

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Time constant: 𝜏 = 𝑅𝐿


3.1.4 Torque and Speed

The best way to decide the maximum speed is by studying the
torque vs. step-rate (expressed in pulse per second or pps)
characteristics of a particular stepper motor

The „maximum self-starting frequency‟ is 200 pps. While at
no-load, this motor can be accelerated up to 275 pps.
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3.1.5 Stepper Drives
 Variable reluctance

pulses per revolution =
360/Step Angle
pps = (rpm/60) * ppr
Tdelay = 1/pps

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3.2 Stepper Drives
 Unipolar
 Full step, 1 phase ON

 Full step, 2 phase ON

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 Half step


3.2 Stepper Drives
 Unipolar

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3.2 Stepper Drives
 Bipolar

1 phase ON

2 phase ON
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3.2 Stepper Drives
 Bipolar

H-Bridge
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3.2 Stepper Drives
 MicroStepping

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Single stepping a motor results in jerky movements of the
motor, especially at lower speeds. Microstepping is used to
achieve increased step resolution and smoother transitions
between steps
If we move the motor in microsteps, i.e., a fraction of a full
step (1/4, 1/8, 1/16 or 1/32), then the step-rate has to be
increased by a corresponding factor (4, 8, 16 or 32) for the
same rpm. Microstepping offers some advantages:
 Smooth movement at low speeds
 Increased step positioning resolution, as a result of a
smaller step angle
 Maximum torque at both low and high step-rates

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3.2 Stepper Drives
In full step and half step modes, rated current is supplied to
the windings, which rotates the resultant flux in the air gap in
90 degrees and 45 degrees “electrical”, respectively.

In microstepping, the current is changed in the windings in
fractions of rated current. Therefore, the resultant direction of
flux changes in fractions of 90 degrees electrical. Usually, a
full step is further divided into 4/8/16/32 steps
The magnitude of the current in the windings:
𝐼𝑎 = 𝐼𝑃𝐸𝐴𝐾 𝑠𝑖𝑛𝜃
𝐼𝑏 = 𝐼𝑃𝐸𝐴𝐾 𝑐𝑜𝑠𝜃

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where, 𝐼𝑎 : instantaneous current in stator winding A
𝐼𝑏 : instantaneous current in stator winding B
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θ: microstep angle; 𝐼𝑃𝐸𝐴𝐾 : rated current


3.2 Stepper Drives

The resultant stator current is the vector sum of the individual
winding currents.
I sum  I a 2  I b 2  I PEAK
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3.2 Stepper Drives
But in practice, the current in one winding is kept constant over
half of the complete step and current in the other winding is
varied as a function of sinθ to maximize the motor torque.

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I sum  ( I PEAK ) 2  ( I PEAK sin  ) 2  I PEAK 1  (sin  ) 2

3.2 Stepper Drives
Table for full step (bipolar)

Table for microstep (bipolar)
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