hệ thống điều khiển động cơ Servo

3.1.2 Classification  Variable reluctance motors  Permanent magnet motors  Hybrid motors 4 Types of Stepping motors:  Variable Reluctance VR Motors VR stepping motors have thr

 Quick starts, stop, and reverse capability

 High reliability because there is no brush or physical contact required for commutation

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Stepper motors convert electrical energy into discrete mechanical rotation Stepping motors have the following advantages and disadvantages

 Advantages:

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 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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 Disadvantages:

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3.1.2 Classification

 Variable reluctance motors

 Permanent magnet motors

 Hybrid motors

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Types of Stepping 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

4 teeth and 3 independent windings (six phases), creating 30 degree steps

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VR Stepper Motors

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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

12 steps per revolution

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PM Stepper Motors

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 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

 Stepping motors with magnetized rotors provide greater flux and torque than motors with variable reluctance

3 rotor pole pairs and

PM Motors

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Operation: Rotation of a PM stepping motor is produced by

energizing individual windings in a positive or negative direction

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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

12 steps per revolution

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PM Motors

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12 steps per revolution

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

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

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 Hybrid Motors

 Hybrid stepping motors combine a permanent magnet and a rotor with metal teeth to provide features of the

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3.1.3 Motors Connection and Wiring

 Identify the motor leads

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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

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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

 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

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 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

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 Torque

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

 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

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 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

Time constant: 𝜏 = 𝐿

𝑅

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3.1.4 Torque and Speed

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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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3.2 Stepper Drives

 Unipolar

 Full step, 2 phase ON

 Full step, 1 phase ON

 Half step

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3.2 Stepper Drives

 Unipolar

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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

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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 = 𝐼𝑃𝐸𝐴𝐾𝑠𝑖𝑛𝜃

𝐼𝑏 = 𝐼𝑃𝐸𝐴𝐾𝑐𝑜𝑠𝜃 where, 𝐼𝑎 : instantaneous current in stator winding A

𝐼𝑏: instantaneous current in stator winding B θ: microstep angle; 𝐼𝑃𝐸𝐴𝐾: rated current

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3.2 Stepper Drives

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

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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 Closed-loop control: high precision in trajectory control

 Brush maintenance: limiting their use in clean rooms, and other environments where brush dust is not acceptable

 Poor thermal performance: all the heat is generated in the rotor, from which the thermal path to the outer casing is very inefficient

 Increased installed cost: the installed cost of a DC servo system is higher than that of a stepper due to the requirement for feedback components

 Disadvantages:

where Te: electrical time constant

Tm: mechanical time constant normally, 𝑇𝑚 ≫ 𝑇𝑒

Then we can approximate eq (3.1) by a first order function

( )( )

3.2.3 DC Servo Drive

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 The structure of a driver

Controller Saturation Power

Amplifier DC Motor

Encoder

Microcontroller H Bridge Circuit

 The diodes (D1 D4) are called catch diodes and are

usually of a Schottky type

3.2.3 DC Servo Drive

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 Operation

 Q1, Q4 are turned on,

 Current starts flowing through the motor which energizes the motor in the forward direction, for example

3.2.3 DC Servo Drive

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 Q2, Q3 are turned on,

 Current starts flowing through the motor which energizes the motor in the reverse direction

 In a bridge, we should never

turn Q1 and Q2 (or Q3 and Q4) on at the same time

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 Using MOSFET Driver (IR2184) + MOSFET-N (IRF3205, IRF540, )

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 H-Bridge Design

3.2.3 DC Servo Drive

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Rotary encoder is a sensor attached to a rotating object (such as a shaft or motor) to measure rotation By measuring rotation we can determine any displacement, velocity, acceleration, or the angle of a rotating object

3.2.3 DC Servo Drive

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 4X Encoding State Transition

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3.2.3 DC Servo Drive

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 Measuring Speed and Position

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 To prepare the variable for fractional operations performed

by control algorithms, you need to convert the position counter result into a signed fractional number

𝐴𝑛𝑔𝑃𝑜𝑠 0 = 𝐶𝑜𝑢𝑛𝑡𝑉𝑎𝑙𝑢𝑒 × 32768

𝑀𝐴𝑋𝐶𝑂𝑈𝑁𝑇

MAXCOUNT

3.2.3 DC Servo Drive

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 Code example:

 Interrupt Period Calculation

The velocity calculation is performed in a periodic interrupt

This interrupt interval must be less than the minimum time required for a ½ revolution at maximum speed

The sample rate is called sampling time, 𝑇 (𝑠)

3.2.3 DC Servo Drive

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 Code example:

3.3 AC Servo Motors

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Currently, majority of AC servomotors are of the squirrel

cage two-phase induction type and utilized in low power applications But recently three phase induction motors has

been modified so that they can be used in high power servo applications

3.3 AC Servo Motors

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 Introduction to Yaskawa AC servo driver

Servopacks are divided into the following two types according

to usage:

 Speed/Torque Control:

SGDA-□□□S Type This type uses speed or torque reference input Reference input is by analog voltage

 Position Control: SGDA-□□□P Type

This type uses position reference input Reference input is by pulse train

undertakes the speed control loop and subsequent control processing

Speed Control

3.3 AC Servo Motors

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 Servopack for position control can be used as below

The host controller can send

a position reference (pulse train) to the Servopack to perform positioning or interpolation

This type of Servopack contains a position control loop

For more information about AC servo drivers, students should read manual documents of their manufacturers, such as: Yaskawa, Panasonic, Mitsubishi,…