servo motor(7)

7/30/2019 Servo Motor(7)

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Implementation methods for Vector controlled PMSM Drive System

Control of Servo Motors


Power Circuit

Detection (Current, Voltage, Speed, Position)Control (Analog , Digital)

Power Circuit

PMSM

P W M

Inverter

Full bridge rectifier ( Six diodes)

- 3-phase AC voltage DC voltage

- Harmonics : 6th times to source frequency

Filtering capacitor

- For suppressing the harmonics in DC Voltage- Electrolytic Capacitor : High capacity & Low cost

Initial charging circuit for filtering capacitor

- To prevent the rush current in the capacitor at start-up of power circuit

- Charging through resistor at initial condition


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PWM inverter

Driver circuit- To drive the switching device

The structure of driver circuit- Opto-coupler : Isolation between the signal ground and the power ground

- Driver circuit : Increasing current or changing voltage level

- Separate dc power source is required

-Six switching devices in anti-parallel with diodes.

a

b

c

A+

C-A-

C+

B-

B+

a

b

c

PWM

+Vcc

Driver

Circuit

+VSSOpto-Coupler

(A+)

A+

Driver circuit


3/18



Switching device

Discrete - type

Module - type

IPM (Intelligent Power Module)

-Six or seven switching devices

(one switching device for

dynamic braking)

- Drive circuit

- Protector circuit for overcurrent,

under or over voltage


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Detection method for PMSM drive system


Sensing devices

Current sensors

Three-phase ac currents & dc link current

Voltage sensorsThree-phase ac voltage & dc capacitor voltage

Speed & position sensor

Requirement of sensor

Accuracy

Range (Measurement)

Nonlinearity

Isolation (Ground of signal is separated from that of power circuit)

Type of output signal

(voltage, current, number of pulse, duty cycle of pulse)


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Current sensor


Current sensor

- Sensing ac and dc currents

- Hall-effect current sensor

Hall element

B

I C

V H

I Cd

BBId

KV CH

Output voltage of hall element

- The Hall voltage is proportional to flux

- The polarity of hall voltage Direction of flux

IAMP.

Vout

Hall device

Hall effect current sensor

Structure of hall effect current sensor

- DC power supply : +15V, -15V

- Measuring both ac and dc current

- Isolation from power circuit


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Current sensor


Output voltage versus current

( Ex. 4V/100A Hall-effect sensor)

- Magnitude of output voltage is proportional to current

Block diagram for current measurement circuit for digital control

Current100 A-100 A

+4V

-4V

Output voltage

Type of Hall-CT

Hall-CTLow-Pass

FilterAmplifier

A/D

Converter


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Voltage sensor


Voltage sensor : Measuring three-phase ac voltages and dc capacitor voltage

Measuring three-phase ac voltages

Transformer for isolation and magnitude control

Vab+

-

Vo+

-

N1 N2

abc

Transfomer

- Signal ground is isolated from power ground

- Output voltage (Secondary winding voltage) of transformer

aboV

NNV

12

- Frequency response of transformer is not good

- Ferrite core is used to improve the frequency performance

- The transformer cant be used to measure the dc voltage

Output voltage is adjusted by winding ratio.

C l f S M


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Voltage sensor


Isolation amplifier

- Measuring both ac and dc voltage

- Isolation- Amplification factor : 1:1 or 1:8 It is difficult to control the magnitude of voltage

Block diagram for voltage measurement circuit using isolation amplifier

Internal structure of isolation amplifier

(AD 202)

Isolation Low-Pass

FilterAmplifier

A/D

ConverterAmplifier

R1

R2 Vi

+

-

Vab

+

-

abiV

RR

RV

21

2

C t l f S M t


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Speed measurement methods


Tachogenerator

Speed measurement methods

- Tachogenerator

- Incremental encoder

- Linearity is not good

- LPF, amplification, A/D converter are required for digital control

- DC generator type

- DC Output voltage is generated by speed

Speed

Output voltage



10/18


Structure of incremental encoder

- Output signals : A, B, Z pulses

- A and B pulses : Many pulses are outputted at one revolutionEx. 2048 P/R (pulses per revolution)

Phase difference between A and B is 90

- Z pulse : One pulse is outputted at one revolution

A

B

Z

Output signals of encoder

Speed measurementusingIncremental encoder



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Direction of rotor speed

- Checking whether the rotor rotates at forward direction or reverse direction using D

F/F.

A

B

D F/FD Q F / R

[1] Forward rotation

- A pulse leads by 90 to B pulse The output of D F/F is 1A

B

F / R

[2] Reverse rotation

- B pulse leads by 90 to A pulse The output of D F/F is 0A

B

F / R




12/18


Calculation of speed

- To increase the frequency of encoder pulse

Improvement of resolution of speed calculation- AB The frequency of encoder pulse is increased by twice.

- Monostable The frequency of encoder pulse is increased by four times.

A

B

A B

Monostable

Method of speed calculation by encoder pulse T method

M method M/T method




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T method

- The one period of encoder pulse is measured by counting the high frequency clock & speed is calculated.

- Motor speed The period of encoder pulse Resolution of speed calculation - Motor speed The period of encoder pulse Resolution of speed calculation

Overflow in counter may be generated

Encoderpulse

High frequencyClock

1 Period

M method

- The speed is calculated by counting the encoder pulse during speed sampling time.

- Motor speed The number of encoder pulse Resolution of speed calculation - Motor speed The number of encoder pulse Resolution of speed calculation

* Resolution is dependent on P/R of encoder, speed sampling period, and speed.




14/18

f

M method

- The speed is calculated by counting the encoder pulse during speed sampling time.

- Motor speed The number of encoder pulse Resolution of speed calculation - Motor speed The number of encoder pulse Resolution of speed calculation

* Resolution is dependent on P/R of encoder, speed sampling period, and speed.

Encoderpulse

Speed Sampling Time(T )s

Ex.) 1024 P/R encoder, Speed sampling time = 10ms, Motor speed =1800rpm .

What is no. of encoder pulse for counter ?

* # of encoder pulse for sampling period = (1800/60)[rps] * (1024*2) * 0.01 = 614

Equation of speed

)(#*2*)/(

60][ pulseencoderof

TRPrpm

s

r

Where Ts

= speed sampling time.




15/18

Incremental encoder for Speed measurement

f

Resolver

- 2-axis stator winding and rotating transformer

- sin t costare applied totwo stator winding, respectively. Output voltage of rotating transformer

R/D converter for digitization

Position measurementmethods

)sin()sincoscos(sin)sincos( 4_32_12_1 tKttKtEtEKE ssR



16/18


Incremental encoder

- Frequency of encoder pulse is increased by twice using exclusive OR

- Z-pulse is generated at the center of N-pole of rotor flux. (at the position =0) External interrupt is generated by Z-pulse

Advantage

- Digital value of position can be obtained

- The low cost and simple structure

- Both the position and speed are measured with one incremental encoder

Disadvantage

- The absolute position of rotor can not be available at any time

- The position should be compensated, whenever the Z-pulse is generated


A

B

A B

Z

N



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Absolute encoder

- Multi layer : 12-bit absolute encoder12 layers

Structure of absolute encoder


Advantage

- Digital value of position can be obtained

- The absolute position can be available at any time Disadvantage

- The number of output lines is too many

- The cost is high

- Number of bit Cost and resolution



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Output data of absolute encoder

- Grey code is used to prevent the measurement error


310000111111

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

110111011010101111001010

91011010010

80011000010

70010011100

61010001100

51110010100

40110000100

30100011000

21100001000

11000010000

00000000000

DecimalGray CodeNatural Binary Code

Grey code

- Only one bit is changed

At transition from 7 to 8

* binary code :

00111b(7) 01111b (15) or 00000b(0)* Grey code :

00100b(7) 01100b(8) or 00100b(7)

Table for binary code and grey code (5-bit)

servo motor(7)

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