servo motor(2)

7/30/2019 Servo Motor(2)

1/15

Choice of gear ratio in geared drives

Control of Servo Motors

The load is coupled with motor

Gear ratio : 1 : n

* Gear ratio n Load torque Load speed

Jm

rmMotorem

1

nL

rl

Load

JLSpeed of load side

n

rmrl

Motor power & Load power

rmemm TP rlLL TP

,,

No power loss in gear

rmLrm

LrlLLrmemmn

T

nTTPTP

)(

Torque of load side

emL nTT

Moment of inertia of load referred to the motor shaft'

LJ

2

2

222'

2

1)(

2

1

2

1

2

1rm

LrmLrlLrmL

n

J

nJJJ

Mechanical energy of motor2

'

n

JJ LL

* Gear ratio n '

LJ


2/15

Choice of gear ratio in geared drives


Mechanical equation

-Neglecting the friction term

Simple acceleration of pure inertia load

dt

d

n

JJTB

dt

d

n

JJT rmLmLrm

rmLmem

)()(

22

Motor acceleration

)(2

nJJ

T

dt

d

Lm

emm

rm

Load acceleration

)(

)(

2nJJ

Tn

dt

d

Lm

emrl

)(

)(

2nJJn

T

dt

d

Lm

emLrl

rl2

n

JL* When Gear ratio n : Load speed Transient response Transient response

rl2

n

JL* When Gear ratio n : Load speed Transient response Transient response Maximum load acceleration with n at = constantemT

0dnd

m

Lopt

JJn moptL JnJ 2

Maximum of load acceleration

optm

emMaxL

nJ

T 1

2

1)(

Motor acceleration at optimal n

m

emMaxm

J

T

2

1)(


3/15

Basic principle of Torque production


Basic torque equation

rs

m

s

s

e iid

dL

id

dL

T

2

2

1

= Reluctance torque + Alignment torque

where, is, ir:Stator and rotor current

Ls,Lm: Self inductance for stator and mutual inductance between stator and rotor

: angular position of the rotor relative to arbitrary reference position

[1] Salient-pole type

Rotor

Stator

N

S

is

(1) Stator winding is excitedThe rotor tends to take up position which minimizes the reluctance

of stator winding. Reluctance torque

Reluctance Torque2

21

ss

e iddLT


4/15

Basic principle of Torque production


(2) Both the stator winding and rotor winding are excited

Alignment torque :

Excited rotor poles N and S are aligned with excited stator poles N and S

+ Reluctance torque :

rsm

ss

e iiddLi

ddLT

2

21

Torque

= Reluctance torque + Alignment torque

Rotor

Stator

N

S

N

S

is

ir

[2] Cylindrical type

Rotor

Stator

N

S

N

S

is

ir

Both the stator winding and rotor winding are excited

- No reluctance torque

Torquers

me ii

ddLT

- Only alignment torque Smooth torque


5/15

Chapter 3. Permanent-magnet materials and circuit


Permanent-magnet materials and characteristics

B-H hysteresis loop of Permanent magnet materials

o-a : External ampere-turns (Field intensity H) are increased

a-b : External ampere-turns are switched off

b : At H=0, B = Br : Remance

(Flux density when the external field intensity is 0)

b-c : External ampere-turns are applied in the opposite direction.

c : At B = 0, H=Hc (Coercive force)

c-d :: External ampere-turns are continuously

applied in the opposite direction

The flux density is in the opposite direction.

d-e : External ampere-turns are switched off

e-a : External ampere-turns are applied in the forward direction


6/15



Demagnetization curve

Hc

Br

Hd

Bd

H [Oe]

B [G]

Q

Second quadrant operation for B-H hysteresis loop

Parameters for permanent magnet

Br (Remanence) [T] : 1[T] = 10,000[G]

- Maximum flux density that can be retained

by the magnet without external ampere-turns

- Remanence Br Flux Torque Power

Hc (Coercive Force) [Oe] : 410-3[Oe] = 1[A/m]

- Negative external ampere-turns (field intensity)

when the flux density becomes 0.

- Hc (Coercive Force)

Possibility of demagnetation by external field intensity

Maximum Energy Productdd HBBH max

- Maximum energy stored at permanent magnet

[MGOe : 106GOe]


7/15



Permanent magnet materials

Ferrite (Ceramic)

Alnico (Aluminum Nichol Cobalt)

SmCo (Samarium Cobalt)

NdFeB (Neodynium Ferrum Boron)

History for developing the PM

Ferrite

Alnico

SmCo

NdFeB

C l f S M


8/15



Demagnetization curve for permanent magnet materials

- Comparsion of the performance of permanent magnet materials

NdFeB

SmCo

Bonded

NdFeB

Alnico

Ferrite

C t l f S M t


9/15



Characteristics of permanent magnet materials

LowNormal4.0 MGOe300 CFerrite

NormalLow7.5 MGOe550 CAlnico

Very HighVery High32 MGOe300 CSmCo

HighHigh48 MGOe150 CNdFeB

CostCoercive ForceMaximum EnergyProductMaximum operatingTemperaturePermanent-MagnetMaterials

Temperature effect for permanent magnet material (Sm2Co17)

9.785.4666.59500 C

12.76.4387.38400 C

14.77.1397.91300 C

19.28.618.8525 C

BHmax

[MGOe]Hc [KOe]Br [KG]Temperature



10/15

Chapter 4. Squarewave Permanent-Magnet Brushless Motor Drives

[ BLDC (Brushless DC) Motor Drives]


Criteria of Motor Drive system for both the speed and position control

Motor

- Cost, Size, Weight

- Output Power, Maximum speed

- Torque characteristics

- Maintenance Requirement

- Input type : dc voltage, single-phase ac voltage, three-phase ac voltage

Power semiconductor circuit

- DC-to-dc converter, Controlled rectifier, Inverter

Sensor :

- ac current, dc current, ac voltage, dc voltage, position, speed, torque

Control loop :

- Open loop or Closed loop control

- Position, Speed, and Current (Torque) control loop



11/15

Structure and Operating Principle for DC Motor


Structure of dc motor

Directaxis

Field Coil N S

a

f

a

Field

AmartureAmarture Coil

Commutator

Brush

Operation of dc motor

Quadratureaxis

Directaxis

Field Coil NS

N

S

(MMF)

e

Amarture

CoilCurrent

Brush position Brush position


12/15



13/15

Structure and Operating Principle for DC Motor


Torque-Speed curve

- Armature voltage Motor speed

- Load torque Motor speed at fixed armature voltage

)

Speed control of DC Motor Controlling armature voltage

Constant flux

Te

rm

Va

Va'

Va''

Va'''

Decrease

Va

TL

TL'

Closed control for maintaining the desired speed



14/15

Closed loop control of DC Motor

f

)

Speed Control loop

*r

r

Converter

DC

Motor

*a

a

Vc

*r

r

*e

e

p

Converter DCMotor

*a

a

Vc

Position Control loop

Torque Control loop

ConverterDC

Motor

*e

e

Vca

aCaluclate

Torque



15/15

Criteria of DC Motor

f

)

Motor

- High cost, size, and weight Brush, Commutator, Armature & Field coil

- Limited power, maximum speed Brush & Commutator

- Torque characteristics : Maximum torque

- Maintenance : Brush, Commutator

- Input voltage type : One variable dc voltage source for armature voltage control

+ One unchangeable dc voltage source for dc field current

or Variable dc voltage for adjustable field current (flux)

Power semiconductor circuit

* 3-phase ac input voltage

DC-to-DC DC

Motor

3ac

Converter DC

Motor

3ac

Controlled Rectifier

* DC input voltage

DC-to-DC DC

MotorConverter

+Vdc

Sensor : one dc current, dc voltage, position or speed sensor

Control loop :

Position, speed, current control loop Simple

servo motor(2)

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