Mechatronics

Brushless Motors

Mechatronics

A servosystem is capable of transforming any mathematical function

into a mechanical movement it can replace mechanical elements, such as cams

and cam shafts, indexing gears, differentials, etc.

A servosystem consists of a servomotor with its control unit.

Servosystems can be used for:

• Positioning; the position, linear or angular, follows a predetermined position function.

• Speed control; the motor speed follows a predetermined speed function.

• Torque control; the torque of the motor follows a predetermined acceleration function.

• Hybrid control; the system alternates between different kinds of control

Mechatronics

The servomotor, which is a permanently magnetized brushless

AC motor, is arelatively new type of motor.

24 slots 2 rotor poles

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Electro-Magnetic Flux Distribution

21 slots 8 rotor poles

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Brushless Solution Pros&Cons:

• Velocity (no sparks at the commutator)• Efficiency (Torque/Inertia)• Weight• Dimensions• Thermical Dissipation• Acoustic Noise• Maintenance• MTBFDisadvantages: Cost

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1. BL-Servo with MP

Cold

• Only the stator is warm.

Warm

Warm 2. DC-Servo with MP

• Rotor and commutator are warm

3. AC-Servo (ASM squirrel cage)

Heiß

• Both rotor and stator are warm

Warm

Warm

Warm

Thermical Behaviour

Mechatronics

Two brushless motors types exist

(they differ in stator windings, permanent magnets lay-down, statoric field wave

shape):

• AC brushless:AC brushless: with sinusoidal field (fcem)

• DC brushless:DC brushless: with trapezoidal field (fcem)

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Permanent Magnets (PM) types:• FerriteFerrite: low cost,

low Kt, torque loss: 0.2%/K, demagnetization temperature: 150C

• Samarium Cobalt (SmSamarium Cobalt (Sm22CoCo1717)): high cost, high Kt, torque loss: 0.04%/K, demagnetization temperature: >150C(example: Danaher HD series, Rockwell MPG series)

• Neodimium Iron Boron (NdFeB)Neodimium Iron Boron (NdFeB): medium cost, higher Kt, torque loss: 0.09%/K, demagnetization temperature: >150C(example: Danaher HR series, Rockwell H, Y, 1326, MPL series)

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Magnetism

NdFeB (vs. SmCo)

• higher energy content• worse thermical reversibility• lower Curie temperature• lower cost • corrosion (not present with

SmCo)• bigger resistance in ,

that in turn limits eddy currents

400

300

200

E [kJ/m3]

1860 1880 1900 1920 1940 1960 1980 2000

100

0

Year

SteelSteel

Al Ni CoAl Ni Co

Sm CoSm Co

Nd Fe BNd Fe B

FerriteFerrite

Mechatronics

8,3Sm2 Co17

density[g/cm^3]

7,4

Curie Temp.[°C]

Br temp. coeff.[% 1/°C]

Nd Fe B

825

315

-0,03 (20°C ÷ 200°C)

-0,1 (20°C ÷ 150°C)

SmCo vs. NdFeB

Temperature that causes para-magnetic behaviour (i.e. weak magnetisation)

Ca

mp

o c

oe

rciti

vo J

Hc

[kA

/m]

50

0

1

00

0

1

500

20

00

25

00

50 100 150 200 250 300 350 400

Max Energetic Product (B*H)max [kJ/m^3]

SECo5

qualitàspeciali

SECo5

Sm2Co17

NdFeB

(Fie

ld t

hat

can d

em

agneti

ze t

he m

agnet)

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The permanent magnets lay-out on the rotor surface depends on: AC or DC brushless, Kt, Cogging Torque (coppia di impuntamento).

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Servo System Electrical Scheme

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PTC Resistor

Positive Thermal Coefficient resistor, used as a sensor inside the motor,in order to stop the driver/controller in case of too-high temperature (before to burn the motor)

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Servo Motor

Shapes and

Air Cooling

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Direct Drive Solution Direct Drive Solution

Problem:

Motor shaft elastical torsion (i.e. its flexibility):

resonance frequencylimited band-widthlow gains in the control looppoor kinematic performances

Mechatronics

Direct Drive Solution (cont’d)

Possible Solutions:

• Digital FiltersDigital Filters (only for constant resonance frequencies)

• High Stiffness MotorsHigh Stiffness Motors (a high inertia would not solve the problem)

• Torque MotorsTorque Motors (Motori Coppia) with:– low velocity– high stiffness– they don’t need gearboxes (i.e. they are direct-drive)

Mechatronics

Once solved the resonance frequency problem, the control loop gains can be increased and thus a good accuracy in the position sensor becomes mandatory:

ResolverResolver: 6 arc min = 0.1 degrees

SinCos EncoderSinCos Encoder: 0.01 arc sec = 2.8E-6 degreesor 1nm for linear encoders (righe ottiche)

Direct Drive Solution (cont’d)

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Position Sensors used in the

Brushless Motors

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ResolverResolver

Characteristics:

linearity: 0. 1 - 0.5% resolution: 0. 1 - 0.5° sensitivity: 5 - 10mV/° (Vref =20V) frequency: 20KHz

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Resolver (cont’d)

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Resolver (cont’d)

Pro absolute in one turn low cost robust

Cons sinusoidal 20KHz reference voltage non-linear output brushes in some (old) versions

It has been the standard position sensor on brushless motors

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EncoderEncoder

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Encoder (cont’d)

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Encoder (cont’d)

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Encoder (cont’d)

Encoder types:

AbsoluteAbsolute Battery Back Up One-Turn Absolute Multi-Turn

IncrementalIncremental

SinCosSinCos

(resolver output, encoder design,

precise as an encoder)

e.g.: Stegmann mounted on Rockwell MPL motors

Mechatronics

Degree of ProtectionIPXY (e.g.: IP65) 

Digit 1 (X): Solid Objects Protection 

0 Non Protected1 Protected against solid objects > than 50 mm2 Protected against solid objects > than 12 mm3 Protected against solid objects > than 2.5 mm4 Protected against solid objects > than 1 mm5 Dust Protected6 Dust Tight

 Digit 2 (Y): Water Protection 

0 Non Protected1 Protected against dripping water2   Protected against dripping water

when tilted to worse case opening3 Protected against spraying water4 Protected against splashing water5 Protected against water jets6 Protected against heavy seas7 Protected against the effects of immersion8 Protected against submersion