Brushless Motors

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Brushless Motors
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• 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

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• The servomotor, which is a permanently magnetized
  brushless AC motor, is a
• relatively 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 ProsCons

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

Disadvantages Cost
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Thermical Behaviour
1. BL-Servo with MP
2. DC-Servo with MP
3. AC-Servo (ASM squirrel cage)
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• Two brushless motors types exist
• (they differ in stator windings, permanent
  magnets lay-down, statoric field wave shape)
• AC brushless
• with sinusoidal field (fcem)
• DC brushless
• with trapezoidal field (fcem)

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Permanent Magnets (PM) types

• Ferrite low cost,
• low Kt, torque loss 0.2/K,
• demagnetization temperature 150C
• Samarium Cobalt (Sm2Co17) high cost,
• high Kt, torque loss 0.04/K,
• demagnetization temperature gt150C
• (example Danaher HD series, Rockwell MPG series)
• Neodimium Iron Boron (NdFeB) medium cost,
• higher Kt, torque loss 0.09/K,
• demagnetization temperature gt150C
• (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 W,
• that in turn limits eddy currents

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

• Problem
• Motor shaft elastical torsion (i.e. its
  flexibility)
• resonance frequency
• limited band-width
• low gains in the control loop
• poor kinematic performances

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Direct Drive Solution (contd)

• Possible Solutions
• Digital Filters (only for constant resonance
  frequencies)
• High Stiffness Motors (a high inertia would not
  solve the problem)
• Torque Motors (Motori Coppia) with
• low velocity
• high stiffness
• they dont need gearboxes (i.e. they are
  direct-drive)

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Direct Drive Solution (contd)

• Once solved the resonance frequency problem, the
  control loop gains can be increased and thus a
  good accuracy in the position sensor becomes
  mandatory
• Resolver
• 6 arc min 0.1 degrees
• SinCos Encoder
• 0.01 arc sec 2.8E-6 degrees
• or 1nm for linear encoders (righe ottiche)

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Position Sensors used in theBrushless Motors
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Resolver

• Characteristics
• linearity 0. 1 - 0.5
• resolution 0. 1 - 0.5
• sensitivity 5 - 10mV/ (Vref 20V)
• frequency 20KHz

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Resolver (contd)
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Resolver (contd)

• 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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Encoder
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Encoder (contd)
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Encoder (contd)
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Encoder (contd)

• Encoder types
• Absolute
• Battery Back Up
• One-Turn Absolute
• Multi-Turn
• Incremental
• SinCos
• (resolver output, encoder design,
• precise as an encoder)

e.g. Stegmann mounted on Rockwell MPL motors
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Degree of Protection IPXY (e.g. IP65)  Digit 1
(X) Solid Objects Protection   0 Non
Protected 1 Protected against solid objects gt
than 50 mm 2 Protected against solid objects gt
than 12 mm 3 Protected against solid objects gt
than 2.5 mm 4 Protected against solid objects gt
than 1 mm 5 Dust Protected 6 Dust Tight   Digit 2
(Y) Water Protection   0 Non Protected 1 Protecte
d against dripping water 2   Protected against
dripping water when tilted to worse case
opening 3 Protected against spraying
water 4 Protected against splashing
water 5 Protected against water jets 6 Protected
against heavy seas 7 Protected against the
effects of immersion 8 Protected against
submersion