Digital Motion Control System Design From the Ground Up Part 2

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Digital Motion Control System Design - From the
Ground UpPart 2
2
Introduction

• Hardware Design Options
• High level overview of Field Oriented Control
  (FOC)
• Software Implementation
• Introduce D3 Engineerings Motor Control
  Development Kit

3
Hardware Design Options

• Choose Feedback Method
• Rotary Feedback
• Current Feedback
• Choose Communications interface
• Isolation requirements
• Isolation between control and power electronics
• Isolation between control electronics and outside
  world
• Digital I/O
• Analog I/O
• Pulse Width Modulation (PWM)
• Putting it all together

4
Rotary Feedback Choices

• Incremental or Absolute
• Resolution requirements
• Environmental considerations
• Sensor must be aligned (zeroed) to Rotor and
  Stator for FOC commutation
• Mechanically
• Software offsets

5
Incremental Optical Encoder

• Code disk with optical transmitter and receiver
  on either side
• Outputs two quadrature signals, A and B, and an
  index pulse
• Multiple options for output configuration
• Open collector
• Differential Line Driver
• 5V-24V
• Each edge is counted giving 4x resolution
• Commutation tracks also available
• Available in high resolution (gt100K counts per
  rev)
• Easy to interface, no analog hardware

6
Incremental Optical Encoder

• Standard products not typically good for harsh
  environments
• No absolute position data
• Need extra commutation signals or an
  initialization routine to use for FOC

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Resolver

• A rotating transformer
• Input AC excitation
• Output Sin and Cos of rotor angle modulated at
  excitation frequency

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Resolver

• Typically considered rugged, good for harsh
  environments
• Absolute within 1 revolution

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Resolver

• Requires Resolver to Digital Converter (RDC)
• Separate ASIC
• Implement in DSP
• Requires careful analog design
• Resolution is a function of RDC

10
Current Sense Feedback

• Shunt resistor
• Current is measured as voltage drop across a
  current sense resistor
• Hall-effect device
• The magnetic field of a current carrying wire is
  sensed and converted to a voltage

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Shunt Resistor

• Place between low-side power device and DC Bus N
• Current sense when low-side is ON and high-side
  is off
• Cant achieve 100 duty cycle, need some OFF time
  to sense current
• Because of power loss, becomes less practical as
  current gets higher

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Shunt Resistor

• Place shunt resistor in motor phase
• Need isolated measurement circuitry
• Able to sense currents at 100 duty cycle

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Hall-effect Current Sensor

• Inherently and isolated sensor
• Usually able to be powered from logic supply
• Less power dissipation, able to sense higher
  currents
• Typically more expensive than shunt measurement
• Available in fixed sensitivity ranges

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Communications

• CAN
• Host Controller
• External Sensors
• DeviceNet
• LIN
• Host Controller
• Automotive
• RS-232
• Host PC
• Display/Keypad

• RS-485
• Multi-drop
• SPI
• Interprocessor
• Absolute Encoder
• EEPROM
• I2C
• EEPROM
• Display/Keypad

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Digital I/O

• Allow drive to interact with the outside world
• Sensors
• Limit Switches
• Relays
• Enable Signal
• Fault Output

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Analog I/O

• To/From the outside world
• Velocity command
• Torque command
• External sensor
• Potentiometer
• LVDT
• Monitor Output (DAC)
• /-10V
• 4-20mA
• Within the drive
• Current sensing
• Voltage sensing
• Temperature sensing

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Pulse Width Modulation (PWM)

• Modulate the duty cycle of a square wave to
  generate an output waveform
• Generate the switching pattern of power
  transistors in a motor drive
• Regulate Current flow
• Generate AC motor voltages

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High Performance DSP

• TMS320C28x Family
• Up to 150MHz or 300MHz
• Internal Flash Memory (Up to 512K)
• Internal RAM (Up to 68K)
• Floating Point Unit (300 MFLOPS)
• Includes peripherals needed for motor control

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High Performance DSP

• ADC 12-bit, 12.5 MSPS
• Current Sensing
• Voltage Sensing
• Resolver
• Analog Inputs
• 300MHz Delfino parts require external ADC

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High Performance DSP

• Enhanced Quadrature Encoder Pulse Module (eQEP)
• Implement incremental encoder feedback
• Use as Pulse/Direction input

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High Performance DSP

• Enhanced PWM Module (ePWM)
• Control switching of the power hardware
• Digital to Analog Conversion (DAC)
• Generate resolver excitation signal

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High Performance DSP

• Communications Peripherals
• SPI
• SCI
• I2C
• CAN
• LIN

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Overview of Field Oriented Control

• Permanent Magnet Synchronous Motor (PMSM)
• Overview of FOC transforms
• TI Digital Motor Control (DMC) Library

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Permanent Magnet Synchronous motor (PMSM)

• Permanent magnet rotor
• Three-phase Y-connected stator
• Sinusoidal phase currents
• Each phase is 120º displaced from the others
• Phase currents must sum to 0

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Background

• Vector Control
• What is a vector?

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Background

• Vector Control
• What is a vector?
• Mathematical representation of physical
  quantities having magnitude and direction
• Velocity
• Acceleration
• Forces

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Field-Oriented Control

• Think of phase currents as vectors
• Overall stator current vector is the vector sum
  of the phase currents

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Field-Oriented Control

• Set up another coordinate axis on the rotor
• q-axis is orthogonal to the Rotors magnetic
  field
• d-axis is parallel to the Rotors magnetic field
• Look at Stator current vector from Rotors frame
  of reference
• Align Stator current vector with Rotors q-axis
• Maximize torque and efficiency

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Physics Problem

• A projectile is launched with initial velocity V0
  at an angle ? with the ground. How far will it
  travel?
• How did we solve this problem?

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Physics Problem

• Resolve the initial velocity vector into two
  components
• Treat the problem as two separate motions

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Field-Oriented Control

• Use measurements of
• Motor currents
• Rotor Angle
• Obtain quadrature components of Stator current
  vector in the Rotors frame of reference.
• Control Isq to desired torque
• Control Isd to 0
• Isq and Isd are non time varying in the Rotors
  frame of reference

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Field-Oriented Control
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Clarke transform

• Transform from three-phase system to a two-phase
  quadrature system
• Simple implementation because
• Align ia phase with a axis
• iaibic0
• Still in the Stators frame of reference
• Still a time-varying system

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Park Transform

• Obtain the quadrature components of the Stator
  current vector in the Rotors frame of reference
• We now have two non time varying signals
• Knowledge of the Rotor angle is key

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Current Loop Regulation

• q and d components are regulated by PI
  compensators
• isqref is torque command
• d component produces no useful torque so isdref
  is regulated to 0
• Outputs of the PI regulators are the quadrature
  components of a voltage vector to be applied to
  the motor
• Voltage vector is in the Rotors frame of
  reference
• Need to transform this voltage vector back into
  three phase quantities in the Stators frame of
  reference

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Inverse Park Transform

• Move from Rotors frame of reference to Stators
  frame of reference
• We have orthogonal components of the voltage
  vector in each frame of reference
• Once again need Rotor angle information

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Space Vector PWM

• Motor connects to a 3-phase voltage source
  inverter
• Constructed of 6 IGBTs or power MOSFETs

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Space Vector PWM

• Think of each transistor as a switch
• Do not allow vertical conduction
• Only eight possible combinations of on and off
  states

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Space Vector PWM

• Eight basic voltage space vectors
• Desired voltage vector will be in one of six
  sectors
• Generate desired vector by applying the two
  adjacent basic space vectors in a time weighted
  manner

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Space Vector PWM

• Need to determine which sector our desired
  voltage vector is in
• Use inverse clarke transform to switch from two
  phase orthogonal system to three phase system
• Look at the sign of each phase to determine
  sector

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Space Vector PWM

• Approximate the reference vector as a time
  weighted combination of adjacent basic vectors
• TPWM period

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Space Vector PWM

• Symmetric PWM switching pattern
• Only one phase switching at a time

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TI Digital Motor Control (DMC) Library

• Contains all of the modules necessary for FOC
• Clarke
• Park
• PID
• IPark
• Space Vector
• More
• Fixed and Floating point options

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Motor Control Hardware/Software Interface

• Information about the system is acquired through
  the ADC
• The system is controlled by the PWMs
• Both information exchanges happen through
  peripherals in the 28x DSPs
• Other feedback is acquired through logical
  interfaces like GPIO, QEP, Capture and Comm.
  peripherals

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ADC Sampling

• For a quality motion control algorithm, accurate
  current information is required
• Noise can be reduced by synching current sampling
  with PWM frequency
• Some phase delay between PWM switching edge and
  ADC sample should be applied to allow for signal
  to settle
• If sampling more than one phase of a motor
  simultaneous Sampling should be used to acquire
  signals at same point in time.
• Proper capacitance on ADC inputs should be used
  to allow for good charge transfer. A good rule is
  200x the ADC capacitance

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ADC Sampling for FOC

• Current can be sampled in leg of switch or inline
  with motor phase
• If sampled in leg of switch a time when all
  Switches are switched to ground must be allowed
• Leg sampling will not allow for 100 duty cycle
  operation
• Depending on worst case slew rate as much as 10
  duty cycle might be lost
• Sampling in line with phase requires either a
  floating reference point or the use of hall or
  other non intrusive current sensors.

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PWM

• Sampling should be synched to PWM frequency
• System torque/current loop should also run at PWM
  frequency and should be able to be
  processed/executed in the same period
• The main control loop should also run at this
  frequency or some even multiple of this frequency
  to keep system synchronous.

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FOC Controls Diagram
Sample Custom Designed Blocks
TI DMC Library Blocks
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IQ Math Library
Near Floating Point Precision with Fixed Point
Performance

• TI provided IQ math Library is just one tool
  available to TI customers.
• Library is available in both Mathworks and as a C
  library.
• TI, its customers and 3rd Parties like D3 have
  worked together to optimize available tools and
  algorithms like the IQ math Library.
• More info available at www.ti.com/iqmath

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Digital Filtering For Feedback

• Observer Tracking filter
• Performance adjusted by changing Alpha and Beta
• Possible application as a resolver angle filter
• Can be related to basic 2nd order Transfer
  function (TF)
• Alpha and Beta can be expressed in terms of a
  Damping Coefficient and a Natural Frequency

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Communications

• CAN
• SCI
• I2C
• SPI
• I/O

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Modular Design With Simulink
Mathworks and TI Tools
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Motor Control Development Kit

• A platform for D3 and our customers to begin
  development of motor control applications
• Include many common features of a motor control
  application
• Allow expansion and flexibility
• A two board design, control board and power board
• Allows mix and match of control and power boards
• Allows control board to be a stand-alone product

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Motor Development Kit

• Control board based on TMS320F2806 DSP
• Isolated from power board and outside world
• 5V input from power board or wall pack
• All peripherals come to headers for expansion

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Motor Development Kit

• Feedback
• Encoder
• Resolver
• Communications
• RS-232
• USB
• CAN
• Digital I/O
• Inputs (4)
• Outputs (3)
• Power Board Interface
• PWM (6)
• Motor Phase Current Sense (3)
• DC Bus Current Sense
• DC Bus Voltage Sense
• Power Board Fault signal
• 5V

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Motor Development Kit

• Power board designed to accept Smart Power
  Modules from 3A to 30A
• DC Bus rectified from 110V or 220V AC
• Voltage Doubler
• Separate control power and DC bus
• Isolated from control board
• Sense three phase currents and DC bus current
  through shunt resistors
• Bootstrap high-side supplies
• DC Bus voltage sense

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Motor Development Kit

• Come see the MDK in action at our booth