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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

D3 Engineering

Introduction

- Hardware Design Options
- High level overview of Field Oriented Control (FOC)
- Software Implementation
- Introduce D3 Engineering’s Motor Control Development Kit

D3 Engineering

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

D3 Engineering

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

D3 Engineering

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 (>100K counts per rev)
- Easy to interface, no analog hardware

D3 Engineering

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

D3 Engineering

Resolver

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

D3 Engineering

Resolver

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

D3 Engineering

Resolver

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

D3 Engineering

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

D3 Engineering

Shunt Resistor

- Place between low-side power device and DC Bus N
- Current sense when low-side is ON and high-side is off
- Can’t achieve 100% duty cycle, need some OFF time to sense current
- Because of power loss, becomes less practical as current gets higher

D3 Engineering

Shunt Resistor

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

D3 Engineering

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

D3 Engineering

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

CommunicationsD3 Engineering

Digital I/O

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

D3 Engineering

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

D3 Engineering

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

D3 Engineering

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

D3 Engineering

High Performance DSP

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

D3 Engineering

High Performance DSP

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

D3 Engineering

High Performance DSP

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

D3 Engineering

Overview of Field Oriented Control

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

D3 Engineering

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

D3 Engineering

Background

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

D3 Engineering

Field-Oriented Control

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

D3 Engineering

Field-Oriented Control

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

D3 Engineering

Physics Problem

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

D3 Engineering

Physics Problem

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

D3 Engineering

Field-Oriented Control

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

D3 Engineering

Field-Oriented Control

D3 Engineering

Clarke transform

- Transform from three-phase system to a two-phase quadrature system
- Simple implementation because
- Align ia phase with α axis
- ia+ib+ic=0
- Still in the Stator’s frame of reference
- Still a time-varying system

D3 Engineering

Park Transform

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

D3 Engineering

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 Rotor’s frame of reference
- Need to transform this voltage vector back into three phase quantities in the Stator’s frame of reference

D3 Engineering

Inverse Park Transform

- Move from Rotor’s frame of reference to Stator’s frame of reference
- We have orthogonal components of the voltage vector in each frame of reference
- Once again need Rotor angle information

D3 Engineering

Space Vector PWM

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

D3 Engineering

Space Vector PWM

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

D3 Engineering

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

D3 Engineering

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

D3 Engineering

Space Vector PWM

- Approximate the reference vector as a time weighted combination of adjacent basic vectors
- T=PWM period

D3 Engineering

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

D3 Engineering

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

D3 Engineering

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

D3 Engineering

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.

D3 Engineering

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.

D3 Engineering

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

D3 Engineering

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

D3 Engineering

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

D3 Engineering

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

D3 Engineering

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

D3 Engineering

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

D3 Engineering

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