Download Digital Motion Control System Design

Digital Motion Control System
Design - From the Ground Up
Part 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
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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
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Digital I/O
Analog I/O
Pulse Width Modulation (PWM)
Putting it all together
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Rotary Feedback Choices
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Incremental or Absolute
Resolution requirements
Environmental considerations
Sensor must be aligned (zeroed) to Rotor and
Stator for FOC commutation
– Mechanically
– Software offsets
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Incremental Optical Encoder
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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
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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
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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
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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
– Can’t 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
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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
DC Bus P
Hall Effect
Current
Sensor
U
V
W
DC Bus N
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Hall Effect
Current
Sensor
Motor
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
• I 2C
– EEPROM
– Display/Keypad
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Digital I/O
• Allow drive to interact with the outside world
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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
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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
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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
ia 0, ib120, ic240
   
is  ia  ib  ic
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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 qaxis
• Maximize torque and efficiency
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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?
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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 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
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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 α axis
– ia+ib+ic=0
Still in the Stator’s 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 Rotor’s 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
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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
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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
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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
• T=PWM 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
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Clarke
Park
PID
IPark
Space Vector
More
• Fixed and Floating point options
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Motor Control Hardware/Software
Interface
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d p (t )
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
r (nT )
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eˆ(nT )
D(z)
u (nT )
D/A
yˆ (nT )
A/D
Sensor
d s (t )
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u (t )
G(z)
y (t )
ADC Sampling
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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
ADC Sampling for FOC
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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
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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
Control Logic
(State Table)
Profile
Generator
Direct
Current PID
Vd
Velocity
PID
Vq
Inverse
Park
Transform
3 Phase
BLDC
Motor
Space
Vector PWM
Generator
Quadrature
Current PID
AD
Voltage
Supervisory
TI DMC Library Blocks
Velocity
Vd
Park
Transform
Vq
Id
Iq
Current
Phase A
Clark
Transform
Current
Phase B
Rotor Position
PWM
Velocity
Calculator
from
Estimated
Position
Rotor
Position
Estimator
Vds
Vqs
Phase Voltage
Reconstruction
AD
Voltage
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Motor Bus
Voltage
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
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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
A
Alpha
1
Input
B
1
Derivative of Output
Beta
Unit Delay1
1
z
Unit Delay
1
z
2
Output
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Communications
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CAN
SCI
I2C
SPI
I/O
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
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Feedback
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Communications
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RS-232
USB
CAN
Digital I/O
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Encoder
Resolver
Inputs (4)
Outputs (3)
Power Board Interface
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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
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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
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