Download Digital Motion Control System Design

Digital Motion Control System
Design - From the Ground Up
Introduction
• Break Motion Control Design into three parts
– Digital Hardware Design
– Power Hardware Design
– Software Design
• Introduce D3 Engineering’s Motor Control
Development Kit
Control Hardware
• Choose Feedback Method
• 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
Feedback
• Incremental or Absolute
• Resolution requirements
• Environmental considerations
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
– Open collector
– Differential Line Driver
– 5V-24V
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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
Incremental Optical Encoder
• Standard products not
typically good for
harsh environments
• No absolute position
data
Resolver
• A rotating transformer
• Input – AC excitation
• Output – Sin and Cos
of rotor angle
modulated at
excitation frequency
Resolver
• Typically considered
rugged, good for
harsh environments
• Absolute within 1
revolution
Resolver
• Requires Resolver to
Digital Converter
(RDC)
– Separate ASIC
– Implement in DSP
• Requires careful
analog design
• Resolution is a
function of RDC
Absolute Encoder
• Serial or Parallel interface
– Typically up to 17-bit single turn resolution
• Absolute over single or multiple revolutions
– 12-bit multi-turn resolution typical
• Available user memory
• Currently popular among commercial industrial
servo drives
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
Digital I/O
• Allow drive to interact with the outside world
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Sensors
Limit Switches
Relays
Enable Signal
Fault Output
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
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
High Performance DSP
• TMS320C28x Family
• Up to 150MHz
• Internal Flash Memory
(Up to 512K)
• Internal RAM (Up to 68K)
• Floating Point Unit (300
MFLOPS)
• Includes peripherals
needed for motor control
High Performance DSP
• ADC – 12-bit, 12.5
MSPS
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Current Sensing
Voltage Sensing
Resolver
Analog Inputs
High Performance DSP
• Enhanced Quadrature
Encoder Pulse
Module (eQEP)
– Implement incremental
encoder feedback
– Use as Pulse/Direction
input
High Performance DSP
• Enhanced PWM
Module (ePWM)
– Control switching of
the power hardware
– Digital to Analog
Conversion (DAC)
• Generate resolver
excitation signal
High Performance DSP
• Communications
Peripherals
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SPI
SCI
I2C
CAN
Power Hardware Design
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DC Bus
Inverter
Control power
High-side supplies
Current Sense
DC Bus
• The DC Bus supplies power to the motor
• Supply can be from a DC source or rectified AC
• An AC source is typically single or three-phase
DC Bus – Single Phase AC Input
• Rectifier
• Inrush current
limiting
• DC Bus capacitors
• Voltage doubler
DC Bus – Rectifier
• Single-phase for up to 1-2KW
• Higher power requires three-phase input and
three-phase rectifier
DC Bus – Inrush Current Limiter
• During a “cold start” DC Bus capacitors initially
look like a short circuit
• Need to limit inrush current to prevent damage
to rectifier and DC Bus capacitors.
DC Bus – Inrush Current Limiter
• Classic approach is to use a resistor in series
with the DC Bus
• Once capacitors are charged resistor is shunted
by a relay
• Resistor doesn’t need to carry full DC Bus
current
DC Bus – Inrush Current Limiter
• Resistor and Relay inrush current limiter is a
common failure point in motor drives
• Relay can’t be used in some hazardous
environments
DC Bus Inrush Current Limiter
• Alternative – Negative Temperature Coefficient
Thermistor (NTC)
• Starts out at high resistance when cold, resistance
decreases to a few milliohms as current flows and device
heats up
• No need for shunt relay
• Limited range of continuous current ratings
• May not work when ambient temperature requirements
are high
DC Bus – Inrush Current Limiter
• Replace relay with a solid state device
• OK for hazardous environments
• Requires more hardware to turn the device ON
DC Bus – Inrush Current Limiter
• Need to extensively test whatever method you
choose
– At max ambient temperature
– At max load
– Power cycle testing
DC Bus – Voltage Doubler
• Ability to obtain 300V DC
Bus from 110VAC source
• Each capacitor charges
separately on opposing
half cycles of the AC
input
• Rectified DC Bus is equal
to 2 times the peak AC
input
• Output power must stay
the same so max
continuous current is cut
in half
Inverter
• A three-phase bridge
made of IGBTs or
MOSFETs that switch
power to the motor
• Usually implemented
as 6 discrete devices
or 1 Intelligent Power
Module
Inverter - IPM
• Intelligent Power Modules are typically designed
to directly interface to a DSP or microcontroller
• Integrated high and low-side gate drive
• Integrated UVLO
• Integrated Over-current/Short-circuit protection
• Limited packaging options
• Limited current/voltage ratings
Inverter – Discrete Implementation
• More packaging flexability
• Greater variety in voltage/current ratings
• Need to design external gate drive, UVLO, and
over-current detection
Control Power Supply
• Minimum of two supplies
– Gate Drive supply
– Logic supply
• Regulated from DC Bus or separate control
power input
• Isolated or Non-isolated
Non-isolated Buck Converter
• Usually used in lowcost designs
• Regulate control
supplies directly from
DC Bus
• Digital supply
regulated from Gate
Drive supply with LDO
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Isolated
Flyback
Converter
Powered from DC bus
or separate control
power input
• Generate multiple
voltages
High-Side Supplies
• Why do we need separate high-side supplies?
• Boot-strap supplies
• Separate floating supplies
Why High-Side Supplies
• IGBT needs
VGE >
VGEsat to turn
completely on
• MOSFET needs VGS >
VGSsat to turn
completely on
Why High-Side Supplies
• Emitter (or Source) of
High-Side device “floats”
with motor phase
Bootstrap Supplies
• High-Side Gate Drive
powered by bootstrap
capacitor
• Capacitor charged
through diode when
low-side device is ON
Bootstrap Supplies
• Can’t run at 100%
PWM duty cycle
indefinitely
• Need some low-side
ON-time to charge
bootstrap capacitor
• Inexpensive
Bootstrap Supplies
• Some considerations
for sizing bootstrap
components
– Minimum Vboot
voltage
– Gate driver quiescent
current
– IGBT Gate charge
– High-side On-time
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Separate
Floating
Supplies
Add three additional
windings to flyback
transformer
• No more limitations on
duty cycle
• Bigger transformer
• More expensive
Current Sense
• 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
Shunt Resistor
• Place between low-side
power device and DC
Bus N
– Current sense when lowside 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
Shunt Resistor
• Place shunt resistor in motor phase
– Need isolated measurement circuitry
– Able to sense currents at 100% duty cycle
Hall-effect Current Sensor
DC Bus P
Hall Effect
Current
Sensor
U
V
W
DC Bus N
Hall Effect
Current
Sensor
Motor
• 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
Motor Control Hardware/Software Interface
d p (t )
r (nT )
+
+
eˆ(nT )
D(z)
u (nT )
D/A
yˆ (nT )
A/D
Sensor
d s (t )
u (t )
G(z)
y (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
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.
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.
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
Voltage
Phase Voltage
Reconstruction
AD
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
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
Communications
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CAN
SCI
I2C
SPI
I/O
Modular Design With Simulink®
Mathworks and TI Tools
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
Motor Development Kit
• Contol 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
Motor Development Kit
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Feedback
– Encoder
– Resolver
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Communications
– RS-232
– USB
– CAN
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Digital I/O
– Inputs (4)
– Outputs (3)
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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
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
Motor Development Kit
• Come see the MDK in action at our booth