Digital Motion Control System Design - From the Ground Up

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

• 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

• 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

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

• I2C– EEPROM– Display

Digital I/O

• Allow drive to interact with the outside world– 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– 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– SPI– SCI– I2C– CAN

Power Hardware Design

• 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 low-

cost designs• Regulate control

supplies directly from DC Bus

• Digital supply regulated from Gate Drive supply with LDO

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

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 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

Shunt Resistor• Place shunt resistor in motor phase

– Need isolated measurement circuitry– Able to sense currents at 100% duty cycle

Hall-effect Current Sensor

Hall EffectCurrentSensor

DC Bus P

DC Bus N

U

WV Motor

Hall EffectCurrentSensor

• 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(z) D/A G(z)

A/D Sensor

++

-

)(ˆ nTe)(nTr

)(ˆ nTy

)(nTu )(tu)(ty

)(td p

)(tds

• 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• 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• 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 DiagramSample Custom Designed Blocks TI DMC Library Blocks

3 Phase BLDC Motor

AD

Clark Transform

Park Transform

Quadrature Current PID

DirectCurrent PID

VelocityPID

InversePark

Transform

Space Vector PWMGenerator

Current Phase A

CurrentPhase B

Rotor Position

Estimator

Id

IqVq

Vd

Vq

Vd

VelocityCalculator

from Estimated Position

ProfileGenerator

Control Logic(State Table)

VoltageSupervisory

ADMotor Bus

VoltageVoltage

Phase Voltage Reconstruction

Vds

Vqs

PWM

Velocity

Rotor Position

IQ Math LibraryNear 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• 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

2

Output

1

Derivative of Output

z

1

Unit Delay1

z

1

Unit Delay

B

Beta

A

Alpha

1

Input

Communications

• 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• 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

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