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Digital Motion Control System Design - From the Ground Up.

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Presentation on theme: "Digital Motion Control System Design - From the Ground Up."— Presentation transcript:

1 Digital Motion Control System Design - From the Ground Up

2 Introduction Break Motion Control Design into three parts –Digital Hardware Design –Power Hardware Design –Software Design Introduce D3 Engineering’s Motor Control Development Kit

3 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

4 Feedback Incremental or Absolute Resolution requirements Environmental considerations

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 (>100K 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

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

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

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

10 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

11 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 2 C –EEPROM –Display

12 Digital I/O Allow drive to interact with the outside world –Sensors –Limit Switches –Relays –Enable Signal –Fault Output

13 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

14 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

15 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

16 High Performance DSP ADC – 12-bit, 12.5 MSPS –Current Sensing –Voltage Sensing –Resolver –Analog Inputs

17 High Performance DSP Enhanced Quadrature Encoder Pulse Module (eQEP) –Implement incremental encoder feedback –Use as Pulse/Direction input

18 High Performance DSP Enhanced PWM Module (ePWM) –Control switching of the power hardware –Digital to Analog Conversion (DAC) Generate resolver excitation signal

19 High Performance DSP Communications Peripherals –SPI –SCI –I 2 C –CAN

20 Power Hardware Design DC Bus Inverter Control power High-side supplies Current Sense

21 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

22 DC Bus – Single Phase AC Input Rectifier Inrush current limiting DC Bus capacitors Voltage doubler

23 DC Bus – Rectifier Single-phase for up to 1-2KW Higher power requires three-phase input and three-phase rectifier

24 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.

25 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

26 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

27 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

28 DC Bus – Inrush Current Limiter Replace relay with a solid state device OK for hazardous environments Requires more hardware to turn the device ON

29 DC Bus – Inrush Current Limiter Need to extensively test whatever method you choose –At max ambient temperature –At max load –Power cycle testing

30 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

31 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

32 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

33 Inverter – Discrete Implementation More packaging flexability Greater variety in voltage/current ratings Need to design external gate drive, UVLO, and over-current detection

34 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

35 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

36 Isolated Flyback Converter Powered from DC bus or separate control power input Generate multiple voltages

37 High-Side Supplies Why do we need separate high-side supplies? Boot-strap supplies Separate floating supplies

38 Why High-Side Supplies IGBT needs VGE > VGEsat to turn completely on MOSFET needs VGS > VGSsat to turn completely on

39 Why High-Side Supplies Emitter (or Source) of High-Side device “floats” with motor phase

40 Bootstrap Supplies High-Side Gate Drive powered by bootstrap capacitor Capacitor charged through diode when low-side device is ON

41 Bootstrap Supplies Can’t run at 100% PWM duty cycle indefinitely Need some low-side ON-time to charge bootstrap capacitor Inexpensive

42 Bootstrap Supplies Some considerations for sizing bootstrap components –Minimum Vboot voltage –Gate driver quiescent current –IGBT Gate charge –High-side On-time

43 Separate Floating Supplies Add three additional windings to flyback transformer No more limitations on duty cycle Bigger transformer More expensive

44 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

45 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

46 Shunt Resistor Place shunt resistor in motor phase –Need isolated measurement circuitry –Able to sense currents at 100% duty cycle

47 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

48 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

49 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

50 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.

51 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.

52 FOC Controls Diagram Sample Custom Designed BlocksTI DMC Library Blocks

53 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 3 rd Parties like D3 have worked together to optimize available tools and algorithms like the IQ math Library. More info available at

54 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

55 Communications CAN SCI I2C SPI I/O

56 Modular Design With Simulink® Mathworks and TI Tools

57 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

58 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

59 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

60 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

61 Motor Development Kit Come see the MDK in action at our booth

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