Digital Motion Control System Design - From the Ground Up

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 • I2C • 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 • I2C • 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.