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Digital Motion Control System Design - From the Ground Up Part 2

Digital Motion Control System Design - From the Ground Up Part 2. Introduction. Hardware Design Options High level overview of Field Oriented Control (FOC) Software Implementation Introduce D3 Engineering’s Motor Control Development Kit. Hardware Design Options. Choose Feedback Method

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Digital Motion Control System Design - From the Ground Up Part 2

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  1. Digital Motion Control System Design - From the Ground UpPart 2 D3 Engineering

  2. Introduction • Hardware Design Options • High level overview of Field Oriented Control (FOC) • Software Implementation • Introduce D3 Engineering’s Motor Control Development Kit D3 Engineering

  3. 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 • Digital I/O • Analog I/O • Pulse Width Modulation (PWM) • Putting it all together D3 Engineering

  4. Rotary Feedback Choices • Incremental or Absolute • Resolution requirements • Environmental considerations • Sensor must be aligned (zeroed) to Rotor and Stator for FOC commutation • Mechanically • Software offsets D3 Engineering

  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 D3 Engineering

  6. 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 D3 Engineering

  7. Resolver • A rotating transformer • Input – AC excitation • Output – Sin and Cos of rotor angle modulated at excitation frequency D3 Engineering

  8. Resolver • Typically considered rugged, good for harsh environments • Absolute within 1 revolution D3 Engineering

  9. Resolver • Requires Resolver to Digital Converter (RDC) • Separate ASIC • Implement in DSP • Requires careful analog design • Resolution is a function of RDC D3 Engineering

  10. 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 D3 Engineering

  11. 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 D3 Engineering

  12. Shunt Resistor • Place shunt resistor in motor phase • Need isolated measurement circuitry • Able to sense currents at 100% duty cycle D3 Engineering

  13. 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 D3 Engineering

  14. 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/Keypad Communications D3 Engineering

  15. Digital I/O • Allow drive to interact with the outside world • Sensors • Limit Switches • Relays • Enable Signal • Fault Output D3 Engineering

  16. 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 D3 Engineering

  17. 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 D3 Engineering

  18. 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 D3 Engineering

  19. High Performance DSP • ADC – 12-bit, 12.5 MSPS • Current Sensing • Voltage Sensing • Resolver • Analog Inputs • 300MHz Delfino parts require external ADC D3 Engineering

  20. High Performance DSP • Enhanced Quadrature Encoder Pulse Module (eQEP) • Implement incremental encoder feedback • Use as Pulse/Direction input D3 Engineering

  21. High Performance DSP • Enhanced PWM Module (ePWM) • Control switching of the power hardware • Digital to Analog Conversion (DAC) • Generate resolver excitation signal D3 Engineering

  22. High Performance DSP • Communications Peripherals • SPI • SCI • I2C • CAN • LIN D3 Engineering

  23. Overview of Field Oriented Control • Permanent Magnet Synchronous Motor (PMSM) • Overview of FOC transforms • TI Digital Motor Control (DMC) Library D3 Engineering

  24. 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 D3 Engineering

  25. Background • Vector Control • What is a vector? D3 Engineering

  26. Background • Vector Control • What is a vector? • Mathematical representation of physical quantities having magnitude and direction • Velocity • Acceleration • Forces D3 Engineering

  27. Field-Oriented Control • Think of phase currents as vectors • Overall stator current vector is the vector sum of the phase currents D3 Engineering

  28. 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 q-axis • Maximize torque and efficiency D3 Engineering

  29. 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? D3 Engineering

  30. Physics Problem • Resolve the initial velocity vector into two components • Treat the problem as two separate motions D3 Engineering

  31. 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 D3 Engineering

  32. Field-Oriented Control D3 Engineering

  33. 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 D3 Engineering

  34. 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 D3 Engineering

  35. Current Loop Regulation • 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 D3 Engineering

  36. 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 D3 Engineering

  37. Space Vector PWM • Motor connects to a 3-phase voltage source inverter • Constructed of 6 IGBTs or power MOSFETs D3 Engineering

  38. Space Vector PWM • Think of each transistor as a switch • Do not allow vertical conduction • Only eight possible combinations of on and off states D3 Engineering

  39. 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 D3 Engineering

  40. 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 D3 Engineering

  41. Space Vector PWM • Approximate the reference vector as a time weighted combination of adjacent basic vectors • T=PWM period D3 Engineering

  42. Space Vector PWM • Symmetric PWM switching pattern • Only one phase switching at a time D3 Engineering

  43. TI Digital Motor Control (DMC) Library • Contains all of the modules necessary for FOC • Clarke • Park • PID • IPark • Space Vector • More • Fixed and Floating point options D3 Engineering

  44. 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 D3 Engineering

  45. 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 D3 Engineering

  46. 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. D3 Engineering

  47. 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. D3 Engineering

  48. FOC Controls Diagram Sample Custom Designed Blocks TI DMC Library Blocks D3 Engineering

  49. 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 D3 Engineering

  50. 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 D3 Engineering

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