Sensorless Field Oriented Control (FOC) of a Permanent ...

1 2010 Microchip Technology 1AN1078 INTRODUCTIOND esigners can expect environmental demands tocontinue to drive the need for advanced motor controltechniques that produce energy efficient airconditioners, washing machines and other homeappliances. Until now, sophisticated motor controlsolutions have only been available from proprietarysources. However, the implementation of advanced,cost-effective motor Control algorithms is now a reality,thanks to the new generation of Digital SignalControllers (DSCs). An air conditioner, for example, requires fast responsefor speed changes in the motor . Advanced motorcontrol algorithms are needed to produce quieter unitsthat are more energy efficient.

2 Field Oriented Control (FOC) has emerged as the leading method to achievethese environmental demands. This application note discusses the implementation ofa Sensorless FOC algorithm for a Permanent MagnetSynchronous motor (PMSM) using the MicrochipdsPIC DSC Use the FOC Algorithm?The traditional Control method for BLDC motors drivesthe stator in a six-step process, which generatesoscillations on the produced torque. In six-step Control ,a pair of windings is energized until the rotor reachesthe next position, and then the motor is commutated tothe next step. Hall sensors determine the rotor positionto electronically commutate the motor . Advancedsensorless algorithms use the back-EMF generated inthe stator winding to determine the rotor position.

3 The dynamic response of six-step Control (also calledtrapezoidal Control ) is not suitable for washingmachines because the load is changing dynamicallywithin a wash cycle, and varies with different loads andthe selected wash cycle. Further, in a front loadwashing machine, the gravitational power worksagainst the motor load when the load is on the top sideof the drum. Only advanced algorithms such as FOCcan handle these dynamic load changes. This application note focuses on the PMSM-basedsensorless FOC Control of appliances because thiscontrol technique offers the greatest cost benefit inappliance motor Control . The Sensorless FOCtechnique also overcomes restrictions placed on someapplications that cannot deploy position or speedsensors because the motor is flooded, or because ofwire harness placement constraints.

4 With a constantrotor magnetic Field produced by a Permanent magneton the rotor, the PMSM is very efficient when used in anappliance. In addition, its stator magnetic Field isgenerated by sinusoidal distribution of windings. Whencompared to induction motors, a PMSM is powerful forits size. It is also electrically less noisy than a DC motor ,since brushes are not Use Digital Signal Controllers for motor Control ?dsPIC DSCs are suitable for appliances like washingmachines and air conditioner compressors becausethey incorporate peripherals that are ideally suited formotor Control , such as: Pulse-Width Modulation (PWM) Analog-to-Digital Converter (ADC) Quadrature Encoder Interface (QEI)When performing controller routines and implementingdigital filters, dsPIC DSCs enable designers to optimizecode because MAC instructions and fractionaloperations can be executed in a single cycle.

5 Also, foroperations that require saturation capabilities, thedsPIC DSCs help avoid overflows by offering hardwaresaturation dsPIC DSCs need fast and flexibleAnalog-to-Digital (A/D) conversion for currentsensing a crucial function in motor Control . The dsPICDSCs feature ADCs that can convert input samples at1 Msps rates, and handle up to four inputssimultaneously. Multiple trigger options on the ADCsenable use of inexpensive current sense resistors tomeasure winding currents. For example, the ability totrigger A/D conversions with the PWM module allowsinexpensive current sensing circuitry to sense inputs atspecific times (switching transistors allow current toflow through sense resistors).

6 Authors: Jorge Zambada and Debraj DebMicrochip Technology Field Oriented Control of a PMSMAN1078DS01078B-page 2 2010 Microchip Technology Control WITH DIGITAL SIGNAL CONTROLLERSThe dsPIC DSC motor Control family is specificallydesigned to Control the most popular types of motors,including: AC Induction motor (ACIM) Brushed DC motor (BDC) Brushless DC motor (BLDC) Permanent magnet synchronous motor (PMSM)Several application notes have been published basedon the dsPIC DSC motor Control family (see the References section). These application notes areavailable on the Microchip web site( ).This application note demonstrates how the dsPICDSC takes advantage of peripherals specifically suitedfor motor Control ( motor Control PWM and high-speedADC) to execute Sensorless Field Oriented Control of aPMSM.

7 The DSP engine of the dsPIC DSC supportsthe necessary fast mathematical Monitoring and Control Interface The Data Monitor and Control Interface (DMCI) pro-vides quick dynamic integration with MPLAB IDE forprojects in which operational constraints of the applica-tion depend on variable Control of range values, on/offstates or discrete values. If needed, application feed-back can be represented graphically. Examples includemotor Control and audio processing applications. The DMCI provides: Nine slider controls and nine boolean (on/off) controls (see Figure 1) 35 input controls (see Figure 2) Four graphs (see Figure 3)The interface provides project-aware navigation ofprogram symbols (variables) that can be dynamicallyassigned to any combination of slider, direct input orboolean controls.

8 The controls can then be usedinteractively to change values of program variableswithin MPLAB IDE. The graphs can be dynamicallyconfigured for viewing program generated HighlightsThe purpose of this application note is to illustrate asoftware-based implementation of Sensorless , fieldoriented Control for PMSM using Microchip digitalsignal Control software offers these features: Implements vector Control of a PMSM. Position and speed estimation algorithm. eliminates the need for position sensors. Speed range tested from 500 to 17000 RPM. With a 50 s Control loop period, the software requires approximately 21 MIPS of CPU overhead (about 2/3 of the total available CPU).

9 The application requires 450 bytes of data memory storage. With the user interface, approximately 6 Kbytes of program memory are required. The memory requirements of the application allow it to run on the dsPIC33FJ12MC202, which is the smallest and most cost-effective dsPIC33F device at the time of this writing. An optional diagnostics mode can be enabled to allow real-time observation of internal program variables on an oscilloscope. This feature facilitates Control loop :The characteristics of the DMCI tool aresubject to change. This description of theDMCI tool is accurate at the date ofpublication. 2010 Microchip Technology 3AN1078 FIGURE 1:DYNAMIC DATA Control INTERFACEFIGURE 2:USER-DEFINED DATA INPUT CONTROLSAN1078DS01078B-page 4 2010 Microchip Technology 3:GRAPHICAL DATA VIEW 2010 Microchip Technology 5AN1078 SYSTEM OVERVIEWAs shown in Figure 4, there are no position sensorsattached to the motor shaft.

10 Instead, low-inductanceshunt resistors, which are part of the inverter are usedfor current measurements on the motor . A 3-phaseinverter is used as the power stage to drive motorwindings. Current sensing and fault generation circuitrybuilt into the power inverter protects the overall systemagainst over currents. Figure 5 illustrates how the 3-phase topology, as wellas the current detection and fault generation circuitry,are first transistor shown on the left side of the inverteris used for Power Factor Correction (PFC), which is notpart of this application hardware that is referred to in this application noteare the dsPICDEM MCLV Development Board(DM330021) (up to 50 VDC) and the dsPICDEM MCHV Development Board (DM330023) (up to400 VDC), which are available from the Microchip website ( ).