Sensorless BLDC Control AN1160B - Microchip …

1 2008-2012 Microchip Technology 1AN1160 INTRODUCTIONThis application note describes a Sensorless BrushlessDirect Current ( bldc ) motor Control algorithm that isimplemented using a dsPIC Digital Signal Controller(DSC) or a PIC24 microcontroller. The algorithm worksutilizing a majority function for digitally filtering theBack-Electromotive Force (BEMF). Each phase of themotor is filtered to determine when to commutate themotor drive voltages. This Control technique excludesthe need for discrete, low-pass filtering hardware andoff-chip comparators.

2 It should be pointed out that allthe discussions here, and the application software,assume a 3-phase motor has to be used. The motorcontrol algorithm described here has four main parts: Sampling trapezoidal BEMF signals using the microcontroller s Analog-to-Digital Converter (ADC) PWM ON-side ADC sampling to reduce noise and solve low-inductance problems Comparing the trapezoidal BEMF signals to VBUS/2 to detect the zero-crossing points Filtering the signals coming from the comparisons using a majority function filter Commutate the motor driving voltages in three different modes.

3 - Classic Open Controller- Classic Closed-Loop Controller- Proportional-Integral (PI) Closed-Loop ControllerThis new Control method is a single-chip 16-bit PIC MCU or dsPIC DSC device-based solution. The onlyexternal hardware required is a few resistors, used toreduce the BEMF signals to the operational voltagerange of the device s ADC Control VERSUS Sensorless CONTROLThe bldc motor is used for both consumer and indus-trial applications due to its compact size, controllabilityand high efficiency.

4 Increasingly, it is also used in auto-motive applications to eliminate belts and hydraulicsystems, to provide additional functionality and toimprove fuel economy, while reducing maintenancecosts to zero. Since the electrical excitation must be synchronous tothe rotor position, the bldc motor is usually operatedwith one or more rotor position sensors. For reasons ofcost, reliability, mechanical packaging and especially ifthe rotor runs immersed in fluid, it is desirable to run themotor without position sensors, which is commonlyknown as Sensorless is possible to determine when to commutate themotor drive voltages by sensing the BEMF voltage onan undriven motor terminal during one of the drivephases.

5 There are some disadvantages to sensorlesscontrol, however: The motor must be moving at a minimum rate to generate sufficient BEMF to be sensed Abrupt changes to the motor load can cause the BEMF drive loop to go out of lockIf low cost is a primary concern, if low-speed motoroperation is not a requirement, and if the motor load isnot expected to change rapidly, Sensorless trapezoidalcontrol may be a better choice for your , there are specific algorithms to overcome allof the above listed disadvantages.

6 The BEMF zero-crossing technique described here isrecommended for several reasons: It is suitable for use on a wide range of motors It can, in theory, be used on both Y and delta-connected 3-phase motors It requires no detailed knowledge of motor parameters It is relatively insensitive to motor manufacturing tolerance variationsAuthor:Adrian Lita and Mihai ChelesMicrochip Technology bldc Control with Back-EMF Filtering Using a Majority FunctionAN1160DS01160B-page 2 2008-2012 Microchip Technology (Trapezoidal) CommutationThe method for energizing the motor windings in thesensorless algorithm, described in this applicationnote, is six-step trapezoidal or 120 1 shows how six-step commutation works.

7 Eachstep, or sector, is equivalent to 60 electrical sectors make up 360 electrical degrees or oneelectrical 1:SIX-STEP COMMUTATIONThe arrows in the winding diagram show the directionin which the current flows through the motor windingsin each of the six steps. The graph shows the potentialapplied at each lead of the motor during each of the sixsteps. Sequencing through these steps moves themotor through one electrical COMMUTATION Step 1- Red winding is driven positive.

8 - Green winding is driven negative. - Blue winding is not driven. Step 2- Red winding remains positive. - Blue winding is driven negative. - Green winding is not driven. Step 3- Green winding is driven positive. - Blue winding is driven negative. - Red winding is not driven. Step 4- Green winding is driven positive. - Red winding is driven negative. - Blue winding is not driven. Step 5- Blue winding is driven positive. - Red winding is driven negative. - Green winding is not driven.

9 Step 6- Blue winding is driven Green winding is driven negative. - Red winding is not every sector, two windings are energized and onewinding is not energized. The fact that one of the wind-ings is not energized during each sector is an importantcharacteristic of six-step Control that allows for the useof a Sensorless Control 123456 Blue WindingGreen WindingRed WindingSector+VDCGND+VDCGNDGND+VDC 2008-2012 Microchip Technology 3AN1160 Generating and Sensing BEMFWhen a bldc motor rotates, each winding generatesBEMF, which opposes the main voltage supplied to thewindings in accordance with Lenz s law.

10 The polarity ofthis BEMF is in the opposite direction of the energizingvoltage. BEMF is mainly dependent on three motorparameters: Number of turns in the stator windings Angular velocity of the rotor Magnetic field generated by rotor magnetsBEMF can be calculated in terms of these parametersand angular velocity using Equation 4:EQUATION 1:BACK-EMF (BEMF)If magnetic saturation of the stator is avoided, or thedependency of the magnetic field on temperature isignored ( , B is constant), the only variable term is therotor s angular speed.