Optimum Vector Control for Brushless Motors

Optimum Vector Control for Brushless Motors Hardware and Software Design for Highest Performance and Lowest Whole-Life Cost Field Oriented Control, or Vector Control, is preferred in systems rotor, several commutation tech- niques are applicable to adjust the using brushless motors; a number of microcontroller vendors offer current in each phase to produce a FOC software as an aid to motor-control development. A new gen- net stator field in quadrature with eration of MCUs that incorporate hardware-based FOC processing U now simplifies design challenges as well as achieving higher performance at lower operating frequencies. ° 1 0 2 2 0 1 ° Torque control for a brushless mo- > Brushless Motor Control tor seeks to maximise torque by adjusting the current in the stator Brushless DC motors offer several windings to produce a net mag- advantages over traditional netic field that is orthogonal – or in Brushless W DC motor brushed AC and DC motors, in- quadrature - to the rotor field. Any V cluding lower materials costs, component of the stator field act- greater reliability, and longer life- ing parallel to the rotor’s field will time. However, since brushless produce a force that has no turn- 120° motors do not self commutate, ing effect. This direct component torque control, which is fundamen- wastes energy and places addi- Figure1: Typical prinzip of a 3-phase tal to successful operation of any tional stress on the rotor bearings. BLDC motor; each phase is servo system, presents a more While maximising the quadrature positioned on a 120° interval arround complex challenge. Several strate- component, torque control aims to the axis. gies have evolved for controlling minimise or, ideally, eliminate the torque in brushless motors, which direct component to ensure opti- the rotor field. Common to each perform commutation on the mo- mal efficiency and reliability. method of commutation, the motor tor’s behalf as well as calculating For controlling three-phase brush- current is sensed and compared the optimal current for each stator less motors, having three stator with the desired torque, and a pro- to produce the maximum torque. phases positioned at 120-degree portional-integral (PI) function then intervals around the axis of the acts on the resulting error signal to generate a correction. This correction signal is subsequently pulse- width modulated and used to control the output bridge of the motor driver. In trapezoidal motor control, also known as 6-phase motor control, the stator currents have equal magnitude in the two phase pairs either side of the rotor, while the third stator is disconnected from the power source. Rotor-position data from three Hall sensors lo- cated in between each pair of stator phases determines which phase is to be disconnected. As the rotor turns the current in each phase is switched between the maximum positive value, zero, and the maximum negative value. The resulting trapezoidal current ap- proximates to a sinusoidal wave- Figure2: ARM® CortexTM-M3 microcontroller with integrated hardware based form. Although the average stator vector engine and analog circuit. field in any period is in quadrature - 4 - TMPM370FYFG with respect to the rotor field, the instantaneous net stator field can Field Oriented Control (FOC): lead or lag by up to 30 degrees. At low rotor speeds this results in Mathematical technique utilized for achieving a separate control imprecise control, as well as high (decouple) of the field producing and the torque producing portions of levels of audible noise. the currents in a three-phase machine. In this scheme Stator current IS is decomposed into: Q-axis > Sinewave Control > Magnetizing current Id, or ) Sinusoidal control produces at (I s producing a magnetic field St nt re smoother torque by applying sinu- ur soidal current waveforms to the C > Quadrature current I , which Torque (I ) stator windings. The currents are q q controls torque. mutually phase shifted by 120 degrees, so that the vector sum of Flux (Id) D-axis the stator field is orthogonal to the rotor field. Compared to trapezoi- cient operation. generate signals in the D-Q refer- dal control, more accurate rotor- ence plane. These must then be position information is required to transformed into the stator domain generate the sinusoidal current > Field Oriented Control to generate the PWM signal for waveforms. This may be achieved each stator phase. Figure 4 illus- Field Oriented Control (FOC), also trates the functional blocks of a Voltage known as Vector Control, over- generic FOC function. comes the poor low-speed accu- racy of trapezoidal control as well Because the inputs to the PI func- as the high-speed inefficiency of tions are constant, FOC maintains Current sinusoidal control. By manipulating high efficiency at all rotor speeds the motor currents and voltages regardless of any limitations on PI- with reference to the rotor’s direct controller bandwidth. However, to and quadrature axes, FOC main- perform FOC in real time requires Figure3: PWM motor supply voltage tains a constant stator field in and sinusoidal winding current. fast execution of the functions that quadrature with the rotor field irre- first transform the sensed stator using an angular encoder or, alter- spective of any bandwidth limita- current signals into the rotor do- natively, using sensorless position tions of the PI controllers. main and subsequently transform detection based on analysis of the static PI values into the volt- instantaneous motor current. How- In FOC, the sensed stator currents age-control signals for the output ever, accurate torque control is are translated into rotor direct (D) bridge. Software-based FOC dependent on rapid computation of and quadrature (Q) components places maximum demands on the required current value as soon by a transform function. To CPU performance and operating as the rotor position is sensed. At achieve maximum torque, the D frequency, to complete the loop high rotor speeds the limited band- and Q currents are then compared within an acceptable time period in width of the PI function results in respectively with zero and the relation to rotor speed. Other fac- an increasing lag between the cal- torque requested by the applica- tors such as integration challenges culated stator current and the ac- tion. The resulting error signals are and any licensing issues must also tual rotor position, leading to ineffi- input to the two PI blocks, which be borne in mind when developing a motor controller using software- based FOC. > Hardware-Based FOC Performing time-critical FOC com- putations in hardware can in- crease the speed of the control loop, as well as reducing operating frequency and freeing valuable processor cycles to be used for application-level functions. Figure 5 illustrates a re-partitioned FOC function taking advantage of the hardware-based vector-control engine embedded in the Toshiba Figure4: Functional block of a generic FOC with sensorless back EMF detection. - 5 - TMPM370 and TMPM372 MCU for brushless-motor control. In this Oscillation Frequency Detector (OFD): scheme, all FOC processing tasks that are fixed and independent of TMPM370 group is equipped with an Oscillation Frequency Detector the application are performed in (OFD) which supports IEC60730 class B regulation for Conformity hardware. To perform these func- Testing to Standards for Safety of Electrical Equipment. The oscilla- tions the MCU’s embedded vector tion frequency detector engine implements functions in- function is a hardware to 32kHz Detection reset cluding decoding, a scheduler for monitor CPU clock. This L-freq OFDOUTn event and priority control, and cal- function automatically de- OSC OFD culation resources including a mul- tects abnormal clock op- CPU tiply-accumulate (MAC) block for eration without a complex 80MHz computationally intensive opera- software and secures save H-freq tions. Two vector-control units im- operation. OSC plement the PI controllers and as- sociated functions. budget and EMI. By complying grated analogue IP fulfils specific By offloading the complex and with the one-MCU-for-one-motor requirements for FOC such as 2x time-critical processing to the vec- convention, designers can use the 11-channel 12-bit ADCs for fast tor engine, the TMPM370 restricts TMPM372 operating at 40MHz to current sensing and shutdown ca- the software component of FOC to take advantage of cost and size pability. An ADC-timing network application-dependent tasks such savings without affecting perform- including op-amp and comparator as ω calculation and speed con- ance. functions is also integrated, which trol. These are performed in the enables precise measurement device’s 32-bit ARM Cortex™-M3 Both M372 and M370 are suitable over the full positive and negative core. With these processing re- for high-end motor control applica- current range of the motor without sources, the TMPM370 is able to tions including next generation of requiring an external op-amp to complete the control loop within appliances, pumps, industrial ma- perform level shifting. each PWM period, resulting in bet- chinery, compressors, and HVAC ter control stability for PWM fre- (heating ventilation air- quencies up to 100kHz. Even conditioning) systems. Permanent > Conclusion when operating at 40MHz (max. magnet brushless AC/DC, stepper 80MHz), this MCU is not only ca- and 3-phase AC induction motors FOC/VC overcomes the low-speed pable of controlling two brushless are all suitable for both devices. imprecision of trapezoidal motor control, as well as the high-speed Vdd inefficiency experienced with con- Motor 1 TMPM370 ventional sinusoidal control. in ad- dition to reducing energy con- About x 1.5 ~10 sumption, FOC delivers advantages such as lower audible noise, ADC reduced wear, constant torque Unit A over the complete speed range including zero-speed operation, SW SW and good velocity control under CMP varying load conditions.

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