Hardware and Software Design for Highest Performance and Lowest Whole-Life Cost

Field Oriented Control, or Vector Control, is preferred in systems using brushless motors; a number of microcontroller vendors offer FOC software as an aid to motor-control development. A new generation of MCUs that incorporate hardware-based FOC processing now simplifies design challenges as well as achieving higher performance at lower operating frequencies.

Introduction: Brushless Motor Control

Brushless DC motors offer several advantages over traditional brushed AC and DC motors, including lower materials costs, greater reliability, and longer lifetime. However, since brushless motors do not self commutate, torque control, which is fundamental to successful operation of any servo system, presents a more complex challenge. Several strategies have evolved for controlling torque in brushless motors, which perform commutation on the motor’s behalf as well as calculating the optimal current for each stator to produce the maximum torque.

Torque control for a brushless motor seeks to maximise torque by adjusting the current in the stator windings to produce a net magnetic field that is orthogonal – or in quadrature - to the rotor field. Any component of the stator field acting parallel to the rotor’s field will produce a force that has no turning effect. This direct component wastes energy and places additional stress on the rotor bearings. While maximising the quadrature component, torque control aims to minimise or, ideally, eliminate the direct component to ensure optimal efficiency and reliability.

For controlling three-phase brushless motors, having three stator phases positioned at 120-degree intervals around the axis of the rotor, several commutation techniques are applicable to adjust the current in each phase to produce a net stator field in quadrature with the rotor field. Common to each method of commutation, the motor current is sensed and compared with the desired torque, and a proportional-integral (PI) function then 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 phases either side of the rotor, while the third stator is disconnected from the power source. Rotor-position data from three Hall sensors located 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 approximates to a sinusoidal waveform. Although the average stator field in any period is in quadrature with respect to the rotor field, the instantaneous net stator field can lead or lag by up to 30 degrees. At low rotor speeds this results in imprecise control, as well as high levels of audible noise.

Sinusoidal control produces smoother torque by applying sinusoidal current waveforms to the stator windings. The currents are mutually phase shifted by 120 degrees, so that the vector sum of the stator field is orthogonal to the rotor field. Compared to trapezoidal control, more accurate rotor-position information is required to generate the sinusoidal current waveforms. This may be achieved using an angular encoder or, alternatively, using sensorless position detection based on analysis of instantaneous motor current. However, accurate torque control is dependent on rapid computation of the required current value as soon as the rotor position is sensed. At high rotor speeds the limited bandwidth of the PI function results in an increasing lag between the calculated stator current and the actual rotor position, leading to inefficient operation.

Field Oriented Control

Hardware-Based FOC

Performing time-critical FOC computations in hardware can increase 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 2 illustrates a re-partitioned FOC function taking advantage of the hardware-based vector-control engine embedded in the Toshiba TMPM370 and TMPM372 MCU for brushless-motor control. In this scheme, all FOC processing tasks that are fixed and independent of the application are performed in hardware. To perform these functions the MCU’s embedded vector engine implements functions including decoding, a scheduler for event and priority control, and calculation resources including a multiply-accumulate (MAC) block for computationally intensive operations. Two vector-control units implement the PI controllers and associated functions.

By offloading the complex and time-critical processing to the vector engine, the TMPM370 restricts the software component of FOC to application-dependent tasks such as ? calculation and speed control. These are performed in the device’s 32-bit ARM Cortex™-M3 core. With these processing resources, the TMPM370 is able to complete the control loop within each PWM period, resulting in better control stability for PWM frequencies up to 100kHz. When operated at 40MHz, this MCU is not only capable of controlling two brushless motors simultaneously, but also has been shown to outperform software-based vector control using a conventional MCU operated at 80MHz, thereby reducing challenges associated with thermal management, system power budget and EMI. By complying with the one-MCU-for-one-motor convention, designers can use the TMPM372 operating at 40MHz to take advantage of cost and size savings without affecting performance.

Both M372 and M370 are suitable for high-end motor control applications including next generation of appliances, pumps, industrial machinery, compressors, and HVAC (heating ventilation air-conditioning) systems. Permanent magnet brushless AC/DC, stepper and 3-phase AC induction motors are all suitable for both devices. Additionally, the TMPM370 and M372 both feature an Oscillation Frequency Detector (OFD), which enables them to meet the IEC60730 class B safety standard.

As well as the vector engine, integrated analogue IP fulfils specific requirements for FOC such as 2x 11-channel 12-bit ADCs for fast current sensing and shutdown capability. An ADC-timing network including op-amp and comparator functions is also integrated, which enables precise measurement over the full positive and negative current range of the motor without requiring an external op-amp to perform level shifting.

Conclusion

FOC/VC overcomes the low-speed imprecision of trapezoidal motor control, as well as the high-speed inefficiency experienced with conventional sinusoidal control. in addition to reducing energy consumption, FOC delivers advantages such as lower audible noise, reduced wear, constant torque over the complete speed range including zero-speed operation, and good velocity control under varying load conditions.

Hardware-based execution of the computationally intensive FOC calculations, as well as built-in analogue IP optimised for motor control, avoids the complications and performance limitations experienced when implementing FOC in software.

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