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Sensorless FOC for cordless power tools

Sensorless FOC for cordless power tools

Shawn_Wu
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Sensorless FOC for cordless power tools

Sensorless FOC used to be considered too complex to implement and the performance was not good enough for power tool applications e.g. power drill, power wrench. It is now possible with high performance motor controllers and advanced motor control algorithms.

The trend of power tool

As many batteries become to have higher power density, they can last longer. And they are lighter. Many works can be done without recharging to the battery. The power tools don’t need to be plugged into the power source and can work with their built-in batteries. Without the restriction of the wire and the power source, mobility is an obvious advantage of the battery powered or cordless power tools.

Meanwhile, compared to brushed DC motors, BLDC/PMSM motors become more and more popular. With proper control technique, BLDC/PMSM motors are more efficient. They have high power to size ratio. They are lighter and more compact. There is no brushes and commutators in the motor, hence have longer life span and lower noise and EMI. And regular maintenance is not necessary. 

Trapezoidal control method is a simple one that is used to drive the BLDC motor. However, it cannot keep the rotor flux and the stator flux at 90 degree during every commutation sector. And in order to have maximum torque, 90 degree should be the perfect angle between the rotor flux and the stator flux. FOC methodology can maintain the angle at 90 degree and thus result in better efficiency and smoother torque.

Torque with a wrench.pngFigure 1. Torque with a wrench

FOC methodology

FOC is a method that generates a three-phase sinusoidal modulation, which can easily be controlled in frequency and amplitude to minimize the currents and therefore maximize the efficiency. The essential idea is the transformation of three-phase sampled current signals into two rotor-fixed signals. In the rotor-fixed reference frame, the currents can be treated as stationary values and are relatively easy to control.

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Figure 2. Stationary and rotating reference frames

The transformation from the three phase system to the two phase system is called the Clarke transform, whereas the one from stationary to rotating two-phase system is called the Park transform.

Feedback on rotor position and rotor speed is required in FOC motor control. The feedback can come from a rotor position estimator algorithm or from rotor position sensors. When the feedback comes from a rotor position estimator algorithm, it is known as sensorless FOC motor control.

PMSM FOC block diagram.png

 Figure 3. PMSM FOC block diagram

Thanks to the powerful microcontrollers and advanced algorithms, sensorless FOC can be used for many applications. For example, power tools, gardening tools, and home appliances.

Infineon’s MOTIX™ IMD700A

Infineon’s MOTIX™ IMD700A is a controller specifically designed for three-phase BLDC or PMSM drive applications. MOTIX™ IMD700A integrates a fully programmable XMC1404 Arm® Cortex®-M0 microcontroller from Infineon’s XMC1400 family with MOTIX™ 6EDL7141, a 70 V three phase smart gate driver with integrated power supply. It can be used for sensored and sensorless motor control.

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Figure 4. Block diagram of Infineon’s MOTIX™ IMD700A/IMD701A

The XMC1404 microcontroller is ideal for PMSM FOC motor control systems. It has dedicated motor control peripherals POSIF, MATH, CCU8, ADC and CCU4.

6EDL7141 is a flexible three-phase gate driver. It has complete power supply as required in the system, three current sense (CS) amplifiers, protections and a remarkable set of configurations to adjust to specific needs.

By implementing Infineon’s advanced motor control algorithm, IMD700A is ideally fit for power tool applications. Thanks to the high integration of the device and sensorless control method, the applications can benefit from flexible form factor. Some algorithms, e.g. electrical clutch (torque control) and MTPA, can further reduce the size and improve the performance of the power tool.

PMSM FOC motor control using MOTIX™ IMD700A.pngFigure 5. Block diagram of PMSM FOC motor control using MOTIX™ IMD700A

With the powerful motor controller IMD700A and the advanced motor control algorithm, Infineon helps customer to ease the development task and to reduce time to market. In additional, the reliability and robustness can be greatly increased with Infineon’s high-quality standard devices and firmware.

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