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Simplifying BLDC motor control: A tailored solution delivers precision and efficiency

Thu, 05/16/2013 - 2:04pm
Wen Chien, Fairchild Semiconductor www.fairchildsemi.com

From the small vibrating motors in cell phones to the more complex motors used in household washing machines and air conditioners, motors have become everyday fixtures in the consumer world. The energy consumed by all these motors is significant: studies have shown that in China alone motors represent as much as 60 to 70 percent of industrial energy use, and fans and pumps make up nearly a quarter of China’s overall power consumption.

Traditional alternating current (AC) motors, which have been in widespread use for more than a century, are the simplest type of induction motor to design with, but they can waste a significant amount of energy. In direct current (DC) motors, the speed can vary and can be controlled, by changing the voltage, to work faster or slower depending on the needs of the application. This can save a considerable amount of energy, since the motor only needs to work as hard as the situation requires. In general, DC motors are more efficient than AC motors.

The benefits of BLDC motors
DC motors can be designed to operate with or without brushes. Brushless DC (BLDC) motors are usually the best choice for most applications. They’re more reliable, quieter, produce fewer electromagnetic emissions, and are safer because they eliminate the sparks associated with the brush and its commutator. BLDC motors are also smaller and more efficient, which means they use less energy.

The downside of using a BLDC motor, though, is that the system requires more complex electronics to manage the motor. For an increasing number of manufacturers, however, the added complexity of using a BLDC motor is more than offset by the increased demand for energy-saving appliances.

Sensorless field-oriented control for advanced BLDC functionality
The traditional way to control a BLDC motor uses a six-step process for driving the stator and thereby generating ripple on the produced torque. The so-called “six-step square-wave” process uses Hall effect sensors to detect the position of the permanent magnet in the BLDC motor.

The six-step process is relatively straightforward, but it can be prone to acoustic noise and isn’t responsive enough for more advanced applications that need to change motor speed quickly in response to changing conditions. In the case of a washing machine, for example, the load varies according to the wash cycle selected, and changes throughout the cycle. The case is more extreme in a front-load washer, where gravity works against the motor when clothes spin to the top side of the drum.
In these situations, a more advanced algorithm is required. Field-oriented control (FOC) delivers the response times needed for quick speed changes, and has become the motor-control method of choice in today’s more advanced energy-saving appliances.

There are different ways of implementing FOC. One way is to use sensors (in a method similar to the six-step square-wave process), but sensors can be difficult to mount and maintain, especially if the application involves a complex wire harness or the motor is exposed to water. The simpler, more cost-effective way to implement FOC is to eliminate the sensors. Sensorless FOC involves a constant rotor magnetic field produced by a permanent magnet on the rotor, and is a very efficient method of control.

The FOC method lets the motor operate smoothly over the full speed range, is able generate full torque at zero speed, and is capable of fast acceleration and deceleration. In fact, the many benefits of sensorless FOC have made it a popular choice for applications with lower requirements for performance, due to the small size of the motor, the cost, and the power consumption.


Still, though, implementing sensorless FOC requires complex, math-intensive algorithms that may not be familiar to the average designer. In the past, designers have typically had to rely on complex digital signal processing (DSP) chips to implement sensorless FOC.

An application-specific solution
For systems that employ sensorless field-oriented control (FOC), Fairchild offers the FCM8531, an application-specific control device with parallel-core processors. Shown in Figure 1, the FCM8531 consists of an Advanced Motor Controller (AMC) processor and an 8-bit, 80C51-compatible MCU processor.

The AMC is a core processor specifically designed for motor control. It integrates a configurable processing core and peripheral circuits to perform FOC  motor control without sensors. The system control, user interface, communication interface, and input/output interface can be programmed through the embedded 80C51 for different motor applications.

The advantage of a motor control IC with parallel-core processors is the two processors can work independently and complement each other. The AMC processes the tasks dedicated for motor control, such as the motor control algorithms, PWM controls, current sensing, real-time over-current protection, and motor angle calculation. The embedded MCU provides motor control commands to the AMC to perform motor control activities through a communication interface. This approach reduces the software burden and simplifies the control system program because complex motor control algorithms are executed in the AMC.

Conclusion
Transitioning consumer appliances and industrial equipment away from traditional AC motors to smaller, more efficient BLDC motors make good sense from the standpoint of energy consumption, but the complexity of designing BLDC control algorithms can be enough to discourage engineers from making the switch. Dedicated ICs for BLDC motor control make it easier for developers to work with BLDC motors, and can help speed the transition to higher-efficiency formats.

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