From CD / DVD players and computer cooling fan to industrial machinery, and includes hybrid cars, the famous Segway electric scooters and many other electric scooter, electric bike (or "electric bicycle"), including electric transport tools, a variety of brushless DC motor is replacing the field rectifier type motor. Because of their growing popularity, especially in Asia, such a "personal transport equipment" to the manufacturer describes the huge potential market, and may help reduce global energy consumption.

Even if the North American market also shows the possibility of using electric bicycles. Since August 2000, from California, Santa Cruz County Regional Transportation Commission-funded activities of the residents to purchase a Electric Bikes  to be a partial refund. Conducted by the Ecology Action, a survey showed that 62% of the actors from only a single passenger car driving into a road bike riding a week on average 24 to 28 miles.

While this situation may be unique, but it illustrates the usefulness and effectiveness of electric bikes is how to reduce personal car journey, and thus reduce traffic congestion, parking demand and air pollution. If this experience throughout the United States to promote, the impact may be very large. According to the Environmental Protection Agency's argument, in the United States, people around for 900 million times a day car travel, which is less than half the distance and only 5 miles and carried by a passenger.

Electric bicycle parts

Basic battery-powered electric bicycle through the brushless DC motor driven by the electronic control device (ECU) to control. Brushless DC motors are popular because they are running fast, no noise, high efficiency, but also shows the motor longer than the rectification work life. Brushless DC motor in the transmission of torque and the motor size ratio is higher than other motors, which makes them very suitable for the volume / weight applications that demand high.

Brushless DC motors need the controller MCU-based applications are usually limited by the requirements, but common example is the control unit for electric bicycles. This application not only requires a small form factor, and cost requirements of particularly high.

Basic chinese electric bikes is simple in design. It's rear wheel by a three-phase brushless DC motor drives, usually in the hundreds of watts of rated power. Battery voltage is typically 36V or 48V, and the ECU contains almost all electronic devices (see Figure 1), including MCU, inverter motors, temperature sensors, fault detection, SMPS, and I / O. These electronic devices are usually placed in the postcard-sized unit, but also for thermal performance and durability with high demands, which are presented many challenges for the designers.

Design challenges

Cost-sensitive, and the overall function of the number of parts problems by using a powerful, yet low-cost 8-bit micro-controller to solve, for example, Infineon's XC866. ECU designers should look for two clocks per machine cycle, enhanced 8051 (rather than the standard 12), which provides more computational efficiency, speed of execution and enhanced the maximum clock rate, and it can crystal at a lower speed to complete the same work, without sacrificing performance to lower energy consumption.

In order to meet the motor control algorithm, embedded flash memory size requirements may be between 4 ~ 16KB. The motor control MCU peripherals may contain a pulse-width modulation circuit, through to pre-programmed to automatically perform tasks capture / compare unit (CCU) to achieve, it helps to reduce code size and CPU load. In addition, CCU may connect to the 8-channel 10-bit ADC to provide the hardware trigger event-driven, to provide control functions without the sensor.
Sensor-based control

Sensor-based control requires the application of low cost and high reliability is very important, such as electric bicycles and exposed to the harsh natural environment and outdoor temperature among other applications. Brushless DC motor applications used by the Hall sensor is very easy these factors, and long-term exposure in this environment will significantly reduce reliability.

However, in considering the conversion type to sensorless BLDC motor control, the designers are faced with how to make the sensor-based systems to meet the required performance level of the problem. And it has the closest relationship is to start the motor, because the sensor-based control is based on EMF, and EMF in the motor rotation only when it exists.

For the electric bicycle with pedals, the issue is not serious, because the rider can achieve the required speed of the motor activation after electronic control. But in the absence of pedal electric bikes must be equipped with the state to start the motor from the flame algorithm.

As unpredictable situation, the traditional start of forced commutation methods are usually not available with the sensor system, so manufacturers often use a combination of several technical methods to start the motor. For example, because did not know before starting the motor rotor position, so you can use the default phase to determine the rotor position or rotor on the determined position.

Once the rotor in the start position, the motor can be applied to the accelerometer to detect back EMF zero-crossing information. If the microcontroller detects a predetermined number of zero-crossing event, it can switch to automatic mode commutation. This requires the implementation of a large number of algorithms.

Considerations

Performance requirements of some applications will need to use motion sensors; In this case, will need to minimum software overhead for flow pattern built in the CCU is very helpful.

Another major consideration is the back-EMF from the failure to detect or recover from synchronization capability, which may occur after the encounter obstacles such circumstances. Similarly, in order to solve this problem, use the combination of algorithms, which makes micro-controller type can be configured as an important factor.