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MCU for electric bicycles to provide an effective drive (Figure)

Print View , by: iSee ,Total views: 24 ,Word Count: 1629 ,Date: Tue, 25 Aug 2009 Time: 4:02 AM

From the CD / DVD players and computers to industrial machinery as well as cooling fans include hybrid cars, the famous Segway electric scooters and many other electric scooters, electric-powered bicycle (or "electric bicycle"), including electric vehicles, brushless DC motor is being replaced by a variety of fields rectifier motors. Because of their growing popularity, especially in Asia, such a "personal transport equipment" to the manufacturers described a huge potential market, and may help to reduce global energy consumption.

Even if the North American market also shows the possibility of using electric bicycle. Since August 2000 have been made by California, Santa Cruz County Regional Transportation Commission-funded activities on the residents to purchase an electric bike to be a partial refund. By the Ecology Action conducted a survey showed that 62% of the activities of participants from only a single passenger car driving into a road bike ride a week on average 24 to 28 miles.

While this situation may be unique, but it illustrates the usefulness and effectiveness of electric bicycles is how to reduce individual car journeys, thus reducing traffic congestion, parking demand and air pollution. If this experience throughout the United States in the promotion, 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 trips, half of which is less than five miles away and only carried by a passenger.

Electric Bicycle Parts

Basic model through a battery-powered electric bicycle brushless DC motor driven by the electronic control device (ECU) to control it. Brushless DC motors are popular because they run fast, no noise, high efficiency, but also shows the motor than the Rectifier longer working life. Brushless DC Motor in the transmission ratio of torque and motor size is higher than other motors, which makes them ideally suited to the volume / weight requirement of relatively high applications.

Brushless DC motor needed to MCU-based controller is usually limited by application requirements, but the common example is the control unit for electric bicycles. The application requires not only small form factor, but also for the cost requirements are also especially high.

Basic electric bicycle in the design is simple. It is the rear by a three-phase brushless DC motor drives, power rating, usually in the hundreds of watts. The battery voltage is usually 36V or 48V, while the ECU includes almost all electronic devices (see Figure 1), including MCU, motor inverter, temperature sensor, fault detection, SMPS, and I / O. These electronic devices are usually placed in a postcard-sized unit, but also on the thermal performance and durability with high demands, which are put forward a number of challenges for the designers.

Design problems

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

In order to meet motor control algorithms, embedded flash memory size requirements may be between 4 ~ 16KB. The motor control mcu peripherals may contain a pulse-width modulation circuit, the adoption of pre-programmed to perform automated tasks, capture / compare unit (CCU) to achieve, it helps to reduce the code size and CPU load. In addition, CCU can be connected to 8-channel 10-bit ADC to provide the hardware event-driven triggering, to provide no sensor control functions.

MCU for electric bicycles to provide an effective drive (Figure)

Figure 1 the control unit driving three-phase electric bicycle brushless DC motor

No sensor-based control

No sensor-based control require low-cost and high reliability applications is very important, such as electric bicycles and the natural environment and severe exposure to outdoor temperatures among other applications. Brushless DC motor applications commonly used in Hall sensors is very vulnerable to the impact of these factors, but also long-term exposure in such an environment would significantly lower the reliability.

However, in considering the switch to non-sensor based BLDC motor control, the designers faced with how to make the non-sensor-based systems to meet the needs of level of performance problems. With this point the most closely related to the launching of electric motors, because there is no sensor-based control is based on back-EMF, while the back-EMF motor rotation only when it exists.

For the electric bicycle with a pedal, the problem is not serious, because the rider can achieve the required speed of the motor re-activated after the electronic control. However, in the absence of pedal electric bicycle, you must be equipped with the state to start the motor from the flame algorithm.

Due to unpredictable circumstances, the traditional start of forced commutation methods often do not apply to non-sensor system, so manufacturers often use the combination of several technical methods to start the motor. For example, because did not know before the start motor rotor position, so you can use the default phase to determine the location of the rotor or the rotor on identifying the location.

Once the rotor position at start, you can apply to the motor speed tables to detect back-EMF zero-crossing of the information. If the micro-controller to detect the number of zero-crossing events scheduled, it can switch to automatic mode commutation. This requires the implementation of a large number of algorithms.

Considerations

The performance requirements of some applications will need to use motion sensor; in this case, would require minimal software overhead of changing mode built into the CCU is very helpful.

Another major consideration is from the back-EMF detection or from the failure to restore the ability to sync, which may occur in such circumstances, after tripping over obstacles. Similarly, in order to solve this problem, need to use the combination of algorithm, which makes micro-controller can be configured to type as an important factor. In self-managed through the peripheral tasks to perform as much as possible, MCU can reduce the CPU burden and reduce the need for the code space. Both of which produce more durable and more cost-effective design.

Whether or not the sensor with the design, can be achieved through PWM various "ladder" approach to drive the converter BLDC. In the trapezoidal commutation method, the current through each time you use a pair of electric motors to control the terminal, while the motor on the third terminal is always in the electrical and power disconnected; that is, at any given time had only been used in the two-phase three-phase, while the third phase is left vacant.

In the "slow decay" modulation method, in the PWM off period, the load current in the bottom of the flow between the switch and body diode.

On the contrary, in the "fast decay" method, PWM switches off all the time will be disconnected, but this approach has a high load current spikes defects. To make up for this shortcoming, you can use synchronous rectification, so that the load current flows in the switch itself, rather than in the in vivo resistance. This requires the same time, modulation with a bridge at the top and bottom of the switch, not just the top of the switch.

However, care must be taken to avoid direct short-circuit current during the conversion. By inserting an invalid course of the circuit switching time, current through the body diode. But in an invalid time after current flow through the bottom of the switch, so synchronous rectification can be used to quickly decay. The use of discrete power devices, rather than integrated with the built-in protection circuit designed to drive low-cost terms, in the MCU's PWM can be used in the above model is essential. These models and fault detection combined with other functions can be provided for the separation of inverter-driven high-level protection.

In the design and algorithm development is completed, manufacturers need to ensure that their intellectual property is protected. This can be done to prevent unauthorized access to the code to realize special functions. This is very important, because the electric bicycle drive is driven by a relatively normal structure and composition, the main difference between products in the software.


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