Challenges of cost, size, weight, and functionality can be addressed using an inexpensive fast 8-bit MCU
BY RYAN SCOTT
Infineon Technologies
Livonia, MI
http://www.infineon.com
BLDC motors are replacing brushed motors in a variety of applications, from CD/DVD players and PC cooling fans to industrial machinery and electric vehiclesincluding hybrid cars, the Segway Scooter, and a number of other scooters and electric bicycles, or “e-bikes.” Because of their growing popularity, especially in Asia, such “personal transportation devices” represent a huge potential market for manufacturers and have the potential to help reduce worldwide energy usage.
Even the North American market shows potential for e-bike adoption. Since August 2000, a program subsidized by the Santa Cruz, CA, County Regional Transportation Commission has been issuing rebates for residents to purchase electric bicycles. A survey by Ecology Action revealed that 62% of the program participants switched from driving exclusively in a single-occupant automobile to e-biking an average of 24 to 28 miles per week.
While this situation may be unique, it illustrates how the availability and utility of e-bikes can reduce solo auto trips, and thus traffic congestion, parking demand, and air pollution. If this experience were scaled across the U.S., the impact could be significant. According to the Environmental Protection Agency, people in the United States make approximately 900 million car trips every day, half of which are shorter than 5 miles and involve only one passenger.
E-bike components
A basic e-bike runs on a brushless dc motor, is powered by batteries, and is controlled by an electronic control unit (ECU). BLDC motors are popular because they are fast, noiseless, efficient, and exhibit a longer operating life than brushed motors. The ratio of delivered torque to motor size is higher in BLDC motors than in other motors, which makes them an excellent match for space/weight-sensitive applications.
The MCU-based controllers required by BLDC motors are often constrained by the demands of their applications, but a typical example is the control unit used for e-bikes. This application not only requires a small form factor, but is also extremely cost sensitive.
A basic e-bike is simple in design. Its rear wheel is driven by a three-phase BLDC motor, generally rated at a few hundred watts. The battery voltage is usually 36 or 48 V and the ECU contains almost all of the electronics (see Fig. 1 ), including the MCU, the inverter for the motor, temperature sensors, fault detection, a SMPS, and I/Os. These electronics are housed in a unit that is typically the size of a postcard and, with the strong requirements for thermal capabilities and ruggedness, present numerous challenges to designers.
Design challenges
The challenges of cost sensitivity, component count, and overall functionality can be addressed using a powerful, yet inexpensive, 8-bit microcontroller, such as the Infineon XC866. The ECU designer should look for an enhanced 8051 core with two clocks per machine cycle, rather than the standard 12. This provides greater computational efficiency, faster execution times, and increased maximum clock rates or, it allows the same work to be accomplished at a reduced crystal speed, lowering power consumption without sacrificing performance.
To accommodate the algorithms for motor control, embedded flash memory size requirements may range from 4 to 16 Kbytes. Motor control peripherals on the MCU can include a PWM, implemented as a capture/compare unit (CCU) that can be pre-programmed to perform tasks automatically, which helps reduce code size and CPU loading. Additionally, the CCU can be linked to an eight-channel 10-bit ADC to provide hardware-event-driven triggers, providing a sensorless control capability.
Fig. 1. The electric bike control unit drives a three-phase BLDC motor.
Sensorless control
Sensorless control is important in such low-cost and high-reliability applications as the e-bike, as well as in other applications that are exposed to outdoor elements and extreme temperatures. The Hall sensors typically used in BLDC applications are vulnerable to these elements, and long-term exposure decreases reliability dramatically.
However, when considering the switch to sensorless BLDC control, designers are faced with challenges in getting the sensorless system to meet the required performance levels. This most notably relates to starting the motor because sensorless control is based on back-EMF, which is present only when the motor is rotating.
For an e-bike with pedals, this doesn’t present that great a problem, because the rider can get the motor up to the required speed and then activate the electric control. But in e-bikes without pedals, a motor-start algorithm from the stall position is required.
A traditional forced-commutation starting method is not normally appropriate in a sensorless system because of the unpredictable conditions, so manufacturers often employ methods that combine several techniques to start the motor. For example, since the position of the rotor before starting the motor is not known a prepositioning phase can be used to determine the position of the rotor or to place it in a known position.
Once the rotor is in its start position, a ramp table can be applied to the motor, allowing detection of the back-EMF zero-crossing information. When the microcontroller has detected the target number of zero-crossing events, it can switch to auto-commutated mode. This requires execution of numerous algorithms.
Concerns
Some application performance demands will require the use of motion sensors; in this case, it is helpful to have commutation modes that require minimum software overhead built into the CCU.
Another major concern is the ability to recover from a loss of back-EMF detection or synchronization, which may occur after hitting a bump, for example. Again, to address this challenge, combinations of algorithms are required, which makes the configurability of the microcontroller an important factor. By performing as many tasks as possible in self-managed peripherals, the MCU can offload its CPU and reduce required code space. Both lead to a more robust and cost-effective design.
Whether sensor or sensorless, a variety of “trapezoidal” commutation methods may be implemented by the PWM to drive the BLDC. In trapezoidal commutation, current is controlled through motor terminal switches one pair at a time, with the third motor terminal always electrically disconnected from the source of power; that is, only two of the three phases are excited at any given time, while the third is left floating.
In the “slow-decay” modulation method, the load current is allowed to circulate through the bottom switches and the body diode during the off period of the PWM.
Conversely, in the “fast-decay” method, all the switches are turned off during the PWM off period, but this technique is subject to the drawback of high-load-current spikes. To counter this drawback, synchronous rectification may be used, which lets the load current circulate in the switch itself, instead of in the body diode. This requires that both the top and bottom switches of the same bridge, rather than just the top switch, be modulated.
However, caution must be taken to avoid current shoot-through during the transition. By inserting dead time into the circuit transitions, the current circulation is through the body diode, but after the dead time the current circulates through the bottom switch, so synchronous rectification may then be used for fast decay. Having access to these modes in the PWM of the MCU is critical for low-cost designs in which discrete power components are used instead of integrated drivers with built-in protection circuits. Combining these modes with other options such as fault detection complements the discrete inverter drive design with high levels of protection capability.
After completing the design and developing the algorithms, manufacturers need to be assured that their intellectual property is protected. This can be accomplished with special features that prevent code from being uploaded without permission. This is extremely important because e-bike drive circuitry consists of relatively common drive configurations, with the primary product differentiation being in software. ■
For more on electronic control units, visit http://www2.electronicproducts.com/DigitalICs.aspx.
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