The 8-bit microcontroller continues to be a corner stone for embedded systems design where low-cost systems, legacy software and scaleable solutions remain deciding factors for new designs. Innovative enhancements to established 8-bit architectures are even beginning to invade application domains that were once dominated by higher performance, higher cost 16 and 32-bit architectures. This is particularly true in the area of motor control where designers are looking for energy efficient solutions at lowest system cost. Field Oriented Control (FOC), also known as Vector Control allows the manufacturers to reduce the size of the motor required and obtains higher energy efficiencies and better dynamic response. FOC implemented on a cost effective 8-bit µC offers an attractive solution for system designers of fans, compressors and motor drives used in end applications such as air conditioners, refrigerators, washing machines and electric vehicles to achieve higher energy efficiency at lowest cost.
The FOC capable XC886 and XC888 MCUs incorporates an 8051 compatible core with the powerful Infineon Vector Computer. The Vector Computer incorporates several processing elements which include the CORDIC (Coordinate Rotational Digital Computer) unit and an MDU (Multiply and Divide Unit) both of which are capable of 16-bit math coupled with the 16-bit CapCom6E and a Fast ADC. The CORDIC provides a method for doing complex mathematical and trigonometric functions such as Clarke and Park algorithms required for vector rotation using addition, subtraction and shifting and a very short look-up table iteratively. While providing results with up to 16-bit resolution, the CORDIC functions independently from the CPU core thereby freeing up those resources for other control oriented tasks. The MDU is capable of performing 16 and 32-bit math operations and may be used instead of the standard 8051 MUL / DIV instructions.
Energy and Cost Efficiency
In comparison to block commutation control schemes for permanent magnet synchronous motors (PMSM), a field oriented control offers a lot of benefits. The block commutation control scheme is based on driving only two of the three stator coils of a PMSM at the same time with a constant voltage. The supply voltage is usually modulated by a PWM signal in order to control the torque or rotational speed of the motor. As the phase voltage is provided as constant value, the rotor is often misaligned to the driving stator field. Even though Hall sensor based block commutation control scheme is easy to implement, it results in low energy efficiency, low torque-dynamics and high noise levels.
Using a powerful 8-bit microcontroller enables the designers to upgrade to sinusoidal commutation control scheme at low system cost. For sinusoidal commutation, all three phases are used to generate a smooth rotating field which always aligns rectangularly to the rotor’s permanent magnetic field. As a result, the maximum torque can be achieved at any time which leads to a high energy efficiency. From a system perspective, the energy efficient control scheme is also cost efficient because a smaller motor can be used for the same mechanical load requirement.
FIELD ORIENTED CONTROL
Field oriented control (FOC) is a method to generate a three phase sinusoidal signal which can easily be controlled in frequency and amplitude. The basic idea is the transform of three phase signals into two rotor-fix signals and vice versa. The control of the speed and torque is as easy as using a brushed DC motor, because the control is performed in a rotor-fix system.
This method requires the knowledge of the rotor position in the stator field. Acquiring the three phase currents with just one shunt in the DC link, it provides a sensorless FOC with the full benefits of sinusoidal commutation at a minimum system cost. The voltage model of a PMSM motor is taken to estimate the rotor’s position in real time. This requires a very fast and accurate ADC for current measurement. Any jitter in the trigger point will influence the actual rotor’s angle estimation. As a result, the total harmonic distortion of the sinusoidal current signals will increase.
The XC886/888C(L)M perfectly implements the above requirement using an event based hardware trigger from the PWM unit CapCom6E towards the ADC. The event based trigger eliminates any interrupt latency and enables fast and accurate current measurement. The multiple result registers of the ADC holds the DC-link current values used for the calculation till the CPU is ready to read the values . The ADC sample time is lower than 200 ns leading to a minimum of torque ripple and a maximum of energy efficiency.
REALIZATION
The highly efficient programming of the sensorless FOC algorithm in 16-bit arithmetic can only be realized by a nested utilization of the vector computer - MDU and CORDIC - and the 8051 compatible CPU core itself. The efficiency of the Sensorless FOC execution on Infineon’s 8-bit microcontrollers XC886 and XC888 (for instance with 15kHz PWM frequency and 133µs current control response time) takes up only less that 58% of the CPU’s loading which provides sufficient bandwidth for system specific functionality.
The resulting sinusoidal waveform of this implementation results in a very low noise operation of the motor. The system costs are reduced drastically by using an 8-bit standard microcontroller, using just one shunt in the DC-link for acquiring the three phase currents and by the use of a FOC algorithm that makes expensive hall sensors obsolete.
The unique software re-programmability enables an optimized startup phase of the motor by programming a controlled ramp using any signal of the algorithm as well as the implementation of field weakening methods. The free source code is optimized for fast execution on XC886 and X888 MCUs and requires less than 3Kbytes of code space.
APPLICATION KIT
To evaluate Sensorless FOC with an 8-bit microcontroller, Infineon offers the FOC Drive Application kit. This kit integrates the XC886CM MCU (TQFP-48), 3-phase power inverter board, 24V BLDC motor, plug-in power supply, and complete FOC source code making the kit ready-to-use. A unique feature of the kit is the inclusion of a CAN to USB bridge which allows for hex code download which can modify motor parameters such as speed and current control during the operation of the motor offering real-time control. The complete development environment offers a free tool chain allowing users to advance to the next stage of application development and customization using the same kit. For further details on the FOC Drive Application Kit please refer to http://www.infineon.com/XC800-FOC
CONCLUSION
According to the Energy Agency (IEA) in the year 2004, a third of the world-wide energy consumption is accounted by electric energy which amounts to about 15.4 million Giga watt hours. Electrical motors consume approximately 40% of this electricity. It becomes evident that by the adoption of variable speed drives using advanced control techniques like FOC, the world-wide consumption of electricity could be lowered by over 20%. Advanced control schemes like FOC allows manufacturers to reduce the size of the motor required, yielding higher efficiency, better dynamic response and lower torque ripple. The benefits offered by FOC coupled with a favorable price point of an 8-bit µC solution makes Infineon’s offering, a solution of choice for motor control developers seeking ways to meet energy regulations and contain pricing pressure.
Internet Links:
FOC Drive Application Kit http://www.infineon.com/XC800-FOC
XC800 8-bit microcontroller http://www.infineon.com/XC800
Datasheet and usermanual XC886 http://www.infineon.com/XC886
Datasheet and usermanual XC888 http://www.infineon.com/XC888
Infineon Technologies AG
