To control speed, angle or position of the rotor, a feedback loop is needed to sense the actual position during operation. Regardless of the motor type, a precision feedback loop is key for high efficiency. AC or DC motors have wide application fields, ranging from a few tens of horse power up to the 50,000 mark. The feedback loop can either be sensorless through measuring the current and voltage for each phase winding, or based on an active sensor delivering the rotor position. In the active feedback loop, optical encoders, hall sensors or resolvers can be used. The key requirements for such sensors in the feedback loop are precision and robustness in harsh environments. Since the sensors are closely attached to the motor, a high operating temperature is required at low cost. Figure 1 shows the block diagram of a typical BLDC DMC system based on a microcontroller or DSP tailored for high-speed signal processing.The challenge for the designers of the DMC feedback loop is to find the right optimised point in the trade-off between precision and cost. Optical encoders meet the precision targets, however at a price that only high-performance applications can justify. In terms of robustness, resolvers are leading in high-temperature applications. Hall sensor have been challenging the traditional optical-encoder + resolver solution for some time in automotive application due to their possible system integration with signal preprocessing. SoC integration has been used in the new iC-MH to provide a complete and flexible sensor feedback for DMC at a new price/performance ratio.
SoC pushing resolution to 12bit
Hall sensors can meet all the requirements for DMC applications in terms of robustness. By integrating a Hall-sensor bridge with 4 sensors, intelligent signal processing and all interface options on one chip, a complete SoC is created to push resolution to 12bit. Figure 2 shows the functional-block diagram of the iC-MH solution. Besides the Hall sensors with auto-gain amplifiers, the signal processing for the different output options and the RS422 line drivers are all integrated. Due to integration of the RS422 drivers for the three incremental and UVW outputs, the chip needs only a diametric permanent magnet to produce a complete solution for DMC feedback loops running at up to 120,000rpm. The digital bi-directional BiSS/SSI-interface delivers the absolute position to the MCU/DSP DMC controller and the programming link for setting signal compensation to correct for assembly errors. The interface also allows cascading up to 8 axes with just three signal lines, while it is upward-compatible with the SSI interface. Thus customers can always use SSI as a starting point and upgrade to the bi-directional open BiSS standard going up to 10Mbit/s in transmission speed. This capability and the absolute position of each axis remove the feedback-computing burden from the MCU/DSP. More computing power is then available to control more than one axis in parallel. Figure 3 shows the signal output of the different interfaces of the iC-MH. While the incremental A and B signals come from the integrated interpolator, the Z signal is a programmable zero-pulse defined by the designer once assembly has taken place. The commutation outputs UVW allow a direct electronic commutation for brushless motor types. Additional options are available to the designer, such as selection of the ABZ with two additional periodic counter outputs.
A new age
SoC integration for the sensor feedback loop creates a greater level of flexibly for the DMC designer and addresses lower cost solutions with only a small space requirement. The new generation of Hall encoders enables DMC to address the large energy-saving potential in the electrical-motor market. Engine-control applications in the automotive space are addressed as well, challenging existing Hall-sensor solutions. The integrated magnetic-field monitoring adds a new self-checking path for the feedback loop and enables the DMC controller to implement safety features not available before, at minimal additional cost.
