International Appliance Manufacturing 2008 - (Page 24) Fig. 1. A DSC-Based FOC Motor Control Scheme Implemented On a PMSM. control algorithms such as FOC, and also double as system controllers in appliances. This is because DSCs feature peripherals tailored for motor control, such as Pulse Width Modulators (PWMs), Analog-to-Digital Converters (ADCs) and quadratureencoder interfaces. When executing controller routines and implementing digital filters, DSCs help designers optimize code execution by being able to execute the MAC instructions and fractional operations in a single cycle. The system-controller aspect of appliance design can be comfortably handled through DSCs. General-purpose I/O lines on the DSC can be used to interface switches and displays. Serial ports on the DSC can also be used for system calibration and diagnosing system faults. The dsPIC DSCs from Microchip Technology also provide fault and diagnostics interfaces that include input lines, with the ability to shut down the PWMs in case of catastrophic faults in the system. In particular, Microchip’s dsPIC33FJ12MC202 DSC is a good fit for applications requiring PFC control with 3-phase load control, such as appliances using PMSM motors or ACIM motors. The dsPIC DSC’s fast and flexible ADC supports current sensing and offers useful triggering options. The dsPIC33FJ12MC202 features four PWM generators, where three of these PWM generators work on one time base and the fourth on an independent time base. This is extremely handy in applications that require different frequencies for PFC and 3-phase load control. Additionally, these highly capable DSCs come in compact 6 x 6 mm QFN package sizes — enabling designers to put DSCs on compact motor-control boards for placement inside motor housings. Current sensing is a crucial function in motor control, and it requires fast and flexible on-chip ADCs. The dsPIC DSC family features ADCs that are capable of converting input samples at a 1 Msps rate, and capturing up to four inputs simultaneously. Multiple trigger options on the ADCs enable the use of inexpensive current-sense resistors, to measure motor phase-winding currents. Using PWM modules, it is possible to trigger A/D conversions — which results in an inexpensive current-sensing circuit — by sensing inputs at specific times where switching transistors allow current to flow through the sense resistors. PMSM example Given their motor-control specific peripherals, DSCs can be deployed in appliances with variable-speed motor drives. In these applications, DSC-based 24 International Appliance Manufacturing 2008
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