Appliance Design - October 2007 - (Page 28)

MOTORS Fig. 1. A DSC-Based FOC Motor Control Scheme Implemented On a PMSM. ASICs to implement the PFC block. Versatile DSCs are suitable for implementing advanced motor-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 quadrature-encoder 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 con28 applianceDESIGN October 2007 trol. 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 FOC control makes practical and economic sense, because modifying the hardware for implementing motor control is minimal. These appliances feature a three-phase inverter, which is used as the power stage to drive the motor windings. By adding a DSC (see Fig. 1) and current-sensing circuitry (see Fig. 2) through the help of two low-cost resistors, software on the DSC improves the motor efficiency through FOC control. The sensor-less control technique implements the FOC algorithm by estimating the position of the motor without using posi- tion sensors (as shown in Fig. 1). The FOC algorithm — which is executed at the same rate as the PWM — is configured by ensuring that the PWM triggers ADC conversions for the two windings, using two shunt resistors. Then, a potentiometer is used to set the reference speed of the motor. The ADC Interrupts are enabled, to execute the algorithm. The position estimator (see Fig. 1) is based on the currents and voltages of the motor, and uses a motor model to measure the motor position indirectly, via an observer. The motor position can be estimated by assuming that the PMSM model is the same as that of a DC motor. A current-observer model aids in the measurement of the back EMF, indirectly, by feeding the motor and its model with the same input. Since the motor model has a closed-loop observer, it works to ensure that the estimated value matches the measured value. FOC is accomplished by keeping the stator magnetic field ninety degrees ahead of the rotor, at all times. This requires constant rotor-position information. In FOC, designers need to deploy a different algorithm to detect or estimate rotor position. FOC results in better torque production and less torqueripple generation by the motor. Using the 3-phase voltage, the FOC algorithm generates a vector to control the 3phase stator current. Converting the physical current into a rotational vector using transforms makes the torque and flux components time-invariant. This time invariance enables control with conventional Proportional and Integral (PI) controllers, as with a DC motor. Using the FOC technique, the motor curwww.applianceDESIGN.com http://www.appliancedesign.com

Table of Contents Feed for the Digital Edition of Appliance Design - October 2007

Contents Editorial Shipments/Forecasts News Watch Applying Powder Coatings to Plastic Parts is No Longer a Pipe Dream New Technology Platform Provides the Ability for a single IC to Control Two- Motors in a Single Appliance, Simplifying Motor Control Design Advanced Motor-Control Techniques have Become More Accessible to a Wider Array of Appliances, Helping to Improve Efficiency and Reduce Noise Adding USB Host Capability to Appliances Permits In-Field Programming and Acquisition of Test Data ZigBee Modules Make it Easier to Design Wireless Connectivity into Applications New Concepts such as Rapid Tooling and Rapid Injection Molding Provide Stepping Stones Between Prototyping and Full Scale Production A Number of Factors and New Developments Affect the Decision on Whether to Build Prototypes In-House or Outsource to a Service Bureau DesignMart Advertiser’s Index Association Report: CEA

Appliance Design - October 2007

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