Power Electronics
DSCs Reorient Low-Cost Motor Control

DSCs Reorient Low-Cost Motor Control

Microchip Technology is presenting designers with a multi-faceted portfolio of hardware, firmware and evaluation tools to bring field-oriented control (FOC) to permanent-magnet synchronous motors (PMSMs) in low-cost applications. The dsPIC33FJ12MC family represents both the smallest and lowest-cost 16-bit DSCs specifically designed for motor-control. Microchip is also releasing free firmware to implement FOC algorithms in these devices, in addition to a starter board that supports all of Microchip’s 16-bit 28-pin components. The portfolio therefore has the potential to fuel a new generation of greener and quieter appliances as well as a host of unforeseen applications, such as precision low-cost robotics.

The DSC family consists of the dsPIC33FJ12MC201 and dsPIC33FJ12MC202. These devices include 12 kBytes of Flash, 1 kByte of RAM, and support for SPI, I2C and UART interfaces (see Figure). To support FOC calculations, the devices also feature a MAC, single-cycle execution of instructions (running at 40 MIPS), and a fast ADC with selectable 10-bit or 12-bit resolution and six input channels that are capable of up to four sample-and-holds.

The DSCs are well suited for space-constrained applications. Housed in 6-mm x 6-mm packages, the devices feature I/O pins that can be custom mapped to optimize pc board layout.

According to Jorge Zambada, senior applications engineer for Microchip’s digital signal controller division, “The FOC firmware is optimized for speed and includes motor start-up subroutines. The FOC algorithm uses the currents and voltages of the individual windings to extrapolate the rotor position without the use of sophisticated sensors, such as optical shaft encoders. To provide further savings, a three-phase motor requires only two shunt resistors, each in series with a single winding. The current in the third winding without a shunt is calculated from the shunt currents of the other two windings.”

Specifically, the FOC algorithm runs on the dsPIC33FJ12MC202 (and larger dsPIC DSCs). Internal variable buffering for data monitoring and control interface (DMCI) debugging is supported. The program code size is 7 kBytes of Flash memory and requires 400 bytes of data RAM memory. The algorithm requires only 11 MIPS performance, leaving the remaining 75% of the DSC’s available instruction-execution capacity for other functions, such as power-factor correction.

The 16-bit 28-pin starter board can be used for developing applications for the dsPIC33FJ12MC family, as well as any of Microchip’s other 28-pin 16-bit microcontrollers and DSCs. The board can be powered from either a 9-V source or a USB plug. It also includes regulators for 3.3-V and 5-V operation, as well as a circuit prototyping area. Microchip’s FOC algorithm can also be evaluated using the dsPICDEM MC1 Motor Control Development Board (part # DM300020) and the dsPICDEM MC1H 3-Phase High Voltage Power Module (part # DM300021), which are optimized for motor-control applications.

FOC is a process by which the orientation and strength of the magnetic field in a PMSM is carefully controlled in relation to the position of the rotor. The two critical and related functions needed to accomplish this are the precise sensing of the rotor position, and the precise control of the stator winding currents.

This has traditionally required complex hardware and software. Therefore, the benefits of FOC, including improved efficiency and acoustical noise reduction, usually come at the expense of high cost and complexity. However, these offerings are priced to speed the development and evaluation of FOC technology for low-cost motion-control projects.

The dsPIC33FJ12MC201 is available in 20-pin SOIC and SDIP packages. The dsPIC33FJ12MC202 is available in 28-pin QFN, SOIC and SDIP packages. Pricing starts at $1.99 each in 10,000-unit quantities. The free FOC algorithm is available now for downloading, as part of application note AN1078, at www.microchip.com/motor. The 16-bit 28-pin starter board (part # DM300027) is also available for $79.99.

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