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issue: May 2007 APPLIANCE Magazine

Motor Technology
Advanced Control

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New digital signal controllers (DSCs) are designed to reduce cost, provide greater design flexibility and offer advanced features to appliance motor control applications.

Part of the 56F8000 family of digital signal controllers, the new 56F8011 and 56F8013M are available in a 32-pin low-profile quad flat pack (LQFP) package with flash memory sizes ranging from 12 KB to 16 KB.

The 56F801x DSCs from Freescale Semiconductor (Austin, Texas, U.S.) are said to provide the design flexibility needed to reduce board space and total system cost for motor control applications such as refrigerator compressors and dishwashers. They have also been tested at 125°C for use in high-temperature applications.

According to Charlie Wu, senior system and application engineer, one of the key features of DSCs is that they combine both digital signal processors (DSP) and microcontrollers (MCU) functionality in a single central process unit with a single instruction stream. “Generally, DSCs can continuously feed the data to the arithmetic unit in single clock cycle without stalling,” he explains. “DSCs can perform rapid context switching for peripheral control functions.”

The new Freescale DSCs are said to employ intelligent peripherals, such as high-speed analog-to-digital converters (ADCs). Wu says that by using the controllers, ADC conversion start can be issued by either a pulse width modulation (PWM) module or CPU, and a high-speed motor control PWM module, to avoid starving the computation engine.

In motor controller applications, the DSCs also enable current control loop control to be implemented. “MCU [technology] doesn’t have the bandwidth for this intensive calculation, so many MCU-controlled motor control systems…only have a speed feedback loop,” Wu notes. “Calculation of a current control loop, which typically is less than 100us per calculation, is 10 times faster than a speed control loop.”

Wu says the purpose of the current control loop is to make the actual current flow in the motor terminals, whose magnitude is proportional to the motor shaft torque, follow the current reference signal, the output of the closed-loop speed controller. This, he explains, is so the current control loop can effectively and instantly control the motor shaft torque without waiting for the speed to change.

The DSCs are also said to allow appliance engineers to implement advanced features that need intensive mathematics computation such as sensorless field orientation control for PMSM and ACIM, adoptive control and repetitive control within a single controller, without increasing the system cost. “Freescale’s 56F8011 is an example of a cost-effective controller with on-chip digital-to-analog converters (DACs), analog comparators and a relaxation oscillator, reducing external component usage,” Wu tells APPLIANCE. “It offers very high-performance peripherals, 32 MIPS core performance and a variety of selections. The core supports program execution from internal memories, and two data operands can be accessed from the on-chip data RAM per instruction cycle.”

Because of the controllers’ advanced technology, Wu says manufacturers can also realize significant savings in component costs. “For example, in a washing machine, the gear box and belt pulley can be eliminated,” he says. “For compressor control, if a low-cost single rotary compressor or linear compressor uses DSC control, the performance—especially operation noise—can reach the same level of twin rotary compressor.”

Suppliers mentioned in this article:
Freescale Semiconductor Inc.

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