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More sophisticated motor control requires more sophisticated MCUs

Microchip has recently expanded its pic32 range

Motors are everywhere; from the huge devices driving industrial processes to the ones driving the compressor in your fridge. It’s no surprise to discover that electric motors consume something like half of all the electricity generated globally.

Many of these motors are inefficient, so there continues to be a focus on providing better control of their operation. But it’s not a new initiative; electronics technology has been applied for the best part of two decades in an attempt to boost efficiency.

Around the turn of the Millennium, systems were developed which relied upon DSPs. “These certainly had the necessary calculation power,” said Erlandur Kristjansson, a product marketing manager with Microchip, “but the problem was that DSPs weren’t any good as microcontrollers.”

Since then, much work has been focused on developing MCUs that can provide the control required to improve electric motor efficiency. This has seen the development of the so called digital signal controller and, more recently, MCUs with integrated DSP capabilities.

Microchip was probably the first company to introduce a digital signal controller, or DSC, when it launched the 16bit dsPIC range in 2002. Kristjansson said the main use for DSCs in the early days was for running advanced algorithms, including field oriented control (FoC) and sensorless control. “That takes a lot of horsepower,” he said. “If you want to stay within the interrupt cycle, you have to do all the calculation before the window is closed.”

Times have changed, of course. Now, a range of 32bit MCUs is available that address motor control. While Microchip has chosen to use MIPS cores for its pic32 portfolio, almost every other MCU developer is using ARM’s Cortex core – either the M4 or the more recent M7.

Danny Basler is a product marketer with Freescale’s industrial group. He said: “Motor control is a big market and people are looking for more advanced control topologies. While DSCs were traditionally targeted at appliances and some industrial applications, today’s consumers need sophisticated MCUs.”


Fig1: Block diagram of the Kinetis KV5x

Freescale, one of the adopters of ARM technology, has recently launched the Kinetis KV5x range of MCUs, which is aimed at two key markets, said Basler – motor control and digital power. “All Cortex-M cores perform well in these segments; even the M0+ can be used to control brushless DC motors. But larger motors need more performance and the M7 core is suited to that – the core is more powerful and faster.”

Power and speed are important factors in this market. Kristjansson said that dsPICs do well in applications which ‘spin things’ – pumps and compressors, for example – rather than position loops. “These products are more cost optimised solutions and are targeted at the lower end of the market.” Mindful of this, Microchip has developed the pic32 range and offers a choice of MIPS cores. “The range started with the M4K, with some interrupt and behavioural features. This was enhanced with the M14K, making it more like an MCU, but users still asked about DSP functions, so the next step was the microAptiv core; a variation of the M14K that competes with the Cortex-M4. But while its performance brings a competitive advantage, it’s the peripherals, libraries and drivers that make the difference.”

The microAptiv core features the MIPS DSP Application Specific Extension (ASE) and is said to retain the features available in the M14K core.

The DSP ASE provides the microAptiv MCU core with high performance, single cycle throughput DSP and SIMD capabilities to address the requirements of such applications as industrial/motor control.

The core integrates a memory protection unit and secure debug functionality, suiting systems requiring a high level of security. Meanwhile,

microAptiv cores are configurable, with the ability to run in MIPS32, MIPS32 and microMIPS or microMIPS only modes.

According to developer Imagination Technologies, microAptiv cores have higher performance than competing converged MCU/DSP solutions, with a CoreMark of 3.44CoreMark/MHz in microMIPS mode.


Fig 2: The microAptiv core

Meanwhile, the KV5x family blends the Cortex-M7’s processing power with analogue and timing peripherals and new connectivity, security and safety features. Running at 240MHz, KV5x MCUs have four 12bit A/D converters, each capable of 5Msample/s, and an IEEE1588 Ethernet controller. According to Freescale, the KV5x family can support fully asynchronous dual three phase motor control. And, because there are four A/D converters, it is possible to control two 3phase motors independently from one KV5x.

With features such as a six stage pipeline, the core is said to have a performance in excess of 5CoreMark/MHz. “What we’re targeting with the KV5x are high end motor control applications, including AC synchronous motors and permanent magnet synchronous motors,” Basler asserted. “But it’s not just pure motor control; we have blocks wrapped around the core, including connectivity, Ethernet and security. We are going to see more demand for this type of product.”

Two ‘flavours’ of the KV5x range are anticipated; one focused around USB connectivity; another addressing higher end connectivity requirements. “The latter needs will mainly be met by Ethernet, with protocol support for things like EtherCAT,” Basler explained. “Motor control over Ethernet has huge potential to monitor systems in the field.”

Kristjansson agreed that communications are important for this kind of MCU. “Automotive motor control applications will use LIN, but industrial applications are more likely to use CAN and/or UART, as well as providing support for Fieldbus communication.” He noted that IEEE1588 is part of the motor control solution, but said that some of this functionality is best handled by off chip solutions.

DSP functionality remains central to the performance of devices aimed at motor control. The Cortex-M7 core includes DSP extensions which support such features as single cycle 16/32bit MACs, single cycle dual 16bit MACs,
8/16bit SIMD arithmetic, and hardware divide. Microchip’s microAptiv based parts offer four 64bit accumulators and a single cycle MAC.

Kristjansson noted: “The microAptiv core has SIMD instructions, including a dual 16bit MAC performed in a single cycle. But for applications at the higher end that need more dynamic range, we’re beginning to see the need for floating point performance.” The Cortex-M7 core provides similar functionality, with single and double precision available. “The floating point unit is important for mid to high end motor control,” Basler claimed.

For applications where price is important, Kristjansson said dsPICs remain relevant. “If it’s ‘just a fan’,” he said, “price will be everything and the customer will push towards dsPICs.” Microchip has recently expanded the portfolio with the dsPIC33EV family. With the ability to run from a 5V supply, the parts offer a performance of 70MIPS and feature DSP acceleration.

Freescale’s KV5x was previewed at Embedded World earlier in 2015 and, for the moment at least, will be the top end of the Kinetis series. Basler said it’s likely the products will launch in October. “For applications like motor control, where there’s a lot of fast control loops and the need for DSP functionality, the KV5x will be a great fit,” Basler concluded. “Motor control needs performance and the M7 core is suited to that.”


Author
Graham Pitcher

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