Low Cost Motor Control

Massimo Incerti - White Goods Application Specialist and Cristian Ionescu - White Goods Marketing Manager, NXP



Advanced motor control techniques like: Brushless DC and Brushless AC are already used on large scale in industrial applications as they are offering several advantages compared to universal AC motor control like: better efficiency, higher robustness and lower motor cost. On the other hand, the electronic portion of driving the motor has become more complicated also driving up the total system cost. The White goods market, as an extremely price driven market segment has been very careful in implementing "new" motor control methods in appliances like Washing Machines and Dish Washers. The traditional and well-known control methods have been preferred so far, but since several years, also supported by year over year semiconductors price erosion the technology used in this market has started to change. NXP as a semiconductors supplier for industrial applications has a very good coverage on various product families like General application (Rectifiers, Zener diodes, etc), Logic and Power (Triac's, Power IC) as well as Interface and Microcontroller products. Nowadays Brushless DC motors (BLDC) are widely used in many applications replacing the traditional brushed DC (BDC) motors. BLDC gains popularity in markets like White Goods (WG), HVAC applications and industrial applications thanks to higher performance in terms of efficiency and reliability, reduced noise and weight, longer lifetime, elimination of sparks created by the commutator and overall reduction of Electro Magnetic emissions. The BLDC control consists of a control unit and a power unit, with NXP offering competitive solutions for both units. This article will focus on a demonstration board developed by NXP, designed for BLDC motors over 300W at 12V to 30V. Rotor orientation feedback is determined using Hall sensors and interfaces to the outside world using a PC using either CAN or UART connection.

The Cortex-M0 core is one of the latest cores released by ARM in 2009. Cortex-M0 is the smallest, lowest power and most energy-efficient ARM processor available on the market able to achieve 32-bit performances at an 8-bit price level ARM's Cortex-M0 is based on ARMv6-M architecture and uses the so-called Thumb instruction set including Thumb-2 technology. The Thumb instruction set is able to operate 32 bit operations out of 16 bit instructions thus enabling a smaller code footprint. Thumb ISA (Instruction Set Architecture) is made by 56 instructions only, all with guaranteed execution time, from this point of view Cortex-M0 has a pure deterministic response; for example Data processing instructions are completed in one cycle, data transfer is done in two cycles and branches take three cycles to be executed. Regarding data transfers, the M0 core can handle 8, 16 or 32bit data in one instruction only. Apart from the core, Cortex-M0 integrates a NVIC (Nested Vectored Interrupt Controller) that is able to handle both interrupts and system exceptions. The Cortex-M0 core is characterized to have a fully deterministic behavior of the interrupt handling which is 16 cycles by default with no jitter. The maximum amount of vectors the NVIC is able to handle is 32 with prioritization. Tail chaining and late arriving interrupts are supported as in Cortex-M3 architecture. In 2009 NXP Semiconductors released the first members of the LPC1100 family based on the Cortex-M0 core. From a computational point of view, LPC1100 family is able to deliver 0.9 DMIPS/MHz according to Dhrystone benchmarks. An additional benchmark (http://www.coremark.org) which is more dedicated to embedded systems performance analysis positions the LPC1100 at 1.4 Coremark/MHz which is extremely high compared to the actual 8 and 16bit Microcontroller solutions. At the same time, the users can save around 40% of the flash memory needed using Cortex-M0 LPC1100. Thanks to the extremely low gate count, Cortex-M0 based devices can be used in low power sensitive applications such as medical devices, e-metering, motor control and battery powered sensors. ARM's Cortex-M family processors integrates support for multiple power modes; sleep, deep sleep, power down modes. LPC1100 family supports up to 50 MHz clock speed, it is a zero latency architecture, integrates a simple AHB-Lite interface. The block diagram is shown in the following picture:

The LPC111x integrates all necessary peripherals for embedded control systems in industrial, consumer and white goods applications. The flash content is up to 32KB and the price is starting (for 8K flash based devices) from 0.65$. For controlling BLDC motors the LPC1100 family incorporates four timers, two 16-bits and two 32-bits, with a total of 13 match outputs where each match output can be configured as PWM. Six PWM signals are used in the demonstration board driving the high and low side MOSFETs. The general-purpose inputs/outputs (GPIO) on the LPC1100 are highly configurable and can be used as external interrupts triggering on the rising, falling or both edges. Rotor orientation feedback is captured through these GPIO interrupts.

The LPC1100 has an 8 channel 10-bits Analog to Digital Converter (ADC) from which one channel is used as e.g. over-current protection by measuring the motor current through a shunt resistor. Measuring the voltage on the floating phase during BLDC commutation, the rotor orientation can even be determined without use of any sensors. This requires accurate timing in capturing the floating phase voltage. In the LPC1100, an ADC conversion can be triggered by a match event of two of the four timers. This decreases CPU load and allows accurate capturing of the floating phase at the right moment. For interfacing to the outside world, the LPC1100 has the UART and/or CAN interface. On the other hand, NXP has introduced in 2009 a new MOSFET generation (6th) with Trench technology to support various applications such as motor control in industrial segment. The new Trench 6 Mosfet has the following benefits: reduce the Rspec - m? / mm2 to lower RDS(ON) and allow fast switching; reduce gate-charge & switching loss; lower QG(tot) & FOM for best efficiency; increase Tj(max) to 175C for reliability and high performance application support. The continuing extension of the product portfolio represents a very good fit into the motor control applications. In the below table you will see an overview of the BLDC board characteristics:

Table 1: BLDC board setup and product key features Further developments based on our Cortex-M products will address BLAC with Field oriented control and U/f control. These developments are linked to our microcontroller family concept that has proved its continuity in offering similar peripheral IP, software compatibility and easy migration within various architectures like ARM7, Cortex-M0, Cortex-M3 and the new Cortex-M4. This strategy allows us to offer for different motor control methods not only the appropriate mix between CPU performance and required peripherals but also tool and software re-use among all the various projects (for example software modules written for Cortex-M0 can be re-used on Cortex-M3/M4 based micros). Our customers can therefore dramatically reduce the time to market and keep the tool investment at a minimum level (same IDE, debugging and programming tools). www.nxp.com