MPUs & MCUs
Homerun
bandwidth limitations of the PI controllers. In FOC (Figure 1), the sensed stator
currents are translated into rotor direct (D) and quadrature (Q) components by a transform function. To achieve maximum torque, the D and Q currents are then compared respectively with zero and the torque requested by the application. The resulting error signals are input to the two PI blocks, which generate signals in the D-Q reference plane. These must then be transformed into the stator domain to generate the PWM signal for each stator phase. Because the inputs to the PI
Frank Thimm looks at how embedded processing has evolved in terms of domestic appliance design
A
s well as delivering innovative features to attract buyers, embedded system design in the
home appliance market is essential to satisfying environmental standards and addressing consumer concerns relating to the consumption of resources such as energy and water. And with in-house design teams looking to focus on their core competencies, demand is growing for technologies and tools that speed the implementation of these embedded systems. The embedded systems deployed in
domestic appliances today are expected to handle everything from the management of user interface functions to system-level features. The need to focus on core design competencies is fuelling the need for microcontrollers that help to simplify designs – not only by offering the appropriate level of processing power but also by minimising the need for external components and memory, ensuring suitable levels of connectivity, and delivering more and more integrated functions. Designers may be looking to migrate from more traditional 8-bit solutions to 32-bit architectures without price compromises and, unlike most consumer designs, there may also be the need for compatibility with 5V (rather than 3.3V) operation. Controlling the motors used in
products such as washing machines, dishwashers and refrigerators is one of the key design tasks for many
34 April 2010
appliance designers. In this respect these systems have similarities with those developed for robotics, fans, pumps and other industrial motion control applications. In particular, the challenges of implementing control of the three-phase BLDC motors deployed to provide high accuracy, high- efficiency operation are common to both.
Motor Control
Unlike fixed-speed alternatives, brushless, electronically commutated DC (BLDC) motors require complex control techniques. Typically real-time rotor position information is needed to adjust the direction of the applied excitation field. This can be detected using discrete sensors such as Hall- effect devices, or through sensorless implementations based on rotor current measurement. Moreover, relatively sophisticated speed control methods can minimise harmonics, optimising energy efficiency and preventing distortion entering the mains supply. The goal of any electronic
commutation system is to adjust current in each stator phase to produce a net stator field in quadrature with the rotor field. This maximises useful torque and helps reduce unwanted rotor bearing stress. The underlying function contained in any sensorless control unit compares the motor current with the desired torque and applies a Proportional-Integral (PI)
Components in Electronics
function to the resulting error signal to generate a correction signal. This signal is subsequently pulse-width modulated and used to control the output bridge of the motor driver. Applying a sinusoidal current
waveform to the stator windings produces the smoothest motor torque characteristic, but requires the desired value of stator current to be computed quickly as soon as the rotor position is detected. For high rotor speeds, the PI function must have a high bandwidth in order to provide timely computation of the stator current. Field Orientation Control (FOC), or Vector Control, overcomes the speed limitation of sinusoidal control by manipulating the motor currents and voltages with reference to the rotor’s direct and quadrature axes. This is achieved by ensuring the stator field remains constant and in quadrature with the rotor field irrespective of any
functions are constant, FOC maintains high efficiency at all rotor speeds regardless of any limitations on PI- controller bandwidth. However, to perform FOC in real time requires fast execution of the functions that first transform the sensed stator current signals into the rotor domain and subsequently transform the static PI values into the voltage-control signals for the output bridge.
Moving up
Embedded systems for the highest- performing appliances have evolved from early microcontrollers based on 8- bit processor architectures to later models based around 16-bit cores. A general increase in CPU clock speeds has also allowed developers to access more of the chosen processor’s ultimate performance potential. The 32-bit ARM Cortex-M3 processor
core has arrived as a disruptive element in the embedded space. Answering demands from multiple industry sectors for increased capabilities and an improved processor price-performance trade-off, the ARM Cortex-M3 takes a ‘clean-sheet’ approach to CPU design – not limited by the constraints of legacy architectures - to deliver a step change in performance, efficiency and cost. As a licensable core, it has been widely
Figure 1. Field Orientated Control
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