Consumers can now purchase the newest line of laundry room appliances that handle not only bulky, dirty items but also fine, delicate fabrics. Home appliance manufacturers have added this increased flexibility and reliability in products in part through advances in motor control technology.
Originally, AC motors powered washing machines. Mechanical transmissions provided the various agitation levels needed for different wash cycles (normal, gentle, delicate), and higher drum speeds achieved more effective water extraction. This transmission permitted a single AC motor without electronic controls to be used in the washer. But transmissions have severe limitations when it comes to cost-effectively providing greater flexibility in agitation cycle and speed control. As the transmission becomes more sophisticated, its complexity (and cost) increases, and its reliability plummets. Thus, manufacturers introduced machines using brush DC motors that provided low-cost speed control with less expensive transmissions to achieve extended control of the machine wash/extraction cycle. However, brushes and transmissions wear out, making for a unit with poor reliability. Thus, the need for greater reliability led to the next phase of washing machine motor control: the integration of microcontroller units (MCUs) with a brushless DC (BLDC) motor. By introducing MCU electronics and BLDC motors, washing machine manufacturers were able to gain even better control of the machine cycle. The MCU was able to electronically commutate the motor to obtain accurate control over motor speed without having to use motors needing the unreliable brushes. MCUs are still a viable choice in many of today's washing machine applications. But tomorrow's high-end washing machine users want even more sophisticated control that only digital signal processor (DSP) technology can offer. Motor control circuit designers increasingly use this new-phase DSP technology to meet the challenge of providing numerous features, energy efficiency, reduced noise, and more reliable operation at a cost consumers can afford. Furthermore, DSP technology makes them even easier and less expensive to manufacture because the mechanical transmission can be reduced to a simple pulley and belt.
MCUs and DSPs have many features in common, and both can be used to provide viable motor control solutions. As long as the proper hardware is present, any motor controller chip can execute any motor control algorithm, but the performance levels available differ greatly. The designer makes choices based on the level of control required. The speed of execution, for example, determines whether sensored or sensorless feedback is needed. With sensored feedback, the designer can use an MCU to do a simple open-loop, volts-per-hertz control. But for closed-loop, sensorless feedback, control is based on the rotation and phase angle of the motor and on its torque requirements. The MCU isn't fast enough to dynamically calculate all of these requirements, so in this case a DSP is required. MCUs can generally offer the best cost/performance choice for open-loop (no feedback) or opto/electromechanical closed-loop feedback systems used in applications controlling motor speed and position. However, a DSP may be the best option with a high-speed or high-torque control motor, where a very high-performance controller is needed, or when sensorless control can further reduce system costs. One solution has been to create DSP controllers that combine the best features of microcontrollers and DSPs in a single architecture. Traditional DSPs were designed to execute signal-processing algorithms efficiently, which leads to compromises between developing a good DSP architecture and a good microcontroller architecture. Chips for motors are often called on to execute a range of algorithms and traditional microcontroller code as well; thus they need the abilities of both DSPs and MCUs. Consequently, most DSP motor control applications used both a DSP and a microcontroller. The use of extra chips, however, adds substantially to a product's material costs, reliability, and development time. Combining the functions of separate circuits in a single chip therefore offers significant reductions in cost and development time.
Washing machines impose a wide range of speed and torque requirements on their motors, with a high torque on the low-speed wash cycle and a low torque on the high-speed spin cycle. Speed ripple and load torque of the washing machine motor provide valuable information on the washing load that allows the machine controller to automatically select the wash program. The speed ripple can also be used to estimate a load unbalance before starting the spin cycle, thereby improving the mechanical reliability of the machine. The capability to vary speed and torque instantaneously gives manufacturers great flexibility in their designs. Torque developed by the motor has to be precisely regulated, which is usually accomplished by controlling the current phases. Washing machines benefit not only from variable speeds but also from greater control of the motor's torque. Thus, advanced washing machine features require precise control and fast response times, and because DSP controllers use digital rather than analog current loops, they're ideally suited to meet these requirements. For that reason, these machines have been among the first appliances to implement DSP controller-based control solutions. Additionally, the DSP-controlled motor provides a more effective and gentler agitation cycle that allows the drum containing the clothes to be rotated first in one direction, then stopped, then rotated in the opposite direction without requiring any additional mechanical device. This forward/reverse agitation cycle provides a more effective means of cleaning clothes without damaging the fibers used to make them. Finally, when the wash cycle finishes and the spin cycle begins, the same motor can then be sped up to a higher speed than previously attainable to thoroughly eliminate water in the clothes and reduce drying times (saving even more energy in the process). The more sophisticated the controller, the better the control of the motor that can be achieved. Using DSP controllers to incorporate this kind of rotational control allows designers to provide washing machines with features that were previously cost prohibitive or even impossible to perform. High-speed water extraction, gentle agitation cycles, and out-of-balance correction are already found in the latest high-end washing machines. As DSP controllers become increasingly more affordable, manufacturers will begin designing these features into lower-end models as well. DSPs allow us to incorporate new features that weren't possible before, as well as significantly lower energy consumption. For example, DSPs can match motors to the specific load at any given time, and they allow a "soft start" of the motor. That is, the motor can be started slowly and build up to speed, which eliminates the current surge at the beginning and makes the washer run much more quietly. And because the bulky mechanical transmission has been eliminated, you can now readily reverse direction.
DSP controllers use sophisticated algorithms to execute functions "on the fly," enabling complex multivariable functions for today's feature-rich appliances. DSP controllers are also extremely accurate because they can calculate actual rotor position and speed with field-transformation equations rather than with just an electromechanical feedback system and a lookup table. System operations based on algorithms rather than on electromechanical and table lookup processes result in smoother operation, which in turn reduces mechanical resonance excitation, reduces power consumption, and improves the reliability of drivers and motors. Motorola's DSPs make extremely efficient use of algorithms as well as memory management, which means designers have a choice: lower cost by using smaller memory or use the freed-up memory to include more functions. Another strength of DSP controllers is that they can handle complex, real-time computations; they're well suited to meet speed demands of up to 40 million instructions per second with a single-cycle, multiply-and-accumulate function; and, with a multibus architecture, they can fetch instructions and data simultaneously. The peripherals needed to support an application are generally integrated onto a single DSP controller such as integrated flash memory, pulse-width modulation (PWM) modules optimized for motor control, analog-to-digital converters, onboard voltage regulation, controller-area network modules, and both synchronous and asynchronous serial communications interfaces. DSP controllers allow performance to be scaled for different motors simply by changing software, which results in development cost savings as well as faster time to market.
無感測控制 (Sensorless Control )
DSP controllers execute control algorithms that use input signals from motors—including phase currents, voltages, flux (rotor or stator), rotor position, rotor speed, and rotor temperature—to generate the required output control signals. These algorithms make it possible to eliminate Hall-effect and optical sensors and implement true sensorless control, which calculates velocity and position in real time from known current and voltage values, and field-oriented control, which converts all variables to a coordinate system relative to the rotor's magnetic field. Although mechanical devices for sensing position and velocity can supply the input variables for motor-control circuits, implementing such sensors is often technically difficult or expensive. The alternative is a DSP-based mathematical option that calculates velocity and position in real time from known current and voltage values, which eliminates the need for sensors. DSP controller-based sensorless control is ideal for washing machines and any other applications where reduced system size and cost are a priority.
脈寬調變控制 (Pulse Width Modulation Control )
Implementing a DSP controller's advanced algorithms and flexible PWM can control mechanical resonances that cause stress on the motor. PWM is frequently used to control switching-power converters in motor control systems, which eliminates the need for digital-to-analog converters and reduces component count, power dissipation, and motor drive system size. DSP controllers carry out electronic commutation of AC motors, and space-vector modulation implemented on DSP controllers executes intensive calculations within nanoseconds. DSP controllers thus eliminate undesired harmonics in motor currents and improve the condition of the signal. Space vector modulation is a leading-edge algorithm that can be implemented only with a DSP. It's important because it allows the use of low-cost AC motors instead of BLDC motors. By implementing space-vector modulation with PWM, much more sophisticated AC motor control is possible. The first choice a designer has to make is how fast he wants the motor to run. No motor control increases the top speed; the true objective of motor control is to control a motor's operation over its entire speed range.
磁場導向控制 (Field-Oriented Control )
Using DSPs for field-oriented control (FOC) is one of the best ways to control an AC induction motor. FOC provides excellent dynamic control behavior by converting all variables to a coordinate system relative to the rotor flux. It uses the current component that is parallel to the rotor flux to hold the flux constant and controls the motor torque by the orthogonal current component. The DSP controller calculates the rotor flux requirement and performs the necessary coordinate transformations of variables from the stator frame of reference to the rotor flux frame of reference. It continuously calculates the difference between them to be able to drive the motor. It must perform a real-time conversion from field coordinates to stator coordinates and back during each controlling cycle. DSP controllers make it possible to derive the speed and position of the motor's rotor indirectly from changes in current. A DSP controller provides instantaneous FOC without the need for position/ speed sensors, which is less expensive than microcontroller-based FOC because the latter requires such sensors. In an AC induction motor, the FOC scheme is necessary when a wide-speed range and high dynamic performance are required. Flux and torque currents can be independently controlled to provide performance close to that of a permanent magnet synchronous motor. For low-speed operation, the flux is kept constant, and torque is directly proportional to the torque current. Flux is reduced to allow operation at higher speeds when the DC bus voltage limits the motor voltages.
由於三相無刷直流馬達(brushless DC motor)具有高效率、易於控制、不需維護(無碳刷)等優點， 藉由DSP控制又能使其發揮節約能源、符合多種應用需求的優點， 因此，近年來，在電冰箱、洗衣機、空調機等家電產品的應用以日益增加。 可調速的無刷直流馬達可說是應用於高效率、價廉、安靜、可靠等場合的最佳選擇， 但由於在應用上性能需求的提升，也就需要更多的計算，例如無感測器的磁場方位計算、相電流估測等，這些計算必須在極短的時間內完成， 通常必須低於100微秒，這些大量且負宅的數學運算，是一般微處理器所無法做到的，因此，就有賴於DSP來實現這些馬達控制運算。
近年來由於環境保護的要求水準日益提高，高效率的空調與冷凍系統已成為未來的發展趨勢，為了有效提升整體效率，新一代的高效率壓縮機已採用高效率永磁式無刷直流馬達變速驅動方式，微電腦控制變頻器成為其中的關鍵組件。因此，如何發展低成本、高效率的專用型變頻器，遂成為變頻器廠商重要的研究課題。無刷直流馬達的轉子由永久磁鐵構成，根據轉子磁通的分佈，可分為：梯形與正弦波兩種。由於轉子磁通分佈的不同，無刷直流馬達的驅動方式也隨之而異，其轉速估測的方法也不同，因此發展出了各種不同的無感測驅動技術。 Before DSP controllers became practical solutions for motor control, AC induction and BLDC motors required complex, expensive controllers to contend with cross-coupled torque and speed controls, but the high performance of DSP controllers makes it possible to implement math-intensive control techniques that enable low-cost controllers for these motors.
研究結果顯示，結合DSP與高效率電源轉換技術於洗衣機的馬達控制，可節省約50%的電費。 此外，一個高效率的洗衣機也就意味著需要較少的清潔劑、較少的水，經年累月，可替消費者節省可觀的金錢。 電子驅動的高效率馬達也去除了傳統式洗衣機的傳動機構設計，簡單的機械結構，不僅提高了可靠度，延長了洗衣機的壽命，同時也更易於維護。
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