issue: January 2005 APPLIANCE European Edition
Engineering Motor Control
New Motion Control Technology for Eco-Friendly Appliances
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by Toshio Takahashi, International Rectifier
A new control solution has been designed to reduce the complexity of designing variable-speed motor drives for energy-efficient appliance applications.
Figure 1. A first-generation, variable-speed, frost-free refrigerator
Lower power consumption and more efficient use of natural resources such as water are among the few remaining selling points open to manufacturers of domestic appliances now that penetration is high in most households in developed countries. In recognition of this, various ecological (eco) labelling schemes are now active in territories worldwide, which allow consumers to compare the ecological performance of appliances from various manufacturers. Current eco labels include the EU flower symbol and the ENERGY STAR® program in the U.S.
To gain eco label approval for future generations of washing appliances, for example, designers need to ensure most or all of the following benefits:
• Better efficiency by reducing the quantity of water needed for a washing cycle
• Ability to sense the load and adjust the water level
• Less detergent per load, reducing the amount of soap discharged after each wash
• Higher spin speed to extract more water and decrease drying time
Manufacturers are already bringing these advantages to the market. Energy efficiency (CEE) and ENERGY STAR® designated washers use 35 to 50 percent less water and 50 percent less energy per load, and can save as much as 7,000 gal (approx. 26,499 L) of water per year.
A different set of constraints is relevant to appliances such as refrigerators. The following factors will make it difficult for conventional models to comply with ecolabelling schemes now and in the future:
• On-off thermostat causes wide hysteresis (-14°C to -12°C) and poor energy efficiency.
• Compressor is larger than needed because it needs to reduce temperature below the set point.
• Enclosure heat loss is higher because temperature must be lowered beyond set point.
• Energy usage is higher because of timed defrost cycle, which must be designed for worst-case conditions
To address these challenges, variable-speed motors, closely coupled with advanced electronic motor controls, are the key to reaching the standards set out by the various ecolabelling schemes. Advantages include lower energy consumption, electrical and acoustic noise, and vibration. A refrigerator, for example, driven by a variable-speed motor capable of responding dynamically to temperature sensing data may offer the following advantages:
• Analogue to digital thermostat continuously controls compressor speed in response to heat loss and optimizes energy usage.
• Compressor capacity is optimized for enclosure heat loss only. (Note that rate of cooling is lower than for on-off thermostat refrigerator.)
• Enclosure heat loss is minimized by operating at fixed temperature without hysteresis.
• Defrost energy is minimized by comparing evaporator temperature with enclosure temperature and initiating defrost cycle only when needed.
A typical schematic for a first-generation, variable-speed motor control for a refrigerator is shown in Figure 1.
The schematic of Figure 1 holds the key to many enhancements in domestic appliance performance. The DSP, or digitial signal processor, is charged with executing the motor control algorithm. Developing the control algorithm focuses on implementing Field Orientation Control (FOC), which is essential for controlling three-phase a.c. motors. However, developing, debugging, and maintaining a suitable FOC is complex and usually requires special techniques to ensure adequate performance.
FOC establishes linear control of torque by transforming three-phase a.c. current and voltage into two variables—torque current and field current. As a result, closed-loop current control actually contains separate current control loops for torque current and field current. Each loop is identical and consists of several control elements such as vector rotator, Clark transformation, proportional plus integral, pulse width modulation (PWM), and current sensing. To complete each computation within a specified timeframe using a traditional motion control DSP or microcontroller demands extensive knowledge of real-time control since each of these high-priority tasks must be executed sequentially. These tasks, often driven by specific hardware events/interrupts, require precise execution timing of software, requiring sequencing of instruction coding to manipulate hardware at a specific time in order to control a motor.
In particular, the FOC for servo application and sensorless control is usually written in assembly language rather than a high-level language in order to maximize computation and update rates to meet demands for higher dynamic performance. For example, sometimes special coding techniques to speed up multiply or divide functions are necessary to overcome sluggishness in classic computation power. However, programmers with assembly language skills are even more rare and expensive than those familiar with a high level language such as C or C++.
Regardless of whether assembly or a high-level language is used, and regardless of the chosen processor architecture, implementing a FOC in software requires thousands of lines of instructions. New source code and any existing software modules must then be compiled and linked together to create the executable object code containing closed-loop control, user interface sequencing, network communication, and all other applicable functions. Any errors must be discovered and fixed at source code level, recompiled, and linked again to produce the revised version of the executable object code. This process is usually repeated a number of times to reach the final product. Another trade-off is code maintenance. The code maintenance cost is usually a hidden cost and does not show up at the start of a development phase.
After developing and debugging the motion control algorithm, the next challenge is to integrate the logic-level circuitry performing the algorithm with the power electronic components that actually deliver the continuously varying drive currents to the motor, as well as the sensing components that perform feedback control.
A conventional approach to motor control design calls for a deep understanding of power electronics technology, hardware integration, advanced control algorithm development, flexible user interface development, network communications, and other disciplines. However, one of the clearest messages emerging from the multitude of market research efforts directed at domestic appliance consumers is that buyers of such equipment are not prepared to pay a premium for any performance advantage. In addition, few consumers appear to be enticed by energy savings that offer a payback in the longer term.
Even so, buyers are more inclined to purchase equipment displaying an ecolabel. This puts appliance vendors and their subsystem suppliers in an interesting position. Variable-speed motor drives provide an easy route to the enhanced energy efficiency required by ecolabel administrators, but selling price pressures make the extra cost and design risk to build a suitable controller from scratch unacceptable.
A New Design Methodology
With this in mind, the next best way for vendors of appliance motor subsystems to implement FOC is to use an existing algorithm or find some way of sidestepping or streamlining algorithm development. However, these algorithms tend to be rather application specific, and tailoring an existing algorithm to suit by modifying the code, for example, is no trivial task.
A new option, in the form of register configurable motion control integrated circuits (ICs), implements an entire motion control algorithm in hardware, leaving the designer to complete the design by selecting optimal parameters via a configuration tool. This approach is now permeating the appliance design arena, as new motion control ICs meet the specific functional and cost requirements of the appliance market. These ICs must be able to work with all necessary power electronics and analog components in order to achieve a low selling price, high-energy efficiency, and short time to market.
By implementing a complete FOC algorithm in hardware, and combining it with a speed control algorithm, International Rectifier (IR) has created a block of IP known as the Motion Control Engine™ (MCE) that eliminates most of the complexity from designing variable-speed motor drives. The MCE includes all control elements necessary to perform closed-loop controls, such as proportional, plus integral, vector rotator, and Clark transformation. Since motion hardware peripherals supporting space vector PWM, motor current feedback interface, and encoder feedback are also implemented on-chip, as well as flow control logic for parallel multi-loop control, no multi-tasking is required. Synchronous execution mechanism of closed-loop velocity control and closed-loop current control is included in the logic hardware.
A digital control IC fabricated using the IP technology allows the designer to quickly configure registers using a dedicated PC-based configuration application, and thereby optimizes the chip to suit the selected motor parameters and other system parameters. The digital control IC replaces the DSP and removes the need for code development and subsequent code maintenance.
Basic peripherals such as PWM, encoder counter circuit, and current sensing interface can then be implemented on the same silicon as the digital controller to gain further efficiency and cost advantages. Moreover, it is possible to integrate market- or application-specific peripherals for variable-speed motor drives.
To deliver an even more complete solution for the needs of appliance developers, the digital control IC needs to be a part of a compatible chipset capable of performing all the functional demands of a variable-speed motor controller. Ideally, this calls for compatible analog high-voltage gate drivers and sensors, power silicon, and power modules to be developed together to create an integrated design platform. In addition to easing design and integration, such a chipset will bring the added benefit of reducing the number of external components required.
A significant part of the integration challenge is taken up by the requirement to interface the digital control IC to the power stage. A new high-voltage IC (HVIC) technology was created to build a three-phase inverter-driver IC as a high-speed power MOSFET and IGBT driver with three independent high- and low-side referenced output channels. For the power stage, integrated power silicon and integrated power module technology are desirable for high efficiency and low noise generation. An integrated power module (IPM) technology, for example, enables six NPT IGBT die, each with its own discrete gate resistor, six commutation diode die, one three-phase, monolithic high-voltage gate driver chip in a single module. There are also three bootstrap diodes with a current limiting resistor and an NTC thermistor/resistor pair for over-temperature protection.
With these key functional blocks (digital motion control, analog control, and integrated power stage) available as turnkey components ready for implementation on-board, a potential servo motor drive application may follow the layout shown in Figure 2.
To derive the maximum benefit from consumer awareness of the environment and the escalating costs, manufacturers need to be able to quickly adopt the latest, most efficient drives and controls that support reduced consumption of energy and water, as well as reduce electrical and acoustic noise, which also affect the quality of the environment. A dramatic change in the approach to designing such controls is required if manufacturers are to implement the new drives quickly and cost effectively. The concept of a register configurable digital controller, with associated compatible components, offers manufacturers a way to dramatically enhance the performance of domestic appliances while, at the same time, speeding up development and product turnaround.
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