The contribution of motors to energy consumption in the United States is close to 50%, so reducing motor energy consumption can effectively improve energy efficiency, and the use of advanced microcontroller (MCU) technology to achieve motor control is an effective approach. This paper describes the motor control MCU technology development and its application.
A major target for reducing energy consumption is the electric motor, which consumes about 50% of the total energy consumption in the United States. More than 50 motors can be found casually in a home, typically 70 to 80, and in industry, the use of motors for factory automation is widespread.
Today, recent developments in MCU technology allow motors to operate more efficiently at a lower cost. In some markets, this can accelerate the transition from electromechanical to electronic control, enabling variable speed motor control to optimize motor efficiency and reduce costs at the device level for all applications.
Compared to brushed motors, which are often used in motor control, MCU-controlled brushless DC (BLDC) motors eliminate brush wear and arcing mechanisms so that the life of the motor is essentially limited only by the life of the bearings. In addition, the advantages of MCU-based BLDC motor systems include high efficiency, high torque-to-inertia ratio, higher speed performance, low noise, better thermal efficiency, and low EMI characteristics.
Utilizing high-volume production 8-bit MCUs specifically designed for motor control is a very low-cost approach to solving digital motor control problems. With performance up to 10 MIPS and dedicated hardware for motion control (including a center-collimated 14-bit PWM, a motion feedback module, and a high-speed ADC), applications that previously required very expensive processors can now be solved using low-cost 8-bit MCUs.
Figure 1: Example of MCU application in a washing machine
Three-phase PWM control in some MCUs, such as the PIC18F4431, can provide all three controls of a BLDC in hardware, minimizing the amount of software that must be developed and debugged. With up to eight available PWM channels, only six are typically needed to drive a three-phase motor. Therefore, the remaining two channels can be used for other functions without the need for additional devices. The motion feedback module with integral encoder interface, which is the main part of the MCU, reduces the number of devices and the system cost.
The MCU with an ADC with a sampling rate of 200K per second provides the necessary speed for closed-loop control. The simultaneous use of two different channels makes it possible to sample voltage and current simultaneously. Such fast conversions are needed when measuring the back-end electric potential (EMF) in closed-loop motor control, and the ability to synchronize the ADC with the PWM on the rising or falling edge makes switching noisy. In summary, these modules make external motor control devices such as high-speed ADCs and position encoders unnecessary.
In many motor control applications, fail-safe operation is very important. MCUs with a fail-safe clock monitor (an internal RC oscillator that can be used as a backup clock in the event of a crystal failure) allow design engineers to use digital control that provides high reliability. Programmable deadtime delays like those on PWMs make switching noisy, reduce development time by weeks, and meet critical program deadlines to get new products to market. In all cases, MCUs with reliable flash memory offer the possibility of fast time-to-market and the flexibility to adjust to changing requirements before installation or during use.
Electronic motor control requirements in home electronics, industrial and automotive markets are driving the demand for MCUs with advanced motor control peripherals.
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In the appliance market segment, performance-enhancing motor controls are required to meet government planning standards, such as the U.S. Environmental Protection Agency's Energy Star program, which promotes manufacturers to introduce high-efficiency appliances. Washing machines are an important area for high-performance motor control. Direct-drive washing machines eliminate the drive belt between the motor shaft and the washer agitator, allowing for different speeds and agitator modes.
A manufacturer's completely redesigned washing machine consumes 38% less power and 17% less water compared to a conventional washing machine. The motor control MCU adjusts the power of the motor according to the amount and type of laundry. However, appliance users remain sensitive to the initial purchase price, so manufacturers must continue to reduce their development and production costs to make more advanced appliances accessible to a wider range of consumers.
In appliances, 8-bit MCUs designed specifically for low-cost motor control applications integrate features that minimize the need for additional devices. With motor PWM on the board, fail-safe clock monitor and highly reliable flash memory, the MCU simplifies the design of appliance motor control and can achieve low-cost goals.
In industrial applications, the cost of power and downtime for assembly operations can reduce a manufacturer's bottom line. An industrial example of how control of motor performance improvements directly affects efficiency and profitability is the replacement of a valve in an industrial pumper with a variable speed (VSD) system with an MCU-based unit.
For a pump or fan, power consumption is proportional to the cube root of the shaft speed, and when the shaft speed is reduced by 10%, the flow rate is reduced by 10% and power consumption is reduced by 27%. If the speed is reduced by 20%, the power consumption is reduced by 49%. Reducing flow by using MCU variable speed motor control instead of constant speed motor valves has been shown to yield 25-40% energy savings for centrifugal pumpers, fans and blowers in industrial applications.
The benefits for industrial applications are clear, and the use of MCU-driven variable speed motor control relies on other factors such as flexibility and reliability - factors that can avoid downtime due to failure or maintenance. MCUs with flash memory and EEPROM offer the flexibility to address industrial user requirements through reprogrammable features when upgrades are required or when control program requirements change. MCUs with 16KB Flash and 256B EEPROM provide enough memory in an 8-bit MCU to handle the many changes that may be required in an industrial environment. Equally important, Microchip's flash memory uses PMOS electronically erasable cell processing technology that typically has data storage cells that can withstand 1 million erase/write cycles and data retention for over 40 years.
Established automotive motor applications include the use of motors to open and close windows and doors, and to position car seats. While these applications are not sensitive to inefficiencies due to their low frequency of use, high utilization applications, such as passenger temperature environmental control and engine compartment fans, constantly consume the vehicle's limited electrical power. Motor control MCUs allow the ambient control fan to run only at a speed that maintains a comfortable temperature, which makes noise and reduces power consumption.
In many cases, motor control MCUs must be connected to the vehicle network using a control area network (CAN) or a local interconnect network (LAN). For vehicle body electronics, low-cost LIN protocols are now used to reduce overall system costs. In some MCU families, a USART module can be found that supports LIN1.2 while providing automatic wake-up and baud detection (baud detect) at the start bit.
As control algorithms become more complex in all market segments, the performance of digital motor controllers rises from the MCU to the DSP level. Digital signal controllers (DSC) bring higher performance and affordable, engineer-friendly MCU technology for more sophisticated motor control designs, including those with vector control applications. With DSP and DSC-based electronic motor control, the appliance industry control and automotive not only work more efficiently, but also at an acceptable price.
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