The contribution of electric motors to energy consumption is close to 50% in the United States, so reducing motor energy consumption can effectively improve energy efficiency, and using advanced microcontroller (MCU) technology to achieve motor control is an effective method. This paper introduces the latest developments in motor control MCU technology and its applications.
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One of the main targets for reducing energy consumption is the motor, which consumes about 50% of the total US energy consumption. More than 50 motors can be found in the home, usually 70 to 80. In the industrial field, the automatic control of the factory is also widely used.
Today, recent developments in MCU technology allow motors to operate more efficiently at lower cost. In some markets, this accelerates the transition from electromechanical to electronic control, enabling variable speed motor control to optimize motor efficiency and reduce the cost of all applications at the device level.
Low cost brushless DC motor control MCU
Compared to brushed motors that are often used in motor control, MCU-controlled brushless DC (BLDC) motors eliminate brush wear and arcing mechanisms, so the life of the motor is essentially limited by the life of the bearing. In addition, the advantages of MCU-based BLDC motor systems include high efficiency, high torque-inertia ratio, higher speed performance, low noise, better thermal efficiency, and lower EMI characteristics.
The use of 8-bit MCUs specifically designed for mass production of motor control is a very low cost method for solving digital motor control problems. With up to 10 MIPS of performance and motion control-specific hardware (including center-aligned 14-bit PWM, a motion feedback module, and a high-speed ADC), applications that previously required expensive processors can now be addressed with low-cost 8-bit MCUs.
Figure 1: Example of an MCU in a washing machine: An 8-bit microcontroller drives a three-phase ACIM variable frequency drive.
Three-phase PWM control in some MCUs, such as the PIC18F4431, can provide all three of the BLDC controls in hardware, minimizing the software that must be developed and debugged. Up to 8 available PWM channels, usually only need 6 to drive a three-phase motor. Therefore, the remaining two channels can be used for other functions without the need for additional components. The motion feedback module with integral encoder interface as the main part of the MCU reduces the number of components and system cost.
An MCU with an ADC with a sampling rate of 200K per second provides the speed necessary for closed loop control. Simultaneous use of two different channels makes it possible to simultaneously sample voltage and current. Such a fast transition is required when measuring the back end electromotive force (EMF) in closed loop motor control, and the ability to synchronize the ADC with the PWM on the rising or falling edge minimizes switching noise. Together, these modules eliminate the need for external motor control components such as high speed ADCs and position encoders.
In many motor control applications, fail-safe operation is very important. An MCU 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) allows designers to use digital control that provides high reliability. Programmable deadtime delays on the PWM minimize switching noise, reduce development time by weeks, and meet critical program deadlines to bring new products to market. In all cases, MCUs with reliable flash memory offer the potential for rapid time-to-market and flexibility to adjust to changes in demand before installation or during use.
Market drivers and solutions
Electronic motor control requirements in home electronics, the industrial and automotive markets have driven the need for MCUs with advanced motor control peripherals.
In the home appliance market, motor control with improved performance is needed to meet government planning standards, such as the Energy Star Program of the US Environmental Protection Agency, which promotes the introduction of high-performance appliances. Washing machines are an important area of ​​high performance motor control. The direct drive washing machine eliminates the drive belt between the motor shaft and the washer agitator, allowing for different speed and agitator modes.
A manufacturer's fully redesigned washing machine consumes 38% less electricity and 17% more water than a conventional washing machine. The motor control MCU adjusts the power of the motor according to the amount and type of laundry. However, home appliance users are still sensitive to the initial purchase price, so manufacturers must continuously reduce their development and production costs so that more advanced appliances can be accepted by more consumers.
In home appliances, 8-bit MCUs designed specifically for low-cost motor control applications integrate features that minimize additional components. With motor PWM on the board, fail-safe clock monitor and highly reliable flash memory, the latest MCUs simplify the design of home appliance motor control and achieve low cost targets.
In industrial applications, power costs and downtime of assembly operations can reduce the benefits of the manufacturer. An industrial example of how control of motor performance improvements directly affects efficiency and profitability is to replace a valve in an industrial pump with a variable speed (VSD) system with an MCU.
For a pump or fan, the power consumption is proportional to the cube speed of the shaft. When the shaft speed is reduced by 10%, the flow is reduced by 10% and the power consumption is reduced by 27%. If the speed is reduced by 20%, the power consumption is reduced by 49%. By using MCU variable speed motor control instead of constant speed motor valves to reduce flow, it has been proven that 25-40% energy savings can be achieved for centrifugal pumps, fans and blowers in industrial applications.
The benefits of industrial applications are obvious, and MCU-driven variable speed motor control relies on other factors such as flexibility and reliability - these factors can avoid downtime caused by faults or overhauls. MCUs with flash and EEPROM provide the flexibility to solve industrial user requirements with reprogrammable features when upgrades or control program requirements are required. MCUs with 16KB of flash and 256B EEPROM provide enough memory in an 8-bit MCU to handle many of the changes that may be needed in an industrial environment. Equally important, Microchip's flash memory uses PMOS electronically erasable unit processing technology that typically has a data storage unit capable of withstanding 1 million erase/write cycles and data that can last for more than 40 years.
Existing automotive motor applications include the use of motors to open and close windows and doors, and to position the seats. Because these applications are used at a low frequency, they are not sensitive to inefficiencies, but high-utilization applications, such as passenger temperature environment control and engine box fans, continue to consume limited power from the car. The motor control MCU allows the environmental control fan to operate at a speed that maintains a comfortable temperature, thus minimizing noise and reducing power consumption.
In many cases, the motor control MCU must be connected to the car network using a Control Area Network (CAN) or a Local Interconnect Network (LAN). For body electronics, low-cost LIN protocols are now used to reduce overall system cost. In some MCU families, a USART module that supports LIN 1.2 can be found, along with automatic wake-up and baud detection 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 (DSCs) bring higher performance and affordable, design engineer-friendly MCU technology for more sophisticated motor control designs, including those with vector control. DSCs operate at speeds up to 30 MIPS, with integrated peripherals dedicated to flash memory and motor control up to 144 KB for more advanced, new motor control applications. With electronic motor control based on DSP and DSC, home appliance industry control and automotive not only work more efficiently, provide more functions, and are more affordable.
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