Cost-effective motor drives tackle energy usage

Cost-effective motor drives tackle energy usage

Low-cost 8-bit MCUs can handle efficient motor control

BY RYAN SCOTT
Infineon North America
Livonia, MI
http://www.infineon.com

Computing applications attract a lion’s share of attention in discussions of how the electronics industry can help reduce energy use. However, according to the Electric Power Research Institute (EPRI), motion applicationswhich include the motors driving consumer white goods and industrial machineryaccount for more than 50% of the electrical energy consumed in the world, making this area a prime target for new designs that reduce energy consumption.

In addition to power management ICs, the equation for these motion control applications relies on chip-based intelligence in the form of MCUs. The MCU allows motors to be managed more efficiently and at a lower cost, accelerating the transition from electromechanical to electronic control and enabling the implementation of variable-speed motor (VSM) control.

MCU-controlled brushless dc (BLDC) motors provide higher efficiencies, high torque-to-inertia ratios, greater speed capabilities, lower audible noise, better thermal efficiencies, and lower EMI than traditional dc motors. An intelligent motor’s efficiency can exceed 95%, compared to 85% for an induction motor. Furthermore, in many applications a variable-speed MCU-controlled BLDC motor can yield a 25% to 40% energy savings compared to a constant-speed motor.

Energy efficiency for BLDC motors

The availability of cost-effective MCUs for electric motor control is also leading both traditional drive manufacturers and appliance manufacturers to compare the benefits and tradeoffs of specific control techniques for VSMs. Using the traditional scalar control method for VSMs requires manufacturers to upsize their motors to accommodate large transients or voltage spikes.

To avoid the cost of up-sizing, manufacturers are looking to field-oriented control (FOC) (also known as vector control) techniques that allow them to actually reduce the size of the motor. FOC designs also provide better dynamic response, higher power density, and lower torque ripple, each of which leads to increased system efficiencies. Additionally, FOC can also eliminate position sensors and require only a single shunt resistor, thereby reducing manufacturing costs and increasing reliability.

Field-oriented control attempts to maintain the angle between the stator flux and the rotor flux in a motor at 90° by varying the currents in the stator windings. While the system knows the stator flux angle, it must measure or estimate the rotor-flux angle to calculate the difference between the two.

When the rotor-flux angle is determined, a vector-control algorithm determines the optimum timing and magnitude of the voltages to apply to the stator-phase windings. Because these vector-control algorithms are numerically intensive, typical implementations of sensorless FOC today require a 16- or 32-bit MCU, DSP, or DSC to handle the complex trigonometric equations.

Additionally, maintaining the required accuracy means that lookup tables need to be built, which mandates larger flash memory sizes and complex software to address the aspects of current calculation, vector rotation, space vector modulation, and proportional/integral control. These factors increase the cost of the control system.

Newly available 8-bit microcontrollers with specific architecture enhancements, such as the XC800 family from Infineon Technologies, provide the required functionality in hardware (see Fig. 1 ) to implement a more cost-effective FOC system.

Fig. 1. Low-cost 8-bit controllers with necessary specialized blocks provide all the horsepower needed for field-oriented motor control.

A low-cost 8-bit FOC solution

The FOC-capable MCU combines an 8051-compatible core with a powerful on-chip processing unit, the “vector computer,” that performs multiple calculations on vectors (one-dimensional arrays) simultaneously (see Fig. 2). The vector computer incorporates several processing elements, including a coordinate rotational digital computer (CORDIC) unit and a multiply divide unit, which are capable of 16-bit math when used with a 16-bit capture/compare unit and a fast on-chip A/D converter.

Fig. 2. An FOC-capable MCU combines an 8051-compatible core with a powerful on-chip processing unit that performs multiple calculations on vectors simultaneously.

The CORDIC supports iterative execution of complex mathematical and trigonometric functions, such as Clarke and Park algorithms, using addition, subtraction, and shifting, with a very short lookup table. While providing results with up to 16-bit resolution, the CORDIC functions independently from the CPU core, thereby freeing up those resources for other control-oriented tasks.

The multiply divide unit can perform 16- and 32-bit math operations, and may be used instead of the standard 8051 MUL/DIV instructions. To further reduce flash size and increase access speed, math libraries for fixed-and floating-point instructions may be added to the boot ROM.

As stated previously, a key objective of an FOC algorithm is to ensure that the magnetic field of the stator maintains rectangular orientation with respect to the position of the permanent magnets of the rotor. Their relationship is estimated by a single shunt current measurement requiring fast triggering of the A/D converter by the corresponding PWM pattern. This objective is achieved using an event-based hardware trigger from the CapCom6Ean intelligent PWM unitto the A/D. This event-based trigger eliminates interrupt latency and enables fast and accurate current measurement.

The combination of on-chip math units and peripherals allows implementation of FOC with sufficient headroom for other system control functions using a very-low-cost 8-bit MCU. For example, with a 15-kHz PWM frequency and 133-µs current measurement, FOC control requires only 58% of the CPU performance, providing headroom for application-specific functionality. And unlike hard-coded FOC implementations, an MCU with an integrated vector computer offers software re-programmability. This can be used to optimize the startup phase of the motor by programming a controlled ramp or implementing field weakening methods, such as weakening the ID component of the FOC algorithm.

Evaluation of sensorless FOC

An FOC drive application kit can be used to evaluate an 8-bit MCU-based sensorless FOC. This kit would include a vector computer-based MCU, three-phase power inverter board, 24-V BLDC motor, plug-in power supply, and complete FOC source code.

In addition, a CAN-to-USB bridge allows for hex code downloading, which can modify such motor parameters as speed and current control during motor operation, offering real-time control. Infineon offers a complete development environment with a free tool chain that allows users to advance to the next stage of application development and customization using the same kit (see http://www.infineon.com). ■