Study on Field-Oriented Control of Three-Phase Brushless Permanent Magnet Motor

Next Generation Motor Controller Design

BAE Systems Avionics designs and manufactures military electronics and surveillance systems. To stay competitive, the Avionics division continually evaluates new tools and techniques to reduce the time it takes to design new technologies into production. The time we spend in the lab developing both hardware and software is key to our continued success.

Field-oriented control (FOC), or vector control, is a new technology that can improve the torque-speed characteristics of a wide range of electric motors, and most of our products incorporate at least one DC motor. The Servo Systems Technology Group at BAE Systems in Edinburgh is interested in increasing peak power because the upgraded motor drives will provide additional performance to existing motors and save weight in aerospace products by reducing motor mass in new designs.

At the same time, as FPGA performance improves, we can use FPGA not only for motor control, but also for servo system control. We use NI products for rapid prototyping, which significantly reduces the risks of new technologies in the early stages of design.

ACEIII current

FOC Technology

Motors driven by traditional square wave amplifiers are limited by non-ideal torque-speed characteristics and torque ripple caused by commutation errors. Sinusoidal commutation solves the torque ripple problem and works well for low speed motors. But at higher speeds, the PI current controller must increase the frequency to track the sinusoidal current while overcoming the back EMF problem of increasing frequency and amplitude. This results in a phase delay, which causes a loss of torque per ampere because the torque-producing flux does not act at 90 degrees on the rotor. This effect is represented by the curve in the torque-speed (TS) graph. Basically, the TS curve consists of two lines, the horizontal line is the voltage limit that determines the maximum speed, and the vertical line is the current limit that determines the maximum torque.

We use FOC to improve the TS characteristic. This commutation method uses a transducer to transform the sinusoidal current and encoder position to a dq reference frame of the rotating rotor. The d and q components are DC, so it is easy to control them using a PI controller. The controller output is then inverted to output a voltage waveform with the correct phase and amplitude to maintain a 90-degree angle between the flux and the rotor, thereby obtaining maximum current-to-torque power conversion.

Space Vector Modulation and FPGA Implementation

With fully digital control, we can use space vector modulation (SVM) to unlock 15% more no-load speed. FOC control makes this possible because we are no longer limited to the classic rectifier limit of bus voltage/2. The triangle characteristic of SVM follows the 30, 60 and 90 degree triangles and 1, 2 and the side lengths, changing the relative relationship to bus voltage/. From this ratio, we can calculate that bus voltage/2 divided by bus voltage/ is equal to 1.1547, or a 15% increase.

Traditional FPGA control algorithm implementations come with great risk because the first physical implementation will continue to be in service until the end of the product design cycle. By using NI LabVIEW FPGA Module software for rapid controller prototyping, we can start testing and further developing the actual hardware even before starting FPGA design.

We use the Math Model Toolkit with fixed-point macroblocks to simulate the math functions of the FPGA for algorithm development. We can quickly rewrite fixed-point algorithms in G-code and run them on the NI PXI platform or CompactRIO reconfigurable control and acquisition platform. During the compilation process, hardware description language (HDL) generation, logic analysis, HDL simulation, and placement and routing operations are fully automated. The VHDL code is downloaded to the Virtex VC2V1000 of the NI PXI-7831R through the backplane of the PXI chassis. The PXI-7831R provides eight 16-bit analog-to-digital converters, eight 16-bit digital-to-analog converters, and 96 transistor-transistor logic I/O pins for quick hardware connections using plug-in terminal cards. Debugging is also easy because we can read data from any FPGA register and display the results on the host running NI LabVIEW without affecting the operation of the FPGA.

Rapid system component prototyping

Our rapid prototyping system for researching new technologies consists of a PXI chassis with an NI PXI embedded controller running LabVIEW software and a PXI-7831R reconfigurable I/O module. We use the LabVIEW graphical development environment, LabVIEW FPGA Module, to develop all system components. As described above, we configure and program the PXI-7831R FPGA directly in the host's LabVIEW environment. The compiled LabVIEW code can be downloaded directly to the FPGA. LabVIEW software running under the host's Windows operating system provides system monitoring and visualization capabilities, which were also developed using LabVIEW.

By using the NI PXI-7831R FPGA, we were able to demonstrate new technology to our customers with minimal investment in time and instrumentation. Without any VHDL learning experience, we created a 40kHz real-time controller that far exceeded the performance of the single-point I/O used previously.