Selex-Galileo Develops Field-Orientated Control of a Three-Phase Brushless Permanent Magnet Motor

Author(s):
Brian Mann - Selex-Galileo

Industry:
Aerospace/Avionics

Products:
PXI-7831R, PXI/CompactPCI, LabVIEW, FPGA Module

The Challenge:
Developing rapid prototyping for the next generation of high-performance motor controllers.

The Solution:
Using the NI PXI platform, LabVIEW 7 Express, LabVIEW 7 FPGA Module, and PXI-7831R reconfigurable I/O modules to implement a fully digital motor controller on a Xilinx FPGA.

BAE Systems Avionics designs and manufactures electronic warfare and surveillance systems. To remain competitive, the Avionics division continually evaluates new tools and techniques to reduce lead-time on new technologies. The hardware in our labs and the software in which we invest our time are key to our continued success.

Next-Generation Motor Controller Design

Field-orientated control (FOC), or vector control, is an emerging technology that promises improved torque-speed characteristics from a wide variety of 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 upgraded motor drives would squeeze extra performance from existing motors and save weight in avionics products by reducing the motor mass in new designs.

Moreover, as FPGAs increase in capacity, we can use FPGAs not only for motor control but also for full servo system control. We used National Instruments products to create a rapid prototyping route that significantly reduces new-technology risk early in the design life cycle.

The FOC Technique

Motors driven by traditional square wave amplifiers suffer from poor torque-speed characteristics and torque ripple caused by commutation errors. Sinusoidal commutation solves the torque ripple problem and works well for low motor speeds. At higher speeds, the PI current controller must track a sinusoidal current with increasing frequency while overcoming a Back EMF of increasing frequency and amplitude. This causes a phase lag, which results in a loss of torque per amp because the torque producing flux is not acting at 90 degrees to the rotor. This effect is exhibited by the curve of the torque-speed (TS) plot.

Essentially, the TS plot comprises two lines – the horizontal line, which is the volt limit that governs the maximum speed, and the vertical line, which is the current limit that determines the maximum torque.

We used FOC to improve the TS characteristics. This commutation method uses transforms to convert the sinusoidal currents and encoder position into the rotating rotor d-q reference frame. The d and q components are DC and, therefore, easily controlled by a PI controller. The resulting controller output is subject to an inverse transformation that produces voltage waveforms of the correct phase and amplitude to maintain the flux at 90 degrees to the rotor for maximum current-to-torque power conversion.

Space Vector Modulation and FPGA Implementation

With full digital control, we could use space vector modulation (SVM) to unlock 15 percent more no-load speed. FOC control made this possible because we were no longer restricted by the classical commutation limits of buss voltage/2. The trigonometry of SVM changed the relationship to buss voltage/Ö3 based on a triangle of angles 30, 60, and 90; and sides 1, 2, and Ö3. From this ratio, we calculate that buss voltage/2 divided by buss voltage/Ö3 equals 1.1547, or a 15 percent increase.

Traditional FPGA control strategy implementation can involve significant risk because the first physical realization occurs toward the end of the design life cycle. Through rapid controller prototyping with the National Instruments LabVIEW FPGA Module, we could test and further develop on real hardware even before we started FPGA design.

Algorithm development starts with mathematical modelling packages incorporating fixed-point block sets to simulate FPGA math capabilities. We could immediately rewrite the fixed-point algorithm in G code and run it on the National Instruments PXI platform or CompactRIO reconfigurable control and acquisition platform. Hardware description language (HDL) generation, logic synthesis, HDL simulation, and place and route activities are fully automated into the compilation process. The VHDL is loaded to the Virtex XC2V1000 on the NI PXI-7831R via 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 easy hardware connection via a plug-in terminal card.

Debugging was easy because we could sample data from any FPGA register and display the results on our host PC running NI LabVIEW without disruption to FPGA execution.

Rapid System Component Prototyping

The rapid prototyping system we used to investigate this new technology included a PXI chassis, which hosts a NI PXI embedded controller running LabVIEW  software, and the PXI-7831R reconfigurable I/O module. We used the LabVIEW graphical development environment, and LabVIEW FPGA module, to develop code for all system parts. As described above, we configured and programmed the PXI-7831R FPGA on the host PC directly in the LabVIEW environment, compiled LabVIEW code is downloaded directly to the FPGA. LabVIEW running under windows on the host PC provided system monitoring and visualization, again developed using LabVIEW.

By using the National Instruments PXI-7831R FPGA, we have demonstrated a new technology to our customer with minimal time and equipment investment. Without a VHDL learning curve, we created a 40 kHz real-time controller that far exceeds the single-point I/O capabilities previously available.

Author Information:
For more information on this Case Study, contact:
Brian Mann
Selex-Galileo
2 Crewe Road North
Edinburgh n/a
United Kingdom
Tel: +44 (0)131 343 8791
Fax: +44 (0)131 343 8941
brian.mann@baesystems.com

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