In today’s aircraft, hydraulic and pneumatic actuation systems are increasingly being replaced by electrical systems. Actuators for primary flight control surfaces (such as the aileron and the elevator), as well as actuators in landing gear, braking systems, and fuel delivery systems, are now driven by power electronics. The electric motors that drive these actuators need to be small, light, and inexpensive. They also need to perform reliably for 50,000–150,000 hours of normal flight operation and under a wide range of failure conditions.
To meet these requirements, the Microsemi Aviation Center of Excellence is developing a line of Intelligent Power Solutions™ (IPS) based on a power core module (PCM) designed and tested with MATLAB® and Simulink®. Model-Based Design has enabled us to push our design to the limits because we can simulate failures, optimize performance, and lower risk by conducting real-time reliability tests of motor drive hardware and control software early in the development process.
Modeling the PCM and Running Closed-Loop Simulations
A complete power electrical control unit consists of functions for pulse width modulation (PWM) control, data conversion, and communications; filtering and protection; a three-phase permanent magnet synchronous motor (PMSM) drive; a control module; and a monitoring module (Figure 1). The motor current, motor speed, and actuator position are fed into the monitoring module, and the control module uses this information to direct the PCM to speed the motor up or slow it down. Because this was a new design, we had to develop the PCM without having working versions of the monitoring module or control module available to test it.
We modeled the PCM in Simulink, using Simscape Power Systems™ and Simscape Electronics™ to model the three-phase PMSM drive and electronic components and the control and monitoring modules. We then ran closed-loop simulations to characterize the system’s electrical and mechanical behavior.
Next, we deployed the three models to a Spartan-6 FPGA in the Speedgoat target system using Simulink Coder™ and Simulink Real-Time™ (Figure 2). The modules communicate through a low-voltage differential signaling (LVDS) interface. In one test setup, both the PCM controller and the other modules were run on the target hardware for real-time tests. In a different setup, we deployed our controller to a production ProASIC3 FPGA on the PCM and ran hardware-in-the-loop tests with the target hardware system performing the functions of the control and monitoring modules. We tested normal operation using both test setups. We also tested the controller’s response to several fault conditions (including motor failure) to perform failure mode, effects, and criticality analysis.
Testing Real-World Flight Profiles Under Real-World Conditions
To demonstrate the PCM under realistic flight profiles, we developed Simulink and Stateflow® models that translate flight characteristics into electrical and mechanical requirements for an actuation system. As the aircraft proceeds through the typical phases of a flight―taxiing, taking off, climbing, cruising, descending, approaching, and landing― the motor current demands for an aileron actuator, for example, vary significantly. Simulations that we ran using our Simulink and Stateflow mission and flight profile models enabled us to accurately estimate motor current demands for ailerons and other components on specific aircraft (Figure 3).
For our reliability tests, we generated aircraft-specific motor current demands based on the flight profile simulation results. We use environmental chambers that vary the pressure and temperature. For example, the ambient temperature in Boston is much lower than that of Dubai in summer, and our tests must take that into account. With the environmental chambers, we can expose the systems to temperatures of -55° Celsius and pressures of less than 0.2 bar (conditions that are common at altitudes of 39,000 feet and higher). Long-term reliability tests representative of 150,000 flight hours require careful monitoring and thorough analysis of the results. We conduct this monitoring and data analysis in MATLAB.
What We Learned
Through our extensive modeling and simulations, we established that units equipped with motor drives based on silicon carbide (SiC) MOSFETs operate at a temperature approximately 40° Celsius lower than similar units with IGBTs.
Because active cooling is not possible with today’s smaller and lighter hardware designs, managing the temperature of the device while in operation is vital to ensuring that it will function reliably for 150,000 flight hours. Simulations also showed that power dissipation with IGBTs is considerably higher than with SiC MOSFETs (Figure 4). These insights informed our design decisions for the PCM, and point to SiC MOSFETS as an enabling technology as the industry moves towards increased fly-by-wire controls in the more electric aircraft.
Simulink, Simulink Real-Time, and Speedgoat target hardware have enabled us to demonstrate the application-specific reliability of our early designs without installing the units on an actual aircraft. With Model-Based Design, we can do continuous validation and verification without waiting until all aspects of the power electrical control unit are developed.
The feedback we’ve received from our customers has been very positive. With our real-time simulation results, we are confident that we can meet the PCM’s reliability targets as we continue to reduce the unit’s size, weight, and cost.
Article featured in MathWorks News & Notes