Electronic controls are slowly transforming aircraft engines of all types, bringing improved fuel consumption, improved efficiency, and more power. As the functionality of control units for jet engines increases, digital management is also adding more capabilities for helicopters and small piston engines.
Full authority digital engine controls (FADEC) continue to leverage the advances in semiconductors and software to improve engine management. As microcontroller capabilities soar, engine controllers can handle functions beyond the engines themselves. For example, the Lockheed Martin F-35B’s engine controller also manages the short takeoff/vertical landing system.
“Engine controls can be linked to other electronic systems on the aircraft when the engine control is expected to be the flight control,” said Louis Celiberti, Director of Control & Diagnostic Systems at Pratt & Whitney Engineering. “During STOVL operation, the F135 engine control is the heart of the integrated flight and propulsion control. The engine control software uses advanced methods to de-couple a highly coupled engine system comprised of the main engine and a lift fan with a driveshaft attached in between. The result is wings-level, rock-solid hover and vertical landing performance.”
While the F135 represents the high end for engine controls, digital systems are also expanding their reach into the low end. Rockwell Collins is shipping what it says is the first dual-channel engine control unit for use in light sport aircraft. The controls for the Rotax 912 iS piston engine were developed in conjunction with the BRP-Powertrain affiliate of Bombardier Recreational Products. The ECU automates tasks like adjusting air/fuel mixture and controlling exhaust gas temperatures.
“Now operators can operate the engine with ease, without having to worry about completing manual tasks to optimize performance throughout all operating conditions,” said David Vos, Senior Director, Control Technologies, for Rockwell Collins.
Design teams are also doing more with ECUs in helicopters. Rolls-Royce recently deployed a dual-channel FADEC system on the M250-C47E, the first new helicopter to be launched by the company since 2005. The advanced FADEC helps improve the fuel burn and cut engine operating costs.
Whether FADECs are employed on the simplest recreational plane or the most advanced jets, design teams face myriad challenges. Hardware and software must operate efficiently without glitches or failures, forcing engineers to work overtime to eliminate mistakes.
Software is a primary focus. Many aircraft routinely run several million lines of code. Increasingly, this software is generated by computer, not programmers.
“As controls get more complex, code generation tools are gaining attention,” said Mahendra Muli, New Business Development Director at dSpace. “Advanced automatic code generation tools are being used to convert algorithm models into well-documented C-code used in production controllers with efficiency, repeatability, and high quality, while easing the process to comply with stringent standards such as DO178B/C."
Both hardware and software must continue to run even if other systems on the aircraft fail. Though many systems share information, they must also be segregated from each other so problems in one subsystem don’t cause errors in a related system.
“Engine controls should be isolated from other systems when a level of system redundancy is required to achieve flight safety requirements,” Celiberti said. “Systems require isolation to ensure cross contamination does not take place and allow secondary systems to be deployed when primary systems fail.”
Green Taxiing Machines
The push to trim fuel consumption and emissions may prompt aircraft manufacturers to adopt some of the electric motor technologies used in hybrid cars and trucks. Honeywell Aerospace and Safran have teamed up to design a system that uses an auxiliary power unit (APU) to power planes during taxi operations.
The Electric Green Taxiing System demonstrated by Airbus in Paris, uses a small APU to power electric motors in the main landing gear so an aircraft can push back and taxi without engaging its main engines. In short-haul operations, this can trim fuel consumption an average of 4%.
Honeywell APUs already provide power for ventilation, lighting, and starting engines for many aircraft. In a system set for production in 2016, the APU will drive electric motors placed on the two main wheel units. One wheel on each main gear employs an electric motor, reduction gearbox, and clutch assembly to drive the aircraft.
Dedicated power electronics and system controllers let pilots control an aircraft’s speed and direction. During development, the design team ran several tests before many of the components were ready.
“When you’re modifying the landing gear that much, you need to do a lot with the control circuitry,” said Mahendra Muli, New Business Development Director at dSpace. “Honeywell did a lot of modeling and simulation with hardware-in-the-loop, using our system onboard to control the drive motors.”
Currently, the wheel assemblies add around 300 pounds each. Honeywell and Safran say that this weight can be reduced significantly over time. Some of these gains may come by leveraging the myriad efforts by semiconductor suppliers to help automakers develop electrified power-trains. Many semiconductor suppliers are adding hardware that makes it easier to drive the high-powered motors needed to move vehicles.
“We added 12 models to our Hercules microcontroller line, adding PWM for motor control,” said Dave Maples, Safety MCU Product Line Manager at Texas Instruments. “That will help people move to brushless motors.”
Brushless motors, which are quieter and use less power than direct current motors, are moving to less expensive technologies. NXP Semiconductors is focusing its design efforts on aniso tropic magnetroresistive sensors, which let motor manufacturers use any type of magnet, not just rare earth materials now used in many brushless motors.
Ensuring that software and hardware operate efficiently in all conditions requires plenty of planning and testing. Modeling and simulation have become common tools, facilitated by hardware and software-in-the-loop testing. Models can be used to improve engine performance.
“The benefits come from the precision of scheduling control effectors (actuators, valves, etc.) to meet dynamic performance and steady state scheduling requirements while maximizing system safety and reliability. This is all facilitated by high-fidelity, onboard model predictive control technology,” Celiberti said.
These virtual tests have helped engineers find faults in components and discrete systems for years, but simulations were often limited to fairly small elements. More powerful computers and improved software now make it possible to see how multiple subsystems work together to spot problems before costly physical prototypes are built.
“With capabilities for simulating bus communications over various standard communication bus interfaces such as ARINC 429, 1394B, and Ethernet, the entire system can be simulated without the need for real components,” Muli said. “This is helping reduce costs in development as more and more testing on the test bench is helping catch errors early on.”
As designs move forward, physical testing will be added. Hardware- and software-in-the-loop let test engineers combine the physical components and systems that are ready with models. These tools continue to evolve, addressing changes in regulatory requirements.
“DO-178B regulations have prompted some to revisit some of their tooling,” Muli said. “We’ve got tools that provide a way for companies to qualify their test environments.”
While virtual testing brings many benefits and helps shorten development time, it’s not the end all. Developers strive to reach a point where flight tests serve to prove the validity of virtual tests. While modeling and simulation make it possible to eliminate some testing, these flight tests are in no danger of disappearing.
“Demonstrating this technology in a simulation environment is one thing; flight clearing it for use in a commercial or military aircraft via FAA and DOD software qualification requirements is another,” Celiberti said.