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When electro-hydraulic servo-actuators are used for the actuation of control surfaces in aircraft, linear variable differential transformers (LVDTs) are used for position or velocity feedback. There is a control loop that compares the actual position with the commanded position.

  1. Where is the loop closed? In the flight control computer or in a module near the actuator? By this, I mean where this comparison is taken place?

  2. What are the pros and cons of each of these methods and which one is usually used in modern aircraft?

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Different aircraft and manufacturers take very different approaches.

1. Boeing 777

The B777 has three Primary Flight Computers (PFC) that are responsible for flight control laws computation and four Actuator Control Units (ACE) that are responsible for the closed-loop control of their responsible flight control surfaces.

The ACE is primarily an analog device, including the actuator control portion. Note that analog here refers to any control system that is not a general-purpose computer. Analog devices have a finite number of possible failure modes. For this reason, it is enticing from a common-mode/generic failure perspective that bedevils digital computers. The main disadvantage is that it is less flexible and powerful than a digital computer.

B777 FCS

Ref: http://www.davi.ws/avionics/TheAvionicsHandbook_Cap_11.pdf

2. Airbus A320

The A320 closes the actuator loop directly on the five flight control computers: 2 Elevator and Aileron Computers (ELAC) and 3 Spoiler Elevator Computers (SEC), plus 2 Flight Augmentation Computers (FAC) for rudder controls.

In order to combat against common-mode failures, SEC, ELAC and FAC are designed to have completely different architectures by different design teams. Software is also designed to ensure dissimilarity across the command and monitoring channel of each computer.

An in-depth discussion on common-mode failures can be found in this paper, which cites software, and in extension, general-purpose computing, as a major source of common mode failures.

A320 FCS

Ref: http://www.davi.ws/avionics/TheAvionicsHandbook_Cap_12.pdf

3. Airbus/Bombardier A220

The A220 accomplishes the closed-loop servo control in Remote Electronic Units (REU) located fairly close to the control surfaces and their actuators. For this reason, a total of 10 REUs are located across the aircraft.

The Primary Flight Control Computers (PFCC) normally compute the flight control laws. Since REU is a digital computer, in the scenario where all PFCCs fail (for example, a generic failure) and if sufficient number of REUs remain operational, the REUs can take up the slack to run Direct Mode control laws.

The primary advantage of this setup is the reduced wiring weight compared to centrally located servo-loop controllers, as well as higher failure tolerance in the case of single servo-loop failures.

The primary disadvantage, once again, is the common-mode failure across the REUs. Due to the high number of REUs, it becomes prohibitively expensive to design sufficient dissimilarities into them. For this reason, a third backup control loop, in the form of an analog Alternate Flight Control Unit (AFCU) is needed to cater for this remote failure mode.

A220 FCS

Ref: https://quizlet.com/421493192/a220-lod-ch9-flight-controls-flash-cards/

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  • $\begingroup$ Thanks for the response. Suppose that a PID control is used to adjust the surface to a specific angle. So the true angle is subtracted from the desired angle and the error is sent to the PID and the loop is closed. You are saying that this subtraction and PID module are located in flight control computer for A 320 and for Boeing 777 and A 220 these two functions are located in ACE and REU. Is that true? Besides that do you have any similar information for F/A-18, F-16 or F-22? $\endgroup$
    – shooshool
    Commented Jan 22, 2020 at 8:06
  • $\begingroup$ I am a little confused on what you mean by flight control law computation. By flight control law computation you mean the angle that is computed for each surface or the signal that should be applied to the actuator? $\endgroup$
    – shooshool
    Commented Jan 22, 2020 at 8:10
  • $\begingroup$ @shooshool The end point of flight control laws are typically surface commands. The servo-control loop then closes the command with the actuators at much higher bandwidth. It's a cascade control architecture. $\endgroup$
    – JZYL
    Commented Jan 22, 2020 at 12:48
  • $\begingroup$ Do you have any reference that suggests common mode failures occur more often in digital computers compared to analog modules? $\endgroup$
    – shooshool
    Commented Jan 22, 2020 at 16:19
  • $\begingroup$ @shooshool I've added a link and made a note on "analog". In my understanding, FPGA based finite-state logic system can be regarded as "analog" for the authorities, even though it's technically not analog. $\endgroup$
    – JZYL
    Commented Jan 22, 2020 at 17:50

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