There are an extensive range of applications for which force sensors can be applied in place of more traditional systems, such as in the transition from hydraulic actuation to electric actuation where a pressure sensor is replaced by a force sensor. There are also instances where force sensing can support the safety system within an aircraft, by signalling when failure starts to occur, which will be looked at in more depth using the A350 as an example. Torque sensing will also be explored as a new alternative to tension or compression force sensing, for example in applications with limited space to fit a force sensor.
ELECTRIC BRAKE ACTUATOR
Traditionally, hydraulic actuation has been used for aircraft braking systems, but the conversion from a hydraulic to an electric actuator requires a force sensor in place of a pressure sensor. In the case of hydraulic actuation, the force can be calculated if the pressure and cylinder size are known. However, in the case of an electric actuator, a force sensor must be mounted in the load path or somewhere it can react against the actuator in order to provide the feedback of the force that has been applied. Such feedback is particularly important in the case of electric braking for example, as it will detect any residual force that could result in drake drag after the brake is released. In cases such as this, the sensor would usually be a shallow disc shape with loads applied to concentric or coaxial rings that will react in terms of stress as the force passes through that section. The electronics are contained in the unstrained areas nearest the centre, but the centre itself remains hollow to allow for the bearing bosses that could need to pass through. There are other options in terms of the form of the sensor, for example it could be in a rod or pin or even as part of the structure or housing within which the actuator is fitted.
This kind of aircraft braking application is very challenging. The undercarriage is tucked away under the aircraft throughout flight, therefore causing its temperature to drop to a very low level, and then shortly before landing, it is then lowered into the warm moist airstream. This causes a significant thermal shock, during which the sensor must remain in its no-load error band. Once the aircraft lands, the brake temperature can then rise to 125 degrees Celsius whilst the aircraft is stationary, and the heat can then pass through the brake assembly and reach the sensor.
This is typical for cases where hydraulic actuation is being replaced by electric actuation, with the sensor measuring the force directly in line with the output of the actuator or measuring the reaction force between the actuator and the structure or housing. Regardless of variations in the shape and form of the sensor, the overall concept remains to provide force feedback to the actuator control system.
A350 HORIZONTAL STABILISER
The horizontal stabiliser of the Airbus A350 includes a health and usage monitoring system in the secondary load path. Installed within the primary load path which carries the main flight loads, the secondary load path attaches to the actuator using four load pins, mounted in such a way that, in the event of a failure of the primary load path, the load would be transferred to the secondary path in a detectable way, which then triggers inspection and rectification, thereby ensuring continuing safety of flight. Traditionally, this is an area that can only be inspected in major examinations, meaning that the secondary load path had to be very strong and capable of functioning for thousands of flight cycles, which resulted in having to carry a lot of weight for the life of the aircraft. The purpose of force sensors here is to detect when force begins to transfer from the primary to the secondary load path, which enables the weight of the secondary structure to be reduced along with the number of flight cycles it will have to withstand, given that messages will be sent instantaneously as soon as any hint of failure is detected.
The four load pins within the actuator, which is again an electric actuator, consist of two long and two short pins with integral electronics with current loop output, which tends to be more typical for areas outside of the pressurised zone and flight critical applications. The control system response in the event of a secondary load path detection would be to lock the actuator, so this application carries Design Assurance Level ‘A’ (DAL-A) hazard status. Some special conditions to be noted are firstly, the very high ratio between the working load and the ultimate load (times twelve), and the fact there is no permanent deformation allowed at ultimate load, as if any deformation did occur, it would not be possible to get the assembly out of the aircraft. The main benefit for sensing is that the secondary load path can be lighter, which pays back throughout the lifetime of the aircraft.
PILOT INPUT FORCE SENSING
Control rod sensing is a particularly important area for HITEC as we do a lot of work in this field. With flight controls, the pilot input force must be sensed and then feedback must be provided to various systems, such as the flight control computer or the flight data recorder. Sensors can be built into control rods or have an integrated rod-end bearing to keep the length as short as possible; these are typically powered from the flight control computer or aircraft bus with a range of options for supply voltages in and signal voltages out. These would usually be assessed as Design Assurance Level ‘B’ (DAL-B) as there are multiple sensors and controls in these control systems, including the secondary load paths, because any component separation in the primary path could result in a loss of control, so a secondary load path would be a requirement.
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The use of torque sensing for in-flight applications is relatively new and it can be advantageous when space is at a premium, for example in retrofit applications. The application is essentially the determination of rotary force, typically generated by a rotary actuator such as a motor or gearbox and measured at the output shaft.
There are many different configurations for torque sensors, including the measurement of reaction torque, but typically the sensor is required to determine the force generated by a torque arm that is part of a control system linkage and translates within a narrow arc. This keeps the torque sensor simple and keeps the cost down (rotary torque sensors can be complex and expensive).
PILOT STICK – FORCE SENSING
Force sensing of pilot sticks is a very significant area within sensor solutions for aerospace and it typically involves multi-axis force measurement. The new generation of pilot sticks are active, which means they move in response to a force being applied, with that response being generated by an actuator under the control surface and therefore capable of having variable simulated inertia.
The force sensing for a pilot stick is typically dual axis (pitch and roll) but could also be tri-axial (pitch, roll and yaw). To achieve sufficient redundancy, there are typically four electrically independent channels of measurement in each axis. They also all have to fit into a very small package with a hollow core for the grip wiring to pass through, with the grip including the necessary switches and controls.
The Design Assurance Level for the force sensing would be level ‘B’ (DAL-B) because the active sticks can still function in passive mode without the force feedback, but single stick applications would need to achieve DAL-A for structural aspects, being a flight critical application.