EMA Torque Limiter Control for Droop and End-Stop Protection
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Solution Overview
Problem
Existing electromechanical actuators (EMAs) face challenges in managing torque limits during different operational modes, particularly in the droop zone and near mechanical stops, leading to potential damage and operational inefficiencies when power is lost.
Innovation Solution
A torque limiter device in the EMA is adjustable based on the actuator's mode and position, allowing controlled activation of the anti-extension device to prevent damage from excessive external forces and ensure smooth operation in droop mode.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the anti-extension device is activated to prevent spurious extension on power loss, then reliability is improved, but the device cannot allow controlled droop movement when needed
Solution Approach 1:
The torque limiter device dynamically adjusts the torque threshold based on actuator position. In the droop zone, the torque threshold is set lower to allow droop movement even when the anti-extension device is activated. Outside the droop zone, the torque threshold increases to prevent spurious extension. This dynamic adjustment resolves the contradiction by making the system adaptive to different operational contexts.
Solution Approach 2:
The torque limiter applies different torque thresholds for different positions of the actuator. Specifically, a first torque threshold is applied when the actuator is in the droop zone, and a second, higher torque threshold is applied when the actuator is outside the droop zone. This local differentiation allows the system to permit droop movement in the specific region where it is needed while maintaining protection elsewhere.
2Device complexity
If a fixed torque threshold is used in the anti-extension device, then device complexity is reduced, but damage can occur near mechanical stops due to excessive torque
Solution Approach 1:
The torque threshold is dynamically adjusted based on the actuator's position relative to mechanical stops. When the actuator approaches a mechanical stop, the torque threshold is reduced to prevent excessive torque and potential damage. This dynamic adjustment maintains system safety without requiring complex additional protection mechanisms.
Solution Approach 2:
Different torque thresholds are applied in different operational zones. A lower torque threshold is used when the actuator is near mechanical stops to prevent damage, while a higher threshold is used in normal operating zones. This localized approach to torque management prevents damage without unnecessarily complicating the overall device structure.
3Ease of operation
If the torque limiter allows free movement in droop mode, then ease of operation is improved, but the actuator may extend beyond safe limits without protection
Solution Approach 1:
The torque limiter applies permissive torque thresholds specifically in the droop zone to allow smooth droop movement, while applying restrictive torque thresholds outside the droop zone to prevent unsafe extension. This spatially differentiated control enables both ease of operation in the droop mode and protection against excessive extension.
Solution Approach 2:
The torque threshold dynamically changes based on actuator position. In the droop zone, the threshold is set to permit free movement for smooth operation. When the actuator moves outside the droop zone, the threshold increases to activate protection against unsafe extension. This dynamic behavior resolves the contradiction between operational ease and safety.
Data Source
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Figure 3
Figure 4A~5
AI summary
An electromechanical actuator, EMA, assembly operable in a plurality of modes, the EMA assembly comprising: an electrical motor having a motor shaft (201) extending along an axis (A) of the EMA, the motor driving the shaft to rotate about the axis; a gear assembly (401, 402) mounted around, and in geared connection with the shaft, to rotate with the shaft; an EMA output (300) connected to the gear assembly such that rotation of the motor shaft causes rotation of the output via the gear assembly, the output rotating at a speed which is a predetermined fraction of the speed of rotation of the motor shaft based on the gear ratio of the gear assembly; wherein the output is arranged to have a neutral position and to be rotatable away from the neutral position in a first, positive, extension direction to an extension position and away from the neutral position in a second, negative, retraction direction to a retracted position; a torque limiter (900) having a predetermined torque limit, wherein the torque limiter is arranged to limit torque transfer between the gear assembly and the output; an anti-extension mechanism (14, 140, 145) moveable between an engaged position in which it inhibits rotation of the output in the extension direction and a disengaged position wherein it allows rotation of the output in the extension direction, the anti-extension mechanism comprising a solenoid and a ratchet wheel and an anti-extension spring, wherein the anti-extension mechanism is in the disengaged position when the solenoid is energized to move the ratchet against the force of the anti-extension spring and is in the engaged position when the solenoid is not energized; wherein, in an active mode of the plurality of modes, the solenoid is energized and wherein rotation of the output shaft in either direction of rotation drives the output to rotate; wherein in an anti-extension mode, the solenoid is not energized and, when the output is at the neutral position or in the extension position, the output shaft is connected for rotation, via the ratchet, wherein the ratchet is arranged to prevent rotation of the output shaft in the extension direction of rotation; and wherein the torque limiter disengages the anti-extension mechanism in the event that the predetermined torque limit is reached; wherein in the anti-extension mode and when the output is in the retracted position, the torque limiter is adjusted to disengage the anti-extension mechanism at a torque lower than the predetermined torque limit., and/or when the output position is approaching an end stop position the torque limiter is adjusted to disengage the anti-extension mechanism at a torque lower than the predetermined torque limit.