Marine Steering Actuator Roller Screw for Long Stroke Packaging
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Solution Overview
Problem
Existing electric actuators for marine steering systems face limitations in stroke length and envelope size, which restrict their integration within marine steering systems, and lack effective clutch mechanisms to manage back-driving forces and axial movement constraints.
Innovation Solution
The electric actuator design includes a housing with a reciprocating output shaft, a screw assembly featuring annular rollers and a central screw, and a motor with a stator and rotor, along with an adjustable clutch using electromagnets or permanent magnets to manage rotation and axial translation, and end glands to minimize bending loads, allowing for tuned cogging torque and enhanced stroke length.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of moving object
If a conventional ball screw mechanism is used, then the actuator structure is simple, but the stroke length is limited and the envelope size is large
Solution Approach 1:
The screw assembly is segmented into multiple components: a central screw, multiple annular rollers, and a nut. The annular rollers are distributed around the central screw and engage with both the screw and the rotor's inner bore. This segmentation allows the stroke to be distributed across multiple rolling contact points, enabling longer stroke length while keeping the overall envelope compact.
Solution Approach 2:
The annular rollers are nested within the rotor's inner bore, with each roller fitting inside the hollow cylindrical space. The central screw passes through the center of all rollers, creating a nested configuration where rollers are inside the rotor bore and the screw is inside the rollers. This nesting arrangement maximizes the use of internal space, allowing long stroke without increasing external dimensions.
2Reliability
If no clutch mechanism is provided, then the structure is simpler, but back-driving forces cannot be managed effectively
Solution Approach 1:
The clutch mechanism utilizes the natural cogging torque generated by the interaction between the rotor's inner bore teeth and the annular rollers. When back-driving forces occur, the cogging torque automatically engages the clutch, causing the rotor to lock relative to the housing. This self-service approach eliminates the need for external clutch actuation systems, maintaining reliability while minimizing added complexity.
Solution Approach 2:
The clutch engagement is achieved by changing the torque parameter through the cogging effect. The rotor's inner bore is designed with teeth that create periodic torque variations (cogging torque) as the rollers pass by. By designing the tooth geometry and spacing appropriately, the cogging torque parameter is tuned to automatically engage the clutch at specific back-driving force thresholds, providing reliable force management without complex control systems.
3Ease of operation
If the output shaft is fixed to the housing, then the structure is more stable, but axial movement and compliance are restricted
Solution Approach 1:
The output shaft is designed to reciprocate axially within the housing rather than being fixed in position. The shaft moves back and forth along the axial direction in response to steering inputs and back-driving forces, providing dynamic compliance. This dynamic movement capability allows the actuator to accommodate axial displacements and shocks while maintaining operational stability through the controlled reciprocating motion.
Solution Approach 2:
The screw assembly acts as an intermediary mechanism between the motor and the output shaft. The central screw translates rotational motion into axial reciprocation of the output shaft, while the annular rollers and nut provide a controlled interface that allows axial movement. This intermediary arrangement enables the output shaft to move axially with respect to the housing while maintaining stable mechanical coupling through the screw-roller-nut mechanism.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This design enhances the stroke length of the electric actuator, enabling a smaller envelope and effective clutch functionality to manage back-driving forces, while maintaining structural integrity and compliance, thus improving the integration and performance within marine steering systems.
Implementation Method 1
The clutch may include an electromagnet fixed to the housing and a clutch plate translatable axially relative to the housing. The clutch plate may be preloaded to be spaced-apart from the electromagnet and abutting the rotor. Friction between the clutch plate and the rotor may inhibit relative rotation of the rotor. The clutch may be disengaged by powering the electromagnet to attract the clutch plate towards the electromagnet, creating an air gap between the clutch plate and the rotor, and thereby allowing the rotor to rotate freely.
Implementation Method 2
The clutch may alternatively include a permanent magnet fixed to the housing and a clutch plate translatable axially relative to the housing. The clutch plate may be attracted by the permanent magnet to abut the housing. Friction between the clutch plate and the housing may inhibit relative rotation of the rotor.
Implementation Method 3
The clutch may include an electromagnetic coil. The clutch may be disengaged by powering the electromagnetic coil to cancel the force of the permanent magnet, thereby allowing the clutch plate to rotate relative to the housing and the rotor to rotate freely.
Data Source
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AI summary
An electric actuator comprises a housing and an output shaft reciprocatingly received by the housing. There is a screw assembly disposed within the housing and coupled to the output shaft. The screw assembly includes a plurality of annular rollers and a central screw received by the annular rollers. The annular rollers are rotatable about the central screw. There is a motor which includes a stator and a rotor. The rotor has an inner bore which engages the annular rollers. Rotation of the rotor causes the central screw to translate axially relative to the rotor and the output shaft to reciprocate relative to the housing.