Parallel Robot Cable Tension Control via Articulated Pulleys
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Solution Overview
Problem
Cable-driven parallel robots face challenges due to the unilateral constraint and visco-elastic nature of cables, leading to non-linear behavior, making control and positioning estimation complex, especially under external forces and varying conditions like temperature and aging, which complicates their industrial applications.
Innovation Solution
The robotic arrangement incorporates a loop path for actuation cables with articulated pulleys and a force sensor to measure tension, allowing for better control and adherence to a simplified model by defining fixed kinematic points and measuring cable tension, thereby reducing errors and improving stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If cables are used to actuate the parallel robot, then the robot achieves high payload-to-weight ratio and extensive actuator strokes, but the unilateral constraint and visco-elastic nature of cables cause non-linear behavior that complicates control and positioning estimation
Solution Approach 1:
The patent changes the physical configuration of the cable system by implementing loop paths with articulated pulleys. This transforms the cable arrangement from simple linear actuators to closed-loop mechanisms with defined geometric constraints. The parameter change in cable topology enables the system to maintain high payload-to-weight ratio while reducing control complexity through better adherence to simplified kinematic models
Solution Approach 2:
The articulated pulleys act as intermediaries between the actuators and the moving platform. These pulleys with fixed axes create well-defined kinematic relationships, serving as mediators that translate cable tension changes into predictable platform movements. This intermediary mechanism helps linearize the system behavior and simplifies the control model by establishing fixed geometric constraints
2Speed
If cables are used to actuate the parallel robot, then the robot achieves high speed and acceleration due to reduced inertias, but the visco-elastic nature of cables causes yielding under prolonged loads that complicates positioning estimation
Solution Approach 1:
The patent changes the cable configuration to closed-loop paths with articulated pulleys, which modifies the kinematic parameters of the system. This configuration change creates more predictable elastic behavior that can be better modeled and compensated for in control algorithms, thereby improving positioning estimation accuracy while maintaining high speed performance
Solution Approach 2:
The patent implements force sensors in the cable transmission system to provide feedback on cable tension. This feedback mechanism enables real-time monitoring and compensation of visco-elastic effects, allowing the control system to adjust for cable yielding and maintain accurate positioning estimation even during high-speed operations with prolonged loads
3Device complexity
If the cable path is simplified to direct connections, then the system architecture remains simple and cost-effective, but the lack of defined kinematic points makes it difficult to account for cable non-linearities in control
Solution Approach 1:
The articulated pulleys with fixed axes serve as intermediaries that introduce well-defined kinematic points into the cable path. These pulleys maintain the overall simplicity of the cable-driven architecture while creating fixed geometric constraints that enable easier control implementation. The pulleys act as mediators between the simple cable connections and the complex control requirements
4Reliability
If redundant actuators are used (number greater than degrees of freedom), then the robot achieves better control redundancy and stability, but the number of actuators increases beyond the minimum required for two degrees of freedom
Solution Approach 1:
The patent changes the cable configuration to closed-loop paths with articulated pulleys, which modifies the kinematic constraints of the system. This parameter change in the cable topology enables the redundant actuators to work more effectively together, improving control redundancy and stability while making better use of each actuator's contribution to the overall system performance
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 configuration enables more accurate control and positioning of the end effector, maintaining the standard model assumptions while accounting for cable non-linearities, enhancing the robotic arrangement's stability and performance in industrial applications.
Implementation Method 1
said robotic arrangement comprises for each loop a proximal transmission assembly positioned on the frame downstream of each actuator with respect to the path of the cable, and a distal transmission assembly positioned on the mobile support of the end effector, each one between said proximal transmission assembly and said distal transmission assembly comprising, at the respective anchorage points articulated pulleys... Associated to each actuation-cable loop is just one actuator... Associated to the proximal assembly is a sensor configured to measure the tension of the respective actuation cable
Implementation Method 2
each one between said proximal transmission assembly and said distal transmission assembly comprising, at the respective anchorage points articulated pulleys, which each have a first axis of rotation of the respective pulley, in particular of a rotating disk thereof, that is parallel to the plane of movement, and a second axis of rotation, about which said first axis of rotation turns, that is perpendicular to the plane of movement
Data Source
Figure 1A
Figure 1B
Figure 2A~3B
AI summary
A parallel-architecture robotic arrangement, comprising a frame (11) and an end effector (15) mounted on a mobile support (12), in particular a mobile platform, said mobile support (12) being connected to said frame (12) via a plurality of actuation cables (14), in particular four actuation cables, in such a way as to move according to two degrees of freedom in a rectangular plane movement space (17), said robotic arrangement (10) comprising a plurality of actuators (13) for moving respective actuation cables (14), said actuators (13) being in a number greater than said degrees of freedom of the end effector, each actuation cable (14) coupling a respective portion of said frame (11) and a respective portion of said mobile support (12) according to a loop path (l i ) between at least two proximal anchorage points (Ai) associated to said portion of the frame (11) and two distal anchorage points (Bi) associated to said respective portion of the support (11), wherein said robotic arrangement (10) comprises for each loop (l i ) a proximal transmission assembly (25) positioned on the frame (11) downstream of each actuator (13) with respect to the path of the cable (14), and a distal transmission assembly (26) positioned on the mobile support (12) of the end effector, each one between said proximal transmission assembly (25) and said distal transmission assembly (26) comprising, at the respective anchorage points (Ai, Bi), articulated pulleys (23, 24), which each have a first axis of rotation (23a) of the respective pulley, in particular of a rotating disk thereof (231), that is parallel to the plane of movement (17), and a second axis of rotation (23b), about which said first axis of rotation (23a) turns, that is perpendicular to the plane of movement (17), the exit point of the actuation cable (14) on each pulley (23, 24) identifying the respective anchorage point (Ai, Bi), said proximal transmission assembly (25) being associated to a sensor (30) configured to measure the tension of the respective actuation cable (14).