Force measurement using a spherical joint

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석준
석준 on 1 Dec 2024
Answered: Kothuri on 16 Dec 2024
Hello, I am currently working on a simulation of a Stewart platform in Simscape to measure the forces applied to each position using spherical joints, as shown in the first image. However, since the base and follower frames of the joints are aligned at the same location, the output appears as shown in the second image.
In the graph, the red line represents the force in the x-axis, the blue line represents the force in the y-axis, and the black line represents the force in the z-axis. This result is from performing a pitch motion. As I understand, the x-axis graph should show identical results for Frames 1 and 4, while Frames 2 and 3 should produce results symmetric to Frames 1 and 4.
Is it not possible to set the base frames of the spherical joints at the center of the upper platform and the follower frames at their respective positions to measure the forces correctly?

Answers (1)

Kothuri
Kothuri on 16 Dec 2024
Hi 석준,
When simulating a Stewart platform in “Simscape”, especially when measuring forces at the joints, it's crucial to ensure that the joint frames are correctly defined and positioned.
It is valid to place the base frames of the spherical joints at the center of the upper platform and the follower frames at their respective attachment points. This arrangement better represents the actual physical configuration of a Stewart platform. By aligning the base frames at the platform's center and defining the follower frames correctly, the force measurements should become consistent and symmetric according to the platform's motion.
You can follow the below steps to measure the forces:
  • For each spherical joint, define the base frame at the center of the platform.
  • Define the follower frame at the corresponding attachment point.
  • Ensure the attachment points are correctly positioned relative to the upper platform geometry.
  • Frames 1 and 4 should indeed show identical results for the x-axis forces during a symmetric pitch motion.
  • Frames 2 and 3 should produce results symmetric to Frames 1 and 4 but in opposite directions if the motion is purely symmetric.
  • Use “Force Sensors” in Simscape Multibody at each joint connection. These sensors can measure the forces and torques directly between connected components.
  • Insert a “rigid transform” to define the frames explicitly for better clarity.
You can refer the below documentation links for more info on:

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