Gyroscopic Stabilization of Working Surface with a Hexapod 6-DOF Motion Platform
There are many optics and metrology-related applications which require a stable work surface in an unstable environment, whether that environment is vibrating, in an aircraft, on a vessel at sea, or elsewhere. Under normal conditions, the work surface is subject to these environmental disturbances. Hexapod motion platforms provide a good basis to compensate for motion in 6 degrees of freedom. In previous articles, the use for these parallel kinematic mechanisms has been explained in cameraquality testing applications and for improving image stabilization algorithms.
In a recent test, we used a gyroscope to provide feedback to a hexapod controller in order to compensate for multi-degree-of-freedom disturbances and to maintain a horizontal top plate during the process.
A hexapod (or Stewart Platform) is a six-axis parallel positioner. Most common hexapods are based on six actuators arranged in parallel between a top and bottom platform. By using six actuators, the top plate is able to move in all six degrees of freedom (the linear axes X, Y, and Z; and the rotational axes U (roll), V (pitch), and W (yaw)). A parallel kinematic system, such as a hexapod, also provides several advantages over conventional serial kinematic stages, including lower inertia, improved dynamics, smaller package size, higher stiffness, and less compounding errors.
PI hexapods are controlled from a designated controller (model C-887). The C-887 handles all of the inverse kinematic equations, meaning the user simply inputs a target position for the top platform, and the controller determines and executes the displacement required for each of the six struts. The controller also has several features which make applications easier to perform. For example:
- The coordinate systems are freely definable. They can be translated and/or rotated. This reduces the effort of transforming position values, if the hexapod is mounted in any non-horizontal orientation.
- The pivot point can be freely defined. This is the point about which all rotations occur. By default, the pivot point is at the default origin: radially centered on the top plate, and level with the bottom surface of the top plate. However, the pivot point can be moved to any XYZ point in space, allowing the user to rotate about exactly where they need.
- There is a configurable data recorder. This allows the user to record various aspects of the application, including target position, actual position, the amplitude of an analog signal, etc.
- There is a waveform generator. This allows the user to predefine a motion profile. By default, the C-887 can configure sinusoidal profiles, linear profiles, or ramps. However, the user can also define a completely custom profile point by point.
- The C-887 is capable of storing macros. This allows for semi-autonomy. The macros can be programmed to run at initialization, or can be called/triggered as needed.
The following equipment was used in this test:
- 1x PC with Matlab
- 1x VN-300 gyroscope (etc.) from VectorNav
- 1x H-811/C-887 hexapod system from PI to be stabilized
- 1x H-8xx/C-887 hexapod system from PI to simulate environmental disturbances
- Brackets for mounting, as necessary
During each trial, the gyroscopic position data was used in a control loop for an H-811. A bracket was 3D printed using PET, to hold the VN-300 to the top plate of an H-811. The environmental angular disturbances were generated using a second hexapod. This way, the disturbances would be repeatable, easily measurable, and readily configurable. During the course of the project, several hexapod systems with different size, load and velocity specifications were used...more....
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