Gyroscopic Stabilization of a Hexapod 6-Axis Positioning Platform

Gyroscopic Stabilization of Working Surface with a Hexapod 6-DOF Motion Platform

There are many optical and metrology applications that require a stable work surface in unstable environments, such as vibrating environments, airplanes, and ships at sea. Under normal conditions, the work surface is exposed to these environmental influences. The hexapod motion platform provides an excellent foundation for compensating six degrees of freedom of motion. In a previous article, we discussed the use of these parallel kinematic mechanisms in applications for camera quality testing and image stabilization algorithm improvement.

Recent tests used a gyroscope to provide feedback to the hexapod controller to compensate for perturbations in multiple degrees of freedom and maintain a horizontal top plate during the process.

Video: Stabilization – H-811 miniature hexapod 6-axis stage with gyroscope mounted on H-840 hexapod positioning system. The H-840 is controlled by the C-887.52 controller’s integrated waveform generator. A Matlab program reads the output from the gyroscope on the H-811 platform and sends commands countering the H-840 motions to the H-811 controller.

VN-300 Dual Antenna GNSS/INS (manufactured by VectorNav). Left, surface mount device. Right, ruggedized with interface. (Image: VectorNav Technologies)

Hexapod Background

The hexapod (or Stewart platform) is a six-axis parallel positioner. The most common hexapods are based on six actuators arranged in parallel between an upper and a lower platform. The use of six actuators allows the top plate to move in all six degrees of freedom (linear axes X, Y, Z and rotational axes U (roll), V (pitch), W (yaw)). Parallel kinematic systems such as hexapods offer several advantages over traditional serial kinematic stages, including lower inertia, improved dynamics, smaller size, higher stiffness, and fewer connection errors.

Range of standard hexapod 6-axis stage sizes available at PI

The PI hexapod is controlled by a dedicated controller (model C-887). C-887 handles all inverse kinematic equations. This means that the user simply enters the target position of the upper platform, and the controller determines and executes the required displacement for each of her six columns. The controller also has several features that make running your application easier. for example:

• Coordinate systems can be freely defined. These can be moved and/or rotated. This reduces the effort of converting position values ​​when the hexapod is mounted non-horizontally.
• Pivot points can be freely defined. This is the point where all spins take place. By default, the pivot point is at the standard origin, at the radial center of the top plate, and flush with the bottom of the top plate. However, the pivot point can be moved to any his XYZ point in space, allowing the user to rotate exactly where needed.
• There is a configurable data recorder. This allows the user to record various aspects of the application, such as target position, actual position, and analog signal amplitude.
• There is a waveform generator. This allows users to predefine behavioral profiles. By default, C-887 can be configured as a sine profile, linear profile, or ramp. However, users can also define completely separate profiles point by point.
• C-887 can save macros. This allows for partial autonomy. Macros can be programmed to run on initialization or called/triggered on demand.

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

C-887.52 Hexapod controller with RS232/TCP/IP. The C-887.53 model is EtherCAT® compatible.

In each experiment, gyroscope position data was used in the H-811 control loop. The bracket that holds the VN-300 to the top plate of his H-811 was 3D printed from PET. Perturbations in the circumferential angle were generated using a second hexapod. In this way, interruptions are reproducible, easily measurable, and easily configured. During the course of the project, several hexapod systems with different size, load, and speed specifications were used.