Gyroscopic Stabilization of a Hexapod 6-Axis Positioning Platform

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.

The gyroscope was configured by sending three commands to the VN-300, sent as ASCII strings over the open USB connection:

1. $VNWRG,06,1*XX (This indicates that it is to output Yaw, Pitch, and Roll values.)
2. $VNWRG,07,50*XX (This indicates the frequency at which the Yaw, Pitch, and Roll is to be output (here, 50Hz).)
3. VNRRG,05*XX (This command reads the output, and must be looped to read the values.)

The frequency of the output of the VN-300 could be set to one of eleven different values, from 1Hz to 200Hz. Setting an output frequency of 50Hz or less guaranteed consistently correctly-formatted results from the VN-300. Setting it to 100Hz resulted in roughly 1% of all output responses being incorrectly formatted, which was likely caused by a USB issue.

An example of a correctly formatted response, as output by the VN-300, can be seen in Table 1. It has the prefix $VNYPR (VN-300, Yaw, Pitch, Roll), three numerical values for each angle measurement, and then a suffix beginning with the asterisk. However, when the output frequency was set to 100Hz or higher, the responses would occasionally be incorrectly formatted in one of two ways. The first option has the response truncated and terminated at the decimal point of the Roll value. The second option would truncate the response at the decimal of the Roll value, and then append a complete response. See the examples below.

Multiple tests were run with different hexapods. The first test had two H-811 systems, as the Master and the Follower placed side by side. The VN-300 was mounted on the Master. As the Master executed the motion profile, the Follower would mimic the motion.

By using the waveform generator in the hexapod controller, the motion profile of the Master was configured as a sinusoid for both the Pitch (V) and Roll (U) axes, with a 90˚ phase shift in the two profiles. The gyroscope data was processed, and those values then sent to the Follower as a series of point-to-point move commands.

The mimicking was done as a first attempt, as it was easier to visually judge the performance, and to graphically compare the recorded results at the end.  The profile was a sine wave with a 10˚ peak-valley amplitude and a frequency of 0.1Hz. The graphs below display the results.

Phase 2 used the H-811 6-axis positioning stage system as Follower to a larger, slower hexapod, the H-850.H11. The H-811 mini hexapod was mounted on the H-850 with a custom bracket. The VN-300 gyroscope was mounted on the H-811. As the Master executed the motion profile, the Follower would counter the motion, to maintain a zero-value Pitch and Roll.

This particular H-850 was an older model with a maximum angular velocity of 6mrad/sec and it did not have a waveform generator, so the motion profile was generated using point-to-point motion. As the H-850 was moving through the motion, the position data from both the VN-300 and the Follower were queried. The Follower was then commanded to move to the difference between the two readings, as outlined in Equations 1 and 2.

The graphs below show the results from this test. There was no position following error for each hexapod, so the Target Position and Actual Position lines overlap. The positions of the H-850 Master and the H-811 Follower were equal in magnitude but opposite in direction, resulting in a gyroscopic reading of approximately 0˚.