Progress Report 3 on Addition of a High-Speed Drive to High-Pressure, Rotary-Shear Apparatus

Terry E. Tullis, & Cameron Meyers

Published September 11, 2022, SCEC Contribution #12157, 2022 SCEC Annual Meeting Poster #136

The purpose of this project is to develop the capability to study high-speed frictional behavior and processes at elevated confining pressure. Existing high-speed frictional apparatus only operate at modest normal stresses and consequently cannot access the conditions experienced by natural faults during earthquakes. We have built a high-speed, high-torque servo motor and retrofit it to our high-pressure rotary-shear apparatus. Our new high-speed direct-drive servo motor can be used for slip rates as high as 1.8 m/s and so can be used to study frictional weakening mechanisms at coseismic slip velocities and confining pressures up to 100 MPa. With the appropriate servo-feedback device the servo motor can effectively stiffen the machine response with unlimited rotation and over a large range of slip rates.

We provide servo feedback signals to the servo motor controller via two angular measurement devices. The first measurement device is the motor encoder. It consists of a Heidenhain absolute optical encoder that we built into the motor. This works over 9 orders of magnitude in speed with a resolution of 27 bits/revolution, corresponding to an astounding sample-slip resolution of 1 nm. The second feedback device is the sample resolver which is mounted on the sample grips inside the pressure vessel. Using the resolver stiffens the machine by a factor of ~40 over using the encoder feedback. In the slow-resolver mode we use both the 1X and 32X stages of our two-stage resolver, processing them through intermediate-speed electronics to give a 24-bit/revolution digital signal. In this slow sample-resolver mode the slip resolution is 9 nm and the range of slip speeds over which it works is 7 orders of magnitude from 1 nm/s to 10 mm/s. In the fast sample-resolver mode we can control at slip speeds as high as 1.8 m/s, using faster electronics with only 19-bit/revolution resolution. This only provides 0.5 microns slip resolution, but this is sufficiently precise for speeds of 10 mm/s (10,000 microns/s) and higher.

In summary, we have found that our servo motor can be servo-controlled over 9 orders of magnitude in speed, either in the natural stiffness mode with the motor encoder as feedback, or in an electronically stiffened mode with the internal resolver as feedback. The ability of the single motor to drive the motion over this large range of speeds is remarkable and would not be possible without careful attention paid to processing the feedback signals.

Key Words
Friction, Lab, Equipment

Citation
Tullis, T. E., & Meyers, C. (2022, 09). Progress Report 3 on Addition of a High-Speed Drive to High-Pressure, Rotary-Shear Apparatus. Poster Presentation at 2022 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)