Picomotor Heating Report

Picomotor Heating Issue

Introduction

A critical part of the operation for the Short Range Gravity Experiment is accurate modulation of the motion of the source mass. Operation of the source mass is achieved by the use of two New Focus vacuum compatible picomotor actuators (model #8310-V).11assets.newport.com/webDocuments-EN/images/8310-V_Manual.pdf These motors control the linear (z-direction) and yaw (\(\phi\)) motions. The actuators have a linear travel of 12.7mm with a maximum speed of 1.2mm/min. Capable of pushing a load of 22N, they have a resolution of 63.5nm per encoder count. Although designed for use in vacuum, they are only rated to \(10^{-6}\) torr. While operating at ultra-high vacuum pressures (\(10^{-9}\) torr) we measured pressure rising within the chamber during any use of the actuators. This section details the steps taken in characterizing this phenomenon and the efforts to mitigate the effects.

Inital tests of the source mass assembly focused mainly on the accuracy of the positioners. It wasn’t until tests of an earlier version of the feedback system that a distinct pressure rise was noticed. During tests of the feedback system, the source mass was moved to the greatest linear distance along the z-direction. During one of these tests it was noticed that the pressure had risen dramatically. Further tests found if the pressure was initially in the ultra-high vacuum regime the pressure rose dramatically even if the motors were powered yet stationary. This is problematic for multiple reasons. A serious concern for the experiment is the sensitivity of the balance. Internal damping forces for the fiber are small and easily dominated by external forces such as air pressure. Not only would a higher vacuum pressure reduce the balance’s sensitivity to weak forces, it also raises the level of stochastic noise due to interactions of the air and balance.

Characterization

Initally there was confusion over the cause of the pressure increase. The actuators use a worm drive to operate. It was hypothesised that the drive itself, through the wearing of the drive, emitted particles into the chamber, also known as out-gassing. This did not account for the pressure increase when the motors were powered but kept stationary. It seemed far more likely that the motors, when operated, simply heat up. This explanation would still need investigation as the pressure rise had not been characterized. It was unknown if the pressure rises uniform regardless of the speed or load placed on the motors.

Data collection Methods

Two methods for collection of data were utilized. For the pressure inside the chamber, neither the ion gauge nor the ion pump output data other than on a screen. This forces the experimenter to record the pressure and time by hand. A simple spreadsheet was made and as time passed the pressure reported by the devices was written down. This limited the length of time that data was collected. Later data collection was made by recording the temperature of the motors. Taking place on an optics table, outside of the vacuum chamber, this had the advantage of recording directly to the computer, using a Vernier Stainless Steel Temperature Probe (TMP-BTA)22http://www.vernier.com/products/sensors/temperature-sensors/tmp-bta/.

In Situ Motor Operation

In order to determine the cause of the pressure rise due to motor activity, characterization of the pressure rise was needed. Inside the vacuum chamber, an actuator was run at various speeds. Speeds were varied through the full range possible by the picomotor, from maximum of 2000 encoder counts per second down to zero. During each run, the actuator would cycle constantly between full extension and retraction. As the motor cycled, the pressure was recorded from both the read out of the ion gauge and ion pump. Multiple trials were made at each speed. After the motor had cycled an appropriate amount of time the motor was turned off or disconnected from power entirely. This allowed for monitoring of the influence of a powered motor while cool down between trails took place. Below is a single trial run at 1500 encoder count per second.

Vacuum Pressure vs Time;

Picomotor actuator operating at 1500 encoder counts per second