Stirrer Operation and Construction

The stirrer system is now operational in the continuous motion mode.  We can vary the rotational speed from a maximum of about 8 revolutions per minute to almost arbitrarily slow.  We are in the final stages of cleaning up the system and will shortly make our first measurements with the stirrer operating.

The stirrer system on the reverberation chamber is driven by a stepper motor under the control of Labview Software. The motor is mounted outside the chamber and drives a shaft that extends from the top of the chamber to the floor of the chamber.  This shaft extends outside the chamber after going though a tight fitting bearing in a metal block grounded to the ceiling of the chamber.  The tolerance are kept tight to provide good coupling between the shaft and the chamber walls (in the operating frequency range of the chamber.)  To the pole can be clamped stirrers of various designs allowing for easy modification of the stirrer.  The shaft itself is easily changed, so that a plastic non-conducting shaft can also be used. 

Current Stirrer arrangement. 

Currently we are using three paddles approximately 7 inches by 21 inches long mounted on the pole with an approximate spacing of 120 degrees.  The top and bottom panels are mounted at an angle to avoid a three fold symmetry.  In the picture below you are looking edgewise at the bottom stirrer.  It is angled downward at the same angle that the upperone is mounted upward.

Design Details

The Stirrer rod is driven from the top by a belt drive.  Currently we are using a 1:1 drive ratio to an old rotational platform that was salvaged from an old experiment. 

Below is a picture of the stepper motor and the rotary drive.  The stepper motor has 200 steps per revolution and the rotary drive has a 180:1 ratio giving us 0.01 degrees per step.

The stirrer pole is mounted though the roof of the chamber by passing though two "bearing blocks" which contain rather larger bearings.  The shaft fights tightly in the bearings which are mounted in the block.

One of these can be seen in the picture of the top of the chamber.  The other is mounted on the ceiling of the chamber.

This block will shortly have the copper tape on the edges that contact the ceiling.  The tight fit of the bearing and the metal plate to the shaft should provide good coupling between the shaft and the chamber walls in the operating frequency range of the chamber.

The lower part of the shaft is mounted on the floor in an axial load bearing, shown disassembled with out the ball bearings in the picture below. 

The bearing in turn is mounted to a metal plate on the floor of the chamber.

The block of wood is insulates the pole from the floor so as to not change the topology of the chamber.  There will still be  some capacitive coupling.  In the future we plan on replacing the wood with a metal plate to see if that makes a difference.