Slit/Decker unit, with decker car in beam, slit car out
Decker and slit cars at open position. Note copper thermal straps connecting each of the cars to the baseplate. This was the initial configuration, designed to keep the cars cold. Note also the Silicon diode temp sensor at the bottom of the decker car. We found during cold testing that both cars were as cold as the baseplate, whether or not the thermal straps were present. Based on this, we no longer use the thermal straps to the cars.
Slit car, moving over the aperture.
Slit plate covering the aperture.
Decker car in motion toward aperture.
Decker and slit cars covering the entrance aperture.
Details of the decker car stepper motor, gear system, drive screw, and detent and reed switch sensor system.
Side view of unit, showing bearing end races for drive screws, "Pilons" securing the stainless steel shafts (the shafts are captured at one end and free at the other, to prevent CTE mismatch problems with the aluminum baseplate).
Details of the decker detent system. It uses a 6-pin disk engaging a rolling bearing attached to a short lever arm bar. The pivot base of the bar captures a flex-pivot and the long axis holds a magnet to trigger the reed switch, mounted on a small circuit board under the arm. When the roller bearing comes out of being detented between two of the disk pins, the long arm rises and breaks the magnet's closure of the reed switch, signaling an out of detent condition.
Another view of the detent system
Details of the gear engagement. The spur gear on the stepper motor engages a tall spur gear that captures the drive screw. Relative motion of the drive screw, due to CTE mismatch, does not change the gear mesh. The inverse gear mechanical advantage was dictated by the physical constraints, both for the motor and the drive screw.
Twin reed switches at the "home" end of the slit car. A magnet mounted in each car triggers the reed switches when the car passes over the switches. They establish "software" and "hardware" motion limits. The software limit serves as the home position for each car. The reed switches are positioned so that the home position is guaranteed to keep each of the decker and slit cars fully out of the entrance aperture.