SYSTEM DESIGN NOTE

SDN0002.13 - GNIRS Motor and Mechanism Data

 
 

Prepared by	Date	Approved by	Date	Rev.	Rev Date
Larry Goble/ Jay Elias	5/27/99	N. Gaughan	6/1/99

Introduction

There are nine driven parts internal to the GNIRS dewar. These data assume the use of a Phytron motor for use at 60 Kelvin and vacuum. The motor has 200 steps per revolution. Repeatability is assumed to be ±1/2 step.
Drive strategy is to use no detent mechanism. Each drive will have an accurate home position switch. The home will be acquired during initialization of the system and then step counting will determine the position thereafter. The position count will always be from the same direction to help compensate for backlash. Additional backlash compensation is discussed under individual mechanisms. A drive past and back up is used for positioning in the opposite direction. The systems will incorporate low friction bearings and friction will be limited to the light seals and worm drive interfaces. The slides will glide on ball bearing rollers.

Gear system description

All the gear systems are AGMA quality 10 or better precision cut gears. The gear pairs are of non-similar metals, that is 2024 Aluminum on 303 Stainless, or Bronze on Vespel-SP3. The material used on the larger of the two gears more closely matches CTE with the Aluminum base structure. With this strategy the contraction in cooling to 60 degrees Kelvin can be accommodated by using less gear backlash allowance.
The bearings in the system are of 440-C Stainless Steel.

Filter Wheels

The filter wheels are driven through a ring gear cut into the edge of the wheel. The wheel is 6061-T6 Aluminum and the pinion is 303 Stainless Steel. The gear tooth profiles are 24 pitch. The ring gear is 312 tooth and the pinion is 15 tooth. The pinion part number is BERG P24S28-15. The drive system has no backlash removal, other than friction with light seals on the filters. The repeatability requirement is about 0.022 inch at the ring gear. This is about 10 times greater than the expected maximum back-lash.

Decker and Acquisition slides

The slides are driven using a rack and pinion drive. The rack is Aluminum and the pinion is 303 Stainless Steel. The gear tooth profiles are 32 pitch. The pinion is 14 tooth and is supported from a 0.25-inch shaft with both bearings on the same side. Another spur gear reduction, 2/1, is used between the pinion and the motor. The drive system has no backlash removal.

Slit Slide

The slit slide is driven through a half nut cut into the bottom of the slide. A pair of short screws that turn under the slide body moves the slide. The screws are preloaded with a spring to control backlash. The motor drives the screw shaft through a worm gear reduction. The slide is made from 6061-T6 Aluminum and the screws from Vespel-SP3. The screw pitch is 0.2 inch. The pre-load is 16 lb. The gear reduction is 30/1 and uses a Bronze gear, single lead 14.5 pressure angle, 48 pitch part number BERG W48B29-S30. The worm gear in the reduction is made from Vespel-SP3. The pre-load can be reduced if the slide is counter-balanced.

All Spectrograph Turrets

The turrets are driven using a worm gear on the turret plus an additional spur gear reduction on the motor end. The gear reduction is 180/1 and uses a Bronze gear, single lead 14.5 pressure angle, 32 pitch, part number BERG W32B29-S180. The worm gear in the reduction is made from Vespel-SP3. The spur gear reduction is 2.5/1. Backlash is controlled by a "watch spring" which provides a preload of the balanced turret against the final direction of motion. The turret is balanced. The suggestion has been made to use a Vespel friction caliper in the place of the spring.

Detector Focus

Mounting the detector on a BERG ultra precision linear ball slide controls the focus motion. The slide, part number BERG LPSK-50A, is rated for a 300-pound load. The detector focus is driven using a split screw and nut like that for the slit slide, with an additional spur gear between the screw and the motor. The nut, Vespel-SP3, is fastened to the stage; and the screw, 6061-T6 Aluminum, is anchored to a fixed point using ball thrust bearings. The spur gear reduction is 4/1. The gear is 48 pitch, 100 tooth, 0.125 inch wide, 2024-T4 Aluminum, parts number BERG P48A26-100. The pinion is made from 303 Stainless Steel, 25 tooth, part number BERG P48S21-25. For motor clearance the gear may be 3/1 bevel type in place of the above. The repeatability values given in Table 1 are for this lower precision of motion.

Motor Usage

The following table summarizes maximum motions of the individual mechanisms, based on the data in Table 1. Note that for the slit slide and grating turret the maximum motions are given for two modes of operation each. The range of focus travel given is for adjustments when the instrument is cold rather than the full range of travel.
 
 

From the above table it is clear that the instrument is slightly out of spec if the motors are operated at 600 rpm; operation at 706 rpm would allow all reconfigurations to take place within 30 seconds if motors are operated simultaneously, except for configuration changes involving an IFU which will take place within the specified time for global reconfiguration of 180 seconds.
Changes involving acquisition, which use only filters, slits, deckers and acquisition mirror will take place in 15 seconds or less, because the acquisition slit position is in the center of the slide, so that motion between it and any other slit will be at most half the maximum distance.
 

---

---

If you have any questions or suggestions regarding this website, please contact Melissa Bowersock.

NOAO Intranet Services

NOAO Copyright  Statement