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Advisory panel 2011 beets
- 1. HETDEX Linear Drive Design Tim Beets, PE University of Texas Center for Electromechanics t.beets@cem.utexas.edu (512) 232-4285
- 2. Tracker Linear Actuators 2 Bridge actuators (X) SKF 60x10 precision roller screw w/preloaded nut 18,800kg Payload 4 meters travel 1 Trolley actuator (Y) SKF 60x10 precision roller screw w/non-preloaded nut 35° Inclination 9,000kg Payload 4 meters travel Bridge and trolley ride THK linear bearing rails Upper X Drive Y Drive Trolley Lower X Drive Bridge
- 3. Unique Design Considerations Operation: 1) Slow-speed, precise positioning (tracking) and 2) high-speed, coarse positioning (slewing) Low temperatures atypical for roller screw applications Components: Long roller screws are prone to rotordynamic instabilities and tend to sag along unsupported length HET structure is flexible with limited alignment capabilities and presents transient alignment issues under dynamic loading (i.e., tracker traverse) COTS drive components Upper X drive model and analysis of screw sag
- 4. Roller Screw Concerns Rotordynamic analysis predicted screw behavior Tracking speed is well below critical speed Screw is not rotated during slewing (high-speed traverses) and nut speed is reduced at center of travel Sag of screw adds considerable load to nut Analyses predicted unacceptable stresses at screw ends when nut was aligned at screw center A single d.o.f. was added to allow the nut to pivot while traversing the sag region Resulting stresses are much lower—increased component life, controller consistency Slew drive pivot bearings
- 6. Subsequent discussion with SKF resulted in changing grease in roller screw and slew drive bearings
- 7. Erratic low-temperature operation significantly complicates precision controlSlew drive velocity at 70°F (blue) and 20°F (green) with constant current (2A) applied
- 8. Technical Collaboration MDO HET engineers HET actuator design/operation experience HETDEX operating requirements SKF roller screws, bearings, lubricants THK linear bearings, lubricants Danaher (Kollmorgen) motors, gearheads R+W couplings/torque limiters Branham restraining brakes H&M Mfg. pulleys, belts Upper X slew drive operation during testing
Notas del editor
- “Tracker” is everything above HET hex. The upgrade includes bridge, trolley, hexapod, science instruments, and all actuators. There are three primary actuators 2 X and Y. The X drives move the bridge and thereby the entire tracker. The y drive moves the trolley (along the bridge) and thereby the hexapod and science instruments. The load on the X drives are primarily friction forces from the screw components (bearings, nut preload) and linear bearings (preloaded). This load is reversed when movement reverses, so a preloaded nut (0.5-2.5Nm) is necessary to eliminate backlash. The load on the Y drive is from the 9000kg payload on the 35deg inclination. This load does not reverse, so a non-preloaded nut is used—increases efficiency.5.2X increase tracker mass7.2X increase science payload6.9X increase bridge payload
- 2 distinct drives providedTrack: 3mm/s max at 2micron following errorSlew: 80mm/s max at 20micron following errorTrack drive is coupled directly to screwSlew drive is coupled to nut via belt drive and gearhead (1:4.2 ratio)Min operating temp is 20F. Grease necessitated by application, but limits low-speed performance.
- 1st critical speed is at 500rpm w/nut centered.As the nut moves toward either end of the screw, the critical speed increases and slew speed can be increased to maximum.
- Achieved over 100mm/s slew speed at 70F and only 10mm/sOrder of magnitude difference in peak speed at constant current.