“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.
Advisory panel 2011 beets
HETDEX Linear Drive Design<br />Tim Beets, PE<br />University of Texas<br />Center for Electromechanics<br />email@example.com<br />(512) 232-4285<br />
Unique Design Considerations<br />Operation:<br />1) Slow-speed, precise positioning (tracking) and 2) high-speed, coarse positioning (slewing)<br />Low temperatures atypical for roller screw applications<br />Components:<br />Long roller screws are prone to rotordynamic instabilities and tend to sag along unsupported length<br />HET structure is flexible with limited alignment capabilities and presents transient alignment issues under dynamic loading (i.e., tracker traverse)<br />COTS drive components<br />Upper X drive model and analysis of screw sag<br />
Roller Screw Concerns<br />Rotordynamic analysis predicted screw behavior<br />Tracking speed is well below critical speed<br />Screw is not rotated during slewing (high-speed traverses) and nut speed is reduced at center of travel<br />Sag of screw adds considerable load to nut<br />Analyses predicted unacceptable stresses at screw ends when nut was aligned at screw center<br />A single d.o.f. was added to allow the nut to pivot while traversing the sag region<br />Resulting stresses are much lower—increased component life, controller consistency<br />Slew drive pivot bearings<br />
Low-Temperature Trials<br />Temperature effects cause large variation in drive performance<br />Current requirements doubled for velocity-controlled runs<br />Random behavior such as velocity spikes were observed at lower temperature<br /><ul><li>Test results drove decision to increase drive torque capacity to achieve performance targets at lower temperatures
Subsequent discussion with SKF resulted in changing grease in roller screw and slew drive bearings
Erratic low-temperature operation significantly complicates precision control</li></ul>Slew drive velocity at 70°F (blue) and 20°F (green) with constant current (2A) applied<br />