HETDEX Linear Drive Design

Tim Beets, PEUniversity of Texas

Center for Electromechanicst.beets@cem.utexas.edu

(512) 232-4285

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

Lower X Drive

Y Drive

Bridge

Trolley

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

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

Critical Speeds of Screw vs Nut Position

0

500

1000

1500

2000

2500

3000

-2.5 -1.5 -0.5 0.5 1.5 2.5

Nut Position, meters

Syn

ch

ron

ou

s C

ritic

al S

pe

ed

, rp

m.

Mode1 V

Mode2 H

Mode3 V

Mode4 H

Slew drive pivot bearings

Low-Temperature Trials

• Temperature effects cause large variation in drive performance– Current requirements doubled for

velocity-controlled runs– Random behavior such as velocity

spikes were observed at lower temperature

• 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

0 5 10 15 20 25-0.12

-0.1

-0.08

-0.06

-0.04

-0.02

0

0.02

Time (s)

Vel

ocity

(m/s

)

Slew Drive

Slew drive velocity at 70°F (blue) and 20°F (green) with constant current (2A) applied

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