Design and Functional Validation of a Mechanism for Dual-Spinning CubeSats

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sponsored by the Assistant Secretary of Defense for Research & Engineering under Air Force Contact FA8721-05-C-0002.

1 Design and Functional Validation of a Mechanism for Dual-Spinning CubeSats Eric Peters, Pratik Davé, Ryan Kingsbury, Meghan Prinkey, Anne Marinan, Evan Wise, Christopher Pong, Kerri Cahoy, William Thalheimer, Devon Sklair, Luis Orrego 10 August 2013 This work is sponsored by the Assistant Secretary of Defense for Research & Engineering under Air Force Contact FA C Opinions, interpretations, conclusions, and recommendations are those of the authors and are not necessarily endorsed by the United States Government.

2 Overview / Motivation Scanner assembly design Structural validation Outline Physical models Structural testing of rotor/shaft interface Notes on tolerance rings Thermal validation Test plan Results from thermal testing 10 August 2013 SmallSat 2013 PCW 2

Motivation Dual-spinning CubeSats Payload scans Earth cross-track for coverage Bus fixed in LVLH frame for pointing MicroMAS: Micro-sized Microwave Atmospheric Satellite 3U dual-spinning CubeSat for

3 Motivation Dual-spinning CubeSats Payload scans Earth cross-track for coverage Bus fixed in LVLH frame for pointing MicroMAS: Micro-sized Microwave Atmospheric Satellite 3U dual-spinning CubeSat for remote weather sensing 1U passive microwave radiometer payload, MIT Lincoln Laboratory 2U bus, MIT Space Systems Laboratory Scheduled to launch in winter Talk on Thurs. Aug 10:30am 10 August 2013 SmallSat 2013 PCW 3

Scanner assembly must: Design Requirements Fit in 10cm x 10cm x 3.5 cm volume Rotate 1U payload at ~1 Hz (60 rpm) Have angular position knowledge of < 6 arcmin (0.

4 Scanner assembly must: Design Requirements Fit in 10cm x 10cm x 3.5 cm volume Rotate 1U payload at ~1 Hz (60 rpm) Have angular position knowledge of < 6 arcmin (0.1 deg) Have 8+ electrical feed lines for power/data transfer between bus and payload Operate continuously in space environment Bus Payload MAI-400 ADCS Unit Avionics Stack Scanner Assembly 10 August 2013 SmallSat 2013 PCW 4

COTS Hardware Bearing NHBB RI-3026 thin section bearing with

MicroE M1500V vacuum-rated sensor Glass rotary grating disc with

01 degrees per count) Rotor/Stator Aeroflex Z-0250-050-3-104

5 COTS Hardware Bearing NHBB RI-3026 thin section bearing with vacuum-rated cage and lubricant Scanner Assembly Housing Encoder MicroE M1500V vacuum-rated sensor Glass rotary grating disc with 7200 counts per revolution (0.01 degrees per count) Rotor/Stator Aeroflex Z zero-cogging brushless DC motor Slipring Aeroflex CAY-1398 with 12 lines for power/data transfer 10 August 2013 SmallSat 2013 PCW 5

Design Overview Chassis Cover Plate Shaft

6 Design Overview Chassis Cover Plate Shaft Bearing Retainer Bearing Bearing Shim Housing Stator Base Plate Rotor Encoder Disc Encoder Sensor Encoder Disc Mounting Hub Slipring Retainer Slipring Blue: Rotary Green: Stationary Bold: Custom 10 August 2013 SmallSat 2013 PCW 6

7 Structural Validation of Scanner Assembly 10 August 2013 SmallSat 2013 PCW 7

Scanner Assembly Prototypes 3D Print Unit

procedures Engineering Design Unit Verify

different mating methods work for interface

function, cost, schedule Functional test

8 Scanner Assembly Prototypes 3D Print Unit Check volume Check fit Verify assembly procedures Engineering Design Unit Verify mass properties Verify machining procedures Early functional testing with nonflight components Rotor/Shaft Test Units Test/prove different mating methods work for interface Engineering Test Unit Verify custom parts function, cost, schedule Functional test with flight-like components 10 August 2013 SmallSat 2013 PCW 8

9 Rotor/Shaft Interface Concern: thermal expansion, loss of contact between rotor (stainless steel) and rotor shaft (aluminum) Tested several mating methods Advantages Disadvantages Tests Performed Epoxy Tolerance Ring Press-Fit Flight heritage (ACIS instrument, bond held with ΔT of -200 F) High CTE Estimated elastic modulus Requires careful application, curing Robust to thermal expansion/contraction Known torque and radial load capacities No prior experience No flight heritage Tests needed to get groove fit right No additional parts needed Simple to design Loss of fit with thermal variation Requires additional equipment Torque testing Torque testing Thermal testing 10 August 2013 SmallSat 2013 PCW 9

10 Rotor/Shaft Mating Tests Purpose: Test strength of rotor/shaft interface Use peak torque provided by motor (4 in-lb) on epoxy and tolerance ring Setup: Used identical flight-like shaft + rotor ring sets Shaft Aluminum alloy 6061-T6 Cut grooves for epoxy Found deeper groove needed for tolerance ring Ring in A513 mild steel Smooth internal surface in in Shaft + rotor ring test rig in 10 August 2013 SmallSat 2013 PCW 10

Torque Testing Setup 1/2-13 bolt secured through

while torque applied to the bolt Lock washer Grade

11 Torque Testing Setup 1/2-13 bolt secured through bore in center of test shaft Flat washers placed between shaft-nut and shaft-bolt Fastener installation torqued to 120 in-lb Ring held by vise while torque applied to the bolt Lock washer Grade 8, 1/2-13 bolt Nut Lock washer 10 August 2013 SmallSat 2013 PCW 11

Tolerance Ring Attempts First attempt, tolerance ring forced out of the groove Initial 0.005" groove depth too shallow Machined new shaft with groove depth of 0.022 Designed to fully retain the 0.

12 Tolerance Ring Attempts First attempt, tolerance ring forced out of the groove Initial 0.005" groove depth too shallow Machined new shaft with groove depth of Designed to fully retain the un-corrugated portion and ¼ of the corrugated portion Trimmed length of ring to maintain gap of between edges Gap of recommended by product engineers Image from: USA Tolerance Rings 10 August 2013 SmallSat 2013 PCW 12

13 Torque Testing Results First, test to peak torque provided by motor during nominal operations (4 in-lb) If successful, then test to point of failure Results: Both epoxy and tolerance rings survived up to 65 in-lb torque Point of failure was not reached for either epoxy or tolerance ring Note: At 65 in-lb, lock washers used in test apparatus dug into aluminum shaft Test was halted and not continued to higher torques Tolerance ring: tested to less than max torque capacity of rings (65 in-lb test << 675 in-lb predicted capacity) Epoxy: tested to much less than epoxy shear strength at ~25 C (tested 44 psi << rated 3800 psi) 10 August 2013 SmallSat 2013 PCW 13

14 Thermal Validation of Scanner Assembly 10 August 2013 SmallSat 2013 PCW 14

Thermal Testing Purpose: Characterize encoder + motor assembly performance over operational/survival temperatures in vacuum Analog output from

15 Thermal Testing Purpose: Characterize encoder + motor assembly performance over operational/survival temperatures in vacuum Analog output from encoder Oscilloscope Digital output from encoder to motor controller PC Temperature data w/ 9 x 100-Ω RTDs Agilent DAQ 10 August 2013 SmallSat 2013 PCW 15

16 Thermal Test Plan Encoder specified operational temperature range = [0,70] C Predicted on-orbit temperature range = [-5,25] C Scenario Chamber State Test Procedure TS Test Setup T-0 Functional Checks T-1 Cold Test T-2 Thermal Cycling Room Pressure & Temperature Ambient Vacuum ( 25 C ) Cold Vacuum ( < 0 C ) Cold Vacuum ( 0 C ) Hot Vacuum ( 70 C ) Prep chamber, test rigs, articles, + sensors Check function + calibrate test articles + sensors Collect benchmark data Pump-down chamber to ~1e-05 torr Functional checks on test articles + sensors Command motor to 60 rpm Collect data until thermally stable Continue running motor at 60 rpm Continue collecting data Supply LN2 to bring encoder temperature to 0 C Monitor encoder, slowly lower T to -10 C Command motor to 60 rpm, collect data Complete 1+ full thermal cycles by switching between LN2 cooling and resistor heating 10 August 2013 SmallSat 2013 PCW 16

Encoder Signal Voltage (mv) Encoder Performance Nominal = 800 mv, minimum acceptable = 400 mv 800 780 760 Ch. A (mv) Ch.

17 Encoder Signal Voltage (mv) Encoder Performance Nominal = 800 mv, minimum acceptable = 400 mv Ch. A (mv) Ch. B (mv) Avg Voltage (mv) Encoder Temperature (degc) Encoder V still in nominal range (> 400 mv) despite drop at higher temps 10 August 2013 SmallSat 2013 PCW 17

18 Encoder Signal Frequency (Hz) Scanner Performance Nominal signal frequency = 7200 Hz Encoder Temperature (degc) Encoder signal frequency stable over temp stable scanner performance 10 August 2013 SmallSat 2013 PCW 18

MicroMAS Scanner Summary Validated scanner assembly: design achieves requirements Successful iterative design process Prototypes and physical models for structural and thermal testing Successful

19 MicroMAS Scanner Summary Validated scanner assembly: design achieves requirements Successful iterative design process Prototypes and physical models for structural and thermal testing Successful tests on two rotor/shaft mating methods (1) Epoxy + grooves (2) Tolerance rings Both methods passed torque tests of up to 65 in-lb Selected Epoxy + grooves Successful test of encoder/scanner performance with temperature at vacuum pressure Future work: Thermal test epoxy/tolerance ring 10 August 2013 SmallSat 2013 PCW 19

20 Backup Slides 10 August 2013 SmallSat 2013 PCW 20

21 Temperature ( C) Temperature (degc) Temperature (degc) Temperature Data Temperature (degc) Motor commanded to continuously spin at 60 rpm during entirety of both tests st Day 1 - Cold Test Test 1: Cold Test Test articles 70cooled to Test -8.5 Day 1 C, - Cold then Test returned to ambient Thermal Plate 60 (E) Encoder (N) Bottom Plate 50 (NE) Side Wall (NW) Chassis Plate 40 (SE) Bracket (N) Bracket (W) 30 Bracket (E) Mounting 20 Plate (SW) 10 0 Thermal Plate (E) Encoder (N) Bottom Plate (NE) Side Wall (NW) Chassis Plate (SE) Bracket (N) Bracket (W) Bracket (E) Mounting Plate (SW) Test Day 1 - Cold Test ince Pump-down (sec) Time since Pump-down (sec) Time since pump-down Time since Pump-down (1000 (sec) seconds) Time since Time pump-down since Pump-down (sec) (1000 seconds) Test 2: Thermal Cycling Test articles cooled Test Day to 2 - Full 0 Cold/Hot C, then Cycle heated to 65 C Thermal Plate (E) Encoder (N) Bottom Plate (NE) Side Wall (NW) Chassis Plate (SE) Bracket (N) Bracket (W) Bracket (E) Mounting Plate (SW) Thermal Plate (E) Encoder (N) Bottom Plate (NE) Side Wall (NW) Chassis Plate (SE) Bracket (N) Bracket (W) Bracket (E) Mounting Plate (SW) x 10 4 Motor and encoder operated nominally throughout all tested temperature ranges 10 August 2013 SmallSat 2013 PCW 21

22 Encoder Signal at -5 C Signal is filtered and averaged by oscilloscope, as will be similarly done by on-board avionics Nominal signal amplitude: > 400 mv Nominal signal frequency: khz Active computation of signal frequency Active computation of signal amplitude (voltage) Encoder signal amplitude and frequency are both well within nominal range 10 August 2013 SmallSat 2013 PCW 22

Encoder Signal at +60 C Signal is filtered and averaged by oscilloscope, as will be similarly done by on-board avionics Nominal signal amplitude: > 400 mv Nominal signal frequency: 7.

23 Encoder Signal at +60 C Signal is filtered and averaged by oscilloscope, as will be similarly done by on-board avionics Nominal signal amplitude: > 400 mv Nominal signal frequency: khz Active computation of signal frequency Active computation of signal amplitude (voltage) Though the encoder signal amplitude decreased at higher temperature, the signal amplitude and frequency are both still within nominal range 10 August 2013 SmallSat 2013 PCW 23

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