# Presentation: 219046

## Phased Array Mirror Extendible Large Aperture (PAMELA) Testbed Activities

- jeff.lindner@msfc.nasa.gov
- john.rakoczy@msfc.nasa.gov

## Agenda 

-  Background
-  Mirror Segment Figure Enhancements
-  Structural Dynamics Investigation/Optimization
-  Wave Front Sensor/New Blue Line Computer
-  Performance Achieved
-  Future Plans

## PAMELA Optical Table Layout

**WYCO 6000**

**Spherical Segmented Primary**

**PAMELA Optical Table Layout**

**CCD Cameras**

**Quad Cell **
**Wave Front Sensor**

**Parabolic Mirror**

**HeNe Alignment Laser**

**Reference**
**Flat**

**Return Flat**

**Mirror**

**B.S.**

**B.S.**

**B.S. =  Beam Splitter**

**= Apodization Plate**

- A

- A

## PAMELA Testbed Activities

- _BACKGROUND_:
-  1st telescope to have a fully adaptive primary mirror
-  Each of the 36 segments mounted on 3 voice coil actuators
-  Segment tilts measured via a Hartmann-Shack wave front sensor
-  Inductive edge sensors used to measure piston errors
-  Closed loop tip/tilt and nearest neighbor edge matching demonstrated 3/95
-  Installed Blue Line Engineering Wave Front Sensor 7/97
-  Demonstrated monochromatic phasing 8/98

## PAMELA Testbed Activities

- _MIRROR SEGMENT FIGURE ENHANCEMENT APPROACH_:
-   Acquire in-situ interferometric measurements with the WYCO
-     6000 Interferometer
-   Remove all segments from the telescope backplane and perform
-     radius of curvature measurements with EB52’s ZYGO interferometer
-   Compile data to determine which segments need to be refigured
-   Debond faulty segments from actuator flexures
-   EB52 resurfaces faulty segments
-   Install resurfaced segments and evaluate performance gains

## PAMELA Testbed Activities

- _IN-SITU MIRROR SURFACE CHARACTERISTICS_:

- Note: All data acquired in a double pass configuration

## PAMELA Testbed Activities

- _ACCOMPLISHMENTS 1_:
-   Surface figure for all segments measured in-situ with WYCO 6000
-   Removed all segments and performed radius of curvature
-    measurements with EB52 equipment in Building 4487
-   Chose 8 segments to be refigured
-   Debonded segments from actuators and removed edge sensors
-   Major developments

## PAMELA Test Bed Activities

- _NEW DEVELOPMENTS_:
-   Initial tests showed that as much as 1 wave Peak to Valley surface
-    figure perturbation could easily be introduced with an in-plane
-    actuator displacement of 0.005 inch
-   A stress analysis was conducted to ensure that the flexures were
-    not stressed beyond their yield or ultimate strength
-   Extreme accuracy required to position the actuators

## Segment Schematic Diagram

- <date/time>

-  Voice Coil Actuator

- Complete Segment Assembly

## PAMELA Testbed Activities

- _Refigure Methodology_:
-   Installed a 0.024 thick ring gasket between the actuators and the
-    actuator plate (the gasket served as a stiff spring)
-   Individual actuators can be minutely tilted by adjusting the torque
-    of the three actuator mounting screws
-   Modified the base plate and backplane to provide access to the
-    screws while the segment is installed in PAMELA array
-   The method is therefore an in-situ procedure that incorporates
-    all of the PAMELA optical surfaces

##  Peak to Valley=.490 wave

-  RMS=.078 wave
-  Strehl=.786

-  Peak to Valley=.269 wave
-  RMS=.053 wave
-  Strehl=.894

-  Initial Spot

- First Iteration

- Image Sensor Spot Intensity Profile

##  Peak to Valley=.206 wave

-  RMS=.039 wave
-  Strehl=.941

-  Peak to Valley=.077 wave
-  RMS=.012 wave
-  Strehl=.995

- Second Iteration

- Final Iteration

- Image Sensor Spot Intensity Profile

## Final Iteration

- Full Aperture
- (1 cm flat to flat hexagon)

- Medium Apodization
- (0.64 cm diameter circle)

- Image Sensor Spot Intensity Profile

## Segments 7 & 19 Phased 

- Segments 7 & 19 Phased

- Segments 7, 13 & 19 Phased

- Image Sensor Spot Intensity Profile

## Image Sensor Spot Intensity Profile

- Reference interference pattern obtained from a plane wave front passing through the inner ring of 6 apertures of the apodization plate

- Interference pattern obtained from the inner ring of 6 *phased* PAMELA segments (apodized)

## PAMELA Testbed Activities

- <date/time>

-  _Modal Test Results_

## Segment 14 Open Loop Response Characteristics

- <date/time>

-  100 count step tilt input
-  WFS horizontal response
-  Damping (log decrement) = 0.7%

-  Calibrated hammer impact on
-    secondary support structure
-  WFS horizontal response
-  Damping (curve fit) = 0.7%

## Segment 14 Closed Loop PID Controller Performance  

-  Control loop gain = 15
-  Damping = 4.6%

-  Control loop gain = 4
-  Damping = 1.2%

- The dynamic response of the segment system is constantly aggravated by the “step function” characteristics of the controller inputs and the actuator’s electronic noise floor.

## Segment Response Characteristics 

-  Classic design optimization includes
-    m & k... what about c ?
-  Main drawback is c = hysteresis
-  How could supplemental damping be
-    added to PAMELA ?

## Supplemental Damping Requirements 

-  Experimental damping treatment must be:
-   Low cost (cheaper, better, faster ...)
-   Non-permanent
-   Adjustable damping ratio
-   Easily installed and removed

## Interlaminar Shear Stress Viscoelastic Damper Concept

-  Cantilever beam deflection produces interlaminar shear stress in the
-    viscoelastic bonding material sandwiched between the substrate
-    material
-  Damping factor is proportional to the number of layers

-  Fixed End

- Actuator
- Armature
- Attachment

## Open Loop Damper Performance  

-  Segment 14 (no dampers)
-  100 count step tilt input
-  Damping = 0.7%

-   Segment 15 (5 layer substrate
-     viscoelastic actuator dampers)
-  100 count step tilt input
-  Damping = 7.8%

## Open Loop FRF Comparison 

-  FRF, no dampers
-  Impact on secondary support
-    structure

-  FRF, 5 layer dampers installed
-  Impact on secondary support
-    structure

## Closed Loop Damper Comparison  

-  No dampers
-  Control loop gain = 15.0
-  Damping = 4.6%

-  Viscoelastic shear dampers
-  Control loop gain = 6.3
-  Damping = **9.4**%

## Acoustic Excitation

- Qualitative data for relative comparison

## Response To Acoustic Excitation

-  Open Loop No Dampers

- Open Loop With Dampers

- Closed Loop No Dampers

- Closed Loop With Dampers

##  Closed Loop Damping Objective

-  100 count step tilt input
-  Controller gain at 0.1

-  100 count step tilt input
-   Controller gain at 15.0

-  An optimal damping factor yields a forgiving system that can easily be
-  tuned with the controller gain.

##  Wave Front Sensor Quick Look

-  Original Lateral effect diode
-    WFS calibration
-  Significant scatter
-  Excessive noise floor of
-    50 counts peak to peak

-   New Blue Line Quad cell WFS
-     calibration
-   Linear characteristics
-  40x reduction in the noise floor!

## PAMELA Lessons Learned (the hard way)

-  When working in the nanometer range, nothing is truly fixed
-  All components exhibit a characteristic time constant associated
-    with their respective position stability
-  Acoustic and thermal excitations drastically reduce this time constant
-  Closing the tilt loop couples all of the bench optics to the relative
-    piston position of the primary mirror segments
-  A double pass setup requires a 2x increase in actuator, edge sensor
-    and WFS resolution when compared to a single pass setup
-  The Hartman-Shack WFS is very sensitive to optical noise such as
-    extraneous reflections produced by refractive optics and beam splitters
-  High gain structural modes within the controller bandwidth can create
-    significant challenges...