Tensegrity Telescope Structures
Alt-Az Workshop, August 2019
Don Peckham
Outline
My Evolution
What Is Tensegrity?
String Telescopes
Tensegrity – Self Compensating
Minimum Requirements
Summary
My Evolution
(Scopes)
Barry Leger
(8” F6, With Pedestal)
12.5” F4.5
8” F6
String Angle:
Bigger Angle,
More Robust
Tensegrity – Proof of Concept
(What Will a 0.490” x 0.026” Tent Pole Support?)
Wood dowels (from 4 strut scope)
Mirror box & upper ring
0.490” tent poles, bowed
Scalable
0.625”
0.742”
26 pounds
Bucket of Clamps
Bob Mcgown
Tension Integrity or Floating Compression
Distributed network (All strings
and rods are interdependent)
Strings In Pure Tension
Rods In Pure Compression
No Bending Moments
Ultra Lightweight,
Efficient
“What Is Tensegrity?”
Tensegrity – Nature – Efficiency
Tensegrity exists in nature
Human body is a tensegrity system
Bones are rods
Muscles and tendons are strings
String Telescopes
What Is A String Telescope?
Strings rigidly locate the upper ring with respect
to the mirror box, and maintain collimation
throughout the movement of the telescope
Struts provide compression force between the
mirror box and upper ring, and put all strings in
tension.
Legend
Strings attaching at ends of struts
cause small (or zero) bending
moments
Strings attaching away from strut
ends cause “large” bending moments
Examples
Traditional String
Telescopes
First String
Telescope
Dan Gray
Tensegrity String
Telescopes
Most
Common
Design
3-Strut
Design
4-Strut Designs
3 pairs of strings
Strings directly connect the
upper ring to the mirror box
1 or 2 middle rings with floating tubes
Strings attach at the ends of all tubes
String / Strut Basics
Strings
3 or 4 pairs of non-stretching strings
Tensegrity string telescopes have
middle ring tubes that do NOT bow
Strings are in pure tension
Struts
2, 3 or 4 struts that provide
compressive force between the mirror
box and upper ring
Struts are in pure compression
Strings
“Pure Tension”
Strings attach directly between string anchors
String anchors do NOT deflect
String pairs locate the upper ring
String Assemblies
Bow String:
BCY 450 Plus
~150 pounds/strand
0% stretch
70% Dynemma
30% Vectran
Source,
Stonehill Primitive Bows
String Assemblies
String Tension and Stretch
1 strand = ~150 pounds tension
1 String Assy. = 5 loops (1 strand) = 10 cross-sections
= ~1,500 pounds tension
(use more loops for higher tension) SCALABLE
Stretch = 0%, regardless of string tension
String Tension With Respect to Tube Orientation
Tube Assy. Vert.
All strings, same tension
Tube Assy. Horiz.
Different strings,
different tension
Strings do NOT stretch
String Tension
Parallel strings have the same
tension
Struts
Struts are in pure compression
Struts are pinned, in column
loading
No bending moments
Strut bowing does not impact
collimation
Use larger diameter tubes for
higher compression SCALABLE
String Telescopes Are Counterintuitive
(Dan Bakken’s “Hercules”, 41.2", F3.9)
Struts appear fixed length, but
are variable length,
compression springs
Strings appear
insignificant, but
maintain collimation
How String
Telescopes Work
Strut Force / String Force Relationship
Each strut provides the compression force to tension
a pair of strings.
The strut compression force is equal and opposite
the axial components of the pair of strings.
String Forces
F(string), F(lat.) and F(axial)
are related as shown
Big Angle = Small F(axial)
Small Angle = Big F(axial)
Traditional
String Telescope
My First String Telecopes
(Bigger Angle, More Robust)
Strut Force – Upper Ring Weight
When the tube assembly is horizontal, upper
ring weight is “proportional” to “Lateral Force”
Tube Vertical Tube Horizontal
String Angle / Strut Force
Relative Strut Force
~1.2~1.1
~2.2
10
15
3 Struts
4 Struts
2 Struts
3 Struts
Big string angle =
small strut force
Small string angle =
BIG strut force
4 Struts
Relative strut
force for each
strut
(order of
magnitude)
(Bigger Angle, More Robust)
Benefits of Low Strut/String Force
Smaller diameter,
lighter weight struts
Lower string tension,
less sensitive to
vibration
Greater flexibility to
create interesting
designs
String Force(lat.) - Couples
String forces provide opposing couples that
constrain rotation of the upper ring
Clockwise Couples: Blue and Red forces
Counterclockwise Couples: Green and
Orange forces
Similar opposing couples constrain rotation of the
middle ring(s)
Tensegrity - Self-Compensating
(My Biggest Surprise)
The blue string pair traces an arc
Red and blue string arcs intersect
at middle ring tube ends
The upper and lower strings
determine the location of the
middle ring tubes
The middle ring tubes determine
the location of the upper ring
The upper ring and mirror box are
integral with the string/strut tube
assembly
What Does The Upper
Ring “Feel”?
The upper ring and mirror box
are integral with the string/strut
tube assembly
The upper ring only “feels”
forces
Axial forces are equivalent to
c-clamp forces
Lateral forces are equivalent
to furniture clamp forces
Minimum Requirements (Rules)
(Apply To All String Telescopes)
Collimation is maintained when:
All strings are in tension and
do NOT stretch
Middle ring tubes are
NOT bowed
Strings and tubes can have
the different forces and
lengths
Struts can be bowed
How To Defeat Collimation
(String telescopes inherently hold collimation)
Change string length
Apply external forces
(not associated to typical telescope function)
Overdesign
(I favor overdesign)
Increase strut compression until strings
are somewhat tight
Use larger diameter struts than needed
Add additional loops of bow string
Summary
Why Tensegrity String Telescopes?
Compared to truss tube telescopes
Lighter weight
Fewer loose parts
Quicker setup
Compared to traditional string telescopes
Significantly lower string and strut force
Lighter weight
Zero bending moments at mirror box
Summary
Various tensegrity string
telescopes
Patrick Joyce
OSP 2019
Design Tips (Dos and Don’t’s)
12.5” F4.5
12.5” F4.5
8” F6
10.1” F4.5
3-Strut Design
String / Tube Length
Tubes
Spacers
Thumb Screws
Captured Nuts
Quick Release
Clamps
Coulter Odyssey
Summary - Conclusion
Tensegrity String Telescopes
Scalable
Self-Compensating
Viable option for transportable scopes
Questions?
End