1 Brianna Thorpe Arizona State University Controls for
Polarimeter Converters
Slide 2
What are we doing? 2 Experiments at Jefferson Lab use controls
for polarimeter converters This minimizes problems in the
experiments We want to make a cheap, simple to use control This
will require: Developing hardware Writing software
Slide 3
Converter Issues: Electron Scatter 3 What is this? As an
electron moves through the source, it bumps against other atoms.
Why is this an issue? As the thickness of the source increases, the
ratio of scattering centers to electrons detected increases.
Slide 4
Converter Issue: Delta Rays 4 What is this? As an electron
travels through the source, it bumps against other atoms. This
knocks off other electrons. Why is this an issue? These other
electrons (Delta rays) hit the detector at angles different than
the recoil electron. Because both have similar kinematics, we have
no way of knowing which is the recoil electron.
Slide 5
Converter Issues: Our Solution 5 The Solution: make the target
(converter) really thin. Our converter three different sources:
Beryllium Slightly thicker Beryllium Carbon
Slide 6
6 How Do We Control the Converters?
Slide 7
7 Distance in beam direction between converter and detector is
35 mm
Slide 8
8 Detector supports attached to plate with screws
Slide 9
9 Rack guide supports attached to plate with screws
Slide 10
The Converter Control 10 Motor The motor moves the converter
tray. Control the motor and you control the converters. Carbon
Beryllium Thicker Beryllium Converters
Slide 11
Step One: The Hardware 11 We want cheap and simple: Arduino:
Open source electronics Works with Linux Programs written in C or
C++ Lots of example code on the internet Cost: $99
Slide 12
12 Motor shield Microcontroller The Arduino board is our
microcontroller Our code tells the Arduino what to do The Arduino
relays our commands to the shields Shields: Electronics that are
stacked on the microcontroller Our motor shield speaks to our motor
Cost: $27 Our Ethernet shield allows for Ethernet communication
Cost: $35 Ethernet Shield
Slide 13
13 Hardware Issues: The Arduino motor shield had a pin conflict
with the Ethernet shield This prevented communication between the
shields Our Solution: An inexpensive pin-reassignment shield We
used solder to reassign the pins
Slide 14
14 We hooked our stacked Arduino and shields to a cheap test
motor Bipolar 200 steps per revolution 12 Volt Cost: $20 (including
shipping)
Slide 15
Step Two: The Software 15 Installed on lc64 Linux box Installed
on my Windows laptop Used example code for testing. The code for
Arduino is called a sketch Compiled using Linux and Windows. We are
fully platform independent.
Slide 16
16 The code to control the motor was written in the Arduino
program. The Graphical User Interface was written in Vpython. The
GUI allows for communication between the sketch and the
Arduino
Slide 17
GUI Position: retracted 17
Slide 18
18 GUI Position: One
Slide 19
19 GUI Position: Two
Slide 20
20 GUI Position: Three
Slide 21
Application at Jefferson Lab 21 The GUI will live here
Slide 22
Cost Effectiveness 22 Controller: $595Controller chassis: $4750
Total Cost: $181 Total Cost: $5345
Slide 23
Putting it all Together 23
Slide 24
The Finished Product 24
Slide 25
Acknowledgments Dr. Mike Dugger Dr. Barrie Ritchie Ross Tucker
Todd Hodges Ben Prather 25
Slide 26
26
Slide 27
27 Converter Stick assembly Color code: 1.5 mm thick 3.0 mm
thick 1.4 mm thick Material: Aluminum All units in mm Hole 1-3: 8x8
Hole 4: 8x42 Screw holes on converter tray are threaded Weight ~
0.41 oz Hole 3 Hole 2 Hole 1 Hole 4 1 1 1 16 1 1 33 8 22 42 67 22
Converter trayConverter plate
Slide 28
28 Converter retracted Mounting plate Rack stop Rack Rack stop
Converter tray Converter leg Detector card Teeth on rack not shown
Converter tray 11 mm from beam center Converter tray detail shown
later 11 mm
Slide 29
29 Converter position 1 102 mm 176 mm 5 mm 10 mm
Slide 30
30 Converter position 2
Slide 31
31 Converter position 3
Slide 32
32 Different view
Slide 33
33 Front view 3 mm 10 mm
Slide 34
34 View from downstream, detector removed Weight of rack ~ 0.97
oz Weight of arm ~ 0.26 oz Weight of converter tray assembly ~ 0.41
oz Weight of rack+arm+converter tray assembly = 1.64 oz 91 mm
Slide 35
35 Gear added Gear has outer diameter of 15 mm (can be a bit
smaller) with a minimum of 18 teeth Teeth on gear not shown
Precision in linear motion per step (200 steps per revolution is
standard for stepper motors): ~1/4 mm
Slide 36
36 Rack guide added If rack guide is aluminum then coefficient
of static friction between rack and rack guide is between 1.05 and
1.35. Using 1.35 as conservative estimate of coefficient of static
friction, then the force needed to break static equilibrium is
1.35*1.64 oz ~ 2.2 oz
Slide 37
37 Motor added Since gear has radius of 7.5 mm (~ 0.3 inch) and
force needed to break static friction is 2.2 oz, then minimum
torque required of motor is ~ 0.7 oz-inch Motor that Im interested
in has holding torque of 7.5 oz-inch Motor details shown
later.
Slide 38
38 Motor mount added Motor mount attached to plate with screws
(not shown) Want two cm clearance between upstream chamber wall and
motor Upstream