Controls for Polarimeter Converters
Brianna Thorpe
Arizona State University
1
What are we doing?
• 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
2
Converter Issues: Electron Scatter
• 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.
3
Converter Issue: Delta Rays
• 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.
4
Converter Issues: Our Solution
• The Solution: make the target (converter) really thin.
• Our converter three different sources:
• Beryllium
• Slightly thicker Beryllium
• Carbon
5
How Do We Control the Converters?
6
Distance in beam direction between converter and detector is 35 mm
7
Detector supports attached to plate with screws
8
Rack guide supports attached to plate with screws
9
The Converter Control
Motor
• The motor moves the
converter tray.
Carbon
Converters
Thicker Beryllium
• Control the motor and
you control the
converters.
Beryllium
10
Step One: The Hardware
•
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
11
• The Arduino board is our microcontroller
Ethernet Shield
• Our code tells the Arduino what to do
• The Arduino relays our commands to the shields
• Shields: Electronics that are stacked on the microcontroller
Motor shield
• Our motor shield speaks to our motor
• Cost: $27
• Our Ethernet shield allows for Ethernet communication
• Cost: $35
Microcontroller
12
•
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
13
•
We hooked our stacked Arduino and shields to a cheap test
motor
•
Bipolar
•
200 steps per revolution
•
12 Volt
•
Cost: $20 (including shipping)
14
Step Two: The Software
•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.
15
• 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
16
GUI Position: retracted
17
GUI Position: One
18
GUI Position: Two
19
GUI Position: Three
20
Application at Jefferson Lab
The GUI will live here
21
Cost Effectiveness
Total Cost: $5345
Controller: $595
Controller chassis: $4750
Total Cost: $181
22
Putting it all Together
23
The Finished Product
24
Acknowledgments
•
•
•
•
•
Dr. Mike Dugger
Dr. Barrie Ritchie
Ross Tucker
Todd Hodges
Ben Prather
25
26
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
3
16
3
1
Hole 1
16
1
Hole 2
16
67
Hole 4
42
1
Hole 3
16
16
Converter tray
Converter plate
8
1
22
1
22
27
Converter retracted
Rack stop
Mounting plate
Rack stop
Teeth on rack not shown
Converter tray 11 mm from beam center
Converter tray detail shown later
Converter leg
•
•
•
Rack
Converter tray
Detector card
11 mm
28
Converter position 1
176 mm
5 mm
102 mm
10 mm
29
Converter position 2
30
Converter position 3
31
Different view
32
Front view
10 mm
10 mm
3 mm
33
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
91 mm
→ Weight of rack+arm+converter
tray assembly = 1.64 oz
34
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
35
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
36
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 I’m interested in has
holding torque of 7.5 oz-inch
Motor details shown later.
37
Motor mount added
Motor mount attached
to plate with screws
(not shown)
Want two cm
clearance between
upstream chamber
wall and motor
Upstream
38