1. Controls for Polarimeter Converters
Brianna Thorpe
Arizona State University

2. 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

3. 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.

4. 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.

5. Converter Issues: Our Solution
The Solution: make the target (converter) really thin.
Our converter three different sources:
Beryllium
Slightly thicker Beryllium
Carbon

6. How Do We Control the Converters?

7. Distance in beam direction between converter and detector is 35 mm

8. Detector supports attached to plate with screws

9. Rack guide supports attached to plate with screws

10. The Converter Control Motor The motor moves the converter tray.
Control the motor and you control the converters.
Carbon
Converters
Thicker Beryllium
Beryllium

11. 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

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
Ethernet Shield
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
Motor shield
Microcontroller

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

14. We hooked our stacked Arduino and shields to a cheap test motor
Bipolar
200 steps per revolution
12 Volt
Cost: $20 (including shipping)

15. 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.

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

17. GUI Position: retracted

18. GUI Position: One

19. GUI Position: Two

20. GUI Position: Three

21. Application at Jefferson Lab
The GUI will live here

22. Cost Effectiveness Total Cost: $5345 Controller: $595
Controller chassis: $4750
Total Cost: $181

23. Putting it all Together

24. The Finished Product

25. Acknowledgments Dr. Mike Dugger Dr. Barrie Ritchie Ross Tucker
Todd Hodges
Ben Prather

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 3 16 3 1
Hole 1 16 1
Hole 2 16
Hole 4 42 67 1 16
Hole 3 16
Converter tray
Converter plate 8 22 1 22 1

28. Converter retracted Rack stop Mounting plate Rack Rack stop
Teeth on rack not shown
Converter tray 11 mm from beam center
Converter tray detail shown later
Converter leg
Detector card
Converter tray
11 mm

29. Converter position 1
176 mm
102 mm
5 mm
10 mm

30. Converter position 2

31. Converter position 3

32. Different view

33. Front view
10 mm
10 mm
3 mm

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

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

36. Rack guide added
If rack guide is aluminum then coefficient of static friction between rack and rack guide is between 1.05 and 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

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 I’m interested in has holding torque of 7.5 oz-inch
Motor details shown later.

38. Motor mount added
Motor mount attached to plate with screws (not shown)
Want two cm clearance between upstream chamber wall and motor
Upstream