Controls for Polarimeter Converters Brianna Thorpe Arizona State University

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

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.

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.

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

How Do We Control the Converters?

Distance in beam direction between converter and detector is 35 mm

Detector supports attached to plate with screws

Rack guide supports attached to plate with screws

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

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

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

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

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

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.

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

GUI Position: retracted

GUI Position: One

GUI Position: Two

GUI Position: Three

Application at Jefferson Lab The GUI will live here

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

Putting it all Together

The Finished Product

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

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

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

Converter position 1 176 mm 102 mm 5 mm 10 mm

Converter position 2

Converter position 3

Different view

Front view 10 mm 10 mm 3 mm

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

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

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

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.

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