Building an XRD State Machine¶
X-ray diffraction is a commonly used laboratory technique to study the properties of crystals or determine the makeup of a bulk sample. The video above summarizes the operation of a diffractometer and shows one in operation. An X-ray diffractometer emits X-ray radiation towards the sample at a known angle \(\theta\). The radiation is elastically scattered off atoms in the crystalline lattice. The angle of reflection is equal to the angle of incidence. The reflected radiation is counted by a detector. By sweeping the emitter and detector through a range of angles. we will see peaks in the X-ray spectra characteristic of the material present. Since crystals have regular structures, at certain incidence angles, the reflections from two adjacent layers constructively interfere and create a larger radiation count. In this activity, you will design a simple state machine to control an X-ray diffractometer.
We often use user stories to define how a machine should operate. The user story describes the process that the user or technician will go through to use the machine.
User Story¶
The user approaches the machine in the off condition. The main power is turned on and the machine starts with the X-ray source off in the X-ray shutter closed. The technician opens the safety doors and inserts a sample for analysis. The doors are closed and the technician sets the scan parameters in the control software. The software specifies the minimum and maximum angles at which to scan, the step at which to go between these angles, and how long the radiation should be counted at each angle. Once the start scan button is clicked, the machine must verify that the safety doors are closed and the safety interlock switches are closed. The X-ray shutter is then opened and the X-ray source turned on. The machine moves to its minimum scanning angle and begins to count the radiation for the specified time. After this time, the machine moves by the specified step amount and counts radiation again for the same amount of time. This process is repeated until the maximum scan angle is reached. If at anytime during the operation of the machine the safety interlock switch is open, the machine must immediately turn off the X-ray source, close the X-ray shutter, and go to the shutdown state. When the scan completes successfully, the machine must turn off the X-ray source, close the X-ray shutter, and go to the shutdown state as well.
Assignment¶
Using what we’ve learned in class about state machines and state machine diagrams, draw a state machine diagram that describes the operation of the X-ray diffractometer. Remember to specify start and end points, error sources, and keep the number of states reasonable.
Grading Rubric¶
| Description | Max Points |
|---|---|
| Turned in on time | 5 |
| Number of states | 5 |
| Logical flow of machine | 20 |
| Error sources included | 5 |
| Follows users story | 20 |
| Total | 55 |