Overview of LabVIEW

LabVIEW is our chosen interface to the Spellman high power amplifier and by extension, the

gyrotron (National Instruments, Austin, TX). There are several other controls that influence

microwave output such as the gun coil (TDK Lambda, Tokyo, Japan), main magnet

(Cryomagnetics Inc., Oak Ridge, TN), alignment stages (Thorlabs, Newton, NJ), and most

notably, the arbitrary waveform generator (AWG) channel on the Redstone spectrometer

(TNMR, Tecmag, Inc., Houston, TX). However, LabVIEW protects the electron gun surface by

turning off the high voltage immediately if one or more interlocks are triggered.

The virtual instruments or VIs in the “GyroControl_(Date)” project folder monitor many inputs

by RS232 electrical interface, and command the power supply to generate AC and DC voltage

for the gyrotron. Many of the inputs serve as interlock set points, which turn off the gyrotron in a

controlled and safe manner in case of a system failure. The main interlocks are internal gyrotron

pressure, water temperature of the gyrotron cooling lines, and the temperature of the magnet

surrounding the gyrotron. These interlocks protect the frequency agile gyrotron during electron

decoupling experiments and continuous frequency microwave experiments. Construction and

processing of a new gyrotron lasts six months to 1.5 years, so it is in the DNP spectroscopist’s

best interest to keep the gyrotron functioning optimally and protect the gyrotron from damage

with the interlocks found in LabVIEW.

5.1.1 Program files

A virtual instrument (VI) is a program file (extension “.vi”) that can collect data and trigger

called the “front panel” and a back end called the “block diagram” and can be linked together

into complex units using global variables.

5.1.2 Front panel

The front panel is the VI user interface window with a grey background that first appears when

the VI is opened (Figure 5.1). When the VI is not running, the various controls, indicators, and

formatting objects can be moved around to any orientation and values in control boxes can be

saved as the default value. When the VI is running, only front panel control variables are editable

and the file cannot be saved. The VI can be started with the straight arrow or repeating arrows in

the upper toolbar and if it runs successfully, it can be stopped with the stop sign button.

Figure 5.1 Front panel for “System Status.vi” in in stopped mode (white arrows and faded stop sign). Start, stop

and pause buttons are indicated by the orange box.

5.1.3 Block diagram

The block diagram window can be accessed by navigating in the top menu to “Window>Show

current VI (Figure 5.2). Controls, indicators, functions, and other components are linked

together by “wires” that indicate the order and direction of operations. As with the front panel, it

can only be edited when the VI is not running. The lightbulb button helps troubleshooting by

slowing the VI and highlighting each step as it activates or fails.

Figure 5.2 Block diagram for “System Status.vi”. The global variables reporting pressure and temperature are

brought here. These values are compared with desired safe points to turn “system status” global variable to true or false. If system status reads false, the heater current and DC voltage are set to zero in “Control and Graphing.vi”. Note that that the front panel does not reflect the complexity of the VI function.

5.1.4 Controls and indicators

Controls are variables placed on either window of a VI. They act as a variable writer and can

change the value of true/false (Boolean), string (alphanumerical characters), numerical, and

many other types of variables. Indicators receive the information from the controls after

functions have been performed on that information. Control wires come out of the right side of

control blocks, and indicator wires connect into the left side of the block, so that the entire block

diagram may be “read” from left to right. A simple example using controls and indicators is

shown in Figure 5.3. If a control or indicator is placed on the front end, its corresponding

marker will be placed on the block diagram. However, a “constant” type control will not

generate a front panel equivalent as the value is entered directly in the block diagram.

Figure 5.3 a) Front panel and b) block diagram for a simple VI representing the equations A*3=X and A*3-B=Y.

The front panel is shown in continuous run mode and the block diagram is shown in the stopped mode.

5.1.5 Global variables

Global variables span different VIs. They can be set as either a control or an indicator type, but it

is best practice to “write” only one indicator per global variable. References to a global variable

in the current “GyroControl_(Date)” folder. “Global.vi” is unique because it does not have a

block diagram, but only a front panel (Figure 5.4). The pressure safe point, temperature safe

point, and heater step size are entered directly into “Global.vi”. Heater step size determines the

value by which the heater current can increase or decrease every LabVIEW cycle. Without this

value to slow down the heater value, the electron gun would be damaged by rapid heating and

cooling. The heater step size can be changed under the “Heater controls” tab in “Global.vi”.

Proportional-integral-derivative (PID) values are also entered into “Global.vi” to control the

electron beam current by adjusting the AC heater current, but these values do not need to be

changed once they are optimized.

Figure 5.4 “Global.vi” front panel showing System Status tab.