Lab Automation with LabVIEW
Chris Addiego

During the Spring 98 semester a significant amount of effort has gone into several projects focusing on automating experiments in the department’s photonics laboratory. LabVIEW (Laboratory for Virtual Instrumentation and Electronics Workbench), which is a graphical programming language, was the software of choice for this purpose. This software is both powerful and flexible, and it can be used for a wide range of instrumentation projects. Under the direction of Professor Saeid Rahimi, a diode laser experiment, a temperature control system, and a fringe counter experiments were designed and automated. In each experiment the equipment is controlled by a Macintosh computer, and the user can adjust various parameters in a virtual control panel displayed on the monitor.

In the diode laser experiment the current injected into the laser is stepped up, or ramped, to a certain value and power readings are taken from a power/wavelength meter at each step. These L-I curves were taken at different temperatures. From this information the threshold current of the diode laser and the degree of its temperature tunability can be determined. Before automation, students manually ramped the current and manually took a power reading at each point. In order to automate this experiment LabVIEW and a data acquisition (DAQ) card were used to modulate the injection current into the diode laser. A voltage is set by the program, therefore setting the injection current, and a power reading is taken by the program at each point. The voltage is increased in steps until the maximum current, which is set by the operator, is reached. The temperature of the laser is changed and another L-I curve is taken. The user determines the number of data points and the maximum current value. The data can be saved to a text file and can be graphed and manipulated for determination of the threshold current.

The next project was a temperature control system. This was designed to monitor a thermocouple and maintain a constant temperature setting. Again, the DAQ card and LabVIEW were used to import the data into the computer and also to control a heater. We wanted to have more flexibility than just turning on the heater when the temperature is too low and turning the heater off when the temperature is too high. A system was designed which basically pulses the heater. The pulse duration is determined by how far away the desired temperature is from the actual value. Longer pulse lengths occur when the actual temperature is far away from the desired temperature. If the actual temperature is above the desired temperature then the pulse length is zero. The heater is turned on by setting the voltage output of a terminal of the DAQ card. This voltage is sent to a solid state relay which switches the current sent to the heater. Using this pulsed heater method we were able to achieve very precise monitoring, to within 1 degree Celsius. More precise temperature detectors will result in even higher precision.

The last automation project was a fringe counter. This was done by using LabVIEW to monitor a power meter, taking readings every 10 milliseconds. Upper and lower power thresholds are entered into the program. The program waits for the power reading to exceed the upper threshold, but does not count a fringe until the lower threshold is surpassed. This program is intended for use in interferometry applications. One limitation of the experiment is an upper limit of the computer to take data. Taking readings every 10 ms is sufficiently fast for most of our applications. The program displays in real time both the power reading and the number of peaks as a function of time.

These automation programs allow the users to focus on the nature of the experiment rather than the details of programming and instrumentation. The programs should be very helpful in the lasers and holography lab and fiber optics and detectors lab, as well as in research projects. Several other projects are good candidates for automation; they may be developed by other applied physics students.

Chris Addiego, assisted by Allan Baker, performed the work described above for his senior design project