Creating an Automated Control System for Optical Tweezers Using NI LabVIEW and Data Acquisition

Author(s):
David M. Carberry - Australian National University

Industry:
University/Education

Products:
LabVIEW,

The Challenge:
Designing a system capable of controlling and automating the individual components in a set of optical tweezers while recording experimental results and conditions.

The Solution:
Using National Instruments LabVIEW 6.1 and PCI-6014 multifunction data acquisition (DAQ) to create a graphical interface to enable rapid manual control of the system and create a second program capable of reading in control signals, modifying the desired optical tweezers functions, and recording the results.

LabVIEW Improves Experiment

Optical tweezers are used in numerous fields, including biology, chemistry and physics. They can separate different types of cells, stretch single polymer chains, aid in the study of laser physics, and are used in laser cooling. While the operating principles vary between models and applications, the end result is the same: a particle is held in a focal plane and can then be manipulated. In our lab, we use the optical tweezers that use radiation pressure from a tightly focussed laser beam to hold a microscopic particle within a focal plane. By varying the laser power, we vary how tightly the particle is held in the focal plane.

Recently, our research group (Polymers and Soft Condensed Matter Group) used the optical tweezers to perform the first experimental demonstration of the Fluctuation Theorem. This is an important theory that links statistical mechanics and thermodynamics. However, this experiment proved extremely challenging with the original optical tweezers control program. An operator was required to manually change the system controls, modify the output signals from a function generator, and use the data collection software. Then, the operator repeated the procedure every minute or two. After four months of data collection, we finally had enough data to be experimentally accurate. For the next set of experiments, we clearly needed a better solution.

We identified two areas that would greatly improve our experimental efficiency: automating the optical tweezers controls (e.g., set a sequence where we oscillate the strength of the laser or move the stage following a preprogrammed path) and automating the data acquisition. Furthermore, due to the lack of flexibility in the original control software, we used LabVIEW to create a new manual control program for easy incorporation of new equipment. We chose LabVIEW due to its quick and easy method to build interfaces and virtual instruments, modular design, and excellent trouble shooting functions.

Due to the frequent problems with the original software, we designed a new program. We designed the new optical tweezers control program so that a user can modify the properties of several pieces of equipment. It can accurately control the laser power, position of the stage relative to the laser, height of the focal plane above the bottom of our sample cell and use a micropipette to generate fluid flows. Using the modular design of LabVIEW, we can incorporate further equipment without having to redesign the entire control system.

This control program also provides feedback to the operator about the current position of the stage, stage velocities, fluid flow rate, laser power status, as well as providing useful functions such as storing and modifying coordinates for future reference.

Automating the Optical Tweezers

In addition to the above control system, we have designed a program to carry out the experiments. This program records the input signals from an arbitrary function generator as well as the experimental results. Using an interpretation VI, the program inputs signals to move the microscope stage in a predefined manner along any (or all) of the x-, y-, and z-axes with an extremely fine resolution (to within 1 um with stepper motors and then fine control to within 1 nm with piezo-electric linear translators). They can also be used to dynamically control the intensity of the laser, determining how strongly we are able to trap our particle.

Our second major improvement came by removing the need to manually start and stop our data acquisition. We modified the continuous DAQ recording algorithms generated by the LabVIEW Measurement & Automation Explorer (MAX) to write to multiple files upon receiving a predefined control signal. We found that LabVIEW easily synchronized our data collection with our control signals. Combined with the automated controls, we now start an experiment and come back several hours/days later to collect our results. This means that time spent performing menial experimental tasks can now be applied to solving other problems.

After only two weeks of software development, we were able to check our original experiment. We obtained the same number of data points using our new program overnight as we had in the original experiment, which required four months. Furthermore, we vastly improved the accuracy of the results by removing operator errors. By simply modifying a few input parameters, we perform experiments that were not possible or feasible earlier. Moreover, we are able to do so with fewer errors.

In the future, we plan to make use of NI-IMAQ functions to automatically record experimental images at various time intervals (e.g. every two to five seconds), providing another valuable tool for our data analysis.

LabVIEW Saves Money and Man Hours

By using LabVIEW, we have saved hundreds of man hours by replacing repetitive experimental functions with an automated system capable of performing the desired functions and recording the data. We are now able to examine experimental systems that require several functions to be performed in rapid succession. Furthermore, by using an automated system, we found that the quality of our results improved. It also provides excellent flexibility for an evolving laboratory.

For more information, contact:

David M. Carberry

Research School of Chemistry,

Australian National University,

ACT, 2615, AUSTRALIA

Tel: +61 2 6125 3712

Fax: +61 2 6125 0750

E-mail: carberry@rsc.anu.edu.au

 

Browse All Case Studies »