Modifying a Biaxial Film Stretcher to Measure Spectral Birefringence Using NI Hardware and Software

Author(s):
Marcus Anderson - Roush Industries, Inc.

Industry:
Research

Products:
LabVIEW, Vision, PXI/CompactPCI, Motion Control

The Challenge:
Developing an integrated, PC-based measurement and control system for a biaxial film stretcher with the ability to measure spectral birefringence, as wells as motion and thermal control functions, data acquisition, machine vision, and spectrometer measurements.

The Solution:
Leveraging the intuitive graphical development environment of LabVIEW and tightly integrated National Instruments hardware to reduce development time and cost, we successfully created a flexible control system with special film sample testing and data recording capabilities.

Customizing for Unique Measurement Abilities
The University of Akron Polymer Engineering Department acquired a biaxial film stretcher without motion or thermal controllers. The machine consists of a thermal chamber containing an arrangement of movable clamps for holding polymer film samples. Two servomotors, one on each axis, draw the film sample apart, stretching it. The University of Akron turned to Roush Industries to create a PC-based system with motion and thermal control functions as well as unique measurement capabilities. This completed system, with its flexibility and special abilities, allows the University of Akron to conduct research with an end goal of using spectral birefringence as a polymer film manufacturing process feedback variable.

Creating a More Powerful, Capable System
A servomotor performs the actual polymer film stretching by drawing apart the film clamps through a series of gear boxes and lead screws. A NI PCI-7344 servo/stepper controller and NI UMI-7764 motion interface interact with the servo drives. We wrote a motion control utility in LabVIEW to exercise all of the motion control functions and to aid in system testing and debugging. Through the software interface, the user can define a move through a distance and velocity or through a profile of position verses time. All movements can include one or both axes independently. This, combined with the profiling feature, allows very complex stretching moves. During a move, displacement and stretch ratio are calculated for both axes. Limit and E-Stop switches ensure safety.

High film strains are only possible when the film samples are stretched at elevated temperatures. The system accomplishes heater control with an off-the-shelf PID controller receiving its set point from one of the analog out channels of the NI PCI-MIO-16E-4 E-Series multifunction DAQ card. Digital I/O channels turn on and off fans and blowers as needed. The NI DAQ card also digitizes transducer signals. Four thermocouples measure film temperature and a fifth thermocouple checks heater temperature conditions. If the temperature of the heating element rises above a preset limit, the heater turns off and the user is notified. In addition, X and Y load cells measure the force applied to the film by the clamps. All input and output channels are conditioned with the appropriate SCC modules in a NI SC-2345 shielded carrier. Thermal control and DAQ utilities written in LabVIEW enable sub-system validation and calibration.

The most unique feature of the system is its ability to measure the in-plane and out-of-plane spectral birefringence of the transparent film sample as it changes during stretching. A six-channel spectrometer with NI PCI interface card and a LabVIEW driver make the measurements. We worked with the University of Akron to adapt its birefringence calculation algorithm for the application, scale it from one to six channels, and implement it as a subsystem utility in LabVIEW.

The algorithm uses the intensity versus wavelength information from the spectrometer to calculate optical retardations and spectral birefringence. The spectrometer utility gives the user the ability to tune the various parameters effecting the algorithm to maximize accuracy and stability. It can also measure the optical properties of static samples.

For the spectral birefringence calculation to be correct, the thickness at the center of the film must be known. We can calculate the thickness by using X and Y stretch ratios and the initial thickness using an incompressibility assumption. Because polymer film samples are not always perfectly homogeneous, strain across the sample is not always uniformly distributed. Therefore, stretch ratios calculated from clamp position will not provide the necessary accuracy. To achieve the desired precision, a machine vision system measures the strain field. We developed an image processing routine and a dot array to apply to the transparent film samples before testing.

A 1.3 megapixel, 15 fps digital camera along with a NI PCI-1422 image acquisition board, captures and processes continually during a test. NI-IMAQ software supports subpixel measurements, increasing the effective distance measurement resolution and allowing significant camera cost savings. Input from the motion control subsystem determines the region of interest, preventing dot detection errors, and allowing it to grow as the film stretches. The calculated strain field is interpolated to find the stretch ratios and therefore the thickness at the center of the film. The method employed accounts for the dots not only moving apart and changing size and shape, but the whole array possibly moving and rotating as the film sample stretches. With a vision utility, the user can adjust the parameters of the image-processing scheme to optimize its performance.

With a little forethought and the modular/hierarchical nature of LabVIEW, the overall application applies all the utilities from the various subsystems. This allows development and debugging of the subsystems separately and the ability to use them together to call all the functions of the complete system. The power to validate the individual subsystems, provides an interface for subsystem parameter-setting and algorithm optimization. Then, reuse the code directly in the main application saves significant development time. The hardware components that make up the measurement and control system are mounted in a cabinet and connect to one of the four NI PCI cards mounted in the rack mount PC. Using a common PC and Windows architecture also saves significant time compared to implementing several different proprietary systems.

Testing consists of a series of predefined consecutive stages that may include any number of different actions. A typical test may include preheating the sample, then stretching along one axis, stretching along the other axis, and finally stretching along both axes before a high temperature soak. However, because the direction research can be unpredictable, we made the test definition process as flexible as possible. The system has the ability to use any of the motion control moves or program a new thermal control set point in each stage, during any of up to 10 possible test stages. During each stage the user can enter a wait time after the desired temperature has been achieved or the move has been completed. Data collection occurring before the next test stage or the end of the test, determines the behavior of the film as it “relaxes”. Test definitions can be saved and used for a test at any time or recalled for editing. After preheating and initial data collection is complete, the system automatically goes through the stages in the sequence, collecting data throughout the entire test duration. A postprocessing utility quickly reviews the results of the test and exports the data for further in-depth analysis.

Cutting Development Time and Increasing Utility
The completed system gives the University of Akron a unique testing and research tool. For the first time, they can measure spectral birefringence as a sample stretches. This allows them to correlate birefringence with stretch ratio, sample history, temperature, and applied force. We realized significant development time savings by choosing to create the system based on National Instruments hardware and software in a PC platform. The University of Akron previously had spent more than two years developing less complex and automated testing tools. From start to finish, we completed, verified, and successfully tested polymer film samples with this biaxial film stretcher measurement and control system in only eight months.

For more information, contact:
Marcus Anderson
Roush Industries
28152 Plymouth Rd
Livonia MI, 48150
Tel: 734-466-6228
Fax: 734-466-6905
E-Mail: mjande@roushind.com

Browse All Case Studies »