Automated Tomographic Imaging of Glacier Ice Using LabVIEW RT

Author(s):
Steve Boon - The Rock and Ice Physics Laboratory, Department of Geological Sciences, University College London

Industry:
University/Education

Products:
LabVIEW

The Challenge:
Measuring the mechanical properties of anisotropic ice from the full depth of a borehole in Antarctica.

The Solution:
Developing a tool capable of deforming an ice specimen with National Instruments LabVIEWRT development system for closed loop control of pressure vessel and loading actuators and National Instruments PCI data acquisition (DAQ) products for tomography control.

Introduction
The Antarctic ice sheet plays a crucial role in the behavior of the Earth’s ice-ocean-atmosphere system because it influences the distribution of heat and mass within that system. Recently, there has been concern that global atmospheric warming could lead to substantial rise in sea level, and the greatest uncertainty lies in the contribution of melting ice sheets. Improved understanding of this process requires integration of direct measurements of the ice sheet mass balance from satellite measurements and modeling of ice sheet dynamics. Currently, our understanding of the fundamentals of ice flow is not sufficiently advanced to make quantitative predictions.

Because field measurements cannot define the state of stress when modeling the flow of glaciers and ice sheets, we needed a control system to simulate the stress of the flow of glaciers and ice sheets.

The control system needed to be flexible enough to measure a large range of physical parameters. Using National Instruments real-time PCI boards and LabVIEW RT, we designed a flexible control system for our experiments on ice sheet core.

The complex multi-axis control system with multiple outer loop control inputs was a lower cost solution to the commercial servo-hydraulic turnkey solution. With the RT capabilities, we built our multi-axis control system in the familiar LabVIEW environment with a high level of quality technical support. The RT Series PCI hardware ensures that we develop an interactive solution for our needs. With the analogue inputs on the RT Series acquisition board, we can input multiple signals from other axes into the closed loop control system to ensure a stable environment for the tomography component of our experiments. The handshake system between the main control program and the RT Series board control program combined with soft test limits ensures a safe shutdown in the event of equipment failures.

Control System Application
The control system for this application needed a high level of adaptability to measure a large range of physical parameters. With cross coupling between the loading axes, we carry out several measurement compensations in real-time for total experiment control.

Two PCI-7030 RT Series boards and 6030E 16-bit DAQ boards, each with 16 single-ended analogue inputs and two analogue outputs, made up the multi-axis digital servo hydraulic controller. A LabVIEW RT application provided the error amplifier and valve drive voltage via the analogue outputs to drive a precision hydraulic servo valve attached to each of the 60 kN double acting hydraulic actuators.

The LabVIEW RT application uses a series of layered windows for viewing and setting up of:

• Load or displacement increments, ramp rate, and tomography transducer selection
• Overall test progress of tomography with load and displacement monitoring
• Control loop function (PID and other outer loop controls) for changes in ice sample stiffness
• Mode of control (load or displacement) for individual or paired actuators
• Graphs for stress and strain of each axis
• Ultrasonic transducer pulsing levels and digital oscilloscope controls
• Transducer calibration
• Test data capture and replay

The application loads the LabVIEW RT program on the PCI-7030 control boards after we enter the test parameters. It then awaits the commands from the main PC-based application. Regular handshake commands between the main control program and the RT Series board ensure a safe shutdown in case of main PC hardware or software problems.

The LabVIEW RT control program effectively closes the control loop by taking data from the test setup and modifying the valve drive voltage to the actuator systems. The system provides feedback directly to the RT Series board by the conditioned transducer signals, which are connected to the board’s analogue input channels. The system modifies the transducer signals with calibration polynomials for each transducer.

Tomography Control and Data Capture
A 12-bit PCI-6025 board performs the data acquisition. This board also provides the stepper motor control and timing for the ultrasonic transducer tomography system.

With the control application, we can switch ultrasonic transducer transmitters and receivers to achieve the required pattern of data. We can measure the physical parameter changes in the ice specimen when loading each increment.

Conclusion
This application is a complex multi-axis control system with several outer loop control inputs. With LabVIEW RT, we built a low-cost multi-axis control system using software with which we are familiar and a high level of quality technical support.

For more information, contact:

 Dr. Steve Boon

The Rock and Ice Physics Laboratory, Department of Geological Sciences, University College of London

Gower Street, London, WC1E 6BT

Tel: 0207 380 7277

E-mail: s.boon@ucl.ac.uk

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