UTMB Uses NI LabVIEW and DAQ to Create Total Field Calibration Method for 3D Eye Position Sensors

Author(s):
Michael Shinder - Department of Otolaryngology, University of Texas Medical Branch at Galveston
Randell Riggs - Applied Innovations
Shawn Newlands - Department of Otolaryngology, University of Texas Medical Branch at Galveston
Jonhong Yin - Department of Electrical and Computer Engineering, University of Texas at Austin

Industry:
University/Education, Medical/ Medical Instrumentation

Products:
LabVIEW,

The Challenge:
Moving from a two-axis sensor system calibrated by fixed positions to a three-axis sensor system calibrated by approximate positioning.

The Solution:
Using National Instruments LabVIEW software and data acquisition (DAQ) hardware to implement a total-field-calibrated, three-axis system.

Difficulty Calibrating Sensors

Calibrating many sensor systems for use in unique field environments can be costly and difficult. This is especially true when there are considerable unknowns, as with geologic sensor systems. A similar, inverse problem exists in the laboratory when the environment is constant, but the sensors are unique or contain unknown qualities. Whole or total field calibration techniques have been developed to quickly and effectively calibrate known sensors in complex and unknown environments (Geesing et al. 2004, Estes and Walters 1989). In this instance, we have applied the total field calibration concept to calibrate sensors that have unknown characteristics.

Two Traditional Research Methods

Research into the mechanisms that underlie how we move our eyes to see or fixate objects requires accurate knowledge of where the eyes are pointed and where they move. To measure and study how the eyes move, technicians have traditionally adopted two techniques - the scleral search coil method and video-oculography. Video-oculography, which uses video eye images and image analysis to determine the eye position, has not been able to produce high-temporal and spatial resolution, so that the scleral search coil method is often required for highly accurate measurement. The scleral search coil method requires technicians to place a loop of wire on the eye while the subject is surrounded by a magnetic field. As the loop moves in the field, the system generates a signal that indicates the eye position relative to the magnetic field direction. However, the difficulty is that often, it is impossible or impractical to determine the exact geometric shape that the loop takes once it has been placed on the eye.

If technicians use two magnetic fields that are perpendicular to one another, and the fields are fluctuated at different frequencies, the position signal from the single coil can be demodulated into two movement dimensions. Technicians often refer to this technique as the Robinson method after the original developer. It is often used to interpret horizontal and vertical eye movements for a subject whose head is stationary. When technicians use the Robinson technique, the signal intensity created by coil movements is dependent upon the loop area. There is a problem in predicting the signal relationship to positioning in the field if the area cannot be precisely defined. When technicians determine the initial calibration with a degree of error, the calibration must be corrected empirically.

If eye movements that are more natural and involve gaze shifts (where the eyes and head are moved together to look at an object) must be monitored, then technicians must monitor three motion axes for coils placed both on the head and on the eye. This often requires a different system type that can accurately measure the large coil angle excursions relative to the magnetic fields. For 3D and three-orthogonal magnetic fields, technicians generally prefer a phase-angle method, where the scleral coil position is determined from the difference in the angle between it and a reference coil incorporated into coils driving the magnetic field.

It is difficult for technicians to use scleral search coils in multiple dimensions for accurate eye position measurement and movement because proper calibrations are difficult to obtain. The coil can be placed in a two-axis gimbled vernier so that output voltages can be matched to a lookup table of positions. This calibration method relies on the vernier and the ability of the technician to place the coil precisely. Technicians confirm the calibration results by comparing the coil output in a trained subject performing known behaviors to the output expected from that reference behavior. However, technicians adjusting for errors in the calibration with new or untrained subjects can not be satisfactorily performed so the initial measurements are dependent upon the vernier calibration similarity to the current coil conditions. As noted, coils may not maintain the same shape or home position in the eye as they had in the calibration fixture. In 3D, the problem is further complicated by the fact that while vertical movements might be limited to a single axis (z for instance), horizontal movements are inherently a combination of two axes (x and y). The technician must then interpret how the flaw in the horizontal behavior is derived from errors in the underlying component axes.

Performing TFC Using NI LabVIEW

The total field calibration (TFC) technique reduces the dependency on extreme accuracy during calibration positioning in the production of an accurate calibration (Estes and Walters 1989). TFC simultaneously takes the measurements from the three axes of the multidimensional sensor system. The system then moves the sensor through the planes of the three axes to create a spherical, 3D response profile of demodulated output data. The system digitizes and records the acquired data on the computer by custom LabVIEW software. The data are then represented by a series of equations representing each of the contributions of the magnetic field axes that are crossed by the search coil at any given orientation inside the coil system. The system fits the data by an iterative technique that accounts for the gain or scale of the signals form each axis, the bias or offset of each signal, and the skewedness or alignment error of the contributions of each signal onto each of the other signals. By applying an iterative technique to solve the three sets of simultaneous equations that best represent the measured data points, we can determine calibration values from each axis and minimize the constituent errors when calibrating the search coil.

We validated the TFC technique application to the scleral search coil system for recording eye movements by calibration coil evaluations and responses to behavioral standards. We evaluated calibration coils of varying size with varying numbers of turns, and the TFC results changed in parallel with the inductive coil characteristics. Reflex responses measured by the NI data acquisition system using the TFC-generated calibration values for transforming input voltages to eye positions resulted in eye movement responses appropriate to the reflex being monitored in trained as well as in untrained subjects. Using the TFC method, we have reduced the delay in recording reliable data from new subjects. In addition, it has produced more reliable values across different technicians than the calibrations produced by using a fixed-calibration lookup table method.

References:

Estes R, Walters P. Improvement of azimuth accuracy by use of iterative total field calibration technique and compensation for system environment effects. 64^th Annual Technical Conference and Exhibition of the Society of Petroleum Engineers, San Antonio, TX, October 8, 1989.

Geesing D, Bachmaier M, Schmidhalter U. Field calibration of a capacitance soil water probe in heterogeneous fields. Australian Journal of Soil Research, 42(3) 289-299, 2004.

For more information, contact:

Michael Shinder

Researcher

ENT Research

University of Texas Medical Branch at Gavelston

Tel: (409) 772-5882

Fax: (409) 772-5893

E-mail: meshinde@utmb.edu

 

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