Eye Tracking System Angle Meter

The Angle-Meter is the first generation of Primelec's search coil systems for the linear detection of 3D eye and head movements, which was  introduced in 1989. The system was developed in close cooperation with the Vestibulo-Oculomotor Laboratory of the University Zurich. The system's high accuracy and bandwidth ensures effortless recording of horizontal, vertical and torsional eye and head movements of unrestrained laboratory animals.

The Angle-Meter is based on an enhanced phase detection principle and ensures effortless and reliable recordings under different conditions without the need of any system adjustments by the user.

• Easy to use
• User friendly operation, no system configuration required by the user
• 2 + 2 + 2 recording channels (modular construction)
• Recording range 360° in the horizontal plane, ± 70° in the vertical plane
• Output data rate 400 Hz
• Resolution: < 0.1° in the horizontal and vertical plane
• Noise: < 0.3° peak-peak
• Microprocessor-controlled operation
• Digital synthesized field signals (crystal controlled)
• Digital and analog output data (RS-232 interface and analog outputs)
• Linear output data (angular orientation in the horizontal and vertical plane)
• Various search coils applicable without the need for any system adjustments by the user
• Various coil frames applicable
• Comprehensive customer support by Primelec
• Two years full warranty

The magnetic field search coil technique introduced by Robinson [1] has become the most commonly used method for quantitative studies of eye and head movements in man and in experimental animals. The technique is based on phase-locked amplitude detection of the voltage induced in a search coil in the external ac magnetic field. The angular orientation or displacement of the search coil in three-dimensional (3D) space is detected by using two or three external magnetic fields, which are arranged in space quadrature. Demodulation of the induced signals with respect to the magnetic field directions is obtained on the basis of phase or frequency coding by driving the external magnetic fields in phase quadrature or at different frequencies.

The Angle-Meter uses a modified measuring principle. The system uses three (partly) modulated, digitally synthesized ac voltages for the magnetic field generation. The measuring plane, i.e. the plane in which the angular orientation of the search coil is faithfully detected, is determined by the plane of rotation of the magnetic field vector [2]. The demodulation of the angular orientation of a search coil is based on the phase detection of the induced voltage with respect to the phase of the rotating flux density vector [3]. This ensures almost drift-free, low noise, high-resolution output signals and an excellent system linearity. 

[1] D. A. Robinson, "A method of measuring eye movement ...", IEEE Trans. Biomed. Eng., vol. BME-l0, pp. 137-145, 1963
[2] H.J. Kasper, B.J.M. Hess, N. Dieringer, "A precise and ...", J. Neuroscience Meth., vol. 19, pp. 115 ... 124, 1987
[3] H.J. Kasper, B.J.M. Hess, "Magnetic search coil system for ...", IEEE Trans. Biomed. Eng., vol. BME-38, pp. 466-475, 1991

Basic System

Basic System plus two additional channel pairs

Application examples:

Application examples:

• Simultaneous binocular measurement of horizontal and vertical eye position
• Simultaneous monocular measurement of horizontal, vertical and torsional eye position 

• Simultaneous binocular measurement of horizontal, vertical and torsional eye and head position 

The Angle-Meter is a fully microprocessor-controlled device, designed in a modular form. The Basic System consists of the field signal generation electronics, an external preamplifier, control- and detection-electronics for two measuring channels, i.e. two search coils may be processed simultaneously.

The Basic System is expandable by the use of additional channel pairs. An additional channel pair consists of an external preamplifier, control- and detection-electronics for two measuring channels. Therefore, the Basic System may be expanded to simultaneously process four or six search coils by adding one or two additional channel pairs.

Each measuring channel processes the voltage induced in the search coil and detects the horizontal and vertical orientation of the search coil, where the output signals for the horizontal and vertical angles correlate directly proportional to the angular orientation of the search coil(s) in the external magnetic field. Thus, the system delivers linear outputs and no extra trigonometrical calculations are required. The detected angular orientations in the horizontal and vertical plane of each measuring channel are simultaneously available as linear analog voltages (BNC-connectors) and as a stream of data over the RS-232 interface (D-SUB connectors). The analog outputs allow to monitor the measurement signals on an oscilloscope, Y/t-tracer, A/D converter etc. Using the serial interface makes it easy to transmit the measuring data directly to a computer, which eliminates the use of an external A/D converter.

» See also the list of references for more information...

The serial Interface (RS-232) continuously transmits the measuring data and may also be used to toggle the magnetic field generation. Transmitting the measuring data by the serial interface to an external computer eliminates the use of additional A/D converters, which reduces the total system costs. The scope of delivery of the Angle-Meter includes a software (Win 9x) to save, analyze and display the measuring data in various representations.

Primelec offers coil frames in various colors, arranged as a cube with an edge length in the range from 25 cm to 70 cm. Due to the monocoque construction, the restriction of the visual field is minimal. Our coil frames can be de- and remounted, which may be helpful during the building of user specific setups. To ensure optimal mounting flexibility, the Matching-Box is not fixed to the coil frame.

The picture shows a coil frame with an edge length of 30 cm and the connected Matching-Box.

The Angle-Meter is capable to process up to six search coils simultaneously, which is sufficient to fit the needs of almost any application. For example, the recording of horizontal, vertical and torsional binocular eye and head movements requires six measuring channels.

Various search coils are applicable in terms of dimension, geometry, number of turns and material. Of course scleral search coils are suitable. Torsional recordings require a two-in-one search coil (frontal and sagittal plane), i.e. two recording channels are required.

The Angle-Meter is compatible with search coils of third-party suppliers. Our partner CHRONOS VISION manufactures and distributes 2D (direction) and 3D (torsion) scleral search coils which are perfectly suited for the Angle-Meter.

» Read more about search coils manufactured by CHRONOS VISION...