Eye Tracking System CS681

CS681 System Overview

The CS681 provides 8 recording channels to process up to 8 search coils. A high performance digital signal processor and a high speed field programmable gate array are used to detect the angular orientations of the search coils in real-time and to control the system.

The handling of the CS681 is very user friendly thanks to the automatic tuning of the gain- and offset settings. All settings may also be controlled manually over the touch screen or over the USB interface with the included application software.

CS681 Main Unit
(click here to enlarge)

The typical application of the CS681 is the recording of three-
dimensional (horizontal, vertical, torsional) eye and head movements in neurophysiological, ophthalmological, psychological, psychiatric and visual studies.

The CS681 is intended for research use only and has not been reviewed, certified, or approved for clinical purposes. All use of the system must be in compliance with appropriate procedures.

Example

The picture shows a motorized multi-axes turntable, equipped with our search coil system (by courtesy of Prof. D. Straumann, Neurology Department of the University Hospital Zurich). The long-term goal of this research is to better understand central and peripheral diseases of the ocular motor system in humans and to assess potential treatments.

See also the list of references with some publications, in which the method includes a Primelec eye tracking system.

Turntable with Primelec Coil System,
Neurology Dept. University Hospital Zurich

The CS681 provides gain-, offset- and phase correction for the respective X-, Y- and Z-signals per channel. Internal lowpass filters with user selectable characteristics may be applied to each channel separately. The user selectable search coil polarity reversing per channel is useful to reverse the polarity without the need to change the physical connection polarity of the respective search coil. Similarly, the user selectable magnetic field polarity reversing per axis may be useful to meet a specific coordinate system without rewiring the system.

Automatic Tuning and Configuration Memory

Even though the extraordinary versatility of the CS681, the handling is very user friendly. The system provides automatic tuning of all gain-, offset- and phasing settings, which eliminates the need for laborious and possibly incorrect manual adjustments. In addition, the CS681 comes with non-volatile memory for up to ten user specific system setups to ensure quick and effortless system reconfiguration.

Manual Configuration

The user may control all settings manually over the touch screen of the display. A menu system allows the quick and straightforward system configuration.

In addition, the system is completely controllable over the USB interface. The CS681 comes with an application software, which may be used to control the system over the USB interface.

Touch Screen with Menu System

The CS681 provides analog and digital output data, i.e. 24 analog outputs and a USB interface.

Analog Outputs

The analog outputs may be connected to an external data acquisition system, e.g. an analog data acquisition board for a PC. The user may select the format of the analog output data: linear angular data (horizontal and vertical orientation of the search coils, i.e. no additional trigonometric calculations required) or detected signals X, Y and Z (raw data). The voltage swing of the analog outputs is user selectable, which is useful to match the voltage range of the analog outputs to the input voltage range of the external data acquisition system.

USB Interface

The USB interface continuously transmits data packets to an external computer. Each packet contains the output data with a resolution of 21 bit (linear angular data and detected signals X, Y and Z) as well as a time stamp and a subsequent packet number. Using the USB interface eliminates the need for an external data acquisition system, which improves the system performance and saves costs.

Output Data Rate

The output data rate of the CS681 depends on the number of processed channels. The user may select the processing of two, four or eight channels, depending on the number of search coils used for the experiment. If two channels are processed, the CS681 provides a very high-speed output data rate of 8000 readings per second. Processing all eight channels still ensures a high speed output data rate of 2000 readings per second.

Included with the CS681 is an application software (Windows 2000 / XP), which may be used to control the system and to analyze and capture the system’s output data over the USB interface as well as for firmware upgrades if future versions of the system’s firmware are available.

» Read more about the included application software...

ActiveX Control

Also included is an ActiveX control for Microsoft Windows. The ActiveX control allows a user application software, e.g. a LabVIEW VI, the straightforward communication with the CS681 over the USB interface. The ActiveX control comes with detailed documentation (help file) and example applications (Visual Basic source code and LabVIEW VI).

» Read more about the included ActiveX control...

Application Software

The coil frame consists of three field coils (X-, Y- and Z-coil), composed of aluminium bars with a cross section of 8 mm. Thus, the restriction of the visual field is minimal.

An external Matching-Box contains the required capacitor networks for resonance tuning.

The entire coil frame is dismountable if required for easy integration into user specific setups.

Primelec offers coil frames in various colors, arranged as a cube with an edge length in the range from 250 mm to 700mm. Within this range, the exact dimensions are freely customizable.

Coil Frame (350 mm) and Matching-Box

Removable Bars

The optional available "Removable Bars" allow the effortless removal of one bar per coil X and Z respectively to open the coil frame. The removable bars are plugged onto the frame such that no tools are required for the handling.

This unique feature is essential to facilitate the insertion of large objects into the coil frame, e.g. a primate chair.

Option "Removable Bars"

The CS681 is capable to process up to eight search coils, which is sufficient for almost all applications. For example, recording horizontal, vertical and torsional binocular eye plus head movements requires six recording channels.

Usually the search coils are manufactured by the user to comply the specific requirements. The CS681 is compatible with search coils of third-party suppliers and Primelec offers search coils on request.

Scleral Search Coil

[1] Robinson D.A. (1963), A method of measuring eye movements using a scleral search coil in a magnetic field, IEEE Transactions on Biomedical Engineering 10: 137-145

The CS681 uses an other approach to demodulate the search coil signal. The spectral components of the field frequencies are obtained by calculating the Fast Fourier Transformation (FFT) of the digitized search coil signal in real-time. This technique was introduced by Primelec in 1999 and successfully implemented in our Angle-Meter NT, the previous model of the CS681.

Three digitally synthesized sine wave signals of different frequencies are applied to the coil frame to generate the AC magnetic field, which induces a signal in a search coil. In order to avoid routing the susceptible search coil signals over a long distance, the CS681 comes with an external preamplifier. The preamplifier contains eight amplifiers, each with ultra low noise input stage and digital programmable gain. The inputs of the amplifiers are transformer-coupled for galvanic isolation between the search coils and the system. The outputs of the eight amplifiers are time-multiplexed by two 4:1 analog multiplexers. To achieve optimum noise immunity, the two output signals of the analog multiplexers are buffered and differentially transmitted to the main unit.

The main unit comes with two high speed analog to digital converters to digitize the preprocessed signals of two search coils in parallel. The use of a high performance digital signal processor (DSP) and a field programmable gate array (FPGA) makes it possible to compute the FFT and the subsequent trigonometric calculations in real-time. The complete data processing for two search coils takes less than 125 us, affording a high-speed output data rate of up to 8000 readings per second and a latency of only 500 us. The output data is available over the USB interface and as analog signals over the integrated digital to analog converters.

This approach provides remarkable advantages compared to traditional systems. Very high system performance is achieved, linear angular output data for the horizontal and vertical plane is provided and almost no system adjustments are required by the user.

External Preamplifier

Mainboard of the Main Unit