EM Field Head Tracking System specifications

Schematic of the Sphere Head Tracking System

Introduction

The EM Field Head Tracking System is used for recording head movements of the subject during a trial. The system consists of the following parts:

• Three oscillators at roughly 45, 55 and 65 kHz or 50, 60, 70 kHz for X, Y and Z direction.
• Three signal amplifiers that amplify the oscillator signals.
• six field coils that make a cube of roughly 3x3x3 meters.
• A small pick-up coil mounted on a glasses frame that is worn by the subject during the experiment.
• Three lock-in amplifiers that split the signal from the pick-up coil in the 3 frequency components and measures their envelope. They output analog signals between -10V and +10V.
• An acquisition device converts the analog output signals from the lock-in amplifiers to digital.
• A RZ6 multi I/O processor that records the acquired data.
• Software that converts the recorded data to head orientations using calibration data.

The following table contains the channel convention with respect of the three directions:

Channel	Frequency	Orientation	Color	Polarity
Channel 1	45/50 kHz	Frontal	Red	Front = positive; Back = negative
Channel 2	55/60 kHz	Vertical	Blue	Right = positive; Left = negative
Channel 3	65/70 kHz	Horizontal	Yellow	Up = positive; Down = negative

Parts

Lock-in amplifiers

Three Femto LIA-BV-150-H Lock-in amplifiers are used in order to separate the Frontal, Horizontal and Vertical signals from the pick-up coil. Each Lock-in amplifier has a Sinus Oscillator Module (SOM-1) that is used both as Field Modulation Signal and as Reference Signal. The Lock-in amplifier settings are set at the front of the device (see picture), except for the modulation/reference frequency which is set inside on the SOM-board.

The front panel of each LIA has four dipswitches:

• Mode: 1f means that the fundamental harmonic is measured (2f = second harmonic signal)
• Dyn.Res.: For dynamic reserve the settings Low (L) is preferred because the signal is normally large in comparison with the noise.
• PLL: For the Phase Lock Loop (PLL) the settings Slow (S) is preferred because the frequency is at a fix value. The phase is more stable in this setting.
• Ref.Thr.: Reference Threshold is set to 0V for analog signals (2V is for digital signals).

Time constant	Model „-L“	Time constant	Model „-H“	Hexadecimal switch	6dB/Octave	12dB/Octave
3ms	300µs	0HEX	8HEX
10ms	1ms	1HEX	9HEX
30ms	3ms	2HEX	AHEX
100ms	10ms	3HEX	BHEX
300ms	30ms	4HEX	CHEX
1s	100ms	5HEX	DHEX
3s	300ms	6HEX	EHEX
10s	1s	7HEX	FHEX

recording device

(RZ6 or RA16)