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	6dB/Octave	12dB/Octave
300µs	0HEX	8HEX
1ms	1HEX	9HEX
3ms	2HEX	AHEX
10ms	3HEX	BHEX
30ms	4HEX	CHEX
100ms	5HEX	DHEX
300ms	6HEX	EHEX
1s	7HEX	FHEX

recording device

(RZ6 or RA16)