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).

Kenmerk	LIA-BV-150-H
Model	Single Phase
Werkfrequentie	50 Hz - 120 kHz
Tijdconstanten	300 µs - 1 s (6 of 12 dB/oct.)
Verstelbaar Signaalfilter	Highpass 2 Hz - 10 kHz, Lowpass 100 Hz - 1 MHz
Datasheet	367 kB
Uitgangen	X = in fase, Y = kwadratuur, R = magnitude, ±10 V volledige schaal, kortsluitbeveiligd, Signaalmonitoruitgang
Gevoeligheid (Volledige Schaal)	Voltage: 3 µV - 1 V in 1-3-10 stappen
Spanningsingang	Echte differentiële instrumentatieversterker, ruis 12 nV/√Hz
Stroomingang	Transimpedantieversterker, versterking 100 kV/A, ruis 0.4 pA/√Hz
Referentie-ingang	±100 mV tot ±5 V, schakelbaar naar TTL
Fase	Aanpasbaar 0° - 360°; resolutie: 8-bit @ f ≤ 60 kHz, 7-bit @ f > 60 kHz, Temperatuur drift <0.01°/K
Max. Dyn. Reserve	80 dB
Digitale Controle	16 TTL/CMOS ingangen: 8-bit fase, 4-bit tijdconstante, 4-bit gevoeligheid
Voeding	±15 V, +120 mA / -60 mA
Afmetingen	160 x 100 x 20 mm (L x B x H), gewicht 100 g (0.22 lbs)

recording device

(RZ6 or RA16)