Silva NX2, Руководство по обслуживанию

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10comtrdNX2.doc Page 1 Latest update: 10 Feb -03
10. COMPASS TRANSDUCER
Principle of operation
The magnetic field is measured by use of two coils 90
°
perpendicular with one
ferrite rod in each coil that are forced into saturation by a known current. The
known magnetic field is mixed with the outside unknown magnetic field, and the
difference are converted into a amplified voltage signal that is compared to a
known reference voltage. This reference is created by an internal triangular
voltage that is passing a comparator input at a rate of 13 Hz. When the magnetic
field signal is compared to the reference, a PWM (Pulse Width Modulation) signal
is created. The ratio or variation of each PWM signal are related to the magnetic
field strength. The advantage is that the high resolution signal can easily be
transmitted up to 100 m with a shielded cable without degradation.
The fluxgate compass will give one sine and one cosine PWM heading signal
output.
The two PWM signals (Sine/Cosine) are only represented as a ratio between
each other. The pulse frequency is nominal 15 Hz (+/- 2Hz) and a duty cycle of
50% high and 50% low is the same as no outer magnetic field is applied, i.e. the
magnetic field is 90
°
perpendicular to the coil, i.e. sine 0 degree.
The duty cycle quota in percent for the transmitted channels (Q
A%
, Q
B%)
is given
by the formula:
Q
A%
= 50 % + A % x Sinus (
α
)
Q
B%
= 50 % + B % x Cosinus (
α
)
(cosinus (
α
) = sinus (
α
+ 90°))
α
= Compass heading
A and B are the magnetic flux in percent where
√
(A² + B² ) is constant at an
undisturbed geografic spot.
The value of
√
(A² + B²) is dependent of the magnetic flux on the actual
geographical spot.