"Strapdown" induction compass transmitter with means to compensate for heading errors during turns and during dives and climbs due to the vertical component of the Earth's magnetic field and due to two cycle error

• US 4,112,755 A
• Filed: 12/20/1976
• Issued: 09/12/1978
• Est. Priority Date: 12/20/1976
• Status: Expired due to Term

• Current Assignee: Ametek Aerospace Products, Inc. (Ametek Incorporated)
• Original Assignee: Gerald Leslie Sullivan
• Inventors: Sullivan, Gerald Leslie
• Primary Examiner(s): Kunin, Stephen G.
• Application Number:
  US05/752,064
• Time in Patent Office: 631 Days
• Field of Search: 33/326, 33/356, 33/357, 33/361, 73/178 R, 235/150.2, 235/150.25, 235/105.26, 235/186, 244/175, 244/179, 318/629, 318/633, 318/647, 318/654, 318/661, 340/197, 340/347 SY, 324/43 R, 324/47, 324/48
• US Class Current: 73/178.00R
• CPC Class Codes: G01C 17/30 Earth-inductor compasses

Abstract

The invention relates to an apparatus for compensating a "strapdown" induction compass transmitter to eliminate heading errors during turns, and during climbing and diving maneuvers which are due to the vertical component of the Earth'"'"'s magnetic field as well as those due to the horizontal component of the Earth'"'"'s magnetic field perpendicular to the axis of tilt. Instabilities and anomalies such as "Northerly Turning Error" which are due to the vertical field are thus eliminated or minimized as is two cycle error which is due to the Earth'"'"'s horizontal field during these same maneuvers. The vertical field compensating signal during turns is proportional to the product of the Earth'"'"'s magnetic field, M the sine of the dip angle D and the sine of the bank or roll angle φ. The vertical field pitch compensating signal while climbing or diving is proportional to the product of the sine of the pitch angle θ, the Earth'"'"'s magnetic field M and the sine of the magnetic dip angle D. These compensating signals are used to drive current through the transmitter sensing windings to produce flux which opposes and cancels the flux sensed by the compass transmitter windings due to the vertical component of the Earth'"'"'s field. Compensating signals for two cycle errors due to the horizontal component of the Earth'"'"'s field are proportional to cosine of the tilt angles, i.e., the roll angle φ and the pitch angle θ are provided to control the feedback current through the sensing windings to cancel the two cycle error.

Claims

1. In a strapdown induction compass transmitter system compensated for errors due to the Earth'"'"'s vertical field and for two cycle errors occurring during turns and during climbing and diving maneuvers, the combination comprising:

(a) magnetic sensing means for producing a first modulated signal responsive to the strength of the Earth'"'"'s magnetic field along one axis of an aircraft, and a second modulated signal responsive to the strength of the Earth'"'"'s magnetic field along another axis of the aircraft,

(b) means to compensate said signals for errors due to the Earth'"'"'s vertical magnetic field during turns and while climbing and diving including;

(1) means to generate a first correction signal proportional to the product of the Earth'"'"'s magnetic field M, the magnetic dip D, and the roll angle φ

, said first correction signal being equal to the vertical field error in the magnetic field sensed along one of the axes of the aircraft during a turn,

(2) means to generate a second correction signal proportional to the product of the Earth'"'"'s magnetic field M, the magnetic dip angle D and the pitch angle θ

, said second correction signal being equal to the vertical field error in the magnetic field sensed along the other axis of the aircraft during climbing and diving maneuvers,

(3) means responsive to said first and second correction signals to produce flux fields which cancel the flux sensed by said magnetic sensing means along the respective axes of the aircraft due to the Earth'"'"'s vertical field during turns and while climbing or diving,

(c) means for demodulating the compensated first and second modulated signals including,

(1) first and second demodulators each having inputs and outputs associated with the magnetic sensing means to produce respectively, first and second demodulated signals from said compensated first and second modulated signals,

(2) feedback paths from the outputs to the inputs of said first and second demodulators for producing negative feedback signals,

(d) and means to correct said first and second signals for two cycle error including means to reduce the negative feedback in one of said paths by a factor proportional to the cosine of the bank angle 100 and the negative feedback in the remaining paths by a factor proportional to the pitch angle θ

whereby the demodulated signals from the outputs of the demodulators are increased correspondingly to cancel two cycle error.

2. The strapdown induction compass transmitter according to claim 1 wherein said first and second modulated signals from said magnetic sensing means are respectively responsive to the Earth'"'"'s magnetic field along the fore/aft and athwartships axes and said first correction signal is coupled to said magnetic sensing means to produce flux to cancel the Earth'"'"'s field sensed along the fore/aft axis during climbing or diving and said second correction signal is coupled to said magnetic sensing means to produce flux to cancel the Earth'"'"'s field along the athwartships axis during a turn.

3. The strapdown induction compass transmitter according to claim 2 wherein the feedback in the feedback path associated with the said second demodulator is reduced by the cos φ

factor and the feedback in the feedback path associated with the said first demodulators is reduced by the cos θ

factor.

4. The strapdown induction compass transmitter according to claim 1 wherein the means to reduce the negative feedback includes;

(a) multiplying means in the negative feedback paths for the first and second demodulators,

(b) means to apply the demodulated first and second signals to the mutlipliers in the negative feedback paths of the first and second demodulators and means to apply a signal proportional to cos φ

to one of said multipliers and signal proportional to cos θ

to the other multipler to reduce the negative feedbacks by cos φ and

cos θ

.

5. The strapdown induction compass transmitter according to claim 4 wherein the signal proportional to cos φ

is applied to the multiplier in the negative feedback path associated with said second demodulator and the signal proportional to cos θ

is applied to the multiplier in the negative feedback path associated with said first demodulator.