# US 4,394,831 A · Helmet metal mass compensation for helmet-mounted sighting system > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 4,394,831 A - Document ID: US4394831A - Status: Granted - Kind: A - Application number: 06/233,747 - Filed: 1981-02-12 - Granted: 1983-07-26 - Earliest claimed: 1981-02-12 - Assignees: Honeywell Inc. - Inventors: Werner H. Egli; Jeffrey M. Setterholm; E. Jack Weir - Web page: https://patentgrep.com/patent/US4394831A ## CPC - G01B 7/004: for measuring coordinates of points - G01S 5/0247: {Determining attitude} - H01F 2005/027: {wound on formers for receiving several coils with perpendicular winding axes, e.g. for antennae or inductive power transfer} - G01B: MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS - G01S: RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES - H01F: MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES ## Abstract An electromagnetic system for determining the orientation including position of a helmet worn by a pilot is disclosed having a transmitting antenna for transmitting electromagnetic field vectors, a receiving antenna for sensing the electromagnetic field vectors generated by the transmitting antenna, and a control apparatus responsive to the sensed electromagnetic field vectors and the transmitted electromagnetic field vectors for determining the compensated orientation of the helmet, the compensated orientation relying upon the rotation matrix determined by a previous solution for the orientation of the helmet. ## Claims ### Claim 1 An electromagnetic system for utilizing electromagnetic field vectors in determining the orientation of a helmet, and for compensating for distortions of said electromagnetic field vectors caused by the metal mass of the helmet, the system comprising: transmitting means for transmitting electromagnetic field vectors, said transmitting means including a transmitting antenna having at least two transmitting coils; a receiving antenna having three non-coplanar receiving coils fixed to the helmet, said receiving coils sensing the electromagnetic field vectors transmitted by said transmitting antenna; and, control means responsive to said electromagnetic field vectors received by said receiving antenna for iteratively determining helmet orientation compensated for distortions of said electromagnetic field vectors caused by helmet metal mass, said control means having an output for supplying said compensated orientation to a utilization means. ### Claim 2 (depends on 1) The system of claim 1 wherein said control means comprises a multiplexer having three inputs, one input connected to a corresponding receiving coil and having an output. ### Claim 3 (depends on 2) The system of claim 2 wherein said control means comprises an analog-to-digital converter means having an input connected to the output of said multiplexer and having a converter output. ### Claim 4 (depends on 3) The system of claim 3 wherein said input of said analog-to-digital converter means comprises a bandpass filter having an input connected to the output of said multiplexer and an output. ### Claim 5 (depends on 4) The system of claim 4 wherein each of said inputs of said multiplexer comprises a corresponding preamplifier. ### Claim 6 (depends on 5) The system of claim 5 wherein said input of said analog-to-digital converter means further comprises a gain changeable amplifier having an input connected to the output of said bandpass filter and an output. ### Claim 7 (depends on 6) The system of claim 6 wherein said control means further comprises computer means having an input connected to the output of said analog-to-digital converter means and further having said first output connected to said utilization means and a digital-to-analog converter having said second output. ### Claim 8 (depends on 1) The system of claim 1 wherein said control means further comprises computer means having an input connected to the three coils of said receiving antenna and further having said first output connected to said utilization means and a digital-to-analog converter having said second output. ### Claim 9 (depends on 1) The system of claim 1 wherein said control means comprises an analog-to-digital converter for converting the analog signals received by the receiving antenna into digital form for use by said control means. ### Claim 10 An electromagnetic system for utilizing electromagnetic field vectors in determining the orientation of a helmet worn by the pilot of a vehicle, and for compensating for distortions of said electromagnetic field vectors caused by the metal mass of the helmet, the system comprising: transmitting means for transmitting said electromagnetic field vectors, said transmitting means including a transmitting antenna having at least two transmitting coils; a receiving antenna having three non-coplanar receiving coils fixed to said helmet for sensing said electromagnetic field vectors transmitted by said transmitting antenna; orientation determining means connected to said receiving antenna and to said transmitting means for iteratively determining a compensated orientation matrix for the receiving antenna by relying upon the rotation matrix for the rotation of the receiving antenna from the transmitting antenna of a previous iteration, said compensated orientation matrix being comprised of an orientation matrix which is compensated for distortions of said electromagnetic field vectors caused by helmet metal mass, said orientation matrix and said rotation matrix being based upon both the transmitted electromagnetic field vectors and the sensed electromagnetic field vectors; and, output means connected to said orientation determining means for providing an output signal for the control of a vehicle apparatus. ### Claim 11 (depends on 10) The system of claim 10 wherein said orientation determining means comprises means for compensating the orientation of the receiving antenna according to the formula .DELTA.=-S.delta.HM'/(R').sup.3 where .DELTA. is the compensation factor, S is a scalar based upon electronic and system gain, .delta. is a matrix constant, H is the rotation matrix of a previous iteration, M is a matrix based upon the unit direction vectors between the receiving antenna and the transmitting antenna, and R' is the range from the metal mass to the transmitting antenna.