# Presentation: 108850

## Challenges in the Development of the Automated Coil Calibration System at the Boulder Magnetic Observatory

**Challenges in the Development of the Automated Coil Calibration System at the Boulder Magnetic Observatory**

- Timothy C. White    Edward A. Sauter

_**Objectives**_
** **Establish and explore different methods for measuring orthogonality

_**Introduction**_
**The USGS is developing a coil calibration system at the Boulder Magnetic Observatory, with the goal of achieving 1 nT accuracy for calibration of a tri-axial, fluxgate magnetometer.  Achieving such accuracy requires accurate measurement of the angular relationships between the coil axes.   **

_**Alignment of D and H axes to Z axis**_
-  The D ring pairs are adjusted until the spacing between the rings is constant about the entire circumference
-  A sine wave is generated with the Z axis
-  The D rings are connected to a chart recorder, operating in an induction coil fashion
-  The rings are adjust until no signal is induced
-  The procedure is repeated for the H ring pair
_**Alignment of D axis to H axis**_
** **A total field sensor is placed in the sweet spot of the coils
-  Using the dIdD method, the D axis is aligned with the ambient field
-  The coils are then rotated exactly 90°, using a theodolite as a reference
-  The dIdD method is now used to find the misalignment of the H coils to the ambient field
-  The coils are adjusted until the H axis is aligned with the ambient field
_**Verification**_
-  A precise calibration of the coils constants is done
-  The PVM method is used to find the ambient components of H and Z
-  The computed components are also found which are heavily dependent on the coil constants
-  The method is done using both H and D coils for the ambient field alignment

_**The Problem**_
_**Established measurement technique**_
** **DI Flux Measurement
-  A theodolite is placed with the sensor centered in the sweet spot of the coils
-  Square waves are generated with one axis
-  The theodolite is then adjusted until orthogonal with the axis, and the angle is recorded
-  The procedure is repeat for all three axes
-  Angular data reveals orthogonality
-  BOU Coils are physically too small for this procedure

_**The Solution**_
** **Established magnetic measurement methodology was studied to find a feasible solution
_**dIdD (delta inclination, delta declination) method**_
-  A total field sensor is placed in the sweet spot of a set of coils
-  A bias current is generated in the coils, in both the positive and negative direction
-  The system is then rotated until the total field reading are the same for both the positive and negative current
-  The system is then aligned with one component of the ambient earth’s field
_**PVM (proton vector magnetometer) method**_
-  A total field sensor is placed in the sweet spot of a set of tri-axial coils
-  The coil system is aligned with the D component of the field, using the same methodology utilized in the dIdD method
-  An ambient measurement of the total field is recorded
-  Current is generated in the negative H coils until the same total field reading is achieved
-  The current is divided in half, leaving just the ambient vertical component of the total field
-  With proper equipment, the measurement can be repeated to find the horizontal component

_**Procedure**_
_**Z axis alignment**_
-  The coil system has been designed such that the vertical coils can be adjusted to rotate about a true vertical gravitational axis, a plum bob is used to establish this relationship
-  A total field sensor is then placed in the sweet spot of the coils and a large field is generated in Z
-  The coils are then rotated in accurate increments, and the total field reading at each increment are recorded
-  From this data, the vertical misalignment is revealed, and can then be corrected
-  A model of this procedure, with a 1° misalignment, is shown

_**Progress**_
-  The structure is being adjusted and precisely leveled to rotate about the Z-axis
-  The structure is now on a five point leveling system, one point in the center and four on the outer corners
-  Adjustments are complicated and time consuming