The present invention relates to methods and systems for setting, adjusting, offsetting, compensating or calibrating in association with instruments which measure physical phenomena, more particularly to methods and systems for aligning magnetic gradiometers.
Magnetic gradiometers are sensors which measure magnetic gradients. A magnetic gradient is the slope of a magnetic field. Gradiometers are used in magnetic research and are also popular for sweeping areas to detect ferrous materials.
A gradiometer comprises two magnetometers which, typically, are located at or near opposite ends of the gradiometer. Each magnetometer includes at least one magnetometer vector (vectorial axis). In order to function properly, the two opposite magnetometers must be "aligned" with each other.
To elaborate, gradiometer alignment typically entails the collinear or nearly (but parallelly) collinear alignment, along or with respect to a single axis, of one or more pairs of vectors corresponding to opposite magnetometers. When at least one magnetometer includes a plurality of vectors, gradiometer alignment can also entail achievement of a desired vector orientation (e.g., orthogonality) with respect to the collinearly (or parallelly) aligned vectors.
For instance, take the situation wherein a gradiometer has two single-vector magnetometers which are nearly collinearly aligned (i.e., approximately along a single axis). The gradient (slope) is the difference between the two vector points. Theoretically, the true gradient would be the difference between the two vector points as the distance therebetween approaches zero. Realistically, a gradiometer cannot be made with this distance equal to zero, so this distance must be specified, e.g., as equal to one foot. Thus, for example, if the distance between the two vector points is equal to 1 foot (ft), the first vector measures a magnetic field of 50,000 nanoTesla (nT) and the second vector measures a magnetic field of 50,109 nT, then the gradient would be -109 nT/ft (or +109 nT/ft, depending on measurement convention).
Gradiometer alignment is critical in many applications. Take, for instance, the situation wherein a gradiometer has two single-vector magnetometers which are not collinearly aligned. The second vector is tilted, say, 0.1.degree. with respect to the first vector, and earth's magnetic field is perpendicular to the first vector at 50,000 nT. The first vector will measure 0 nT, but the second vector will measure 50,000(sin 0.1)=87 nT; hence, the second vector will be 87 nT "off" (in error). Therefore, the gradient would be -87 nT/ft (or +87 nT/ft, depending on measurement convention) instead of the desired 0 nT/ft. This is poor alignment because, in the context of detecting ferrous materials, many magnetic signatures are &lt;20 nT/ft.
A gradiometer which is perfectly aligned can be "swinging" (i.e., moving freely at varying orientations) in earth's non-gradient field, and still maintain a reading of 0 nT/ft. However, if the gradiometer alignment is off by 0.1.degree., any motion of the gradiometer in earth's field will show a false gradient, thereby rendering difficult the differentiation between a ferrous material and a false gradient.
A typical test of gradiometer alignment involves rotation of the gradiometer spherically in the earth's 50,000 nT gradient-free field. The peak-to-peak gradient error will then reveal the misalignment angle.
Conventional approaches to aligning a gradiometer are mechanical in nature. Several gradiometers have been developed which are mechanically aligned to a precision of 0.01.degree.. A gradiometer aligned to 0.01.degree. will produce an error of 8 nT/ft in a 50,000 nT field. This is the best alignment which current fluxgate magnetometers can achieve over a 40.degree. F. temperature range.
Mechanical alignment of a gradiometer axis is a long and tedious process. It is an arduous, time-consuming struggle to mechanically align a gradiometer to a reasonable degree of precision. Furthermore, temperature stability has to be taken into consideration for the mechanical mounting scheme as well as for the electronics. Moreover, the mechanical alignment typically degrades over time, whereupon the gradiometer must be realigned, usually with greater difficulty than for the initial alignment. In addition, commercially available gradiometers are generally expensive, with single axis gradiometers and triaxial gradiometers running about the same price; achievement of the requisite precision for a trixial gradiometer could obviate the need to purchase three single axis gradiometers in its stead.