Sensors and Methods for Mobile Robot Positioning

48 Part I Sensors for Mobile Robot Positioning
steel. In actuality, due to hysteresis, µ depends not only on the current value of H, but also the
history of previous values and the sign of dH/dt, as will be seen later. The important thing to note
at this point in the discussion is the B-H curve is not linear, but rather starts off with a fairly steep
slope, and then flattens out suddenly as H reaches a certain value. Increasing H beyond this “knee”
of the B-H curve yields little increase in B; the material is effectively saturated, with a near-zero
permeability.
Figure 2.14: The slope of the B-H curve, shown here for cast iron and
sheet steel, describes the permeability of a magnetic material, a
measure of its ability (relative to air) to conduct a magnetic flux.
(Adapted from [Carlson and Gisser, 1981].)
When a highly permeable material is introduced into a uniform magnetic field, the lines of force
are drawn into the lower resistance path presented by the material as shown in Figure 2.15.
However, if the material is forced into saturation by some additional magnetizing force H, the lines
of flux of the external field will be relatively unaffected by the presence of the saturated material,
as indicated in Figure 2.15b. The fluxgate magnetometer makes use of this saturation phenomenon
in order to directly measure the strength of a surrounding static magnetic field.
Various core materials have been employed in different fluxgate designs over the past 50 years,
with the two most common being permalloy (an alloy of iron and nickel) and mumetal (iron, nickel,