Biomedical Engineering Reference
In-Depth Information
Sensor 1
Sensor 2
moving
N
a
S
b
Magnetic
Source
b
Position of
the source
a
t
Output
signal of
sensor 1
t
Output
signal of
sensor 2
t
Fig. 3.6.
A moving magnetic source
two sensors would vary with different phases, as shown in Fig. 3.6. The phase
difference depends on the relation of the positions of and directions between
the sensors. If the sensors are close together, the phase difference becomes
very small and it is dicult to make a decision as to whether or not the source
is moving. In this case we need more sensors to learn about the source. From
this simple example, we can see the possibility of knowing the dynamics of
the source, such as movment, rotation, or a combination of both, if we have
more than two sensors. To analyze such data, we need to consider not only
the distribution of the magnetic field but also the time variation. Therefore,
this can be called this spatiotemporal analysis. Practical data and methods
about this will be described in later sections.
3.2
A High-Sensitivity Magnetic Field Sensor
3.2.1 The SQUID
The fundamentals of a dc SQUID are described in this section. A dc SQUID
consists of a superconducting loop with two Josephson tunnel junctions. Each
