Biomedical Engineering Reference
In-Depth Information
Fig. 7.13
(
a
) Photograph of planar electromagnet and flux guides. (
b
) Measured magnetic field
versus current applied to the electromagnet. (
c
) Time domain signal from the GMR nanosensors
before and after applying the digital filter. (
d
) Transfer function of the 113th order digital finite
impulse response (FIR) filter [
17
]
In contrast, reducing the form factor and power consumption to create a
handheld, ultraportable device is essential for POC application, but posed several
engineering challenges. To accomplish this miniaturization, a planar electromagnet
was designed using 1.27-mm (50 mil) traces on a four-layer PCB (Fig.
7.13
a). The
orientation of the current flowing through the coil alternates between clockwise
and counterclockwise to avoid the need for any crossover traces that would reduce
the number of available routing layers. The magnetic field is generated out of
the plane (perpendicular to the PCB) and reoriented by soft magnetic flux guides
manufactured out of cold rolled steel. The flux guides concentrate the field over
a smaller region, acting as a form of passive amplification, and are used as heat
sinks for the electromagnet. In addition to the flux guides above the coil, there
are flux guides below the coil to close the flux loop and increase the efficiency.
Due to the off-axis nature and the use of magnetic flux guides, analytical models
are not tractable for design. Instead, finite element modeling (FEM) is needed to
determine the required number of turns (11 turns per layer) and current, which
includes both field strength and frequency. The miniature electromagnet is driven by
a custom-designed class-A power amplifier. Figure
7.13
b illustrates the relationship
between the current through the electromagnet and the measured field across the
GMR SV. Power consumption was minimized by cycling the power amplifier and
electromagnet when they are not being used.
