Biomedical Engineering Reference
In-Depth Information
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Mass
Housing
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Mass
Active
element
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Housing
Active
elements
(a)
(b)
FIGURE 7.10
Accelerometer implementations using PZT in (a) 3-3 mode and (b) 3-1 mode.
often used to sense acceleration. Figures 7.10(a) and 7.10(b) are representations of
two approaches. Figure 7.10(a) depicts the most basic embodiment, a piezoelectric
ceramic element, such as lead zirconium titanate (PZT), in line with a mass. When
the base is accelerated vertically, the inertia of the mass causes strain in the active
element that, when electroded, generates a charge proportional to the strain. Damping
of the first-order system may be tuned to provide a nearly flat mechanical response
over a desirable range below the system resonance frequency. Since piezoceramic
materials are brittle and cannot support significant tension loads, a mechanical or
electrical preload must be applied in this embodiment to avoid such a situation.
Figure 7.10(b) shows a slightly more common approach for an acceleration
sensor. A mass is suspended from three sides by piezoelectric elements that are
affixed to the boundary of the accelerometer enclosure. In this case, as the enclosure
is vibrated, the active elements undergo a shear strain, which also generates a
proportional voltage. The piezoceramic material can support shear loads in either
direction, so in this embodiment no vertical preload is necessary.
In either of the conventional embodiments with realistic active element sizes,
low-frequency response (<1 Hz) is generally poor without the use of additional
electronics. Piezoelectric elements are largely capacitive and, when connected
directly to an oscilloscope (
input impedance), charge tends to bleed off
quickly at low frequencies in the effective RC circuit. Voltage followers or charge
amplifiers are usually included in the circuit to lower this frequency and eliminate
measurement sensitivity to cable noise and environmental parameters. However, low-
frequency performance comes at the expense of size, weight, and complexity. Fur-
thermore, useful performance at frequencies below 1 Hz is extremely difficult to
achieve. For this reason, piezoelectric accelerometers make a poor choice for near-
DC acceleration measurements. Here, we investigate the use of a fairly new material,
IPMNC, for use as an active element in near-DC accelerometers.
≈
1 M
Ω
7.3.2
B
ACKGROUND
Recently, IPMNCs consisting of a thin Nafion-117 sheet plated with gold on both
sides have received much attention for their possible applications to sensing and
