Biomedical Engineering Reference
In-Depth Information
actuation (see Shahinpoor and Kim, 2000, for an extensive review). The materials
work via internal ionic transport phenomena, requiring water internal to the system
and providing a coupling between electrical field and bending deformation. Not only
do such materials exhibit relatively large deformations with a mild voltage input,
but they also have a significant electrical response to bending deformation, even at
subhertz frequencies. The long response time of IPMNCs—on the order of 4-10
sec—limits their use in applications requiring a short response time. Although this
limitation may be mitigated to some extent through the use of feedback control, it
becomes an advantage when considering the material for low-frequency applications.
The usefulness of IPMNC material in a near-DC accelerometer application, as well
as potential problems and design considerations, is examined here.
7.3.3
E
XPERIMENT
S
ETUP
The complex impedance of the IPMNC was measured by the well-known voltage
divider method. The impedance was measured over a range from 0.05 to 5000 Hz,
using a 15-k
resistor in series and a Siglab data acquisition system to record the
magnitude and phase of the voltages.
For the dynamic experiments, a Ling 5-lb shaker was mounted on an optical
table in the vertical orientation with the stinger pointing down. A machined alumi-
num block was mounted on the end of the stinger to provide a flat movable surface.
An eddy current probe was mounted such that the head was held beneath the block,
near one edge, with a 0.25-mm standoff distance. The probe was calibrated to
provide 1.2-mm/V displacement response near an aluminum surface. For the control
case, a piezoelectric patch (PZT-5A with nickel electrodes, 11
Ω
0.26 mm,
0.646-g mass, 20-nF capacitance) was used as the active element. In the experiment
case, an IPMNC patch (with gold electrodes, 0.237-g mass) was used as the active
element and cut to the same planar dimensions as the piezoelectric patch, but with
0.32-mm thickness.
For the control case, the mounting apparatus consisted of an aluminum-base
block with a piezoelectric patch (PZT) sandwiched between the base block and the
shaker block. The shaker and block were lowered onto the optical table mounting
post to generate an initial compressive preload in the patch and to ensure that it
remained in compression over the entire range of motion applied to the shaker.
Both blocks were covered with Kapton tape where they contacted the piezoelectric
element to prevent current bleed-off into the optical table ground. Copper shims
were cut and affixed to the top and bottom of the piezoelectric patch with conductive
grease to provide external leads. When the shaker was actuated, the piezoelectric
element was compressed in the 3-3 direction, generating a voltage between the
copper leads. The leads were connected via BNC cable to an input channel of a
HP digital signal analyzer (DSA 35665A) with 1-M
×
29
×
input impedance. No condi-
tioning circuitry was applied. The DSA source channel was used to actuate the
shaker with a 5-V peak-to-peak sine sweep input over a frequency range from 0.015
to 30 Hz. The frequency response function from eddy current displacement to
piezoelectric element voltage output was measured over this frequency range and
converted to a volts/strain spectrum.
Ω
