User Manual

Page 5
Figure 3. DT1 Element in [mm]
Bimorph configurations (like a bimetal strip) allow the
small differential displacement of two reverse-
connected elements to be translated into substantial
flexural motion. Small fans or optical deflectors can
thus be created. Such devices consume very little real
power (being capacitive in nature). Large devices may
be difficult to drive due to high capacitance, especially
when transformers are used to step up the drive
voltage. Good amplifier design is important.
Nevertheless, conventional fan and blower
technologies generate higher flow rates and back
pressures than piezo bimorphs.
Although the forces involved are small, the film can be
used to excite other mechanical structures over a very
wide frequency range. If a second element of film is used to receive the induced vibration, the
system can possess a very high dynamic range, even though the overall "insertion loss" due to the
film is about -66 dB typically for a structure at resonance. If sufficient gain is applied between these
elements, the structure will self-oscillate at its natural frequency. For these resonant mechanical
systems, high voltage drive is not required. The amplifier circuit may function adequately from a
normal dual rail op-amp supply, or even from a single 9 volt battery. For analysis purposes, even
lower applied voltages, e.g., the noise source of a spectrum analyzer at 70 mVrms, are sufficient to
insert the mechanical energy into a structure when piezo film is also used to monitor the result.
Mechanical to Electrical Conversion
The sensitivity of piezo film as a receiver of mechanical work input is awesome. In its simplest
mode the film behaves like a dynamic strain gage except that it requires no external power source
and generates signals greater than those from conventional foil strain gages after amplification.
Frequency response is thus free from any limitations imposed by the need for high gains and will
extend up to the wavelength limit of the given transducer.
The extreme sensitivity is largely due to the format of the piezo film material. The low thickness of
the film makes, in turn, a very small cross-sectional area and thus relatively small longitudinal forces
create very large stresses within the material. It is easy to exploit this aspect to enhance the
sensitivity parallel to the machine axis. If a laminated element of film (for example an LDT1-028K)
is placed between two layers of compliant material then any compressive forces are converted into
much larger longitudinal extensive forces. In fact, this effect tends to predominate in most
circumstances since most substances are compliant to some extent and the ratio of effective
sensitivity in the 1 (length) vs 3 (thickness) directions is typically 1000:1.
Piezo film transducers may often cover a much larger area than normal strain gages so any direct
comparisons should be performed in a uniform strain field for meaningful results. Obviously
"point"-type transducers could be used where required although the capacitance of a very small area
will require consideration. The low frequency limit of operation will be defined by the greatest
resistive load achievable, or by the largest capacitance load that still allows the signal to be easily
detected. Operation down to fractions of Hz can be achieved using either conventional charge
amplifiers or, since signal levels are relatively high, simple high impedance FET buffer circuits.
Pyro to Electrical Conversion
Piezo film absorbs strongly in the region of 7 to 20 µm which corresponds to well beyond both
operating temperature limits of the film. It thus makes a sensitive pyroelectric detectors for,
say, human body radiation. Since the pyro sensitivity is strong, care must be taken when designing
low (<0.01 to 1Hz) frequency mechanical sensors to avoid ambient temperature changes swamping