Piezoelectrics are the active material of choice in a wide range of electromechanical applications including SONAR,
medical ultrasound and non-destructive evaluation. However, designers of high power piezoelectric systems have
suggested that a discrepancy exists between mathematical modelling predictions and measured transducer performance.
In most high power applications piezoelectric materials are operated under large compressive stresses. Manufacturers of
piezoelectric materials publish a wide range of performance data, however, the majority of the data is acquired under no
bias stress. In this paper, a new technique that facilitates the characterisation of piezoelectric materials over a wide range
of operating stresses (0-140MPa) at their resonant frequency is described. It builds upon the IEEE techniques for
piezoelectric characterisation and utilises measurement equipment found in the majority of piezoelectric development
laboratories. The technique therefore offers a low cost extension to existing facilities for the accurate determination of
piezoelectric properties under high stress loading. Results gained using the new technique confirm that substantial
variation in electromechanical properties of piezoelectric materials occurs under stress loading. Using this derived data,
a more informed evaluation of transducer materials and more accurate predictions of transducer performance can be
made.
Traditional solutions to vibration problems often employ viscoelastic materials which can be heavy, temperature-sensitive
and bulky. Active solutions can provide useful damping but are often complex and expensive. This paper
outlines a passive piezoelectric damping system with an adaptive controller capable of not only providing useful
damping levels, but of modifying the components so as to change the circuit resonant frequency and thereby the damping
effort. Experiments on simple beams and more realistic structures are described and the potential benefits and power
requirements of such a system discussed. Increases in loss factor up to a factor of 10 and a high level of tuning
repeatability were seen.
Shape memory alloy actuators, with their simple operation through heating, and high solid state strain and force output
are ideally suited to a range of robust engineering applications within the oil & gas industry, such as down-well flow
control valves. Because reservoir temperatures can reach up to 250°C for very deep wells, a range of alloys with high
transition temperatures are required. For a specific 'single-shot' valve application, with an operational temperature
requirement of 110°C, a robust shape memory alloy (SMA) actuator, capable of delivering a stroke of 2 mm with high
force output (4 kN), was developed and tested from Nitinol alloy H (Ni:Ti ratio of 49.5%:50.5%). The inducement of
mechanical stresses within nickel-titanium alloys can influence the transitional temperature range of the alloy. This
characteristic was exploited to raise the martensitic-to-austenitic transition temperature (Ap) of the alloy from 90°C to
130°C, through the application of large compressive cold-working stresses (equivalent to 10% compressive strains). The
actuator acted as an electrically-activated trigger within a hybrid SMA-hydraulic valve. Once activated the actuator
releases a high pressure seal allowing stored hydraulic pressure to operate the main mechanism of the valve. The
operation of the SMA actuator and complete valve assembly, inclusive of battery pack and control electronics to activate
the SMA, was successfully tested within a test-well environment at depths up to 900 meters and under hydrostatic
pressures of 7,500 psi (51 MPa). The reliability of the valve and the SMA actuator demonstrates the applicability of this
technology to down-hole oil & gas applications.
A comparative study has been made to explore the potential benefits of newly available single-crystal ferroelectric materials
when used in a practical device, in this case an ultrasonic micro-motor. This type of micro-motor exhibits exceptional power-to-weight characteristics, which could be exploited beneficially, for example, in unmanned air-vehicle (UAV) systems. The
operating principles of a range of commercial and experimental motor designs were evaluated objectively in order to identify
areas of performance that can potentially be enhanced using PMN-PT single-crystal piezoelectric ceramics. Based on this
analysis a practical motor design was selected for construction and experimentation. Detailed numerical analysis indicated
that a motor constructed from single crystal PMN-PT could be expected to provide an improvement in motor stall-torque by
up to a factor of 2.8 and a no-load speed improvement by a factor of 1.5 when compared with motors based on standard
polycrystalline lead-zirconate-titanate (PZT) ceramics. In practice single-crystal versions of the motor were found to produce
double the power output of their polycrystalline counterparts. Overall efficiency was found to be improved two-fold. There
were significant discrepancies between the numerical predictions for the single-crystal devices and their measured
performance, whereas the polycrystalline devices were found to perform closely in line with predictions.
A new, low cost technique for excitation and control of high density, actuator arrays has been developed. Primarily aimed at the tactile display of universal text, Braille and graphics for the Blind community, magnetic forces are utilised to actuate and hold individually addressable pins. Current tactile displays that allow Blind people to interact with the world via electronic media are expensive and due to their inherent complexity can only display the Braille language, which on average only 10% of visually impaired people in Western society are able to read. There is a need for a more flexible communication device. Using multilayer printed circuit board techniques to minimize production and assembly costs, large arrays of magnetic solenoid actuators were fabricated on a single substrate. They were electrically interconnected to allow matrix addressing of any single element in the array to reduce electronic component count. A bistable mechanism was produced using a permanent magnet layer allowing the solenoid actuator to be toggled between an 'up' state and a 'down' state. Besides being necessary for matrix addressing, this bistability gives good energy efficiency as power is only required when the system is updated and not when holding the static display. An error correction technique was developed that automatically corrected a bistable actuator if it had mistakenly moved into an incorrect position through mechanical shock. A 360 actuator demonstration unit was produced that displayed alphanumeric text, Braille or graphics in a tactile form.
Battery-free sensor systems would benefit from the availability of a stress or strain sensor that exhibits a large enough property change to allow simplification and power reductions in sensor interface and data transmission circuitry. A new sensor has been developed specifically for this purpose, which uses the large stress induced impedance changes exhibited by ribbons of amorphous magnetic alloy. In comparison to semiconductor strain gauges, which show a change in resistance of 15% when strained to their maximum recommended stress level, the amorphous alloy sensor demonstrates a change in inductance of 315%, when strained to its maximum working level. Although, amorphous magnetic alloys are inherently sensitive to external magnetic fields, a simple, biasing technique renders the stress-sensing device insensitive to modest field strengths. The amorphous magnetic alloys are produced in large volumes and realize an extremely low cost sensor. A low cost and low power analogue electrical circuit has been designed that, in combination with the amorphous alloy sensor, functions as a battery-free sensor 'tag'. The sensor tag can transmit stress data to a transceiver system via an inductive link, negating the need for battery power or a hardwire connection. The system’s range is directly related to the transceiver and tag antenna dimensions; however a system with 20cm diameter antennas has been demonstrated operating over a range of up to 60cm. This range is achieved through the extremely low power demands of the sensor tag (<1mW). A demonstration unit has been developed for vehicle tyre pressure monitoring applications.
The concept of using embedded or surface-bonded solid-state actuators to effect shape change in carbon fibre composite laminates continues to have technical merit and appeal. Conventional laminate design methods tend to lead to stiff structures, whilst it is easiest to impose a change of shape on a compliant structure. This presents a possible conflict of design and suggests that the useful performance of solid- state actuators will always be limited by the stiffness of the host laminate. One possible solution is to increase the in-plane work capacity of the actuators either by using improved materials such as phase change perovskites like PLZT or improved eletroding techniques such as inter-digitated electrodes (IDEs). In this study, the performance of several different actuator/laminate systems have been modelled to determine a baseline capability in pure bending. Four cases have been considered for different panel thicknesses and lay-up sequences. The materials performance and IDE design issues have also been addressed. Modelling indicates that even with conventional actuator materials, structural displacements can be produced which could provide useful shape change in applications such as missile roll control.
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