Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
1995-10-10
2001-06-26
Budd, Mark O. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S316010, C310S317000, C310S330000, C310S326000
Reexamination Certificate
active
06252334
ABSTRACT:
BACKGROUND
Being able to accurately determine and control the movement of structures, particularly in aerospace is increasingly important. Being able to easily program and vary the degree of control and movement is particularly valuable.
This invention relates to vibration, alignment control, and health monitoring of foundation members and surfaces. In one particular aspect, the invention relates to the control of such members in aerospace applications such as the control of struts in spacecraft. The invention also has particular application in the static control of foundation members and the control of deformity of structures, for instance in aircraft and automobiles.
Space missions during the next decade will require lightweight, cost effective, high performance material systems which can achieve enhanced satellite pointing capabilities. These material systems will employ smart structure technologies. This includes active damping, passive damping, and leveraging advanced metallic and plastic composites to shape and tailor mechanical behavior.
Also, there is a need to reduce noise and vibration in vehicles, for instance, to improve comfort for passengers. By being able to alternate vibration, the structure borne sound in cars, trucks, and other passenger vehicles can be reduced.
Smart Structure Technology
Vibration control and suppression includes use of an active technology. This approach uses materials with properties that can be altered with externally applied signals. These materials are sometimes referred to as smart materials. Smart materials include wafers of piezoelectric elements such as ceramics, for example, lead-zirconate titanate (“PZT”) elements. These elements are embedded into advanced composite structural foundation members composed primarily of graphite fibers with several kinds of matrix systems—epoxies, polycyanates, and thermoplastics. The members thereby become smart structures.
By applying an electric field, &egr; across the PZT wafer thickness, a strain, ∈ is induced into the structural member. The relationship of strain to electric field is defined as the piezoelectric strain to electric field coefficient, d
31
and is defined as ∈=d
31
&egr;. This determines the efficiency in the evaluation of PZT. The strain constant together with the PZT material modulus of elasticity, E, measures the lateral stress, &sgr;, generated per unit field and is an indicator of the actuation force on the smart structure. This can be expressed as &sgr;=E∈ or &sgr;=Ed
31
&egr;.
Vibration damping requires a direct method to sense strain. Piezoceramics have a high strain sensitivity. For precision spacecraft applications where nano-strains must be measured, the viable choice is PZTs. Sensing micro-strains with PZTs requires measurement of milli-volts, which is relatively easy. PZTs are also favored for environmental factors such as insensitivity to temperature. Piezoelectric sensors enjoy the further advantage that they are self-generating, producing a signal directly from strain. This makes them more power efficient, and drift is less significant.
Vibration and shape control may be achieved with a number of actuators. The specific requirements of space and vehicle vibrations make PZTs the preferred choice. Most piezoceramics produce a similar blocked force per unit field, about 0.56 lb/V-in. At maximum working field, roughly 2,000 psi of applied stress deforms a graphite layup about 50 micro-strains. This is sufficient for almost all bending vibration control applications, and some shape control requirements. Linearity of actuation is also excellent. In previous research, it has been shown that embedding PZTs in graphite virtually eliminates hysteresis due to creep. Similar to PZT sensors, actuation strength over a wide temperature range was also shown to be very uniform.
Various assemblies built for structural applications contain unacceptably low levels of damping. These structures could benefit immensely from the addition of active damping. One drawback is that the structure may already be manufactured and assembled before the need for additional damping is identified. Metallic structures that require active damping are one example.
There is a need for a discrete piezoelectric sensor/actuator assembly that can be bonded in, on or mechanically attached to a foundation member.
Throughout the evolution of smart structures using piezoelectric elements, control electronics have been a major implementation problem. Their inherent size, weight, interface cabling needs, and environment requirements have been a substantial application roadblock.
Also it has not been possible to provide for easy adjustment of the control of the smart structure after setting up and deployment of the structure.
There is a need for miniature, discrete electronic controllers which can be adjusted in operation for use with piezoelectric elements.
SUMMARY
The invention seeks to fulfill the needs of smart structures.
According to the invention, there is provided a modular assembly control member, namely a patch, as a preformed assembly of encapsulated piezoelectric sensors and actuators. The patch may be embedded in a foundation member device during structure fabrication as it may be or it can be bonded on to an existing foundation member. A smart structure is thereby obtained.
Local digitally-based control electronics for the assembly is attached through a button board interface to the assembly in either an axial or transverse arrangement relative to the actuator and scissor in the assembly. Other electrical connecting methods between the local control electronics and the patch may be used.
One local electronic controller is designed to slave to one sensor/actuator assembly as a single input, single output system. Thus, n-controllers are required for an n-assembly strut system.
The electronics applied locally to the assembly is fed by a serial electrical interface, laced throughout the structure to a central control. The local electronic control for each assembly eliminates large control wire harnesses, eliminates sensor noise contamination, and reduces electronic signal lag (limiting bandwidth). The local control electronics is digitally based and allows for programmable independent operation by each local controller for maximized system reliability and simplicity.
The patch effects active damping of the foundation member by detecting local strains in the foundation member. Strain detection by the patch is effected with one or more piezoelectric sensors in the patch. Strain actuation is done with a piezoelectric actuator in the patch. The sensors can be colocated with the actuator, or nearly colocated with the actuator.
The control electronics includes three basic components to provide electronic digitally based control signals into a smart structure. The first part, or input stage, uses analog charge amplifiers that convert piezoelectric (PZT) charge to a voltage appropriate for a digital compensator. The second stage, a digital compensator, processes the signal with gains and phase shifts to dampen or cancel vibrations in the foundation member.
The digital compensator is responsive to variable operation by remotely generated signals. As such, the digital compensator is programmable, and the smart strut can be set up to be variably responsive according to the requirements determined remotely from the strut. The digital compensator can be varied in its operation. The digital compensator stage generates a digital feedback signal for the purpose of controlling the foundation member. This signal is then fed into the third part, or analog drive amplifier stage, which drives the PZT actuators. The drive amplifier is a linear voltage amplifier device with phase compensation to enhance stability when driving PZT devices at high frequencies.
The invention addresses active damping, shape control and health monitoring control electronics using digital compensators.
The invention is now further described with reference to the accompanying drawings.
REFERENCES:
patent: 4363991
Bronowicki Allen J.
Dvorsky George R.
Kansaku Claude I.
Nye Theodore W.
Steffen Nicholas R.
Budd Mark O.
TRW Inc.
Yatsko Michael S.
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