Wires make wireless strain gauge

By Ted Smalley Bowen, Technology Research News

Buildings may seem to embody inertia, but in reality they bristle with all manner of tension and compression -- dead loads like interior walls, live loads like people and furniture, and locked-in loads like those in pre-stressed concrete.

Then there are dynamic forces -- everything from the drop of a shoe to high winds and earthquakes. The effects of these forces can be difficult to predict and to detect. For example, even when a quake fails to bring a building down, there can be extensive damage, some of it invisible to the naked eye.

In recent years, designers have incorporated increasingly sophisticated mechanisms for dampening and countering violent shifts. Seismic retrofits and new construction place buildings on rollers and put dampers on joints and walls.

And embedded sensors in key places within walls, frames and foundations allow designers, building managers, and inspectors to measure the conditions that cause a building to topple, or gauge the soundness of a standing structure that's been subjected to strong forces but does not bear visible signs of strain.

Today's sensors measure forces like peak strain, peak displacement, peak acceleration, story drift, absorbed energy and accumulated plastic deformation, but can be expensive and difficult to read.

Researchers at Keio University in Japan have developed peak strain and displacement building sensors that do not require a constant power source, and that can be read using a wireless device. The sensors are designed to be embedded in concrete and fire-protection coatings.

Peak strain and displacement are measures of maximum distortion and shifting. Peak acceleration is the swiftest portion of distortion, story drift measures the side to side displacement of a given story of a building, absorbed energy is one measure of a jolt to a structure and accumulated plastic deformation is the cumulative warping of a structure.

A strain sensor must change when it is subjected to stress and must provide some way to read the change. Changes in electrical properties like resistance, capacitance and inductance can indicate that a sensor has undergone strain.

Resistance is a measure of how easily electricity flows through a material. Capacitance is the amount of electric charge a material can briefly store. Inductance is a measure of the voltage that can be induced through a circuit when a magnetic field produces an electric charge in a material.

The Keio design uses thin aluminum wires that stretch and remain elongated when subjected to strain. The stretching changes the resistance, capacitance and inductance of the wires, which can be measured relatively easily. And because the change to the sensor is mechanical, the device does not have to be powered. The researchers' design measures capacitance. "Capacitance is our preference [because] the system is simple and cheap," said Akira Mita, a researcher at Keio University. Such sensors can be made of any conductive material, he said.

Today's common embedded strain sensor designs work differently. Some measure peak strain with transformation-induced plasticity (TRIP) steel wires, which become magnetized when subjected to significant strains. This type of sensor can only be used once, however, and measuring its magnetism is relatively complicated.

Other embedded sensors are made from plastics reinforced with carbon and glass fiber, but the sensors record both peak and residual strains and untangling the two is difficult.

Two other approaches use carbon powder or shape-memory alloys in place of carbon fiber, but these have drawbacks as well. Carbon powder sensors are difficult to read and shape memory alloy sensors are tricky to design.

The Keio researchers have made several versions of their sensor. In one configuration, a thin wire is attached at one end to a block of conductive material and sandwiched between layers of conductive material at the other end. When pulled, the wire stretches, changing its electrical resistance.

An alternate design uses a variable capacitor, variable inductor or variable resistance element. Under strain, the wire in this scheme does not change length, but the variable element records the strain.

The variable capacitor version was made of a pair of aluminum cylinders separated by a nonconductive material and a 0.22-millimeter-thick fluorocarbon wire.

The researchers added an inductor or capacitor, depending on the sensor's design, that made it possible to sense the state of the wires using a wireless receiver. The addition created a closed circuit, and a change in the circuit's frequency indicated the change in capacitance or inductance and thus the level of stress encountered.

The test data transmitted from the variable capacitor sensor compared well with laser sensor readings of the same peak strain, according to Mita.

The researchers also tested a variable resistor design consisting of two potentiometers, thin wires, pulleys, a spindle and a spring. Potentiometers indicate displacements side-to-side. The researchers device was able to measure side-to-side shifts of up to 300 millimeters in each direction.

The next step is to make a version of the sensor that will record several types of strain at once. The design has the potential to produce sensors that read strain, stress, acceleration, cumulative displacement and integrated displacement, Mita said. Integrated displacement measures, which take into consideration multiple displacements, are used to estimate the durability of an element.

The sensors are currently in use.

Mita's research colleague was Shinpei Takhira. The work was published in the Feb. 17, 2003 issue of Smart Materials. The research was funded by the Japan Iron and Steel Federation.

Timeline:   Now
Funding:   Corporate
TRN Categories:  Materials
Story Type:   News
Related Elements:  Technical paper, "A smart sensor using a mechanical memory for structural health monitoring of a damage-controlled building," Smart Materials and Structures, Feb. 17, 2003


June 18/25, 2003

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