What Is a Strain Sensor?
A Strain Sensor is a measuring instrument that detects strain in an object to be measured as a change in electrical signals and measure the amount of strain from this change in electrical signals.
They are capable of extremely accurate and fast-response measurements and can be used for both static and dynamic strain measurement.
Uses of Strain Sensors
Strain Sensors are used to check the strength and safety of automobiles, railroads, aircraft transporters, buildings, and other structures. They are also used in clinical medicine, rehabilitation, ergonomics, and other fields, where they are valuable for measuring physical quantities in a wide range of fields.
In addition, sensors based on Strain Sensors include acceleration sensors, load cells, pressure sensors, displacement sensors, and torque sensors.
Principle of Strain Sensors
The general structure of Strain Sensors is a zigzag wiring consisting of a thin metal film on a thin insulating film. The metal film wiring is formed by a photo-etching process, and the Strain Sensor is attached to the sample using a special adhesive.
The Strain Sensor measures the strain on the surface of the sample by converting it into an electrical signal. The sensor changes the resistance of the entire electrical circuit in response to the strain on the sample, so the strain can be quantified by measuring the resistance. When the Strain Sensor is attached to the sample to be measured and the sample is distorted, the zigzag-shaped wiring of the Strain Sensor expands and contracts in accordance with the strain.
When the wiring is stretched, the cross-sectional area of the wiring becomes smaller and the electrical resistance value increases, and when the wiring is shrunk and the cross-sectional area becomes larger, the electrical resistance value decreases. From this change in electrical resistance, the amount of strain can be measured and quantified. A Hoiston bridge circuit is used to detect this minute change in resistance of Strain Sensors.
In the Hoiston bridge circuit, voltage is applied to the input side of the bridge, and the voltage changed by the strain of the object to be measured is measured at the output side. From this difference, the change in electrical resistance of the wiring of the Strain Sensor is detected, from which the strain of the object to be measured can be quantified. The wiring of Strain Sensors is mostly made of metal foil, but some are made of semiconductors. Strain Sensors utilizing the piezoelectric effect of ceramics are also available.
Types of Strain Sensors
Strain Sensors are available in both uniaxial types, which have only one element with zigzag-shaped wiring, and multiaxial types, which have more than one element with zigzag-shaped wiring. In the multi-axis type, two elements are arranged in such a way that some of them overlap.
Some are suitable for Poisson’s ratio measurement, and some are suitable for stress analysis with three elements overlapping each other. The one with multiple elements arranged in parallel is suitable for strain analysis of stress concentrations, etc.
How to Select Strain Sensors
It is important to select the type of Strain Sensor and the length of the wiring section in the direction of strain (gage length) according to the object to be measured, as well as to match the linear expansion coefficient of the Strain Sensor to the linear expansion coefficient of the object to be measured.
Other Information on Strain Sensors
1. Surface Treatment of the Object to be Measured
In order to make accurate measurements, it is necessary to remove rust, paint, etc. from the object to be measured. It is also important to prepare the surface of the object to be measured with sandpaper, etc., and to degrease the surface to which the Strain Sensor is to be bonded.
2. How to Apply the Strain Sensor
To make accurate measurements using Strain Sensors, first determine and mark the exact position of the object to be measured. Then, apply the specified adhesive to the back side of the Strain Sensor, align the mark on the object to be measured with the center mark on the Strain Sensor, and crimp the adhesive.
Even when instant adhesive is used, it is important to allow sufficient drying time.
3. Accuracy of Strain Sensors
Strain Sensors measure the amount of strain using the change in electrical resistance of the zigzag-shaped wiring portion, but measurement errors can occur due to errors in resistance values and errors specific to the wiring of Strain Sensors. In addition, the resistance of the lead wire connecting the Strain Sensor and the power supply, as well as the temperature and humidity of the test environment are also factors in measurement error.
Therefore, some devices automatically correct these errors by using a specific combination of Strain Sensors and data loggers.