Resistive displacement sensors

Table 4.22 shows impact resistance of the fly ash containing WPC boards (ASTM D 3763, high-speed puncture properties of plastics using load and displacement sensors), which was measured by the rate sensitivity of the material to impact (load versus deflection response). Test speed was of 3.3 m/s (10.8 ft/s). 

FIGURE 2.1 Examples of displacement sensors (a) variable resistance sensor, (b) foil strain gauge, (c) linear variable differential transformer (LVDT), (d) parallel plate capacitive sensor, and (e) ultrasonic transit time-displacement sensor. 

A different type of displacement sensor based on an electrical resistance change is the strain gauge [2]. If a long narrow electrical conductor, such as a piece of metal foil or a fine gauge wire, is stretched within its elastic limit it will increase in length and decrease in cross-sectional area. The electrical resistance between both ends of the foil or wire can be given by... 

Stimulus mechanical measurands Position and displacement (dimension) Absolute (for position) and relative (for displacement), odometers, strain and displacement sensors Optical, capacitive, resistive, inductive. Hall, magnetostrictive effect... 

Position transducers are also very important in the plastics industry. Both linear and rotary displacement sensors are widely used. In injection molding, screw position for shot size and cushion must be very accurately measured. Mold open and close positions as well as ejector plate travel are also controlled with input from linear position sensors. Generally, it is recommended that linear position sensors provide an accuracy of better than 0.01%, a response time of less than 1 ms, and good vibration resistance [9]. 

Resistive sensors are devices that transduce the measured physical parameter into a change in resistance. This change is then measured by electrical means. Typically, resistivity change is induced by some kind of a displacement. There are two major groups of resistive sensors potentiometers and strain gages. 

All microfabricated pressure sensors derive their sensitivity from the elastic deformation of a membrane. Measuring the elastic deformation provides the desired signal information. In the broad class of microfabricated pressure sensors, the two most widely used phenomena are the change of resistance due to stress in boron-doped (p" ") resistors and the direct measurement of the membrane displacement using optical or capacitive methods. 

Figure 14.6 Force—displacement plot against normalized resistance variation for the two sensors inside laminated composite specimen tested until fracture at constant loading rate of 1 mm/min.

Figure 14.8 Multicyclic three-point bending test of glass laminated composite, (a) Force-displacement plot (b) normalized resistance curves for the two sensors against displacement by Nauman, S., Cristian, I., Koncar, V., 2011. Simultaneous application of fibrous piezoresistive sensors for compression and traction detection in glass laminate composites. Sensors 11 9478—9498 Licensee MDPl, Basel, Switzerland.

In addition to testing to breaking point, a specific test was undertaken on the sensor-containing composite by varying the strain applied on the composite in steps. The results from this test are shown in Fig. 16.20. A change in resistance is noted to follow closely the applied strain. Because the sensors reacts to very small displacements placed upon the composite structure, this indicates its validity for application as sensors for the monitoring of composite parts structural health. 

Sensors that are part of the textile and that are resistant to mechanical, chemical and thermal influences must react to all different kinds of forces. Therefore, various sensor concepts that rely on physical, chemical and thermal mechanisms of action are suitable for application. They help detect forces, displacements, thermal energy, humidity, chemicals, UV radiation and other influences. 

---