Ferroelectret force sensor

Cellular ferroelectret foil Ferroelectret force sensor ... 

The traces on the oscilloscope in the photo of Fig. 8 depict four typical measurement signals from the curves, it is obvious that the sensors measure the force rate. The winner is immediately obvious from the signal traces. The game is also useful to teach students the generation and interpretation of signals from ferroelectret sensors. The proposed approach to start experimenting with ferroelectrets enables students to not only fabricate a simple ferroelectret force sensor but also to play with the sensor... 

Equivalent circuit of ferroelectret film sensor Force rate measurements Oscilloscope... 

The commercially available ferroelectret film is based on polyolefin foam manufactured in a continuous biaxial orientation process, followed by an expansion process to adjust the mechanical and the piezoelectric properties (Fig. 2 left). The equivalent circuit of a ferroelectret film sensor consists of a current source in parallel to the sensor capacitance (Fig. 2 right). The sensor capacitance Cp follows fromCp = eoeA/s, where sq is the permittivity of vacuum, e is the dielectric constant of the ferroelectret foam, A is the electrode area, and s is the thickness of the film. With a typical thickness of S = 70 pm and a dielectric constant of e = 1.5, the sensor capacitance is 19 pF/cm, a small capacitance in comparison to other piezoelectric sensor elements. The ferroelectret sensor element produces a current i(t) when a force F(t) is applied to the sensor element, according to i t) = where the dot... 

Fig. 7 Force measurement with a ferroelectret sensor coupled to a charge amplifier top). A rectangular varying compressive force is applied to the sample. Force rate measurement, when the ferroelectret sensor is connected to a digital oscilloscope bottom). Measurement signals can be calibrated to determine the piezoelectric coefficient of the ferroelectret foam sensor...

Ferroelectrets are best used in the thickness mode, wifli negligible responses upon bending. They are thus ideal sensor materials for force sensing, for example, in keypads and in bed sensors for sleep monitoring (EMFIT 2015). 

Fig. 6 Experimental setup for testing ferroelectret sensors. The arrangement allows for measurements of the elastic modulus (when the LCR meter is applied) or for the characterization of the piezoelectric 1 33 coefficient (when the electrometer is employed). Force is applied to the sensor by a Teflon bar the force magnitude is adjusted by pressurized air (Redrawn with permission from (Dansachmuller et al. 2005))...

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