User:Raduran/sandbox/electromechanical film

The Electromechanical Film (EMFi) is an elastic material developed by the Technical Research Center of Finland that is biaxial oriented, permanently charged and is coated in electrodes that can convert mechanical energy into electrical energy and vice-versa. Along with the conversion, an EMFi can be used as either an actuator or a sensor.

Mechanics
An external force applied to the surface of the film will cause the thickness of the air voids to change. This change in thickness causes the air voids found in the polypropylene interfaces to move with respect to each other, creating a mirror charge in the process. This charge that is created is proportional to the change in thickness of the film.

Algorithm
Since the electromechanical film can be used as a sensor to identify changes in pressure, it can been implemented into tablets, however, an algorithm had to be created to determine the coordinates of where and how much pressure was applied. With the help of a data acquisition card, the algorithm functions as followed:


 * 1) Using a moving average filter, smooth the raw data of each channel.
 * 2) Calculate the standard deviation of the smoothed channels in a span of five samples after each sample.
 * 3) If the calculated standard deviation surpasses a preset sensitivity threshold value, find the position in which the signal started to increase and mark it as a viable starting point.
 * 4) Mark the end of the interaction as a point in which any of the remaining four signals started to decrease.
 * 5) If the endpoint follows within the five samples following the initial position, take consideration of the interaction, otherwise ignore the interaction.
 * 6) Calculate the average force used for the interactions as the average of the samples between initial and final positions.
 * 7) Calculate the x and y coordinates of the interaction with the following formulas:

$$x = D_{31}(\left ( \frac{\sqrt{F_1}}{\sqrt{F_1}+\sqrt{F_3}} - 0.5 \right )) $$

$$x = D_{24}(\left ( \frac{\sqrt{F_4}}{\sqrt{F_2}+\sqrt{F_4}} - 0.5 \right ))$$

In this equation, $$D_{31}$$and $$D_{24}$$are the distances between opposing corners and $$F_1,F_2,F_3$$and $$F_4$$are the average forces that are calculated on each channel by using the equation:

$$F_k = \frac{s_k}{N} \sum_{n=start}^{end}x_k(n), k = \{1,2,3,4\}$$

Where, xk, is the value of the force on each channel, k, at time instance n, and N is the length of the interaction. To compensate for a slight differentiation in sensitivities between the electromechanical film sensors and their respective amplifiers, different variables were used for the distances between the opposite corners of D31 and D24 along with a different scaling factor, sk for each channel.

Manufacture
The two basic types that have been manufactured were given the labels O01 (37 μm thick) and HS01 (70 μm thick). The electromechanical coefficient, ks, for O01 is 30 pCN-1 and HS01 has a coefficient of 170 pCN-1(charge developed/applied stress). Upon applying an external voltage, the EMFi operates as an actuator and its effectiveness is dependent on variation in thickness and the films motion in an air gap.

Real World Implementation
Some examples of how the EMFi is used by the public is that the film is commonly found in microphones for stringed instruments, keypads, speakers and variations of both small and large sensors.

A great example to focus on is the use of electromechanical films in keypads, specifically keypads that are meant to withstand harsh environmental changes or vandalism. Since the main focus on these devices is durability, they are engineered to have multiple protective layers and contain a certain distance between keys, which causes the device to rely on changes in pressure. Each key is placed above an EMFi sensor and upon pressing, a charge is generated in the layers of electrodes. Once the key is released, an equal charge of opposing signs is created in the same layer.