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The use of Thera-Band allows for a wide range of objects of various shapes and sizes to be secured. By attaching Thera-Band across two rigid frames opposite one another, the object can be centered between these two planes, which are then actuated shut. The elastic force of the Thera-Band will increase as it stretches to fit around the object . With and this , the elastic force creates a strong enough acts against the object as a Normal force. This normal force creates friction with the object in the horizontal direction, which will correspond to a higher friction force between on both sides which opposes the object’s gravitational force, which is what holds the object in place. The following graph shows a projection of the estimated elastic force in the Thera-Band and the object thereby opposing the gravitational force of the object.
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The approximate coefficient of static friction of the theraband was found experimentally. Using known values provided by TheraBand for the force generated for a given percent elongation of the material, an approximation for the friction force generated to hold the object in place can be found. This was then used to decide what range of elongation values would be necessary to work with given the competition specifications for up to a 2kg, 20cm^3 objectcorresponding friction force, along with the constant weight of the object which is the force that needs to be counteracted. The %Elongation represents the elongation of the Thera-Band from its original length.
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The friction force was calculated using an experimental coefficient of static friction value between the Thera-Band and silicon however, it should be acknowledged this value would vary depending on the object’s material. Data for the elastic force of Thera-Band at various %Elongations was retrieved from [1].
Actuation
The grabber actuates the two rigid frames allowing them to close around the object being picked up. This is accomplished using a central shaft in combination with two servos which are mounted to the frame. String is attached to either frame, which connects to a pulley attached to each servo on either side of the grabber. Once activated, the two servos winch the frames in opposite directions thereby opening the grabber. Actuation in the opposite direction in order to close the grabber is done mechanically using elastic bands, which will force the frame shut once the object is secured. This was done in order to reduce power consumption during flight, as power will only be drawn initially to open the grabber when picking the object up. An example of this actuation mechanism can be seen in a prototype version in the figure below.
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References
[1] Prohealthcareproducts, “Thera-Band Colors Sequence Resistance Levels,” Prohealthcareproducts.com [Online]. Availiable: https://www.prohealthcareproducts.com/blog/theraband-colors-sequence-resistance-levels/