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The use of TheraBand allows for a wide range of objects of various shapes and sizes to be secured. By attaching theraband across two rigid frames opposite one another, the object can be centred between these two planes, which are then actuated shut. The elastic force of the TheraBand will increase as it encloses to fit the object. With this, the elastic force creates a strong enough normal force with the object in the horizontal direction, which will correspond to a higher friction force between the theraband and the object thereby opposing the gravitational force of the object.// insert image here
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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 object.
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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