For our grabber mechanism, we are designing a claw-like design that uses both a rigid frame and flexible TheraBand Thera-Band to gently and accurately pick up objects of various sizes and weights.
The rigid frame portion is comprised of two flat, four sided, slender frames, tensioned together by Elastics at the start and bottom, and attached to an axle at the top. The frames are then pulled apart with two servos when going to grab an object. When completely closed, the rigid frame supports the object from the bottom and ensures the object is secure. The TheraBandThera-Band, which surrounds the sides of the is attached to each frame, creates a wall in front of the frameflexible wall to contour around objects being picked up. When the frame closes on the object, the TheraBand Thera-Band bends around the object, creating a tight and secure grab on whatever object is being picked up.
We chose to go with this design because it is lightweight and allows flexibility in what can be picked up. Because we don’t know the The specific dimensions and weight of the object we that will be required to pick up at competition are unknown, this design allows for flexibility to gently and accurately grab any object less than 20cm^3 and 2 kg.
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. The following image shows the grabber design when mounted to the bottom of the drone frame.
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The following image shows the grabber opening to pick up the blue cube.
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Features and Capability
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Thera-Band
The use of TheraBand Thera-Band allows for a wide range of objects of various shapes and sizes to be secured. By attaching theraband Thera-Band across two rigid frames opposite one another, the object can be centred centered between these two planes, which are then actuated shut. The elastic force of the TheraBand Thera-Band will increase as it encloses 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 the theraband 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 objecton 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 corresponding 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/. [Accessed January 2022].