For our grabber mechanism, we are designing a claw-like design that uses both a rigid frame and flexible Thera-Band to gently and accurately pick up objects of various sizes and weights.
The rigid portion is comprised of two slender frames, tensioned together by Elastics at the 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 Thera-Band, which is attached to each frame, creates a flexible wall to contour around objects being picked up. When the frame closes on the object, the 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. The specific dimensions and weight of the object 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. The following image shows the grabber design when mounted to the bottom of the drone frame.
The following image shows the grabber opening to pick up the blue cube.
Features and Capability
Thera-Band
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 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 Thera-Band and the object thereby opposing the gravitational force of the object.
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.
