When designing a claw to pick up the large foam balls, two key points need to be considered; stability, and weight. The claw does not need to be complex to complete its task, but the more advanced teams can get creative and have very unique designs. The claw needs to be reliable and consistent because the possible 30 or even 90 points for a water ball needs to be scored every time for the team to do well.
The claw has to be designed to each teams specifications. Some teams will have minimal room to work with, so the claw will have to be smaller; but, other teams may have the claw as the main aspect as their robot and will have plenty of room to work with. Another thing to consider is making the claw slightly oversized to help ensure the ball is in the claw every time. The mechanic also has to consider the ability of his or her programmers and whether or not they will be able to find the ball accurately enough for the claw. A bigger claw can help compensate if the programmer can not be that precise.
There are two main aspects to the claw, the lift and the grab. The simplest method to raise and lower the claw is by just screwing a lift arm to the servo arm and having the serve rotate the arm. The more complex teams could utilize a geared lifting, or even a parallelogram design. The other aspect is in the grab itself. There are two main options for this, the more complex parallelogram system, or the simpler pincer type. Both have advantages, the parallelogram can fold in and be fit into very small spaces, but it can be very challenging to build for new teams. The pincer is good because it is very simple to build and it is easy to repair if it breaks during a run. The key to the design is to make sure it will work before it is built. It is a hassle to build the whole claw to discover it won't lift, or it is too small. These all have to be tested before the final build.
The claw needs to be rigorously tested before put into use in the game. It needs to be tested grabbing the ball in many different places in the claw. It also has to grab it at different heights and lift the ball to different heights. Another option not highly considered is closing at a different speeds. Sometimes closing the claw slower (utilizing the technique in the programming article on servos), can help to secure the ball in the claw. The tests of the claw need to be successful and can pick up the ball every time before it is put into action on the robot itself.
Here is an eDrawings of a basic water claw.
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