Bucket / ball lifting mechanisms / manipulators

Some projects require a suitable method to manipulate and lift items like buckets or balls during testing and competition. Since a variety of mechanisms exist which might be able to accomplish these tasks, it’s important to review the capabilities of the controller used for the project before further investigation. Specifically, the controller is able to control five motors independently: three under relay control (forward or reverse with no velocity control) and two under proportional control (forward or reverse with full velocity control). Assuming the two proportional channels are used to control the wheel motors on the mobile platform (which is quite common), that leaves three relay channels for all other mechanisms on the robot, such as the bucket and/or ball manipulator(s), or a door for releasing balls at the appropriate time. One of the three relay channels will typically be dedicated to the lifting arm to which the bucket or ball manipulator is attached. That means two relay channels remain which can be used to control a release door, a moveable gripper mechanism, or any other mechanism you can think of.

In general bucket and ball lifting mechanisms can be divided into the two categories noted below:

Moveable Gripper:

A moveable gripper is defined as a manipulator that uses a moveable linkage to open and close its “fingers”. Three examples of moveable grippers are shown below in figure 1.

Figure 1: Examples of moveable grippers requiring at least one motor to operate (open/close)

Figure 2 below is another example of a moveable gripper that uses gears and an electric motor to open and close.

Figure 2: Another example of a moveable gripper requiring a motor for operation

Stationary Gripper:

As the name implies, a stationary gripper is defined as a manipulator that requires no moveable parts to grab the item of interest. This type of manipulator is shown in figure 3 below. Stationary grippers are commonly found on fork lifts and work well for items such as buckets that have a large lip on the top for the stationary forks to rest against. One disadvantage with this type of gripper is the driver must be exact in his placement/alignment of the forks or gripper before raising the arm, which requires more sensitivity with the controls and caution with the selection of the drive motors and wheels.

Figure 3: Basic Forklift with stationary grippers/forks