Arms are usually attached to a tower on the robot chassis and are used to lift another manipulator on the end of the arm. Arms can also be used to lift the robot off of the ground if this is part of the game's scoring. Motors are usually mounted to the tower and drive a gear train, chain, and sprocket system which is attached to the arm. Arms frequently use rubber bands to assist with lifting.
EXP Robot arms can be assembled out of C-Channels. Arms can be just a single set of assembled metal or two arms can be paired side by side with a span between them and cross supports such as stand-offs connecting the pair.
See below for examples of a variety of arms that you can build with a VEX EXP Kit.
A single swing arm is perhaps the easiest arm to assemble. The manipulator on the end follows the arc of the swing arm motion. It is possible for a swing arm design to pass over the top of the tower and reach the other side of the robot.
However, this motion could be an issue with a passive fork, scoop, or game piece which needs to remain level.
Linkage arms involve more than one pivoting bar which makes linkages between a tower and an end tower.
- The linkages are typically built to form a parallelogram.
- When these bars and towers have the same amount of distance between their parallel linkages, they remain parallel as the arm lifts. This can keep whatever the arm is lifting relatively level. However, the arm does move in a slight arc as it lifts.
- These arms are limited in how high they lift because at some point the parallel bars will come in contact with one another.
Linkage arms include the: 4-bar, 6-bar, and chain bar. See below for examples of these robot arm variations.
The 4-bar arm is a linkage arm and is usually the easiest type of linkage arm to assemble. They consist of a tower connection, a set of parallel linkage arms, and an end tower/manipulator connection.
An example of the 4-bar arm can be found on the EXP Clawbot.
A 6-bar arm usually can reach higher than a 4-bar arm, however they extend out farther as they swing up and can cause the robot to tip over if the wheelbase is not large enough.
The chain-bar arm uses sprockets and chain to create a linkage arm. This assembly uses a Round Hole insert in a High Strength Sprocket. This sprocket is mounted to the tower and the driving shaft is passed through the tower and insert. The Round Hole inset allows the arm’s shaft to freespin. The arm is attached to a High Strength Sprocket/Chain system or High Strength Gear system with a motor which is used to raise and lower it.
Another free spinning shaft is passed through the other end of the arm. The end manipulator is mounted to a second same-size High Strength Sprocket with a metal Square Insert. This insert is used to fix the sprocket to the second shaft. When the chain is connected between the sprockets of the arm, the chain acts like a 4-bar linkage as a motor system rotates the arm.
The advantage of a chain-bar arm is it does not have two linkages coming together limiting its height, however if the chain becomes unsnapped or has a link break, the arm will fail.
For more information see the Arm Design Video and Lesson Summary in the Up and Over STEM Lab.