How to Decide on a Robot Arm

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, or a Turntable Bearing which is attached to the arm. Arms frequently use rubber bands or latex tubing to assist with lifting.

Robot arms can be assembled out of structural metal such as Rails, C-Channel, or Angles. 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 connecting the pair.

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 they are lifting relatively level, however these arms are limited in how high they lift because at some point the parallel bars will come in contact with one another.

There are a number of different types of arm assemblies some include:

Swing Arm

A single swing arm is perhaps the easiest arm to assemble. This is the type of arm which is found on the Cortex ClawBot build. The manipulator on the end follows the arc of the swing arm motion and this could be an issue with a passive fork. scoop, or game piece which needs to remain level. However, it is possible for a swing arm design to pass over the top of the tower and reach the other side of the robot.

Small swing arms can be attached to the end of a primary arm. These are sometimes referred to as a wrist. An example of wrists can be found on the Cortex Super Claw build and the V5 builds, Flip and Super Flip.

Single Swing Arm Wrist

4-Bar Arm

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 V5 builds, the V5 ClawBot and the V5 Lift.

4-Bar

6-Bar Arm

The 6-bar arm is an extension of the 4-bar linkage arm. This is accomplished by using a longer top bar and an extended end bar on the first set of linkages. The longer bar serves as the bottom linkage for the second set of linkages and extended end bar serves as a “tower” for the top two remaining linkages.

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.

6-Bar

Chain-Bar Arm

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 shaft is fixed to the arm and a motor with a High Strength Sprocket/Chain system or High Strength Gear system 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 chain is connected between the sprockets of the arm, the chain acts like a 4-bar linkage as a motor system rotates the arm.

Chain-bar arms are usually assembled as pairs to equalize the forces on the arms. 

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.

Double Reverse 4-Bar Arm (DR4B)

The double reverse 4-bar arm requires the most planning and time to assemble. They are nearly always assembled as pairs to equalize the forces on the arms. The assembly of these arms starts with a four bar linkage. The end linkage serves as a second tower for a top set of four bars.

Typically an 84T High Strength Gear is mounted to the far end of the top linkage of the bottom 4-bar and another 84T Gear is mounted to the near end of the bottom linkage of the top 4-bar. As the arm is lifted the two gears intermesh moving the top set of 4-bars in the reverse direction to the bottom set, extending the arm up.

When designing a double reverse 4-bar arm it is important to provide clearance so the top 4-bar can pass either to the inside or to the outside of the bottom 4-bar. Providing as many cross supports as possible between the pair of arms will help keep the arms stable.

Many double reverse 4-bar designs mount the lift motor(s) with a 12T gear to the second tower and drive the 84T gears on the lift. Although, they can be lifted with motor(s)/gear systems on the stationary towers attached to the chassis or both locations.

Double reverse 4-bars can have the highest reach of all the arms discussed. Due to the possible extreme height which can be reached with this design, caution needs to be used when driving the robot with the arm fully extended or the robot may tip over.

Double Reverse 4-Bar (Bottom Tower Motor Mount) Double Reverse 4-Bar (Center Tower Motor Mount)

Double Reverse 4 Bar Motion: