There is a variety of ways that you can build a robot arm to add to your VEX IQ Robot. A robotic arm is a mechanism or machine that functions similarly in motion to a human arm. It can be used to pick up, move, and transport objects. Robot arms are usually attached to a tower on the 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.
Motors are usually mounted to the tower and drive a gear train or a chain and sprocket system to move the arm. Arms may also use rubber bands to assist with lifting. VEX IQ Robot arms are usually assembled out of Beams or Large Beams. Arms can be just a single set of assembled Beams or two arms can be paired side by side with a span between them. Cross supports made using Standoffs or Corner Connectors can be used to connect the pair.
See below for examples of a variety of arms that you can build with a VEX IQ Kit.
Swing Arm
A single swing arm is perhaps the easiest arm to assemble. This is the type of arm which is found on the ClawBot IQ (1st gen) build. 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
Linkage arms are arms that 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, chain bar, and double reverse 4-bar. See below for examples of these robot arm variations.
4-Bar
The 4-bar arm is the easiest arm to assemble due to its simple structure consisting of two sets of parallel bars. Its structure also increases its stability and enables that arm to have a large range of motion. The 4-Bar arm contains 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 ClawBot (2nd Gen). To construct the ClawBot, you can follow the steps in the graphic below, or view the 2D Build Instructions.
6-Bar
The 6-bar arm is an extension of the 4-bar linkage arm. It 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 the 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.
Chain-Bar
The chain-bar arm uses sprockets and chain to create a linkage arm. A capped shaft passes through the tower. A sprocket is mounted to the tower and over the cap of the shaft. This allows the shaft to spin while the sprocket remains attached to the tower. The shaft is fixed to the arm and a motor with a sprocket/chain system or gear train is used to raise and lower the arm.
Another free spinning shaft is passed through the other end of the arm. The end manipulator is mounted to a second same-size sprocket. 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.
You may need to use longer pins with spacers or short standoffs to attach the sprockets to the Beams to allow clearance for the chain.
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
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 a large Gear is mounted to the far end of the top linkage of the bottom 4-bar and another large 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. This can be accomplished by mounting the upper 4 bar to the inside of the central gear system and the lower 4 bar to the outside of the gear system
- Upper 4-bar mounted on inside of gear
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Lower 4-bar mounted on outside of gear.
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 large 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 and the most linear lift 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.
Use this link to view a 3D model of a Double Reverse 4-Bar Arm in more detail.
For more information see the Arm Design Video and Lesson Summary in the Up and Over STEM Lab.