Articulated Robots

The most commonly used robot is the articulated arm which comes in 4, 5, and 6-axis versions. Each axis rotates the next segment relative to a previous segment. This gives you varying degrees of motion. A 4 axis generally can only move in space while maintaining roll and pitch. The 4th axis allows for rotation in yaw. This is most commonly used in packing and palletizing/de-palletizing as boxes or bags don’t normally need to be turned other than in yaw. A 5-axis robot is normally a 6-axis robot design with one axis removed. This allows for a more compact arm design than the traditional 4 axis. It does this by removing the linkage that keeps the robot attachment level and instead levels it with a motor. This also allows for a few tricks a 4-axis robot can’t do like picking from a bin that is angled toward the robot. A 6-axis robot is very common as it can be used in almost any application. Some examples include deburring, machine tending, and order picking. They can move in space and rotate about any direction.

Delta Robots

Delta robots specialize in high speed motion. With their light weight arms and low payloads, they can be a blur to watch! They are most commonly used in robotic packing applications and come in 3, 4, and 6-axis versions. However, a 4 axis is the most common.

PAYLOAD

Payload is the term used to describe how much the robot is rated to articulate. There are a few factors to determine how much payload your robot needs. To find your robot payload, ask yourself the following questions:

1. How much does the product weigh? Will the robot need to lift multiples to make rate?
2. How much will the tooling weigh? Normally, the tool will weigh 2-3 times the product, given there is a lower limit when you get into lightweight products (under a few pounds).
3. How far away from the mounting surface will the center of gravity be? A load that is suspended out will require a stronger robot than a load near the face plate.

Also, it is important to note that a robot using half its payload will run faster than the same robot at its payload.

REACH

The other side of the robot selection coin is reach. Payload and reach are normally balanced in that when you want one of more, you get less of the other within each robot series. An example of this is a robot that goes from a payload of 130 kg (286 lbs) at 3.2 meters (126 inches) of reach from the center of the robot to 235 kg (517 lbs) at 2.55 meters (100.4 inches) of reach. This is due to the same principal as holding a dumbbell at you side verses at arm’s reach. It takes more effort to hold that weight out further. Reach is normally measured to the center of the “wrist” of the robot, but this is not a foolproof way to tell if your robot will reach. Other items to take into consideration are:

1. Elevation. If you are reaching from up high to down low, the reach might not be there at both. Most reaches look like spheres or doughnuts (Taurus).
2. Orientation. If you have to angle the part or tool to perform the job, even with the wrist inside the reach, sometimes an interference or even inside reach limit can make it unreachable. This is checked by simulations.
3. Tooling. A long tool can help lengthen the reach of the robot. There is a limit to how much reach can be added this way.

SPEED

The robot must be fast enough to keep up with production, or what good is it? The only way to get a good estimate of how long it will take to complete a task is to simulate it. You can look at the robot’s stated spec for how fast each axis can move, but the robot isn’t always moving at that speed due to acceleration limits. A robot can’t reach full speed if it needs to move two inches (in most cases).

CONCLUSION

The three sections above are the largest factors in selecting which mechanical robot arm to use. Our trained robotic application engineers look at these factors and others when choosing a robot for a given task. If there is concern, a simulation can be done to verify cycle time and reach. If you need assistance with selecting a robot for your next project, please contact us.

Author: Colin Shipley