Why are 6-axis Robot Arms so Common?
In the typical articulated arm structure, six axes of motion are often seen as a standard design target. Why is six that target, and what are the benefits and losses from more or fewer axes?
Industrial robotics has no shortage of makes, models, colors, shapes, and sizes. But when it comes to mechanical designs, many similarities are seen across all brands.
Industrial articulated robot arms are often seen as the image portraying a high-tech manufacturing facility. Even the common collaborative robot (cobot) valentines adhere to the same mechanical structures—transporting payloads, stacking, sorting, palletizing, assembling, and a host of other repeatable tasks.
Video used courtesy of RobotWorx
Across all brands and models, one common characteristic stands out: six axes of motion. Some models have more, and many have less, but 6-axis seems to be the standard target. What makes this number so common? This leads to two related questions: Is fewer than six axes going to be a bad investment; likewise, if six is good, should seven or more axes be preferred?
Ideal 6-axis Range of Motion
In simple terms, a robot arm having six points around which motion can occur leads to the ability for the gripper to reach anywhere inside the total working radius and reach any point with the gripper (end-of-arm-tool or EOAT) at any orientation.
Figure 1. FANUC’s S-6, one of their 6-axis robots. Image used courtesy of RobotWorx
There are a few limitations to this simplified answer. First, the total working area (or envelope) can only be reached with the gripper fully extended, like if you were to stretch out your arm to reach an object at a distance. If the gripper angle changes, this certainly will limit the total reach. Additionally, and probably quite obviously, the gripper cannot extend into the solid metal base of the robot. Hard and soft axis limits may prevent such collisions.
The axes in a typical articulated arm are revolving joints, so they spin around an axis defined by the center of the motor or drive pulley. In contrast, some robot varieties such as a “delta,” or SCARA type robot use linear sliding axes of motion. But articulated arms are rotational, meaning the joint position is an angle measurement rather than a linear distance.
So this “ideal” 6-axis robot can reach any possible point within its work envelope with any EOAT orientation, making them the ideal choice if your workpiece exists at an angle that is not parallel to the robot’s base.
Disadvantages of Robots With Fewer Axes
This question may arise in designing a system where a 4- or 5-axis model may be available for a lower cost: Will five axes be sufficient for my application? This isn’t a simple yes or no answer, but one factor will certainly be cause for investigation.
Suppose the robot will work with a piece or pick/retrieve an item from a location not parallel to the ground on which the robot sits. Then, you should carefully consider whether five axes will work.
Imagine a scenario where a level conveyor feeds product to a robot squarely secured to the floor, then picks it up and transfers it to another level conveyor. In this instance, even a 4-axis robot may be sufficient. If the end gripper must also be rotated to orient the product, then a fifth axis must be employed.
Figure 2. A robot working on a conveyor system.
In a more complicated scenario, a tray sitting at 45 degrees from the ground feeds the product that the robot must retrieve. This might only be possible with a 6-axis robot, unless this try is aligned perfectly so that the sixth axis is not required.
Certainly, the situation is not as easy to define as yes or no for every situation. Still, this test of whether the objects are oriented at anything other than parallel with the robot base would indicate that care should be taken.
If the robot is intended for welding, glue application, or any other path-following application, it almost certainly must be six axes.
Benefits of Robots With More Axes
Add-on axes are usually installed on a case-by-case basis. One common example of a seventh axis is a sliding table on which the robot can traverse, allowing it to perhaps simultaneously tend multiple CNC machines, or perform different steps in a long assembly line process. With a bit of added complexity in programming, it can save massive amounts of investment over the alternative of buying multiple robots.
Figure 3. A 6-axis robot arm working with a high-precision CNC laser.
Sometimes, the sliding axis is vertical rather than horizontal, which might allow a palletizing robot to stack higher, or a storage/retrieve system to pick from bins high off the ground. Other times, rather than the robot itself mounted to a mobile base, the workpiece may sit on a rotating table. This can allow a stationary robot to reach a much wider range.
Another example is when two 6-axis arms are mounted on a pedestal, forming an almost humanoid torso. These robots may boast axis numbers of 12 or more, since they are formed from 6-axis arms.
All of these robots have distinct advantages, and they may prove to be a worthy investment. But if extra axes are not necessary, some might be overly complex and more prone to failures. In all cases, it is worth seeking the advice of professionals to investigate the process and the environment to determine which robot is best for each application.