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Singularity in Robotics

Stanford University_052922A
[Stanford University]


A singularity is a specific point in the robot workspace that causes the robot to lose one or more degrees of freedom (DoF). When the robot's Tool Center Point (TCP) moves into or approaches a singularity, the robot stops moving or moves in an unexpected manner. 

Remember, a robot's degrees of freedom are the number of independently controllable joints it has. So a 6 DoF robot - like most industrial robots - has 6 independently movable joints. When a 6 DoF robot goes into singularity, one or more of its joints will effectively become useless, turning it into a 4 or 5 DoF robot.

  • A robot is a physical device with physical limitations. For example, each motor of a robot has a maximum speed.
  • Robot movement is controlled by algorithms and mathematics without physical constraints. For example, it is mathematically valid for a joint velocity to be "infinity".

The conflict between these two facts can cause a host of problems when you're programming a robot. If you're not careful, the control algorithm can instruct the robot's motors to perform physically impossible movements. This is basically what happens when a robot encounters a singularity. Robots try to do impossible things like move at infinite speed.

There are 3 basic singularity types in industrial robots: wrist, elbow, and shoulder singularities. This classification is a bit simplistic. However, this is useful because these are the 3 types of singularities that you will most often encounter in industrial robotics when you use most standard 6 DoF manipulators.


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