Numerous industrial applications require precise robots to carry out critical tasks, such as aerospace manufacturing and metrological inspection. However, industrial robots typically do not offer high accuracy.
The inaccuracy of robots can be attributed to several error sources. These error sources fall into three categories: active joint errors, kinematic errors, and non-kinematic errors.
Active joints refer to the robot’s articulating and actuating joints, as some joints are articulated but not actuated, for example in parallel robots, these joints are called passive joints.
We have observed that the largest contributors to robot inaccuracy are active joint errors and kinematic errors. It’s conceivable that non-kinematic errors have a greater impact on applications involving high effective loads.
1. Joint Errors
These errors pertain to discrepancies associated with displacement values provided by the encoders of the robot’s active joints. They represent the difference between the motion reported by the sensors (i.e., the encoders) and the actual movement of the joints.
These differences are primarily caused by sensor errors and offsets induced by the zeroing (or homing) of each active joint, which result in errors in the zero position or fundamental location of the active joints.
2. Kinematic Errors
Kinematic factors are related to our understanding of the robot’s motion model. Models that cannot fully represent the actual geometric shape of the robot are the root cause of errors. The primary reasons for kinematic errors can be summarized as follows:
1) The difference between the nominal length and the actual length of a robot’s linkages is primarily caused by manufacturing and assembly tolerances.
2) The geometric characteristics of the robot components, such as parallelism and orthogonality.
3) Positional errors of the reference frames: these include the robot’s base reference frame relative to the workpiece reference frame (also known as the unit frame or world frame), and the tool reference frame relative to the flange frame (i.e., the final reference frame) of the robot.
3. Non-Kinematic Errors
Non-kinematic errors are attributed to the mechanical characteristics of robot components, summarized into the following key elements:
1) The rigidity of mechanical components (such as a robot’s linkages and gearboxes).
2) Mechanical clearances (for instance, gear box clearances).
3) The impact of temperature on robot structure and mechanical components.
For industrial applications requiring precise robots to accomplish critical tasks, understanding where to look for and how to reduce errors while enhancing accuracy will undoubtedly improve production outcomes.
Utilizing laser vision sensors to increase measurement precision may be one method to eliminate these deficiencies.