Robot precision is a concept that appears simple but is, in fact, quite complex. In most vision-guided scenarios, a robot’s own precision far exceeds the precision of the vision system, rendering the impact of robot precision on the vision system negligible.
Robot Zero Point
When suspecting issues with robot precision, it is recommended to first check whether there is any offset in the zero points of the robot’s axes. Robot zero point refers to the position of the encoders’ zero points on each axis of the robot. If there is an offset in the zero points, it can cause discrepancies between the numerical values displayed on the robot’s teach pendant and the actual angles of the robot’s axes.
Robot zero points can be shifted due to external forces impacting the robot during movement or if the robot axes are rotated under power-off conditions due to external forces.
Zero Point Calibration
For some robots like ABB, you can manually align the markers on each axis and input the encoder values corresponding to the zero points from the robot’s surface nameplate into the teach pendant to calibrate the zero points.
For other robots such as KUKA, specialized EMT (encoder measurement tool) equipment must be used for zero point calibration.
Repeatability
Repeatability refers to the fluctuation in position when a robot reaches a specific point in space repeatedly. In conventional industrial robot applications such as spot welding, assembly, and material handling, high repeatability is essential to meet production requirements.
Common robot repeatability values:
Traditional industrial applications Emerging collaborative applications ABB IRB 1100 (4kg&475mm): 0.01mm ABB GoFa 5 (5kg&950mm): ±0.02mm FANUC LR-MATE (14kg&910mm): ±0.01mm FANUC CRX-10iA (10kg&1249mm): ±0.04mm KUKA KR 30 R2100 (30kg&2101mm): ±0.05mm KUKA iisy (3kg&760mm): ±0.1mm
Absolute Accuracy
Absolute accuracy refers to the deviation between the robot’s actual position and the target position when it reaches a specific point in space. One way to roughly measure a robot’s absolute accuracy is by comparing the set motion distance with the actual traveled distance.
Currently, most manufacturers only specify repeatability and do not provide information on absolute accuracy.
Robot absolute accuracy is typically at the sub-millimeter level, and it has minimal impact on most vision-guided applications. During extrinsic calibration, errors related to robot absolute accuracy, camera accuracy, thermal drift, etc., are consolidated into the camera’s extrinsic offset parameters.