The ornamental design for a robotcaster, as shown and described.
1. Robot caster
1.1: Perspective
1.2: Perspective
1.3: Front
1.4: Back
1.5: Left
1.6: Right
1.7: Top
1.8: Bottom
A robot caster is applied to a walking end of a robot, a foot walking of a robot, etc.; it can be used in the fields of service robots, medical robots and industrial robots; the broken lines in the drawings depict portions of the robot caster that form no part of the claimed design.