difference between the target (desired) output of the system and the actual achieved output
NOTE 1 It is denoted by eP
NOTE 2 Usually this is time dependent.
NOTE 3 Depending upon the system, different quantities can be relevant for parameterising the performance error. A degree of judgement is used to decide which is most appropriate.
NOTE 4 For example: The difference between the target and actual orientation of a frame can be parameterised using the Euler angles for the frame transformation or the angle between the target and actual orientation of a particular vector within that frame.