I don't like the two definitions, but let me try to explain them. I could be wrong or others may make improvements or suggestions or not.
1) Non-measured error If you do measure and compare it, then it is not possible to control it with feedback and measure the error
The 3 control systems are : Design, Process and Quality systems each with measurable inputs/outputs and verified outputs with error limits. Hint: all the best designs start with good specs. Good statistics are useful and unavoidable.
2) Measured errors (Steady State or otherwise)
You might be able to measure the error, but unable to control or eliminate it to some acceptance criteria due to undue constraints or you can measure it and have well defined targets for ramp, step, impulse (finite) or sinusoid.
Steady State error assumes there is no input change and the response does not change after an extended period of time and no measurement errors.
Actuator Error is "not specified" because it is to vague and can be caused by anything, measurable or not but presumed to be operating within its design environment and load.
This could be; Position error, velocity Error, voltage error, acceleration error, pressure error, impulse error, timing error, jitter error, phase error.
What is an actuator error?
I say it is the difference between an input and the measured feedback response + undetectable error ( but exists) + uncertainty in the feedback measurement. ( which can be anything from dynamic to steady-state)
My error signal list includes;
- dynamic, steady-state, unstable ( oscillating, or noisy)
- measurable or not measurable but known by effects and deduction
- from measurement error or actual measured error
- a result of a change in desired input or an initial condition.
- a result of an undesirable disturbance , transient, step, impulse or steady-state or initial condition.
- a result of sub-optimal or optimal system design for loop gain and phase response.
- a result of unknown causes or known causes from failures in component(s), software code errors, system failure, design error, cost reduction, compromises in performance, aging, degradation,
- caused by environmental stress, e.g.; ground-benign, automotive, airborne, seaborne or aerospace ( climatic, mechanical, electrical, electromagnetic etc)
- environmental stress such as a pothole, an arc welder, a geomagnetic storm, thermal shock, earthquake or transport vibration, low air pressure from high altitude, radar pulse and ESD discharge, a mobile phone RF sync packet , etc.etc.etc.
It can also be nominal or within error limits with x-sigma deviation or guaranteed within limits under specific conditions like 25'C and other limits for specific environmental limits, or have some error sensitivity with temperature or any other variable.
What did I leave out?
Cost control errors? Quality errors (escapes), MTBF errors?