# Confusion of terminology for control systems

I am confused about the following definitions (if there are any):

• Reference input
• reference signal
• setpoint
• reference

I don't understand specifically what they are referring to.

Do they refer to an external environmental variable, or the value we have assigned to that environmental variable?

For example, for a heating system, does "reference input" or "setpoint" refer to 20 degrees or to the voltage that corresponds to 20 degrees?

I have looked and I haven't found any clear definitions and most seem to be pretty vague.

• It depends on whether the system input is taking voltage or a temperature. All the terms you listed are synonymous in this context. – Eugene Sh. Feb 26 '19 at 19:27
• Ok lets say Im using a thermistor to analyze temperature with a wheatstone bridge. The actual analysis would be done with a voltage, and this voltage would correspond to a specific temperature. Is the reference input the temperature applied to the thermistor, or the voltage measured from the wheatstone bridge. – Colin Hicks Feb 26 '19 at 19:37
• Control system is a system which is supposed to bring the "plant" to the state given by the reference signal, not to analyze anything. Your thermistor is the input sensor and the input is the temperature. But you might want to separate it from the control system and look on it's voltage output as an input to the system. It depends on how you want to represent it. – Eugene Sh. Feb 26 '19 at 19:43
• @ColinHicks you really need to abstract things! For control system theory, it doesn't matter whether what your system deals with are currents, voltages, temperatures, pressures (there's actually pretty impressive control systems just built from hydraulics with no electronics at all!), rotational speeds, forces… What matters is that they are some entity changing over time, i.e. signals. – Marcus Müller Feb 26 '19 at 19:49
• These terms are not strictly applied. The context determines what the variables represent. – Chu Feb 26 '19 at 22:34

Reference input = Stable static reference: a DC voltage, a current source, a weight, gravity=1g, etc

• for creating any stable voltage using gain and/or offset for accurate conversion of drive or feedback signals to voltage in [units/Volt]
• e.g. provide a voltage reference = Max temp. for use with setpoint Min to Max.

Reference signal = a stable signal e.g. a frequency, an RF LO, H sync, GPS clock

Setpoint = Desired target ( or any unit of measure converted to voltage )

• e.g. a target acceleration, velocity and position jump for a servo input

Waypoint = Intermediate target

Some Control System Metrics

Steady-state error: ( null with integrators)
Step error : with specs for overshoot or settling time,
Gain + phase margin