Confusion of terminology for control systems
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
  Step load regulation error  proportional to source/load impedance
  Input regulation error
  Deadband = mechanical, electrical or thermal hysteresis where the  output does not respond to some small input range, This can cause problems as the gain=0 or create solutions such as hysteretic regulators and de-noising logic signals    - e.g. a thermostat may have a deadband of +/- 0.5'C
  Deadtime = time constant due to a deadband for a fixed or variable interval
  Saturation: When an output has reach its min or max level and the gain becomes = zero


If using a digital control system, then everything is numerically controlled (NC)  with analog interface, where required.