Signals and Process

DC current signals are also used in control systems to command the positioning of a final control
element, such as a control valve or a variable-speed motor drive (VSD). In these cases, the milliamp
value does not directly represent a process measurement, but rather how the degree to which the
final control element influences the process. Typically (but not always!), 4 milliamps commands a
closed (shut) control valve or a stopped motor, while 20 milliamps commands a wide-open valve or
a motor running at full speed.
Thus, most industrial control systems use at least two different 4-20 mA signals: one to represent
the process variable (PV) and one to represent the command signal to the final control element (the
“manipulated variable” or MV):

The relationship between these two signals depends entirely on the response of the controller.
There is no reason to ever expect the two current signals to be equal, for they represent entirely
different things. In fact, if the controller is reverse-acting, it is entirely normal for the two current
signals to be inversely related: as the PV signal increases going to a reverse-acting controller, the
output signal will decrease. If the controller is placed into “manual” mode by a human operator,
the output signal will have no automatic relation to the PV signal at all, instead being entirely
determined by the operator’s whim.