Does the deviation in self-control have a direction?

There are two definitions of deviation: the system definition and the instrument definition. System definition of deviation: Deviation = Setpoint – Measurement. Instrument definition of deviation: Deviation = Measurement – Setpoint. This difference arises from the distinctions between the two fields of study. In control systems, the setpoint represents the input to the system, while the measurement represents information within the system itself. For regulators, the measurement is their input, whereas the setpoint (typically an internal setpoint) is the information available within them. Regulators use the instrument definition for their positive and negative actions, whereas system analysis employs the system definition. Some manufacturers have now taken note of this issue; for example, Siemens’ PCS7 (which Siemens claims to be its DCS) uses the system definition for its controllers.

　　The self-control deviation has a direction; the positive and negative actions of the regulator are unrelated to the magnitude of the deviation. It is related to the direction of the deviation. 　　The definition of deviation differs between the follow-up system (externally specified) and the constant-value system (internally specified). The explanation on the 2nd floor is easier to understand. 　　The normal operation of a traditional regulator means that the output increases as the measured value rises. As for the control modules of DCS, some follow traditional concepts, while others are designed such that the output increases when the set value rises, resulting in a positive action. 　　Regardless of the definition, the purpose of setting a feedback mechanism is to make the control loop a negative feedback system. 　

What is the relationship with the direction of the deviation? Assume PV is equal to 20 and SV is equal to 50. As PV continues to increase, if the controller is of the positive-action type, the output increases; the error remains negative at all times ； If the PV starts at 60 and keeps increasing, why does the controller’s output also increase, yet the error remains positive?

Do you mean that when the given value is fixed, the controller’s output depends only on whether the measured value increases or decreases? It has nothing to do with whether the deviation is positive or negative, right?

Positive-action regulator: Error ↑ → Output ↑. Error = Measurement – Setpoint; in other words, Measurement ↑ → Error ↑ → Output ↑. Setpoint ↓ → Error ↑ → Output ↑. Measurement ↓ → Error ↓ → Output ↓. Setpoint ↑ → Error ↓ → Output ↓. The output of a positive-action regulator changes in the same direction as the measurement, but in the opposite direction to the setpoint. Negative-action regulator: Error ↑ → Output ↓. Error = Measurement – Setpoint; in other words, Measurement ↑ → Error ↑ → Output ↓. Setpoint ↓ → Error ↑ → Output ↓. Measurement ↓ → Error ↓ → Output ↑. Setpoint ↑ → Error ↓ → Output ↑. The output of a negative-action regulator changes in the opposite direction to the measurement, but in the same direction as the setpoint

The explanations on the second and third floors were very clear. In actual production processes, positive and negative deviations are likely to occur alternately; this is related to the skill level of the operators as well as the operating conditions, and it is also connected to the settings of the PID parameters. As instrument technicians, our main task is to set the action modes of the regulators and the PID parameters properly, without needing to worry too much about positive and negative deviations.

　　:L, having talked about this *so often, always overlooks this point: the direction of the deviation refers to the variable of the deviation (the direction of change), not its arithmetic value. 　　 The system changes from PV equal to 20 and SP equal to 50 to PV equal to 50 and SP equal to 50. At this point, the deviation (as defined by the deviation meter on the second floor: Deviation = Measurement – Setpoint) increases from -30 to 0, meaning the variation is +30 ； 　　The system changes from PV equal to 50 and SP equal to 50 to PV equal to 80 and SP equal to 50; at this point, the deviation rises from 0 to 30, with the change amount also being +30 ； 　　Under the above conditions, the OP of the positive-action regulator always increases. 　　Since the \"direction of deviation\" is a variable, different definitions of the deviation value arise when discussing given regulation (internally given) and follow-up regulation (externally given); in fact, both of these different definitions of deviation stem from the definition of the deviation variable.

There are things like thermobaric compensation; as for the direction, it depends on what kind of physical quantity it is

Directed: Determine the forward and reverse actions of PID based on the deviation