Discussion on PID deviation – input value – output value
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If the PID is of the direct-action type, the larger the deviation value, the greater the output signal. If the PID is of the reverse-action type, the larger the deviation value, the smaller the output signal. I would like to discuss how to define whether a deviation is large or small. Taking the pressure at the top of the tank (with a set value of 1 MPa) as an example, when the pressure is 0.1 MPa above the set value or 0.1 MPa below it, which of these deviations is larger? By the way, how is the relationship between the deviation and the input signal determined?
In the field of automation, there are two definitions of deviation: the system definition and the instrument definition. 1. The systematic definition of deviation is the one from control theory, and it is used in system analysis. Systematic definition of deviation: Deviation = Setpoint – Measurement, that is, the setpoint of the system minus the measured value of the controlled variable. 2. The instrument definition of deviation is related to the definition of a controller, and it is used in the analysis of the controller’s positive and negative actions. Definition of deviation for instruments: Deviation = Measurement – Setpoint, that is, the value obtained by subtracting the setpoint from the input measured by the controller. The inconsistency in the definition of deviation lies in the different definitions across various disciplines. Because in system analysis, the given values of the system represent its inputs, while the measured values of the controlled variables (i.e., the feedback signals) constitute the internal information of the system ； For a controller, the given values are internal information of the controller, while the measured values are external inputs to the controller. Now some manufacturers are trying to standardize the definition of deviations; for example, in Siemens’ PCS7 system, the forward and reverse actions of the controller follow the definitions set by the system.
Is that description from the book? So he must be treating positive and negative deviations as positive and negative values respectively; therefore, the examples you gave should be +0.1 and -0.1, making it clear at a glance which one is what. What do you mean by deviation and input signal?
Regarding the forward and reverse actions of controllers, most manufacturers use instrument definitions. An analysis can be done as follows: For a positive-action controller, an increase in its error input leads to an increase in the output. Since its error is the measurement minus the setpoint, it can be inferred that an increase in the measurement results in an increase in the output, which is characteristic of positive action; similarly, a decrease in the setpoint also leads to an increase in the output, which is also positive action. It can be expressed as: the output change of a positive-action controller is in the same direction as the measured signal, and in the opposite direction to the setpoint signal ； A feedback controller has such a behavior that as its error input increases, the output decreases. Since its error is the measurement minus the setpoint, it can be inferred that an increase in the measurement leading to a decrease in the output constitutes feedback, or a decrease in the setpoint resulting in a decrease in the output also constitutes feedback. It can be expressed as: the output change of the feedback controller is in the opposite direction to the measured signal, and in the same direction as the setpoint signal.
Why do I feel that your description is incorrect?
Deviations can also be positive or negative, which eliminates any ambiguity as to whether the deviation is large or small.
““Relationship between deviation and input signal”: Answer: Measurement deviation = measured value corresponding to the real-time signal value – given value; Signal deviation = real-time signal value – signal value corresponding to the given value. :lol
I would like to ask: is the relationship between the output signal of 4-20mA and the valve position of 0-100 linear? Are there any other functional relationships?
There is a linear relationship between the output signal of 4-20mA and the valve position of 0-100. There are four types of relationships between the 4-20mA signal and the relative flow rate passing through the valve: linear relationship, logarithmic (equal percentage) relationship, square root relationship, and fast-opening relationship. Among them, linear relationships and logarithmic (equal percentage) relationships are the most common.