Why are differential laws generally not used in automatic adjustment of pressure and flow?
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Why are differential laws generally not used in automatic adjustment of pressure and flow? As for temperature regulation and ingredient regulation, differential laws are mostly used? For pressure and flow, the time constant T of the adjusted object is small, and the load changes quickly. At this time, the differential effect will cause oscillation, and the integral effect cannot be too strong, otherwise it will also cause oscillation, which will have a greater impact on the quality of the adjustment. The time constants of the temperature and components being adjusted are relatively large, so differential adjustment has a leading effect and can achieve better effects on the quality of adjustment. Just like what was said on the second floor! This post was last edited by zlky2005 on 2009-3-24 10:59 ]
The main reason is that temperature adjustment has a large hysteresis, so differential adjustment must be used to make the adjusted quantity reach the set value faster. Differential adjustment also needs to be added to other physical quantities with large hysteresis, such as PH, etc.
You will benefit a lot from reading the articles written by Emerson engineers in this regard.
Can someone upstairs post this article written by an Emerson engineer?
Yes, please post it on the third floor!
Pressure and flow respond quickly, no need to add differential for overshooting
The function of differential is to give the system an effect in advance, and it can be added to systems with slow response. The pressure and flow rate change relatively quickly in the system, so it is not used
It should be because the time constants of pressure, flow and some liquid levels are very small, while the lag of temperature is often serious. Therefore, in principle, there is no need to add D for pressure, flow and liquid level.
Differential regulation is generally not recommended in pressure and flow regulation loops
change: Differential (D) control In differential control, the output of the controller is proportional to the differential of the input error signal (ie, the rate of change of the error). Automatic control systems may oscillate or even become unstable during the adjustment process to overcome errors. The reason is that there are large inertia components (links) or delay components, which have the effect of suppressing errors, and their changes always lag behind the changes in errors. The solution is to make the change in the effect of suppressing errors "ahead", that is, when the error is close to zero, the effect of suppressing errors should be zero. That is to say, it is often not enough to only introduce the "proportional" term in the controller. The role of the proportional term is only to amplify the amplitude of the error. What needs to be added now is the "differential term", which can predict the trend of error changes. In this way, a controller with proportion + differential can make the control effect of suppressing the error equal to zero or even negative in advance, thereby avoiding serious overshoot of the controlled variable. Therefore, for controlled objects with large inertia or hysteresis, the proportional + derivative (PD) controller can improve the dynamic characteristics of the system during the adjustment process.