Averaging (or equalizing) level control

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Presentation transcript:

Averaging (or equalizing) level control Course IIA1117 Control Engineering Fall 2017 Averaging (or equalizing) level control By Finn Aakre Haugen (finn.haugen@usn.no) USN. IIA1117 Control Engineering. Haugen. 2017.

Buffer tank with a level control system 2 Buffer tank with a level control system Aim: Averageing (or equalizing, or attenuating) inflow variations so that the outflow becomes smoother than the inflow. USN. IIA1117 Control Engineering. Haugen. 2017.

USN. IIA1117 Control Engineering. Haugen. 2017. 3 How to tune LC? We need a sluggish or soft or compliant LC so that the liquid volume (the level) can take up the inflow variations. Ziegler-Nichols is useless here since it gives fast or stiff control :-( But Skogestad is excellent, using Tc as tuning parameter :-) Kc = 1/(Ki*Tc) Ti = 2*Tc where Ki = -1/A is the integrator gain or normalized process step response. How to select Tc? ... USN. IIA1117 Control Engineering. Haugen. 2017.

Δhmax <= (Tc/A)*ΔFin 4 ... How to select Tc? As a start, assume P (proportional) level controller. It can be shown, from a mathematical model of the level control system, that Δhmax = (Tc/A)*ΔFin where Δhmax is corresponding maximum allowed level change (in steady state) after max inflow step change, ΔFin. With a PI controller with the same Tc: Δhmax <= (Tc/A)*ΔFin Solving this inequality for Tc gives Tc >= A* Δhmax/ΔFin => Specification of Tc in the PI settings: Tc = A* Δhmax/ΔFin USN. IIA1117 Control Engineering. Haugen. 2017.

Comparison of responses in level h 5 Comparison of responses in level h due to step change in inflow Fin, with P and with PI controllers: USN. IIA1117 Control Engineering. Haugen. 2017.

Example Assumptions: A = 2000 m2 ΔFin = 1 m3/s Δhmax = 0.5 m 6 Example Assumptions: A = 2000 m2 ΔFin = 1 m3/s Δhmax = 0.5 m Resulting Tc: Tc = A* Δhmax/ΔFin = 2000*(-0.5)/(-1) = 1000 s PI settings: Kc = 1/(Ki*Tc) = -A/Tc = -2000 m2 / 1000 s = 2.0 (m3/s)/m Ti = 2*Tc = 2*1000 s = 2000 s USN. IIA1117 Control Engineering. Haugen. 2017.

USN. IIA1117 Control Engineering. Haugen. 2017. Simulation! USN. IIA1117 Control Engineering. Haugen. 2017.