Averaging (or equalizing) level control

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Presentation transcript:

Averaging (or equalizing) level control Course IIA1117 Control Engineering Fall 2018 Averaging (or equalizing) level control By Finn Aakre Haugen (finn.haugen@usn.no) USN. IIA1117 Control Engineering. Haugen. 2018.

Level controller (must be tuned for 2 Example of application (results shown at end of this PPT): Level control of equalization magazine upstreams the VEAS water resource recovery facility (wrrf) or resource recovery facility (wrrf), at Slemmestad, south of Oslo, Norway: Inflow Outflow is smoother than inflow! Level controller (must be tuned for "soft (compliant" control) USN. IIA1117 Control Engineering. Haugen. 2018.

Principal buffer tank with a level control system 3 Principal buffer tank with a level control system Simulator USN. IIA1117 Control Engineering. Haugen. 2018.

USN. IIA1117 Control Engineering. Haugen. 2018. 4 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. 2018.

Δhmax <= (Tc/A)*ΔFin 5 ... 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. 2018.

Comparison of responses in level h 6 Comparison of responses in level h due to step change in inflow Fin, with P and with PI controllers: USN. IIA1117 Control Engineering. Haugen. 2018.

Tc = A*Δhmax/ΔFin = 2000 m2*(-0.5 m)/(-1 m3/s) = 1000 s 7 Example Assumptions: A = 2000 m2 ΔFin = 1 m3/s Δhmax = 0.5 m Resulting Tc: Tc = A*Δhmax/ΔFin = 2000 m2*(-0.5 m)/(-1 m3/s) = 1000 s PI settings: Kc = 1/(Ki*Tc) = - A/Tc = - ΔFin/Δhmax = - (1 m3/s)/0.5 m = - 2.0 (m3/s)/m Ti = 2*Tc = 2*1000 s = 2000 s USN. IIA1117 Control Engineering. Haugen. 2018.

USN. IIA1117 Control Engineering. Haugen. 2018. Simulation! USN. IIA1117 Control Engineering. Haugen. 2018.