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Undergraduate Technology Lab • ELET 285 • Laboratory 2
Laboratory 2: Closed-Loop Level Control via High-Level Switch (LSH-1)
Transition from blind open-loop timed sequence control to sensor-regulated negative feedback. In this laboratory, the open-loop relay timer is replaced by a physical float switch—LSH-1 mounted at the 75% mark (7.5 Gallons). Students verify how discrete closed-loop feedback guarantees exact volumetric fill regardless of supply pressure shifts or leaky downstream valves.
Core Architecture
Closed-Loop Negative Feedback
Compare blind timing with direct sensor feedback. When the physical state of the process dictates control actions, variations in flow velocity, pressure, and initial volume are automatically compensated.
Instrumentation & Sensors
Level Switch (LSH-1) & Pilot Lamp
Integrate discrete process-actuated float switches (LSH-1) and panel pilot indicators (PL-1) directly into hardware interlocking chains and relay ladder logic control rungs.
Engineering Analysis
Disturbance Rejection & Safety
Demonstrate robust immunity to process disturbances: fluctuate supply flow rates and induce continuous drain leaks, proving that the liquid level always halts precisely at the calibrated float height.
Interactive Closed-Loop Tank Level Simulator
In this closed-loop workbench, construct a 3-rung relay ladder circuit using the drag-and-drop matrix below. Wire up a 3-wire Start/Stop seal-in latch, an interlock rung driving the inlet solenoid valve with a Normally Closed contact from LSH1, and an indication rung powering the Tank Full pilot light PL-1.
Closed-Loop Tank Level Control (Level Switch LSH-1)
Closed-Loop Feedback System
READYLSH-1) directly measures the liquid level. Regardless of changes in inlet water pressure or downstream drain leakage, the sensor guarantees cutoff at exactly 7.5 Gallons (75%)!
Relay Ladder Logic Workbench (Sensor Feedback Matrix)
IEC 61131-3 / Closed-Loop Relay Logic- Rung 0: Drop a
NO Contact(PB1_START) into Slot 0, aNC Contact(PB2_STOP) into Slot 1, aNO Contact(CR1) into the Branch Slot, and aRelay Coil(CR1) in the Output. - Rung 1: Drop a
NO Contact(CR1) into Slot 0, aNC Contact(LSH1) into Slot 1, and anOutput Coil(SOL1_VALVE) into the Output. (Because LSH1 is untripped at rest, this NC contact conducts power until water touches 7.5 Gal!) - Rung 2: Drop a
NO Contact(LSH1) into Slot 0 and anOutput Coil(PL1_FULL) into the Output to light up the green “FULL” annunciator when the float switch trips. - Press START and adjust the inlet supply flow or open the drain valve: notice how closed-loop control guarantees accurate fill every single time!
Laboratory 2: Structured Experiments & Disturbance Rejection
Execute the three sequential procedures below to test closed-loop disturbance rejection and record data for your lab report.
Experiment 2.1
Nominal Closed-Loop Fill
With an empty tank and drain valve closed (0%), load the Standard Solution or wire Rung 0 (Start/Stop Latch), Rung 1 (CR1 + LSH1(NC) → SOL1_VALVE), and Rung 2 (LSH1(NO) → PL1_FULL).
Level < 7.5 Gal: LSH-1 Normal → SOL-1 ENERGIZED
Level ≥ 7.5 Gal: LSH-1 TRIPS → SOL-1 CUT OFF, PL-1 ON
Press START. Watch the water rise. The instant it hits the 7.5 Gal mark, LSH-1 mechanically actuates, opening Rung 1 to cut off inflow immediately, while turning on the green Tank Full pilot light PL-1.
Experiment 2.2
Supply Pressure Disturbance
Empty the tank. Use the Inlet Supply Flow Rate slider to simulate upstream pressure changes: test both a low supply rate (0.5 Gal/s) and a high supply surge (1.8 Gal/s).
At 0.5 Gal/s: Fills in 15.0 s → Stops at exactly 7.5 Gal
At 1.8 Gal/s: Fills in 4.17 s → Stops at exactly 7.5 Gal
Unlike Lab 1’s timer (which would have yielded 3.75 Gal or a spill), the closed-loop switch ignores the elapsed time and triggers strictly on physical liquid position.
Experiment 2.3
Drain Disturbance & Residual Level
Repeat the test with the manual discharge valve open to 25%, or start with 5.0 Gal residual level in the tank.
Drain Leaking (0.25 Gal/s): Fills in 10.0 s → Reaches 7.5 Gal
Pre-filled to 5.0 Gal: Fills in 2.5 s → Cuts off safely (NO SPILL)
The closed-loop architecture prevents the dangerous spill observed in Experiment 1.3, shutting off flow as soon as the float switch trips at 7.5 Gal.
Comparative Analysis: Open-Loop vs. Closed-Loop Level Control
Compare key operational and safety metrics between the open-loop timed sequence examined in Lab 1 and the closed-loop feedback architecture examined here in Lab 2:
| Metric / Parameter | Lab 1: Open-Loop (Relay Timer) | Lab 2: Closed-Loop (LSH-1 Switch) |
|---|---|---|
| Feedback Mechanism | None (Operates blindly on time $t$) | Direct process sensor (Float switch LSH-1 at 75%) |
| Fill Cutoff Trigger | Timer reaches Preset value (T1_DN = 1) |
Liquid contacts sensor float (LSH1 = 1) |
| Supply Pressure Variation | Fails: results in underfill or overflow | Compensated: fill time adjusts; level is exact |
| Discharge Leak Disturbance | Fails: level falls short proportional to leak | Compensated: continues fill until level hits 7.5 Gal |
| Residual Starting Level ($V_0 > 0$) | Hazardous: tank overflows and spills liquid | Safe: shuts off immediately upon reaching 7.5 Gal |
| Instrumentation Cost | Lower initial cost (No vessel penetrations) | Requires level sensor mount, wiring, calibration |
| Typical Industrial Role | Batch metering when fluid source is tightly regulated | Critical level regulation, high-level spill prevention |
Relay Ladder Logic Implementation Details
Compare the schematic structure of the open-loop timer circuit with the closed-loop level switch circuit:
Rung 000: 3-Wire Seal-In Control
The momentary green Start button (PB1_START, N.O.) is wired in parallel with an auxiliary contact from control relay CR1. When Start is pressed, CR1 energizes and latches itself ON through the seal-in branch.
Fail-Safe Stop Button: Physical stop pushbuttons use Normally Closed (N.C.) contacts. In software ladder logic, a Normally Closed instruction (--[/]--) evaluates to True when unpressed, and opens when the button is hit, breaking the rung and dropping the latch.
Rungs 001 & 002: Sensor Interlocks
Rung 001 (Valve Solenoid): A normally open contact from CR1 provides running permissive, wired in series with a Normally Closed contact from LSH1 (--[/]--). While the level is below 7.5 Gal, the switch is un-actuated (False), so the contact conducts power to SOL1_VALVE. When 7.5 Gal is reached, the float trips, opening the contact and shutting off the valve.
Rung 002 (Pilot Light): A Normally Open contact from LSH1 (--[ ]--) connects directly to pilot light coil PL1_FULL, providing local panel indication of target level attainment.
Review Open-Loop Timing Control
Need to contrast this behavior with open-loop timing? Return to Laboratory 1 to explore on-delay timer logic, nominal flow timing calculations, and load disturbance failure modes.