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Undergraduate Technology Lab • ELET 285 • Laboratory 1
Laboratory 1: Open-Loop Tank Level Control via Relay Timer
Explore the principles and vulnerabilities of open-loop sequential control. In this lab, students construct a virtual relay ladder circuit that meters liquid into a 10-gallon tank using an On-Delay Timer (TON) without sensory feedback, and evaluate the catastrophic failure modes caused by load disturbances and residual liquid errors.
Core Competency
Relay Ladder Logic (RLL)
Master the foundational logic structures of industrial electrical panels: normally open (N.O.) and normally closed (N.C.) pushbuttons, interlocking control relays (CR), and the classic 3-wire seal-in memory branch.
Control Strategy
Open-Loop Timing Circuits
Learn how on-delay timers (TON) execute automated batch filling without sensory feedback, and evaluate the mathematical relationship between fluid flow rate ($Q$), valve state, and volumetric accumulation ($V = Q \times t$).
Engineering Analysis
Disturbances & Failure Modes
Witness why open-loop systems fail in safety-critical manufacturing: line pressure fluctuations, leaky discharge valves, and non-zero starting levels directly result in underfilled tanks or catastrophic liquid spills.
Interactive Open-Loop Tank Level Simulator
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 on-delay timer (TON), and the inlet solenoid valve. Then test the system under nominal conditions, external load disturbances, and initial level errors.
Open-Loop Tank Level Control Simulator
Process Tank & Instrumentation
READYRelay Ladder Logic Workbench (Drag & Drop Matrix)
IEC 61131-3 / Industrial Relay Logic- Rung 0: Drop a
NO Contact(tagPB1_START) into Slot 0, aNC Contact(tagPB2_STOP) into Slot 1, aNO Contact(tagCR1) into the Branch Slot, and aRelay Coil(tagCR1) in the Output. - Rung 1: Drop a
NO Contact(tagCR1) into Slot 0, leave Slot 1 empty (or pass-through), and drop anOn-Delay Timer [TON](tagT1) into the Output. - Rung 2: Drop a
NO Contact(tagCR1) into Slot 0, aNC Contact(tagT1_DN) into Slot 1, and aRelay Coil(tagSOL1_VALVE) into the Output. - Press START on the Operator Desk to initiate the open-loop timed filling cycle!
Laboratory 1: Structured Experiments & Observations
Execute the three sequential procedures below to evaluate open-loop performance and record data for your lab report.
Experiment 1.1
The Nominal Calibration Run
The tank has a total volume of 10 Gallons. The inlet supply pipe delivers a rapid, calibrated flow of 1.0 Gal/sec. The target fill level is 7.5 Gallons (75%) starting from an empty tank with the drain valve completely closed (0%):
t = Target Volume / Flow Rate
t = 7.5 Gal / 1.0 Gal/s = 7.5 seconds
Set the timer preset slider to 7.5 seconds and click START. Observe the liquid level stop precisely at the amber target marker when T1.DN asserts.
Experiment 1.2
The Load Disturbance Test
Empty the tank using the Empty Tank button. Next, simulate an unmetered downstream draw or leaky valve by moving the Manual Discharge Valve [MV-1] slider to 25% (0.25 Gal/s draw).
Net Inflow = Q_in – Q_out = 1.0 – 0.25 = 0.75 Gal/s
Actual Level at t=7.5s: 5.6 Gal (Severe Underfill)
Notice that because the timer has no sensor feedback, the valve closes on time, but the tank is severely short of the 7.5 Gal target.
Experiment 1.3
Initial Level Error & Spill
Close the drain valve (0%). Click the Fill to 5.0 Gal (50%) button to simulate a scenario where residual liquid was left in the tank from a previous batch cycle.
Final Volume = V_0 + (Q_in * t)
Final Volume = 5.0 Gal + 7.5 Gal = 12.5 Gal (OVERFLOW!)
Press START with the 7.5s preset. Observe the tank overflow warning flash red at $t=5.0\text{s}$, illustrating the safety hazard of open-loop automation.
Relay Ladder Logic Implementation Details
The ladder circuit used in the simulator models industrial safety standards for pushbuttons and timed actuation:
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: Timer & Valve Control
On Rung 001, a normally open contact from CR1 enables the On-Delay Timer (T1:TON). As long as CR1 remains latched, the timer accumulator (ACC) counts up toward the preset (PRE).
On Rung 002, the valve solenoid coil SOL1_VALVE is energized through CR1 and a Normally Closed contact addressed to T1_DN. When the preset time elapses, T1_DN asserts, turning the NC contact into an open circuit and de-energizing the fill valve.
Next Laboratory: Closed-Loop Sensor Feedback
Ready to solve the disturbance and overflow flaws of open-loop timing? In Laboratory 2, we replace the timer with a process-actuated discrete float switch (LSH-1) mounted at the 75% cutoff mark to achieve true closed-loop regulation.