The task is to convert an analog flowmeter signal into a totalized volume on a Mitsubishi PLC by accumulating the flow value at a fixed 5-second interval. Because the PLC executes continuously, you need a time base that fires exactly once per interval; a self-resetting on-delay timer (TON) is the standard way to build it in ladder logic.
Totalization Principle
Volume is the integral of flow over time. In a PLC this becomes a running sum executed once per sampling period: Volume = Volume + Q * Dt, where Q is the instantaneous flow rate in engineering units and Dt is the 5-second sampling period. The method assumes the flow rate is approximately constant during each 5-second window; if the process flow changes faster than that, totalization error accumulates and you must shorten the period. The increment instruction must execute exactly once per pulse, never on every scan while the pulse bit is true.
Building the Self-Resetting TON Pulse Generator
The pulse generator works as follows: the timer input is enabled only while the pulse bit is false, so the timer starts timing. When the preset elapses, the timer done bit sets the pulse bit true. On the next scan the true pulse bit removes the timer input, the timer resets, and the pulse bit returns false, restarting the cycle. The pulse bit is therefore true for exactly one scan every 5 seconds, which is precisely what the accumulator rung needs.
In ladder, the first rung is a normally closed contact of Impulsion in series with the TON coil, and the second rung seals the timer done bit to Impulsion. A third rung uses Impulsionto trigger the addition.
Scaling the Analog Flow Signal
The analog value must be converted to engineering units before accumulation. The multiplication factor per 5-second pulse depends on the flowmeter's time base:
| Flowmeter output unit | Increment per 5 s pulse |
|---|---|
| per second (L/s, m³/s) | Q × 5 |
| per minute (L/min) | Q × 5 / 60 |
| per hour (m³/h) | Q × 5 / 3600 |
Verify against a known volume: run the process at a stable flow, compare the accumulated value over a timed interval against Q × elapsed time, and confirm the raw analog input reads steady in the PLC monitor before trusting the total.
FAQ
How do I generate a repeating 5-second pulse on a Mitsubishi PLC?
Use a TON whose input is the negation of its own pulse bit: the timer times while the bit is false, sets the bit when done, and the bit resets the timer on the next scan. The result is a one-scan pulse every 5 seconds.
How do I convert an analog flow rate into total volume in a PLC?
Add Q × Dt to an accumulator once per sampling period, where Dt is in the same time unit as the flowmeter output. For a 5-second pulse: multiply by 5 for per-second units, by 5/60 for per-minute units, and by 5/3600 for per-hour units.
Why does my timer output only stay on for one PLC scan?
That is the intended behavior of a self-resetting timer: the done bit drives the pulse bit, which immediately removes the timer input on the next scan and resets it. Use that single-scan pulse to trigger exactly one accumulation per interval.