Lessons · Industrial control · an input that will not stay still
An input that will not stay still
A signal at the edge of its range flickers, and anything that needs a condition to stay true breaks.
Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.
What it is for
It presents as an intermittent software fault that will not reproduce on demand, and it wastes days if you start in the program.
How to think about it
Let the program count the evidence for you: an accumulator that keeps returning to zero is a signed statement that the input is not steady. Then go and look at the sensor.
Worked example
[Run] [TON T1 300ms] [T1.DN] (Ready)One flicker is enough. Run on, off for a scan, on again. Ready: 0 0 0 0 1 1
Your turn
What in the program betrays a chattering input?
a timer's returning to zero
Run a rung and watch the scan
The trap
Filtering it in software instead of realigning the sensor is borrowing against the next breakdown, and the filter stays there adding delay long after everyone has forgotten why.