Glossary · PLC and control programming
Debounce
Also known as: Debouncing, Contact debouncing, Input filter
German: Entprellung
In control engineering, debouncing is the filtering of a digital signal so that the rapid, unintended transitions of a mechanical contact while it opens or closes are recognized as a single change of state.
- PLC programming
In one sentence
Debouncing filters the rapid transitions of a bouncing contact so a PLC sees one clean change of state instead of several.
Example
A push button input is filtered with a 10 ms debounce time so that one press starts exactly one machine cycle.
How it applies
- Engineering: Mechanical contacts bounce for a few milliseconds when they switch. Without filtering, a Counter can register several counts and edge detection can trigger several times. Debouncing is done by input filter settings on the I/O module, by hardware circuits or in software with a short timer.
- Commissioning: The filter time is a trade-off: long enough to hide bounce, short enough not to miss real, short pulses. Fast sensors such as encoders or high-speed counters need short or no filtering.
- Maintenance: Worn contacts bounce longer. Intermittent double counts can be a sign that a switch needs replacement.
- Documentation: Record the configured input filter times in the hardware configuration documentation. If a machine function depends on a minimum pulse length, state it in the interface description for connected devices.
Debouncing vs. plausibility check
Debouncing cleans up the timing of a single signal. A Plausibility check compares a signal with other information, such as a second sensor or the expected machine state, to detect wrong but clean signals. Neither replaces the other.