Threshold Switch
The Threshold Switch (HYST), also called a two-point controller in control engineering, turns a numeric tag into a boolean tag. It switches on when a value reaches a turn-on threshold and off when it reaches a turn-off threshold.
Function parameters
| Source | Numeric tag that is monitored (e.g. temperature, pressure or level). |
| Turn-on threshold | Value at which the output switches on. |
| Turn-off threshold | Value at which the output switches off. |
How it works
Between the two thresholds, the output keeps its previous state. This gap is called hysteresis and prevents the output from constantly switching back and forth when the value fluctuates around a single limit.
Depending on which threshold is higher, there are two use cases:
- Turn-on threshold above the turn-off threshold: The output switches on with rising values. Cooling example: turn-on threshold 8 °C, turn-off threshold 6 °C.
- Turn-on threshold below the turn-off threshold: The output switches on with falling values. Heating example: turn-on threshold 19 °C, turn-off threshold 21 °C.
Notes
- A larger gap between the thresholds leads to less frequent switching, but to larger fluctuations of the monitored value.
- To ignore short peaks, a Moving Average can be used as the source, or an on delay can be added downstream.