Provides a hysteresis or deadband function.
When a message arrives, the node will evaluate if the
msg.payload is above a defined
Upper Threshold or below a
Lower Threshold , while taking into account the previous value. Whenever this happens a
msg is send to the output. Following rules do apply:
msg.payload is greater than previous
msg.payload greater or equal
Upper Threshold then send output
msg.payload is lesser than previous
msg.payload lesser or equal
Lower Threshold then send output
msg.payload is greater than
Lower Threshold but lower than
Upper Threshold do nothing
- Once a threshold has been hit, no new output will be send until the respective opposite threshold is triggered
Fixed versus Dynamic Thresholds
In the node settings either fixed or dynamic threshold can be specified.
Fixed thresholds allows to directly specify a
Upper Threshold and
Lower Threshold . Both values have to be valid float numbers.
Dynamic thresholds expect following settings:
Topic Threshold specifies a message topic under which a triggering point is send as
Topic Current specifies a message topic under which the current values are send. This values are then matched against the respective thresholds.
Hysteresis+ is the upper delta for the triggering point. The
Threshold Topic plus the
Hysteresis+ value equals the
Upper Threshold .
Hysteresis- is the lower delta for the triggering point. The
Threshold Topic minus the
Hysteresis- value equals the
Lower Threshold .
Note: The values set in dynamic mode will not survive a node-red deploy or restart.
Send initial message
After starting node-red or deploying the flow, the hysteresis node does not know any previous values nor is able to determine the direction how the values develop.
Send initial message will simply match the first valid value against the lower or upper limit and send an output if any of the levels is exceeded respectively underran.
In the node Output settings either
Original Payload / Topic or custom values can be specified.
In control systems, hysteresis can be used to filter signals so that the output reacts less rapidly than it otherwise would, by taking recent history into account. For example, a thermostat controlling a heater may switch the heater on when the temperature drops below A, but not turn it off until the temperature rises above B. For instance, if one wishes to maintain a temperature of 20 °C then one might set the thermostat to turn the heater on when the temperature drops to below 18 °C and off when the temperature exceeds 22 °C.
Similarly, a pressure switch can be designed to exhibit hysteresis, with pressure set-points substituted for temperature thresholds.