node-red-contrib-hysteresis 0.3.0
Adds hysteresis function to node-red
node-red-contrib-hysteresis
Provides a hysteresis or deadband function.
Details
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 previousmsg.payload
ANDmsg.payload
greater or equalUpper Threshold
then send outputmsg.payload
is lesser than previousmsg.payload
ANDmsg.payload
lesser or equalLower Threshold
then send outputmsg.payload
is greater thanLower Threshold
but lower thanUpper 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
Fixed thresholds allows to directly specify a Upper Threshold
and Lower Threshold
. Both values have to be valid float numbers.
Dynamic Thresholds
Dynamic thresholds expect following settings:
Topic Threshold
specifies a message topic under which a triggering point is send asmsg.payload
.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. Themsg.payload
fromThreshold Topic
plus theHysteresis+
value equals theUpper Threshold
.Hysteresis-
is the lower delta for the triggering point. Themsg.payload
fromThreshold Topic
minus theHysteresis-
value equals theLower Threshold
.Raise error on missing threshold
will create an error object in case the threshold is missing andTopic Current
value arrives. This can be handled via aCatch
node. Also see the included examples
Note: The values set in dynamic mode will typically not survive a node-red deploy or restart. With version 0.3.0 the node will save all relevant settings as context. Using a persistent context store, e.g. file, will allow the node to recover these values and continue from there.
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.
Output Options
In the node Output settings either Original Payload / Topic
or custom values can be specified.
Node Status
The node makes extensive use of status information. These can be used to react on status changes with the Status
node. Also see the included examples for a how-to.
Use cases
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.