Hysteresis

Function

On/off controller that generates a digital output depending on the deviation of the controlled variable from the reference variable.



2PT Example

This example shows how to control a domestic water storage tank with one sensor. The control action opens the hot water valve and releases the charging pump when the actual value of the hot water temperature is less than the setpoint by the amount of the hysteresis. The following diagram illustrates the control strategy for this example:

Fig. 135.   Hysteresis application example


An additional condition is that the system supply temperature of the heat generators is larger than the setpoint plus an increase of 5 K to compensate the losses in the pipes and to prevent the hot water storage tank from cooling down because of an excessively low flow temperature, instead of being charged (summer case). This condition is implemented using an Addition and a Compare statement.


The valve starts immediately after the DHW_Temp is lower than the setpoint by 5 K realized with the Hysteresis statement and if the system supply temperature is higher than the DHW_setpoint plus 5K realized with a compare statement.

The valve will be closed with a switch-off delay of 200 s, after the system supply temperature drops below the DHW_Temp_Sp + 5K.


The pump switches on with a start-up delay of 60 s after the valve has opened. The delayed switch-on of the charging pump prevents the pump from working against the isolation valve.


If the hot water tank is completely charged, the pump will stop immediately and the valve will continue to be open for another 200s. This extended opening switches the pump and valve off with a delay and serves to prevent an accumulation of heat. When the hot water storage tank reaches its temperature and no longer sends a demand to the heat generator, a further rise of the flow temperature can occur because of the prolonged heating effect. This energy is sent to the hot water storage tank through the delayed charging switch-off. The following diagram illustrates this situation with the hot water storage tank.


Fig. 136.   Hysteresis function diagram


Fig. 137.   Wire-sheet Icon

Fig. 138.   Property Sheet

Fig. 139.   Slot Sheet


Inputs and Outputs

Table 97.   Inputs

Inputs

Type

Functional description

In Value

numeric

Controlled variable such as an outdoor air temperature sensor

In Trigger

numeric

Reference variable such as a setpoint

In Enabled

boolean

If the input “In Enabled” is set to false disabled in the slot sheet, the output “Out” will be disabled and is set to the value “0”.


Table 98.   Outputs

Outputs

Type

Functional description

Out Normal

boolean

Binary output

Out Invers

boolean

is an optional inverted output


Parameter List

Table 99.   Parameter List

Description

Value Range

Default Value

Unit

P1 Switching Differential

-...

0,1

-