.Buildings.Controls.OBC.ASHRAE.G36.Generic.TrimAndRespond .Buildings.Controls.OBC.ASHRAE.G36.Generic.TrimAndRespondThis block implements the trim and respond logic according to Section 5.1.14.3 and 5.1.14.4 of ASHRAE Guideline 36, May 2020.
For each upstream system or plant set point being controlled by a trim and respond loop, define the initial values in system or plant sequences. Values for trim, respond, time step, etc. shall be tuned to provide stable control.
| Variable | Value | Definition |
|---|---|---|
| Device | AHU Supply Fan | Associated device |
| SP0 | iniSet | Initial setpoint |
| SPmin | minSet | Minimum setpoint |
| SPmax | maxSet | Maximum setpoint |
| Td | delTim | Delay timer |
| T | samplePeriod | Time step |
| I | numIgnReq | Number of ignored requests |
| R | numOfReq | Number of requests |
| SPtrim | triAmo | Trim amount |
| SPres | resAmo | Respond amount |
| SPres_max | maxRes | Maximum response per time interval |
The trim and respond logic shall reset setpoint within the range minSet to
maxSet.
When the associated device is off (uDevSta=false), the setpoint
shall be iniSet.
The reset logic shall be active while the associated device is proven
on (uDevSta=true), starting delTim after initial
device start command.
When active, every time step samplePeriod, trim the setpoint by
triAmo.
If there are more than numIgnReq requests, respond by changing
the setpoint by resAmo*(numOfReq-numIgnReq), i.e., the number of
requests minus the number of ignored requests, but no more than maxRes.
In other words, every time step samplePeriod:
triAmo; numOfReq > numIgnReq, also change setpoint by resAmo*(numOfReq
-numIgnReq) but no more than maxRes.
Optionally, if the parameter have_hol is set to true, an additional
input signal uHol allows for holding the trim and respond loop output
at a fixed value for the longer of the time the input uHol remains true
and the duration specified by the parameter dtHol.
When uHol switches back to false, the hold is released and resetting
continues from the previously held value (without reinitializing to iniSet
or going through a delay time of delTim).
This is typically used in control sequences to freeze the reset logic during the plant
staging process.
Consider for example the following specification:
"When a plant stage change is initiated, the reset logic shall be disabled and value
fixed at its last value for the longer of 15 minutes and the time it takes
for the plant to successfully stage."
Using this block with have_hol=true and dtHol=15*60
yields the following sequence of events.
The figure below illustrates the trim and respond logic with a negative trim amount, comparing scenarios with and without holding the loop output.
The figure below illustrates the trim and respond logic with a positive trim amount.
The figure below illustrates the trim and respond logic with a negative trim amount, in a scenario where the equipment switches on and off.
