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It is possible to send start/stop signal and the setpoint to MIXIT via the BACnet or Modbus or on an analogue
interface. If the analogue interface is used for sending signals to MIXIT it is still possible to read datapoints
from the BACnet or Modbus fieldbuses.
Use case 2 with the dedicated controller can be a problem as it is designed to control the valve position
directly. However, sometimes the temperature setpoint and on/off signals are available in the bus profile of
the dedicated controller. They should then be transferred to MIXIT via the programmable controller at the
automation level.
Figure 2: Use cases for BMS integration with MIXIT, the field level reflects the case for an air handling unit (AHU).
1.1
Alternative option for interfacing MIXIT to a dedicated controller
If MIXIT cannot be connected at the automation level in use case 2, for example because no spare analogue
outputs are available or there is no BACnet MS/TP or Modbus RTU fieldbus master available at automation
level, there is one additional way of interfacing MIXIT to the dedicated controller.
The analogue valve opening setpoint
𝑢
from the dedicated controller may be interpreted as a temperature
setpoint because of the following steady state relation that is true for any mixing loop:
𝑇
𝑚𝑖𝑥
= 𝑇
𝑟𝑒𝑡𝑢𝑟𝑛
+ 𝑓(𝑇
𝑟𝑒𝑡𝑢𝑟𝑛
, 𝑇
𝑠𝑢𝑝𝑝𝑙𝑦
, Δ𝑝, 𝑞
𝑚𝑖𝑥
, 𝑢)
where
𝑇
𝑚𝑖𝑥
: The secondary supply temperature
𝑇
𝑟𝑒𝑡𝑢𝑟𝑛
: The return temperature
𝑇
𝑠𝑢𝑝𝑝𝑙𝑦
: The primary supply temperature
Δ𝑝
: The differential pressure across the valve
𝑞
𝑚𝑖𝑥
: The secondary flow
𝑓(. )
: A nonlinear function
