User Manual

Control concept
Closed-loop control strategy
5
CM110664en_07 95 | 353
Cascade control
The CAS_CTR block integrated into the Desigo system is a PI master controller
for room supply air cascade control. It delivers three supply air set points on the
basis of the difference between the measured room temperature and the room
setpoint.
The following functions are integrated into the block:
● Facility to select P or PI control
● Gain and integral action time (can be configured)
● Low supply air setpoint for the reverse-acting part of the sequence
● High supply air setpoint for the direct-acting part of the sequence
● Supply air setpoint for energy recovery
● Min/Max setpoint limit control (supply air setpoint)
● Selection of type of operation for heat recovery
● Initial value for the integrator can be defined
Compared with control without a cascade, e.g., cascade control improves the
dynamics of the control process.
If the temperature in a ventilated room is below the setpoint, e.g., the supply air
temperature must be increased, at least for a brief period, in order to raise the
temperature to the room setpoint. This can be achieved by measuring and
controlling not only the room temperature, (that is, the value which actually
concerns the user), but also the supply air temperature, whose setpoint
depends on the difference between the room setpoint and the room
temperature.
If the room temperature is lower than the room setpoint, the supply air setpoint
is adjusted in proportion to the room control differential, and the supply air
temperature is increased via the supply air control loop.
The master controller generates the setpoint for the auxiliary variable (e.g., the
supply air temperature) on the basis of the difference between the primary
setpoint and the primary controlled variable (e.g., the room setpoint and the
room temperature).
The master controller must include an integrator function (I component),
because even under static conditions (that is, when the measured value and
the setpoint are equal) there is generally a negligible control deviation, which
means that the controller output must be at a different operating point. For
improved control dynamics, a P-component should be connected in parallel
with the integrator. This is why the master controller in this case has a PI
control structure.
Even when the primary controlled variable (room temperature) is identical to its
setpoint, the auxiliary controlled variable (supply air temperature) must
generally be at a value other than 0, (that is, setpoint ≠ 0). This is only possible
if the output of the master controller is not equal to 0, even if the P component
= 0. In other words, the master controller must have an I-component which
remains constant when the control differential = 0. This is why the master
controller has a proportional and an integral component. It is a numerical PI
controller for use as a master controller in a room/supply air cascade.