21
Thermostat
Communication objects and ETS parameters
Proportional/Integral (PI) control
This type of control uses a more advanced algorithm that keeps
the temperature inside the room more stable, increasing comfort.
This algorithm works by switching the system on and off so as
to be like a gradual increase or decrease in the system's thermal
(or refrigerating) power. To obtain optimal performance you need
to perform the calibration according to the type of room and
heating system.
Define the variables:
- Ta = room temperature
- Sp = current setpoint
- Kp = coefficient of the proportional component
- Ki = coefficient of the integral component
- Bp = proportional band
- Ti = integrative time
the algorithm is characterised by the following parameters:
•
Proportional band
: used to calculate the coefficient Kp = 100
/ Bp and corresponds to the breadth of the proportional control
band. Starting from the set temperature, this value is the
temperature range in which the system power goes from 0%
to 100%.
For example
: with the (heating) temperature set to
20.0°C and Band (P) =4.0°C, the thermostat runs the heating
system at 100% when Ta is <= 16.0°C; as this temperature
increases, the system power is consequently lowered down
to 0% when Ta reaches 20°C. The value must be set taking
account of the thermal capacity of the room to control; in
general, it is recommended to use small values for rooms with
a good level of thermal insulation and vice versa.
•
Integrative time
: used to calculate the coefficient Ki = Kp / Ti
and corresponds to the time passed which, when equal to the
deviation from the setpoint (error), the integrative component
generates a contribution equal to that generated by the
proportional component. The integral contribution reduces the
error on full operation if thermal energy is lost in the room to
be controlled, as this contribution increases according to the
time during which the setpoint is not reached. If this value is not
set correctly, it can cause transients with variations around the
setpoint or it may take longer to reach the setpoint.
Proportional integral PWM:
PI control with On/Off valve
Continuous integral proportional:
PI control with proportional
valve
The parameters of the proportional and integral coefficients Kp
and Ki are set using the ETS software: the proportional coefficient
Kp for heating corresponds to the “
Heating Proportional Value
”
parameter while the coefficient for cooling is set using the
“
Cooling Proportional Value
” parameter. The integral time Ti
is set with the "
Heating Integral Value
” and “
Cooling Integral
Value
” parameters for heating and cooling, respectively.
The PI control parameters should be set according to the type
of heating or cooling system used, the size of the room and its
thermal insulation.
IMPORTANT
: Generally, when using fan coils, PI control is not
used. The valve is typically managed with On/Off control and On/
Off valve or proportional valve (0%-100%); the fine adjustment is
then made using the fan speed.
COMMUNICA
TION OBJECTS
