TM_i-BX(-N)_NC05_00_06_20_EN
MEHITS S.p.A.
●
TECHNICAL MANUAL
Version NC05 - Translation of the Original Instructions
69
This action is defined using the integral constant Ki (also known as integral gain) calculated with this formula:
𝐾
𝑖
=
𝐾
𝑝
𝑇
𝑖
Ti is the value of the integral time defined by one of the following parameters:
•
0508 and 0511 for the regulation of the heat and cool temperature
•
0514 for the regulation of the DHW temperature
The following table shows the reactions on the system caused by the
decrease
of the integral time and resulting increase of the integral constant.
Parameter
(Decreases)
Constant
(Increases)
Promptness of
response
Overshoot
Balancing time
Error at full
power
Stability
Ti
Ki
Increases
Increases
Increases
Cancels out
Worsens
The integral action can be disabled by setting the integral time to 0.
9.1.3
DERIVATIVE ACTION
The derivative action sets a control action based on the prediction of what will happen in the future. The value of this action is proportional to the error
trend (increase or decrease).
This action helps to improve dynamic performance, while maintaining the high value of the proportional action, but is less simple to adjust. It is useful
in that, if the output deviates too quickly from the setpoint value, the error derivative and, therefore, the derivative action, can be significant.
This action is defined using the derivative constant Kd (also known as derivative gain) calculated with this formula:
𝐾
𝑑
= 𝐾
𝑝
∗ 𝑇
𝑑
The Td time defines the projected time horizon: If too small, it will not have a good anticipatory effect; if too large, its forecast may be completely wide
of the mark.
The following table shows the reactions on the system caused by the
increase
of the derivative time and resulting increase of the derivative constant.
Parameter
(Increases)
Constant
(Increases)
Promptness of
response
Overshoot
Balancing time
Error at full
power
Stability
Td
Kd
Increases (slightly)
Decreases
Decreases (slightly)
Does not change
Improves
The derivative action can be disabled by setting the derivative time to 0.
9.1.4
THE COMBINED EFFECT OF THE THREE ACTIONS
The control action of the system is, therefore, calculated, one moment at a time, as the sum of the three contributions (Proportional, Integral and
Derivative).
The
P
roportional Action represents the
component most sensitive to the current value of the error
:
•
High Kp values entail a significant reaction also for slight error variations.
•
Reduced values transfer on the control variable limited variations also in case of significant errors.
The
I
ntegral action varies in a linear manner following the action of proportionality coefficient Kp/Ti and, as mentioned above,
takes the past trend
of the error into account
:
•
Reduced Ti values lend greater importance to past system history.
•
High values decrease the weight of the integral action by transferring to the control variable variations that are more dependent on the
current error value.
The
D
erivative action varies in a linear manner with the error derivative following the action of proportionality coefficient Kp*Td and, as mentioned
before,
takes the current trend of the error into account
:
•
High Td values place greater importance on what may be the future trend of the error, with greater reliance on the algorithm.
•
Lower values transfer to the control variable variations that are less dependent on future trends.
The image below illustrates what has been said above.
Error
Time
Current
Kp
Past
Ki
Future
Kd
