Derivative Action; The Combined Effect Of The Three Actions; Anti-Windup Control (Control Of Integral Error) - Mitsubishi Electric s-MEXT-G00 006 Technical Manual

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TM_s-MEXT_ME28relC_00_12_19_EN

Parameter

Constant

(Decreases)

(Increases)

The integral action can be disabled by setting the integral time to 0.

21.1.3

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 K

Where T

is the value of the derivative time defined by one of the following parameters:

•

P20.07 (for the adjustment of direct expansion units with inverter compressor)

•

P20.23 (for humidifier adjustment)

•

P20.96 (for the adjustment of direct expansion units with compressors with delivery air adjustment active and Free Cooling).

The T

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

Constant

(Increases)

The derivative action can be disabled by setting the derivative time to 0.

21.1.4

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 Proportional Action represents the component most sensitive to the current value of the error:

•

High K

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 Integral action varies in a linear manner following the action of proportionality coefficient K

•

Reduced T

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 Derivative action varies in a linear manner with the error derivative following the action of proportionality coefficient K

trend of the error into account:

•

High T

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.

21.1.5

Anti-Windup control (control of integral error)

The PID adjuster has an integral error control function called Anti-Windup.

This function is used when the demand of the PID reaches 100%. In this case, without the function, the integral action of the controller would continue to integrate the error

and render the PID adjuster inactive even if the error were to decrease in relation to the setpoint, causing the system to overshoot. This would continue until the end of the

integral period (time T

This function stops the calculation of the integral action until demand falls again below 100%.

Technical manual

Promptness of

Overshoot

response

Increases

(also known as derivative gain) calculated with this formula:

Promptness of

Overshoot

response

Increases (slightly)