PI controller

Short description

A PI controller is a proportional-action and integral-action controller. You can use both proportional action and integral action individually or combined.

Connection

Description

Input A/M

Set the mode of the controller:

1: automatic mode
0: manual mode

Input R

Use the input R to reset the output AQ. As long as this input is set, the input A/M is disabled. The output AQ is set to 0.

Input PV

Analog value: process value, influences the output

Parameter

Sensor: Type of sensor being used

Min.: Minimum value for PV
Value range: -10,000 to +20,000

Max.: Maximum value for PV
Value range: -10,000 to +20,000

Gain
Value range: -10.00 to +10.00

Offset
Value range: -10,000 to +10,000

SP: Set-value assignment
Value range: -10,000 to +20,000

Mq: Value from AQ with manual mode.
Value range: 0 to 1,000

Parameter sets: application-related presets for KC, TI and Dir (see below)

KC: Gain
Value range: 00.00 to 99.99

TI: Integral time
Value range 00:01 min to 99:59 min

Dir: Action direction of the controller
Possible settings: + or -

p: Number of decimal places
Possible settings: 0, 1, 2, 3

Output AQ

Analog output (manipulated variable)
Value range for AQ: 0 to 1,000

Parameters SP and Mq

The actual value of another already-programmed function can provide the set-value SP and the value for Mq:

For the 0BA7 and later version devices, you can additionally use the actual values of the following already-programmed functions:

You select the required function by the block number.

Parameter P (number of decimal places)

Parameter p only applies for displaying the values from PV, SP, Min. and Max. in a message text.

Timing diagram

The nature, manner and speed with which the AQ changes depends on the parameters KC and TI. Thus, the course of AQ in the diagram is merely an example. A control action is continuous; therefore the diagram portrays just an extract.

A disturbance causes the PV to drop, as Dir is positioned upwards, AQ increases until PV corresponds again to SP.

A disturbance causes the PV to drop, as Dir is positioned downwards, AQ decreases until PV corresponds again to SP.

Dir is coordinated to the basic conduct of a control loop. The direction (dir) cannot be changed during the term of the function. The change in Dir here is shown for the purposes of clarification.

As the input R sets AQ to 0, PV changes. This is based on the fact that PV increases, which on account of Dir = upwards causes AQ to drop.

PI controller trend view in simulation or online test

When you view the circuit program in an Online test or in a simulation, LOGO!Soft Comfort displays a trend view of the PI controller showing input PV, output AQ and the parameter SP. The trend view shows the change in AQ and PV over time, in reference to the SP. The time sampling period is configurable, allowing you to see frequent samples over a short amount of time, or less-frequent samples over a longer period of time.

Description of function

If the input A/M is 0, then the special function issues output AQ with the value that you set with parameter Mq.

If the input A/M is 1, then automatic mode commences. As the adopted value Mq is an integral sum, the controller function begins the calculations in accordance with the formulas given in Control and regulate basics. Uss the updated value PV in the formulas:

Updated value PV = (PV * gain) + offset

If the updated value PV = SP, then the special function does not change the value of AQ.

Dir = upwards/+ (timing diagram numbers 1 and 3)

Dir = downwards/- (timing diagram number 2)

With a disturbance, AQ increases or decreases until the updated value PV again corresponds to SP. The speed with which AQ changes depends on the parameters KC and TI.

If the input PV exceeds the parameter Max., then the updated value PV is set to the value of Max. If the PV falls short of the parameter Min., then the updated value PV is set to the value of Min.

If the input R is 1, then the AQ output is reset. As long as R is set, the input A/M is disabled.

Sampling time

The sampling time fixs at 500 ms.

Parameter sets

In order to simplify the use of the PI controller, the parameters for KC, TI and Dir are already given as sets for the following applications:

Parameter set

Application example

Parameter KC

Parameter TI (s)

Parameter Dir

Temperature fast

Temperature, cooling control of small spaces; small volumes

0.5

30

+

Temperature slow

Heating, ventilation, temperature, cooling control of large spaces; large volumes

1.0

120

+

Pressure 1

Quick pressure change, compressor control

3.0

5

+

Pressure 2

Slow pressure change, differential pressure control (flow controller)

1.2

12

+

Full level 1

Vat and/or reservoir filling without drain

1.0

99:59

+

Full level 2

Vat and/or reservoir filling with drain

0.7

20

+

Characteristics when configuring

Observe the control and regulate basics.