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PI-Controller

A PI-Controller is a screen object that operates as a self-contained, closed-loop, automatic process control system that can regulate temperature, pressure, flow rate, tank level, etc. It uses an input to read the data from the PV sensor, and an output for manipulating the actuator or control valve to keep the process variable at a specific set point. This controller features bump-less transition from manual to automatic modes, bump-less transition when changing the Integral gain, and an anti-windup offset bias for use when large errors or large changes in the set point occur.

Once a PI-Controller is placed on the screen, it can be moved to the desired position by dragging on it with the mouse. To set the appearance and operating properties of the PI-Controller, right-click on the object which will bring up the Properties dialog box as shown below. The following is a summery of the user defined attributes in this dialog box.

Properties Dialog Box:

Screen Text:

Type in the Title text that will appear at the top of the PI-Controller and select the font Size and Color. Making changes to the font size will also change the overall dimensions of the screen object frame.

Communications:

The Object ID Label is used for identification so that other objects within ModCom can manipulate the PI-Controller or use the PV sensor data as a variable in another object's function. The Sensor Polling Command is the command string which should be transmitted to the DAQ Module in order to read the data from the PV sensor. This command string will be transmitted automatically by the PI-Controller on a continuous basis.

Sensor Data Conversion:

The data coming from the PV sensor can be converted if necessary to be displayed in the correct engineering units. This can be accomplished with a simple Multiplier and Offset, or a Math Formula containing a complex mathematical expression. If the PV sensor has a non-linear output such as an RTD temperature probe, a Lookup Table can be used to convert the data by matching it with known substitution values.

Kp/Ki Format:

Use the Dec Point selection to set the number of decimal places that the Kp and Ki gain adjustment controls will use. This will also effect the magnitude of each increment and decrement step change when pressing the adjustment buttons. Once the PI-Controller has been tuned adequately, the Lock Settings box can be checked so that the Kp and Ki gain settings are locked in place and can't be accidentally changed by the user.

Positive Correction:

Max Value: Type in the output value which will produce the maximum positive correction of the actuator. For example, if the actuator is a flow controller, this will be the value that causes the flow controller to open to its full amount.

Min Value: Type in the output value which will produce the minimum positive correction of the actuator. For example, if the actuator is a flow controller, this will be the value that causes the flow controller to completely close up.

Append value to these characters when transmitting: Each time the PI-Controller updates and a positive correction is needed, these characters (representing the Header, Command Title, and Channel Number) followed by the output value will be transmitted to the DAQ Module controlling the actuator. If a positive correction is not being used, such as using a cooling actuator without a heating actuator, leave this box blank.

Negative Correction:

Max Value: Type in the output value which will produce the maximum negative correction of the actuator. For example, if the actuator is a flow controller, this will be the value that causes the flow controller to open to its full amount.

Min Value: Type in the output value which will produce the minimum negative correction of the actuator. For example, if the actuator is a flow controller, this will be the value that causes the flow controller to completely close up.

Append value to these characters when transmitting: Each time the PI-Controller updates and a negative correction is needed, these characters (representing the Header, Command Title, and Channel Number) followed by the output value will be transmitted to the DAQ Module controlling the actuator. If a negative correction is not being used, such as using a heating actuator without a cooling actuator, leave this box blank.

Operation:

To set up the PI-Controller for operation, connect the PV sensor to an Analog Input channel of a DAQ Module, and the output actuator to an Analog Output channel. If separate actuators are needed for positive and negative corrections, connect each one to its own Analog Output channel. After setting up the PI-Controller's Properties box, click the green "Run" button at the top of the screen. At this point the Controller will be in manual mode and the "Output" adjustment buttons can be used to raise or lower the driving force of the actuator and manipulate the process variable. Once the process variable shown by the PV sensor data is at or near its desired level, click on the "Auto" button to switch the PI-Controller into automatic mode. At this point the Controller will automatically maintain the set point level provided that the Kp and Ki gains are adequately set (see Loop Tuning below).

Loop Tuning:

The proportional and integral gain settings must be set (or tuned) for proper operation. First adjust the "Output" adjustment buttons while in manual mode so that the PV sensor data is at or near its normal level of operation. Assure that both the Kp and Ki gains are set to their minimum level and then place it into automatic mode by clicking on the "Auto" button. Follow the directions below for tuning the Kp and Ki gain elements.

Kp Gain: To set the Kp gain, first increase it until oscillation occurs. This is best done by making an increase and then bumping the Set Point up and then back down again, trying to induce oscillation. If oscillation does not occur, increase the Kp gain further and make the Set Point bump again. Keep repeating this process until oscillation begins and is sustained. Now slowly decrease the Kp gain to the point that oscillation settles down, then reduce this number to one half of this value.

Ki Gain: To set the Ki gain, make a small change to the Set Point and watch how long it takes for the PV sensor data to come to rest at the new set point level. When first making the change, the PV sensor data will approach the new set point quickly but will slow down as it gets closer and closer. The Ki gain determines the rate of approach while in this slow down region. If the Ki gain is set too low, it will take too long to eventually come to rest at the new set point. If the Ki gain is set too high, it will overshoot the set point and begin to oscillate. Adjust this for optimum performance.