Lambda Tools for VisSim

A VisSim Add-On for Industrial Process Simulation

Introduction

The Lambda Tools for VisSim add-on for VisSim developed by Lambda Controls is intended to provide an operator interface and control system capabilities normally present in a Distributed Control System (DCS) or similar control system used in the process industries, to VisSim users wishing to simulate an industrial process and its control system. The Lambda Tools for VisSim add-on also provides the ability to configure the control system to have the "look and feel" of control systems provided by different vendors of industrial control systems.

Using VisSim and Lambda Tools for VisSim provides a cost effective way to build high fidelity dynamic simulations of an industrial process which are easy to modify and maintain using the VisSim software and also to present a user interface which is familiar to plant operators and engineers by using the Lambda Tools for VisSim add-on.

The Lambda Tools for VisSim add-on can also be used to provide cost effective training simulators to train operators, electrical/instrumentation technicians and engineers.

The Lambda Tools for VisSim add-on to VisSim currently provides the following VisSim UserFunction Blocks:

1. PID Controller - This block provides the functionality of a PID controller as commonly found in industrial control systems.
2. Operator Station - This block provides a configurable interface to modify the properties of a PID control loop but otherwise performs no actual feedback control action. The feedback control action is provided by the PID Controller block.
3. System Setup - This block provides capability to configure permission to access controller tuning setup and controller configuration setup from the Operator Station block.

VisSim Diagram Example

Consider the following Pulp and Paper process:

The Stock Tank Level is controlled using a cascade control strategy whereby the level controller provides a Remote Setpoint to the Stock Flow controller which controls the Stock Flow to the Stock Tank. The Stock Consistency is controlled at the outlet of the Stock Tank by adjusting the flow of dilution water to the suction of the Stock Pump.

The following screenshot shows the VisSim dynamic simulation software with a simulation diagram opened which simulates the above process using the Lambda Tools for VisSim add-on.

The VisSim blocks labelled KC-100, FC-101 and LC-99 are Lambda Tools for VisSim Operator Station blocks used to provide operator access to the three control loops in the simulation.

Right-clicking on an Operator Station block displays the operator face-plate for the selected control loop. The display for KC-100 is shown below.

From the Operator Station block the buttons labeled "Man", "Auto" and "Casc" can be used to select the Mode of the controller and a new Setpoint or Controller Output can be entered as appropriate. The buttons labeled "Loop Tuning" and "Loop Config" can be used to access the controller Tuning Selection and the controller Configuration Setup respectively. The Tuning Selection page is shown below.

The Tuning Selection page shows that the controller has been been configured as a PID (Proportional-Integral-Derivative) controller in Standard Form which executes every 0.1 sec. The PID controller has adjustable tuning paramters of Controller Gain, Reset Time and Derivative Time to adjust the Proportional, Integral and Derivative control actions. There is also a "PV filter" with an adjustable filter Time Constant. The control loop Configuration Setup page is described below, in connection with the PID Controller block.

Controlling Access to Control System Features

The details of the process simulation and other Lambda Tools for VisSim control system UserBlocks are contained in the Compound Block labeled "Simulation Details". This Compound Block can be set to be "Locked" from the VisSim Compound Properties dialog box. This allows only the developer of the VisSim diagram to modify the simulation details but still allow users of the diagram to operate the simulation using the Operator Station block to make changes to the control loops.

The contents of the Simulation Details Compound Block are shown below:

The VisSim blocks labelled KC-100, FC-101 and LC-99 are Lambda Tools for VisSim PID Controller UserFunction blocks which perform feedback control. A PID Controller block receives as an input the process variable (sensor measurement) and calculates a new controller output value which is provided as the input to the process Compound Block. As with any commercial PID Controller block, there are additional inputs and outputs which are used to provide additional information or to provide additional functionality to that of simply providing feedback control action. PID blocks are described in more detail below.

The VisSim block labelled "System Setup" is a Lambda Tools for VisSim System Setup UserFunction block which is used to set the access permissions to the controller Tuning Selection and the controller Configuration Setup pages. Shown below is the dialog box to set access permissions obtained by right-clicking on the System Setup block.

PID Controller Configuration Features

Right-clicking on a PID Controller block displays the control loop Configuration Setup for the selected control loop. The display for KC-100 is shown below.

The PID block supports the usual configuration capabilities such as engineering unit specification, reverse/direct action, control interval selection, etc. In addition, the form of PID controller (Parallel, Standard or Classical) and variations of tuning parmeters, for example whether to use repeats per minute or minutes per repeat to specify the integral action can be changed, to make the PID block look like a particular vendor's PID controller.

The Lambda Tools for VisSim PID Controller also has an Over-Ride Control or Output Tracking feature whereby when the Over-Ride Mode is active the block Output which is normally the output of the PID control algorithm is switched to the control signal provided as the "OverRide Output" which is provided as a PID Controller block input. In the present VisSim block diagram example, Output Tracking is made active on the outer level loop when the inner flow loop is not in Cascade mode. When Output Tracking is active on the level loop, the level loop controller output is made to track the inner flow loop PV expressed in Percent of Span.