Features of PLC-based Process Control

Outline
The PLC-based Process Control is a process control system which is built on the base of CS/CJ series PLC. Loop control functions can be added on the base of general PLC function, by mounting the PLC-based Process Control components. The basic functionality of the PLC can be utilized, too.
In the PLC-based Process Control system, the Loop Controller handles analog operation and CS/CJ series PLC CPU Unit handles ladder operation. Data exchange between the Loop Controller
and CPU Unit is executed through the data table in the memory area.
The operation processing of analog control and ladder operation are separately handled. Therefore, the program can be simpler compared with the program which is built by only ladder program. This feature will contribute to reduce bugs because engineering to build the system becomes easier.

Features

• PLC CPU Unit manages the total system.
• Loop Controller, analog I/O unit, and contact I/O unit can be selected and mounted on this building block-type PLC system.
• Communication type to connect with host system (HMI software, PT, etc.) can be freely selected from Ethernet, Controller Link, Serial communication, and etc.
• The Loop Controller has the Function block data in it, and peforms loop operation.
• Analog data in the Loop Controller is handled in percentage unit, not in engineering units.
• Loop Controller automatically exchanges the data in percentage unit with HMI data area (allocated area in the CPU Unit).
• Based on the CSV Tag File (corresponding table of tags and allocated area address in the CPU Unit with setting data for scaling, etc.), HMI software & special monitoring software (CX-Process Monitor Plus) can access to the HMI data area. (Analog data is converted from percentage unit into the engineering unit depending on the scaling setting which is set with other tag data).
• For NS series PT, screen data can be automatically created from the CSV Tag file by Face Plate Auto-Builder for NS (automatic screen creation software).

 

Overview


For engineers who need to identify system behavior and design controllers to obtain the best possible closed loop system characteristics, National Instruments (NI) provides LabVIEW based software tools and flexible, low cost hardware solutions. This is a superior alternative to expensive fixed-functionality systems. Unlike these embedded, inflexible alternatives, NI products provide an easy to use, integrated graphical environment and built-in compatibility across a broad range of data acquisition and control hardware devices.







Table of Contents

1.1. Introduction

2.2. Identifying the System

3.3. Controller Design

4.4. Simulating the Closed Loop System

5.5. Driving the Motor with the closed loop system

6.6. Conclusions

1. Introduction

This article describes the process of designing a closed loop control system, or plant, using the NI System Identification and Control Design Assistants. A DC Motor will be the plant (Figure 1).





Figure 1: The Quanser Engineering Trainer (QET) will be the plant for which we will design a closed loop controller.





The Quanser Engineering Trainer will be used in velocity mode. A voltage signal commands the motor to move and the tachometer output determines the velocity. The motor system is connected to a National Instruments CompactDAQ device, shown in figure 2, where Analog Input Channel 0 of a 9211 Analog input module (AI0) is connected to the tachometer and Analog Output Channel 0 of a 9233 Analog output channel (AO0) is connected to the motor command input. For demonstration purposes, you can replace the DC motor with an RC circuit.







Figure 2: CompactDAQ system