Instrumentation

Instrumentation is defined as the art and science of measurement and control of process variables within a production or manufacturing area. The process variables used in industries are Level, Pressure, Temperature, Humidity, Flow, pH, Force, Speed etc. Control engineering or control systems engineering is the engineering discipline that applies control theory to design systems with desired behaviors. Control engineers are responsible for the research, design, development and control devices/systems, typically in manufacturing facilities and plants. The practice uses sensors to measure the output performance of the device being controlled and those measurements can be used to give feedback to the input actuators that can make corrections toward desired performance. When a device is designed to perform without the need of human inputs for correction it is called automatic control (such as cruise control for regulating a car’s speed). Multi-disciplinary in nature, control systems engineering activities focus on implementation of control systems mainly derived by mathematical modeling of systems of a diverse range.

Instrumentation and Control Engineering

This term refers to the graduate discipline which many universities provide at graduate and postgraduate level. Instrumentation and Control plays a significant role in both gathering information from the field and changing the field parameters, and as such are a key part of control loops. The Instrumentation Technology, being an inter-disciplinary branch of engineering, is heading towards development of new & intelligent sensors, smart transducers, MEMS Technology, Blue tooth Technology. This discipline finds its origin in both electrical and electronics engineering, and it covers subjects related to electronics, electrical, mechanical, chemical and computing streams. In short, it deals with measurement, automation and control processes. In today’s scenario, there are many people who are willing to make a career in this stream. Almost all process and manufacturing industry such as steel, oil, petrochemical, power and defense production will have a separate instrumentation and control department, which is manned and managed by instrumentation and control engineers. “Automation is the buzz word in process industry, and automation is the core job of instrumentation and control engineers. Hence, the demand for instrumentation will always be there.

Nature of Job

An instrumentation and control engineer is required to

1. Design and develop control systems

2. Maintain the existing control systems

3. Manage the control systems

4. Collaborate with design engineers, purchasers and other staff members involved in the production processes

5. Manage projects within the given restraints including cost and time

6. Troubleshoot

7. Ensure that the instruments comply with health and safety regulations

8. Ensure that quality standards are maintained

9. Provide consultancy support

Requirement

Instrumentation and control engineers have a role to play in all the fields where there is automation. The instruments created by control engineers to automate the processes, thus reducing the involvement of manpower.
An instrumentation and control engineer is expected to learn subjects like

• Industrial Instrumentation

• System dynamics

• Process control

• Control system

Knowledge of PLC programming, Panel view HMI screens, SCADA Application programming preferred.

Goal of Instrumentation and control engineers

Instrumentation and control engineers works with the goal of improving

• Productivity,

• Optimization,

• Stability, dedication

• Reliability,

• Safety,

• Continuity.

These engineers design, develop, and maintain and manage the instruments and the instrumentation systems

Industrial control system (ICS) is a general term that encompasses several types of control systems and associated instrumentation used in industrial production, including supervisory control and data acquisition (SCADA) systems, distributed control systems (DCS), and other smaller control system configurations such as programmable logic controllers (PLC) often found in the industrial sectors and critical infrastructures.^[1]

Industrial control systems are typically used in industries such as electrical, water, oil, gas and data.

Based on data received from remote stations, automated or operator-driven supervisory commands can be pushed to remote station control devices, which are often referred to as field devices. Field devices control local operations such as opening and closing valves and breakers, collecting data from sensor systems, and monitoring the local environment for alarm conditions.^[2]

A historical perspective

A pre-DCS era central control room. Whilst the controls are centralized in one place, they are still discrete and not integrated into one system.

A DCS control room where plant information and controls are displayed on computer graphics screens. The operators are seated as they can view and control any part of the process from their screens, whilst retaining a plant overview.

Overview of control evolution

Process control of large industrial plants has evolved through many stages. Initially, control would be from panels local to the process plant. However this required a large manpower resource to attend to these dispersed panels, and there was no overall view of the process. The next logical development was the transmission of all plant measurements to a permanently-manned central control room. Effectively this was the centralization of all the localized panels, with the advantages of lower manning levels and easier overview of the process. Often the controllers were behind the control room panels, and all automatic and manual control outputs were transmitted back to plant. However, whilst providing a central control focus, this arrangement was inflexible as each control loop had its own controller hardware, and continual operator movement within the control room was required to view different parts of the process. With coming of electronic processors and graphic displays it became possible to replace these discrete controllers with computer-based algorithms, hosted on a network of input/output racks with their own control processors. These could be distributed around plant, and communicate with the graphic display in the control room or rooms. The Distributed Control System was born.

The introduction of DCSs allowed easy interconnection and re-configuration of plant controls such as cascaded loops and interlocks, and easy interfacing with other production computer systems. It enabled sophisticated alarm handling, introduced automatic event logging, removed the need for physical records such as chart recorders, allowed the control racks to be networked and thereby located locally to plant to reduce cabling runs, and provided high level overviews of plant status and production levels. Industrial control system technology and its constituent components have evolved over the decades.

The PLC (programmable logic controller) evolved out of a need to replace racks of relays in ladder form. The latter were not particularly reliable, were difficult to rewire, and were difficult to diagnose. PLC control tends to be used in very regular, high-speed binary controls, such as controlling a high-speed printing press. Originally, PLC equipment did not have remote I/O racks, and many could not perform more than rudimentary analog controls.

SCADA’s history is rooted in distribution applications, such as power, natural gas, and water pipelines, where there is a need to gather remote data through potentially unreliable or intermittent low-bandwidth and high-latency links. SCADA systems use open-loop control with sites that are widely separated geographically. A SCADA system uses RTUs (remote terminal units, also referred to as remote telemetry units) to send supervisory data back to a control center. Most RTU systems always did have some limited capacity to handle local controls while the master station is not available. However, over the years RTU systems have grown more and more capable of handling local controls.

The boundaries between these system definitions are blurring as time goes on.^[3] The technical limits that drove the designs of these various systems are no longer as much of an issue. Many PLC platforms can now perform quite well as a small DCS, using remote I/O and are sufficiently reliable that some SCADA systems actually manage closed loop control over long distances. With the increasing speed of today’s processors, many DCS products have a full line of PLC-like subsystems that weren’t offered when they were initially developed.

This led to the concept of a PAC programmable automation controller or process automation controller, that is an amalgamation of these three concepts.

DCSs

Main article: Distributed Control System

DCSs A Distributed Control System (DCS) is a computerized control system for a process or plant, wherein control elements (controllers) are distributed throughout the system. This is in contrast to non-distributed systems that use discrete controllers. In a DCS, a hierarchy of controllers is connected by communication networks, allowing both centralized control rooms and local on-plant monitoring and control.

The introduction of DCSs allowed easy interconnection and re-configuration of plant controls such as cascaded loops and interlocks, and easy interfacing with other production computer systems. It enabled sophisticated alarm handling, introduced automatic event logging, removed the need for physical records such as chart recorders, allowed the control racks to be networked and thereby located locally to plant to reduce cabling runs, and provided high level overviews of plant status and production levels.

DCS Structure

Functional levels of a typical Distributed Control System.

A DCS typically uses custom-designed processors as controllers, and uses either proprietary interconnections or standard protocols for communication. Input and output modules form the peripheral components of the system.

The processors receive information from input modules, process the information and decide control actions to be performed by the output modules. The input modules receive information from sensing instruments in the process (or field) and the output modules transmit instructions to the final control elements, such as control valves.

The field inputs and outputs can either be continuously changing analog signals e.g. 4~ 20mA dc current loop or 2 state signals that switch either “on” or “off”, such as relay contacts or a semiconductor switch.

DCS systems can normally also support such as Foundation Fieldbus, provirus, HART, Modbus, PC Link and other digital communication bus that carries not only input and output signals but also advanced messages such as error diagnostics and status signals.

PLCs

Main article: Programmable logic controller

PLCs provide Boolean logic operations, timers, and (in some models) continuous control. The proportional, integral, and/or differential gains of the PLC continuous control feature may be tuned to provide the desired tolerance as well as the rate of self-correction during process upsets. PLCs are used extensively in process-based industries. PLCs are computer-based solid-state devices that control industrial equipment and processes. While PLCs can control system components used throughout SCADA and DCS systems, they are often the primary components in smaller control system configurations. They are used to provide regulatory control of discrete processes such as automobile assembly lines and power plant soot blower controls and are used extensively in almost all industrial processes.

Embedded control

Another option is the use of several small embedded controls attached to an industrial computer via a network. Examples are the Plantronics Export^[4] and Digi/ME.

References

“Instrumentation “, The Northern Alberta Institute of Technology., Retrieved 17 October 2012.

Instrumentation engineering in demand”, The Hindu, April 16, 2012

Instrumentation and Control Engineering is for Perfectionists”, Daksha P Gupta