Monday, December 10, 2018

Programmable Logic Controllers (PLC)

Programmable logic controllers (PLC) play an important role in automation sector. Various industries like Food & Beverage, Chemical, Petrochemical, Power generation etc.  use PLC.
We have several types of PLC designs:
Compact PLC: This is built by several modules within a single case. The I/O capabilities are decided by the manufacturer and not the user.
Modular PLC: This is built with several components that are plugged into a common rack or bus with extended I/O capabilities. It contains power supply module, CPU and other I/O modules that are plugged together in the same rack, which are from the same manufacturers or from different manufacturers.
Soft PLC: This is an advanced PLC system that consists of compact, rack mounted components such as power supplies, I/O modules and a CPU which embeds a powerful PLC Control software.
There are several programming languages used to write programs in a PLC. They include but not limited:
  • Ladder Diagram
  • Instruction List
  • Functional Block Diagram
  • Sequential Function Chart
  • Structured Text

So what are some of the components that make up a Programmable Logic Controllers?
Components that make up a PLC system
PLC  System

CPU – This the unit that contains microprocessors
INPUT-OUTPUT SECTIONS – This is where the processor receives information from external devices and communicates information to external devices.
POWER SUPPLY UNIT – It converts the Main AC voltage to low DC voltage.
PROGRAMMING DEVICES – Used to enter the required program into the memory of the processor.
MEMORY UNIT – This is where the program is stored that is used to control actions.
Check the input status: First the PLC takes a look at each I/O to determine if it is on or off.
Execute Program: Next the PLC executes the program one instruction at a time. 
Update output status: Finally the PLC updates the outputs. It updates the outputs based on which inputs were on during the first step. 
How a PLC system works
The Working of a PLC system

Advantages of PLC:
  • More flexibility
  • Lower cost
  • Increased reliability
  • Faster response
  • Easier to troubleshoot
  • Communication capability
  • Remote control capability

  • They can render some jobs redundant
  • They have a high initial cost
  • If a Programmable logic controller stops, then the production stops 

Industrial Applications of PLCs
  • Food and Beverage industry
  • Gas and Water Filling Stations
  • Power Sector
  • Bottling Plants

Some of the Top PLC Brands in the world include:
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Friday, December 7, 2018

How to upgrade your Legacy Equipment for industry 4.0

Scholar and leadership expert Warren Bennis once said, “In life, change is inevitable. In business, change is vital.” This wisdom resonates with every business owner, but none more than the manufacturer.
We are in the midst of a new industrial revolution, one which will significantly impact the manufacturing industry. Experts are calling it Industry 4.0, the fourth wave in the industrial revolution behind steam power, electricity and computing.
According to TechRadar, Industry 4.0 is “the label given to the gradual combination of traditional manufacturing and industrial practices with the increasingly technological world around us.” Industry 4.0 is ushering in a new era of production where automation and data exchange are integrated into the manufacturing process to streamline productivity.
Sounds great, right? It is, if you can upgrade your legacy equipment. Nobody enjoys the process of upgrading, let alone talking about it, but this is a revolution you don’t want to miss. Here’s how you can upgrade your legacy equipment to successfully ride the wave of Industry 4.0. 
Industrial Internet of Things
The Industrial Internet of Things (IIoT) is the interconnection between manufacturing and production equipment. This equipment uses sensors and internet connectivity to communicate with themselves and one another to create a more efficient production output. As a result, equipment can consider factors like stress on the electrical grid and projected weather to determine the most efficient way to operate at any given time.
According to Gartner, a leading research and advisory company, more than half of major new business processes and systems will incorporate some element of the IIoT by 2020. What’s more, McKinsey Global Institute reported that in the last five years, the number of connected machines has grown by 300 percent.
These businesses are onto something; there are many benefits of integrating the IIoT into manufacturing processes. Information gleaned from the IIoT provides access to real-time data and insights on equipment’s performance and use. Operators can also closely track the lifespan of their machinery in order to proactively plan for maintenance and upgrades. IIoT integration also aids in the automation process. Digitally connecting the machinery creates a mesh that seamlessly translates into full automation. Finally, clients can more readily track the progress of their order with insights provided by the IIoT.
Integrating the IIoT with existing equipment can be challenging, but it isn’t impossible. Most legacy equipment can be retrofitted with sensors and other online monitoring devices.
Smart Factories
In the past, many manufacturing facilities relied on Manufacturing Operations Management (MOM) software to integrate the many independent facets of the production process. Unfortunately, this technology is not able to manage production processes in real-time.
Smart Factory software integrates every part of the production process, including production, resources, supply chain, maintenance and human resources, in order to create a single, efficient output.
This technology enables factory managers to examine data once unavailable, informing decisions about production and other business processes. With Smart software, operators can be more responsive to several factors, including resource availability and cost, consumer demand, market fluctuations, and more. 
Wireless HART network
An Example of Wireless HART mesh network

Digital Supply Chains
Digital supply chains aren’t simple A to B, B to C, C to D processes. In these systems, relationships between different parts of the overall production process are affected by changes or events elsewhere in the system and able to adapt to those changes.
To create a truly digital supply chain, the facility must consider all factors that could potentially impact each part of the supply chain, all the while remedying any issues that may impede the supply chain from operating as designed. Insights from a digital supply chain give manufacturers a real-time overview of every link in the supply chain. As a result, they can quickly respond to problems and simulate scenarios to proactively plan for the future.
To do this well, factories must integrate every step of the product life cycle. This includes everything from sourcing and shipping raw materials, to ordering packaging, advertising the product, and scheduling employees on the factory floor. The digital supply chain system acknowledges that creating a product isn’t black and white. It is a highly sophisticated process that involves many interconnected variables.
Industry 4.0 is here to stay. Upgrades can cause growing pains, but in the end, change is almost always a good thing. Be a part of the next industrial revolution. Integrate your equipment and transform your business.
About the Author:
Page Long is the Marketing Operations Director at PDF Electric & Supply, which is based out of Cary, NC. PDF Electric & Supply is an automation supplier specializing in Legacy GE PLCs.
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Wednesday, December 5, 2018

Key Considerations when Specifying Displacer Level Transmitters

Displacer level transmitter is used in Process measurements of liquids across many process industries. In this article we discuss the Principle of operation and industrial level applications that displacer transmitter technology offers.
Principle of Operation
Displacer Level Transmitter

Operation is based on buoyancy force. The buoyancy force works on the displacer which will vertically move in (increasing liquid level) and out (decreasing liquid level) the linear differential transformer (LVDT). Due to this movement, voltages are induced in the secondary winding of the LVDT. These signals are then processed in the electronic circuitry and used to control the output signal.
Industrial Applications
Displacer transmitters are ideal solution for liquids or slurries, clean or dirty and light hydrocarbons to heavy acids with a specific gravity (SG) of 0.23 to 2.20.
Displacer transmitter technology works well in a variety of vessels including process & storage, bridles, bypass chambers, interfaces, sumps and pits up to unit pressure and temperature ratings. They can also handle most liquid conditions including varying dielectric, vapors, turbulence, foam, buildup, bubbling or boiling and high fill/empty rates.
Some of the specific industrial applications include:
Boiler control – The displacer unit provides a stable output signal for valve control on turbulent surface applications, such as feed water heaters, flash tanks, and reactors. 
Interface control on storage tanks – The displacers are tolerant of emulsions. They can track towards middle of emulsion, and are tolerant of unstable interface. They also ignore vapor/liquid interface point above displacer.
Mixing Tank – The displacer transmitter can also be used in harsh production environments like those in a mixing tank. It provides a stable output, easier to configure and is resistant to heavy surging caused by the mixer.
Water elevation – The displacer can be used to maintain a water elevation at a given height by sending out a proportional 3-15 PSIG signal (over a 14 ″ control band) to a control valve to maintain water level at midpoint by regulating the water flow rate out of the separator. The control valve is fully open at 15 PSIG input and fully closed at 3 PSIG input.
In Summary the advantages and limitations of Displacer transmitter include:
  • Stable signal in turbulent applications
  • High Pressure/Temperature capabilities
  • No flexure of pressure boundary part

  • Shifting SG can affect this technology

Some of the manufactures of Displacer level transmitters include:
Sources: Magnetrol
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Tuesday, December 4, 2018

How to Troubleshoot Servo Drives

Servo drives are used to control devices including robotics, model airplanes, aerospace technology and multiple industrial applications. The drive is a component in a closed-loop system, and uses an amplified control signal to send power to the device motor. They are primarily used to control the output torque, speed and position of a motor shaft.
Most servo drives in use in 2017 are digital, and like all other electronic equipment, they can malfunction at the most inconvenient times. A breakdown can disrupt an entire production schedule and cause significant loss of revenue, or it can cause problems like overheating and result in damage to the systems it supports.
Common Problems with Servo Drives
Most of the typical servo drive issues you’re likely to experience have common causes, which helps to make troubleshooting less frustrating. Here are some malfunctions to watch for:
·        System instability: If any of your settings are incorrect, the servo drive may not operate correctly. There are numerous parameters to check related to motor tuning as well as speed and current loops.   If you’re getting noise transmitted into the control wiring, you’re likely to see erratic movement of the motor shaft.
·        Inability to reach the right levels of acceleration or deceleration: This can occur for several reasons, such as when the servo amplifier’s capabilities are insufficient for the system inertia, or the friction is excessive.
·        Not responding to a velocity command: The reason for a lack of response is usually easily identifiable, and common causes are problems with the control interface, system or motor malfunctions, incorrect voltage supply (or none at all), or the motor thermal protection has tripped.
·        Noise on signal wires: This is typically caused by incorrect wiring or grounding, but can also be caused by electromagnetic interference from nearby equipment.
·        System runs uncontrollably: This issue develops when there are problems with the velocity command signal, when the motor speed/position feedback signal is erratic or missing, or when there is an internal malfunction in the servo drive.
Motor Drive

Troubleshooting: What to Look For 
Correct identification of the problem is critical for effective troubleshooting of your servo drives. Here are some tips on how to go about determining causes of common issues, and measures you can take to try and resolve them.
#1: Review the display on the drive. If it doesn’t come up, check the power supply. If you see an alarm on the display, use the instruction manual to investigate the possible causes.
#2: Verify that the feedback device (resolver, encoder, etc.) is functioning properly.  Use an oscilloscope to check waveforms and pay special attention to noise, missing channels, incorrect wave shapes, or low levels. Look for breaks or bad splices in the feedback cable.
#3: Check the line voltage to ensure that the incoming  power to the drive is balanced and the correct voltage. For common DC bus systems, check the intermediate dc voltage as well.  Use an oscilloscope to check for noise, voltage fluctuations, etc.
#4: Don’t forget the possibility of a mechanical problem. Problems such as friction or vibration in the machine can cause issues in the servo drive and motor.
Run all the tests recommended in the manual and record the results for future reference. If these initial measures don’t work, it’s time to consider getting a professional service company to help.
You can also read: Stepper and Servo motors
About the Author
With over 25 years of experience in the industrial automation repair industry, Jeff Conner is the Dallas Service Manager for Control Concepts and serves on the Advisory Committee for the Electronics Technologies Department at Texas State Technical College. Control Concepts offers around the clock service and support anywhere you need it. To learn more, visit
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Tuesday, November 27, 2018

Key Components used in Industrial Control

Understanding the different components involved in a given process control is important especially for its proper application, and troubleshooting. In this article we look at the most common discrete components used in industrial control applications.
Electrical Symbols of Components commonly used in Industrial Control

Power consuming devices like Motors are usually controlled by contactors having heavy duty contacts that can switch the power circuits safely. The contactor will be actuated by an electromagnetic solenoid (coil) which pulls the contacts closed when energized. They have an arc-suppressing cover to quench the arc formed when the contacts open under load. Note that AC contactors should never be used to break the current of a loaded DC power circuit. It is more difficulty to quench the arc from a DC power load because there is no AC  ''zero crossing'' to interrupt the current flow. DC contactors are designed to specifically handle DC current. You will find they have embedded magnets, or special blow-out coils that are used to stretch the arc long enough to break the DC current flow.
Devices using AC solenoids will have one or more single turn coil (s), called a shading ring, embedded in the face of their magnetic armature assembly. When the solenoid is energized, the magnetic field has continuous reversals matching the alternating current that is been applied. This will produce a noticeable hum or chatter because the magnetic structure is not energized continuously. As the magnetic field reverses, it induces a current in the shading ring, which in turn produces a secondary reversing magnetic field. The secondary field is out of phase with that of the applied power, and holds the armature faces sealed continuously between power reversals, and minimizes the noise. Over time, shading rings tend to crack from the pounding of the armature faces. When this happens, the solenoid will become very noisy, coil current will increase, and premature failure will result. In an emergency, one can remove the damaged ring and replace it temporarily with a shorted turn of copper wire.
Solenoids are used to actuate brake/ clutch mechanisms, hydraulic valves, air valves, steam valves or other devices that require a push-pull button. Some solenoids can be quite large, requiring contactors rated for their high current draw. Smaller pilot valves may draw no more current than a simple relay. Some heavy duty units operate on DC current. The DC solenoids are often specified for operation in dirty or corrosive areas because current is controlled by circuit resistance, and will not rise if the air-gap is fouled. AC solenoids depend upon the impedance of the circuit. If the air-gap is not sealed properly, inductance reactance is reduced and coil will draw excess current and over-heat. Shading rings must also be switched for possible failures.
Relays have a similar construction to contactors, but since they switch low-current logic signals, they do not have a requirement for the heavy-duty contacts and arc-suppression hardware. Most relays contacts have AC continuous ratings of no more than 10 A. They can close on an inrush current of 150%, but only break 15% at 120 volts AC (vac). A NEMA A600 rating limits the in-rush to 7200 volt-amperes (va), and a circuit breaking rating of 720 volt-amperes. As higher voltages are used, the current capacity goes down proportionately. This difference in make and break ratings closely matches the typical ratio of inrush and holding currents of AC control coils. AC coils have relatively low resistance, allowing high in-rush currents. As the coil is energized, the AC current builds up inductive reactance. Total impedance (ohms), the vector sum of resistance and reactance, limits the continuous holding current. The ratio of in-rush to holding current is often 5:1 or more. Maximum impedance is attained when the air gap in the magnetic armature assembly has sealed closed. A failure to close this gap will reduce the inductive reactance, allowing more current, which will overheat the coil, causing premature coil failure. A shading ring fracture will also lead to overheating and coil failure.
The same 10A contacts are only rated at 0.4 A DC, at 125V, and 0.2 A DC at 250 V (50 va) because the small air-gaps are not adequate to break a sustained DC arc. Voltages for DC logic controls seldom exceed 24 volts with typical current in the milliamp ranges.
Relays may have multiple coils for latching and unlatching of the contacts. Contacts may be normally open (NO), and or normally closed (NC). The number of contacts usually varies from 1-8. Some relays use contact cartridges which can be converted for either NO or NC operation. Most standard relays will have totally isolated contacts. Some miniature relays have type ”C” contacts where a NO and NC contact share a common terminal. This construction requires careful planning to match the schematic wiring diagram to actual relay construction.
Occasionally the required load on relay contacts may be slightly higher than their normal rating. One can increase the current capacity by connecting contacts in parallel, and improve the arc suppression by connecting multiple contacts in series. When multiple contacts are used this way, they should be on the same relay, because relay coils operating in parallel may have a slight difference in their time-constant.  If one relay closes late, its contacts will take all the inrush punishment. If a relay opens early, it will suffer more damage from breaking most of the full load current.
Timers are a type of relay that have pneumatic or electronic, time contacts to meet various sequencing requirements. They may be, ”stand-alone” relays or attachments to standard relays. On-delay timers actuate the contacts at a preset time after the coil is energized, and reset instantly with power off.  Off-delay times actuate the contacts instantly when energized, and reset a preset time after de-energizing. NO and/or NC contacts may be time-closed or time-opened. Many timers also include instantaneous contacts, actuated with no time delay. Instantaneous contacts may also be added as a modification kit in some cases to stand-alone timers.
The coils of contactors and relay may be rated at a different voltage than the circuits being switched. This provides isolation between the low voltage logic circuits and higher voltages power circuits.
Push Buttons
Push-buttons may have a single contact block, or an assembly of multiple contacts depending upon the complexity of the requirement. Most push-buttons have a momentary change of state when depressed, then return to normal when released. Some may have a push-pull actuator that latches in each position. They are often used as a control-power master-switch with a Pull-on/Push-off action.
Selector Switches
Many selector switches have the same construction as push buttons, except that the contacts are actuated by rotating a handle or key-switch. The rotating cam may be arranged with incremental indices so that multiple positions and contact patterns can be used to select exclusive operations. Contacts of push-buttons, selector switches, and limit switches usually have the same rating as the logic relays 10A continuous at 120 (vac).
Limit Switches
Mechanical limit switches have many configurations. Most will have both NO and NC contacts available. The contacts are switched when the layer arm is rotated a few degrees by a moving cam or slider. The conventional drawing will show the contact conditions when the machine is un-powered and at rest. It is assumed that the cam will normally strike the arm on the switch to change the state of the contact(s).
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Non-Contact Limit Switches
There are a number of electronic limit switches that are used where it is not practicable to have an actuator arm physically contact a product or machine part. The switches include: Photocells, and Proximity switches (inductive, magnetic, and capacitive). In each case, a control signal is activated whenever an object enters its operating field. These devices require additional wiring to energize their power.

Saturday, November 24, 2018

The Basics of a Process Control System

Instrumentation is a science that deals with measurements and control of processes. Instrumentation is applied in almost any field from Medical, Manufacturing of pharmaceutical drugs, to simple processes like temperature control in homes using Thermostats. Note that if we can’t measure it then we have no need of controlling it.
Measurements can be in form of Fluid pressure, Fluid volume, Fluid Flow rate, Temperature, Electrical voltage or current, Chemical concentration etc. Once we have the quantity of the measured value,  we then transmit a signal  representing this quantity to an indicating or computing device where either human or automated action then takes place. If the controlling action is automated, the computer sends a signal to a final controlling device which then influences the quantity being measured. The final control device can be in the form of:
  • Electric motor
  • Control valve-for throttling the flow rate of a fluid
  • Electric heater
The measurement device and the final control device connect to some physical system called the process.
Process Control Basics
Block Diagram of a Process Control System

  • The measuring device senses
  • The Controller decides
  • The final control device influences the process
  • The process reacts to the influence of the final control

You can also read: Basics of a Control Loop
So in a nutshell this is what we call a Process control system.

Friday, November 23, 2018

Key differences between PLC and DCS systems

We have so many people with questions as to what are the similarities and differences if any between PLC and DCS systems. We will try to describe the working of these two systems and in the process help you understand the functions of each system.
Initially DCS was focused only on process control with analog signals that were used as main control system in process industries like Refining, Chemicals, and Petrochemicals etc. while PLC was focused on discrete automation with discrete on-off signals, that were used for example in Factory assembly lines and bottle lines but today DCS supports discrete I/O and some logic functions and PLCs support analog I/O with some control functions. In some instances Both PLC and DCS are used in the same plant i.e. PLCs are used on separate units on a plant floor which are then integrated with main plant-wide DCS for Control and Monitoring.
Let us now look at each system separately to help us understand more how they work.
Programmable Logic Controllers (PLCs) comes in different sizes which means various I/O and program capacities. Smallest sized PLCs are typically referred to as nano PLCs, micro PLCs and mini PLCs. They have fixed I/O and mainly used in stand-alone applications.
Large PLC support redundancy for CPU, power supply and possibly the control network, but typically not for I/O cards though there are large PLCs that support I/O redundancy by using duplicate I/O-subsystems with separate backplanes where the field instruments are wired in parallel to both I/O subsystems. The control network is typically a standard industrial Ethernet application protocol over Ethernet media and IP. The Field cabling comes directly onto the I/O card.
PLC usually support very fast scan times as required in discrete manufacturing but PID loops add to the CPU load, much more than discrete load thus making the scan time slower.
Loops are not handled individually in a PLC. Addition or change to loop requires a download of the entire program which affects other loops in the CPU as well.
PLCs are built around a given native protocol, this maybe: PROFIBUS, Modbus, DeviceNet etc. The PLC comes with its own native interface cards for native protocol supported by the PLC maker but relies on third-party interface cards for other Fieldbus protocols. The engineering software therefore automatically configures the communication interface card for the native protocol.
Key Points to note on PLC
PLCs were designed to eliminate assembly-line relays during model changeovers. PLC is easier to change than relay panels; this has reduced the installation and operational cost of the control system compared with electromechanical relay systems.

A basic block diagram of PLC system

PLC offers the following advantages:
  • Ease of programming and reprogramming in the plant
  • Programming language is based on relay wiring symbols familiar to most plant electrical and instrumentation personnel
  • High Reliability and minimal maintenance
  • Small physical size
  • Ability to communicate with computer systems in the plant
  • Moderate to low initial investment cost
  • Available in modular designs
Distributed Control System (DCS) supports redundancy for controllers, power supply and control network as well as redundant I/O cards including fieldbus interface cards in the same backplane. The control network supports peer-to-peer communication between controllers. The control network is typically a proprietary application protocol over Ethernet media and IP. The field cabling in DCS lands on a Field Terminal Assembly (FTA) where a special system cable with a connector takes the signals to the I/O card.
Loops in a DCS are executed individually. The scan time in a DCS is set individually for each loop. Most loops run at 1000ms although 250ms is common for pressure and flow loops in refining and petrochemicals and even 100ms is also possible. The scan time is constant, and does not change with task loading. This is important for PID control and time-based functions such as integration/totalizing and lead-lag dynamic compensation.
Loops in a DCS are managed individually. A change and download to one loop doesn’t affect the other loops.
A DCS has an integrated development environment where I/O control strategy and operator graphics are created together and stored in a single database. This means once a tag is created in the DCS it automatically becomes available everywhere in the system with the same human readable tag name for use in basic control, advanced control, graphics, faceplates, trending, alarming, and turning etc.  Without mapping data through registers or other tag names makes it easy to do changes or additions.
The Sensor & Actuator level “H1” Fieldbus network supported by DCS is basically FOUNDATION fieldbus for instrumentation and PROFIBUS-DP for motor controls.
The DCS comes with its own native Fieldbus interface cards. The engineering software therefore automatically configures the communication interface cards for the variables used in the control strategy and graphics.
Key Points to Note on DCS
DCS is miniaturized version of the multitasking, multivariable, multi-loop controller used for process control. It is functionally and geographically processing distributed system. Equipment making up a DCS is separated by function and is installed in two different work areas of a processing installation. Equipment for operator to monitor process condition and to manipulate the set point of the process operation is located in a central control room; from where the operator can view information transmitted from the process area and displayed on a video display unit and can change control condition from a keyboard. DCS systems are suitable for the following processes:

  • Where a single centralized system is not adequate i.e. Power, Steel, Pulp & Paper plants, Fertilizer etc.
  • Processes of different level of hierarchy
  • Processes which can be divided into different and functionally independent sections, based on functional scope and geographical distribution
DCS offers the following Advantages:
  • Compact to contain ON/OFF controllers
  • Reduced complexity and easy expandability
  • High Speed of the control processing
  • Control Algorithms changes do not call for hardware changes
  • Continuous trend data is available
  • User friendly but higher data security
  • Plant data are transparent on the network
  • Sequential, batching and feedback control are possible

Differences between PLC and DCS based Systems
Redundancy not possible or limited
Redundancy possible at every level
It is used for low loop count
It is used for any loop count
Performance drops with increasing loop count
No change in performance with increasing loop count
Purely free running mode
Highly efficient multitasking mode
Analog processing simulated through digital computer
Analog processing done in real frequency domain function
No interplant connectivity
Fully Functional Inter-plant connectivity
Individual database for every node
System-wide global database
Typical performance: 100 PID loops/sec
Typical Performance: More than 1000 PID loops/sec.

Do you have comments or questions on PLC or DCS? Feel free to post them in the comments section below.