Tuesday, March 19, 2019

Types of Signal Converters Commonly used in Industrial Control


Signal converters change a signal from one form to another. In most cases, we have standard inputs and output ranges.
Types of signal converters

Most signal converters have two adjustments-zero and range.
Examples of Signal converters
Nozzle-flapper and differential pressure cells
The nozzle-flapper system is widely used in D.P. Cells. An example is shown below, that converts differential pressure (e.g. from a differential Pressure flow meter into a standard pneumatic signal). This is widely used in the control of air operated pipeline valves.
Flapper Nozzle System

The bellows respond to the differential pressure and moves the lever. This moves the flapper towards or a way from the nozzle. The air supply passes through a restrictor and leaks out of the nozzle. The output pressure hence depends on how close the flapper is to the end of the nozzle. The range of the instrument is adjusted by moving the pivot and zero position is adjusted by moving the relative position of the flapper and nozzle. This system is used in a variety of forms. Instead of bellows, a bourdon tube might be used and this is operated by an expansion type temperature sensor to produce a temperature-pneumatic signal converter.
Current/Pressure conversion 
The figures below show typical units for converting 4-20 mA into 0.2-1 bar or 3-15 psi. They contain adjustments for range and zero. They are widely used for converting the standard pneumatic and electric signals back and forth. They can also be adjusted to work with non standard inputs to convert them into a standard form.
Current to Pressure Converter (C/P)
Current to Pressure Converter

Pressure to Current Converter (P/C)
Pressure to Current Converter

Electric D.P. Cells
They provide the same functions as the pneumatic versions but they are given an output of 4-20 mA using electrical pressure transducers. They are typically used with D.P. Flow meters.
Differential Pressure Transmitter
Differential Pressure Transmitter

Analogue -Digital Converters
Analogue to digital conversion is a process of turning an analogue voltage or current into a digital pattern which can read by a computer and processed, this is done by analogue -digital converters.  Lets look at the Binary Numbers, a number may be represented in digital form by simply setting a pattern of voltages on a line high or low. It is normal to use 4, 8, 16 or 32 lines. An 8 bit binary pattern is shown in the below figure:
Binary Numbers
The total pattern is called a word and the one above is an 8 bit word. The pattern may be stored in an 8 bit register. A register is a temporary store where the word may be manipulated. The Bit zero is called the least significant bit (LSB) and the bit with highest value is called the most significant bit (MSB). Each bit has a value of zero when off (LOW) or the value shown when on (HIGH). The maximum value for an 8 bit word is 255.
Digital to Analogue Converters
These are devices for converting a binary number into an analogue voltage. The change in the binary value from zero to a maximum corresponds with a change in the analogue value from 0 to maximum.
One of the manufacturers of signal converters is Omega.
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Sunday, March 10, 2019

The Inputs and Outputs of Process Measurement Instruments Commonly used in Control Systems

Basically when you are doing the troubleshooting of any instrumentation system you assume that every instrument has at least one input and at least one output and that the output(s) should accurately correspond to the input (s). In normal circumstances, if the instrument’s output is not corresponding to its input according to the instrument’s design function, then there could be something wrong with the instrument. Lets consider the inputs of the following examples of  instruments that are commonly used in process control systems:
  • Differential Pressure transmitter
  • Temperature Transmitter
  • Controller

Process measurement instruments


Controller
Each of the above instruments takes in (input) data and generates the (output) data.  In an instrumentation loop, the output of one instrument feeds into the input of the next. Such information is passed from one instrument to another.
By intercepting the data communicated between components of an instrument system, we are able to locate and isolate faults. For us to able to properly understand the intercepted data, we must understand the inputs and outputs of the respective instruments and the basic functions of those instruments. From the above diagrams, we  are able to highlight the kind of inputs and outputs for each of the instruments indicated.
To be able to check the right correspondence between the instrument inputs and outputs, we must therefore use appropriate test equipment to intercept the signals into and out of these instruments e. g. in case of analogue instruments using 4-20 mA signals we can use the electrical meters capable of measuring the current and voltage.
So what are some of the key considerations when using milliameters to measure loop current?
For you to measure the loop current, you have to break the circuit to connect the milliameter, in series with the current, and which means the current will fall to 0 mA until the meter is connected. Interrupting the current means interrupting the flow of information that is conveyed by that current, be it a process measurement or a command signal to a final control element. This can have adverse effects on the control system unless certain preparations are made before hand. The preparations can be in form of:
  • Informing the personal in charge that signal will be interrupted - state the number of times you intend to do the interruption.
  • For case, where the signal is coming from a process transmitter to a controller, the controller should be placed in manual mode, so that it will not cause an upset in the process. 
  • If the current drives process shutdown alarms, these should be disabled on temporarily basis, so that nothing shuts down upon the interruption of the signal.
  • All process alarms should be temporarily disables so that they do not cause panic.
  • If the current signal to be interrupted is a command signal from a controller to a final control element, the final control element either needs to be manually overridden so as to hold a fixed setting while the signal varies or it needs to be bypassed completely by some other devices (s)
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Thursday, February 7, 2019

Ultrasonic Level Switches


This type of level switch uses ultrasonic sound waves to detect the presence of process material either solid or liquid at one point.
The Working Principle
Sound waves pass back and forth within the gap of the probe, sent and received by piezoelectric transducers. The presence of any substance other than gas within the gap detects the received audio power, thus signalling to the electronic circuit within the bulkier portion of the device that process level has reached the detection point.
Ultrasonic Level Switches
Fig 1

Ultrasonic Level Switches
Fig 2

In Fig 1, the sound passes through the gap when filled with liquid (wet gap).
In Fig 2, the sound cannot pass through an air-filled gap (dry gap).
The lack of moving parts makes this probe quite reliable although it may be become fooled by heavy fouling.
Some of the industrial applications of Ultrasonic level  switches include:
  • Leak detection
  • As an overfill alarm
  • Detection of presence of liquids in pipeline
  • Pump Protection
  • Engine oil level control

Tuesday, February 5, 2019

The Components that make up a 4-20 mA Current Loop

The 4-20 mA current  loop is an important aspect of process measurement and control circuits. The important components that make up this loop are:
  • The Sensor
  • The Transmitter
  • The Power Source
  • The Loop itself
  • The Receiver

The following diagram shows each component with its function:
Components in a 4-20 mA current loop

This is a simple illustration of a 4-20 mA current Loop System.
 You can also read:


Thursday, December 27, 2018

Key Applications of Thermal Mass Flowmeters

Thermal mass flowmeters work on the principle that when you place a heated object in the midst of a fluid flow stream and measure how much heat the fluid transfers away from the heated object, then you can be able to determine the mass flow rate. Industrial thermal mass flowmeters consists of a specially designed flow tube with two temperature sensors inside: One that is heated and one that is unheated. The heated sensor acts as the mass flow sensor (cooling down as flow rate increases) while the unheated sensor serves to compensate for the “ambient” temperature of the process fluid.  The following diagram shows an example of a thermal mass flowmeter from Magnetrol.
Magnetrol Thermal Flowmeter
Magnetrol Thermal Flowmeter

An important factor in the calibration of a thermal mass flowmeter is the specific heat of the process fluid. Fluid with high specific heat values make good coolants because they are able to remove much heat energy from hot objects without experiencing great increases in temperature. Since thermal mass flowmeters work on the principle of convective cooling, this means a fluid having a high specific heat value will elicit a greater response from thermal mass flowmeter than exact same mass flow rate of a fluid having a lesser specific heat value. Therefore it is paramount that you know the specific heat value of the fluid you plan to measure with a thermal mass flowmeter and be assured that its specific heat value will remain constant. For this reason, thermal mass flowmeters are not suitable for measuring the flow rates of fluid streams whose chemical composition is likely to change over time.
Another potential limitation of thermal flowmeters is the sensitivity of some designs to changes in flow regime. Since the measurement principle is based on heat transfer by fluid convection, any factor influencing the convective heat-transfer efficiency will translate into a perceived difference in mass flow rate. Turbulent flows are more efficient at heat convection than laminar flows. Therefore, a change in flow regime from turbulent to laminar will cause a calibration shift for this design of thermal mass flowmeters.
So what are some of the applications where thermal flowmeters are used?
Generally thermal flowmeters are used in applications where the composition of the fluid is known especially in purified gases. Having said that, lets look at some of the areas where thermal flowmeters are commonly used:
Natural Gas
Flow measurement of natural gas fuel usage is important for combustion efficiency as well as general energy management projects for both industrial and commercial facilities. Thermal mass flowmeters will monitor the flow to individual combustion sources.
Air Efficiency
In combustion applications, thermal mass flow meters can ensure repeatability of air flow measurements to obtain an efficient air-to-fuel ratio. On compressed air, it is common to measure for plant allocation or to determine leaks.
Tank Blanketing
Nitrogen is frequently used to maintain an inert environment in the vapor space of a tank. Thermal meters are ideally suited for measuring the flow of nitrogen in such applications because they support mass measurement, they are easily installed into the pipe & are excellent at measuring low flow rates.
Flare & Vent Gas
Thermal mass meters are a particularly good fit for flare measurement. Flares can range from vent gases at atmospheric pressure to high flow applications needing extended turn down. Oil and Gas is a common industry here.
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Biogas
Both the landfills and anaerobic digesters at wastewater plants produce a mixture primarily composed of methane and carbon dioxide. Excellent low flow sensitivity, hot tap capabilities and mixed gas calibration make thermal mass flow measurement a popular technology.
Sources: Magnetrol

Thursday, December 13, 2018

Types of Proximity Sensors used in Industrial Control

Almost every automated manufacturing operation has sensors that ensure that the system is working correctly.
Examples of Sensors that are used in industrial control are:
Non-Contact Presence Sensors (Proximity Sensors)
Contact sensors are often avoided in automated systems because wherever parts touch there is wear and a potential for eventual failure of the sensor. Automated systems are increasingly being designed with non-contact sensors. The three most common types of non-contact sensors in use today are:
  • Inductive proximity sensor
  • Capacitive proximity sensor
  • Optical proximity sensor

The above sensors are actually transducers, but they include control circuitry that allows them to be used as switches. The circuitry changes an internal switch when the transducer output reaches a certain value.
Hall Effect Limit Switches
Hall Effect Limit Switch

The inductive Sensor
This is the most widely used non-contact sensor due to its small size, robustness, and low-cost. This type of sensor can only detect the presence of electrically conductive materials.
The DC power supplied is used to generate AC in an internal coil, which in turn causes an alternating magnetic field. If no conductive materials are near the face of the sensor, the only impedance to the internal AC is due to the inductance of the coil. If however, a conductive material enters the changing magnetic field, eddy currents are generated in that conductive material, and there is a resultant increase in the impedance to the AC in the proximity sensor. A current sensor, also built into the proximity sensor detects when there is a drop in the internal AC current due to increased impedance. The current controls a switch providing the output.

Inductive Sensor
Inductive Sensor

 
Capacitive proximity Sensors
These sensors sense the target objects due to the target’s ability to be electrically charged. This works both on conductors and non-conductors.
Inside the sensor is a circuit that uses the supplied DC power to generate AC, to measure the current in the internal AC circuit, and to switch the output circuit when the amount of AC current changes. Unlike the inductive sensor, the AC does not drive a coil, but instead tries to charge a capacitor. The AC can move current into and out of this plate only if there is another plate nearby that can hold the opposite charge. The target being sensed acts as the other plate.
Capacitive Proximity Sensor
Capacitive Proximity Sensor

If this object is near enough to the face of the capacitive sensor to be affected by the charge in the sensor’s internal capacitor plate, it will respond by becoming oppositely charged near the sensor, and the sensor will then be able to move significant into and out of its internal plate.
Optical Proximity Sensors
These are widely used in automated systems because they have been available longer and some can fit into small locations. They are commonly known as light beam sensors of the thru-beam type or of the retro-reflective type. A complete optical proximity sensor includes a light source, and a sensor that detects the light. The light source is supplied because it is usually critical that the light be tailored for the light sensor system. The light source generates a light of a particular frequency which is able to be detected by the light sensor in use. Infra-red light is used in most optical sensors. To make the light sensing system more foolproof, most optical proximity sensor light sources pulse the infra-red light on and off at a fixed frequency. The light sensor circuit is designed so that light that is not pulsing at this frequency is rejected.
Optical Sensors
Optical Proximity Sensors

The light sensor is a semiconductor device such as a photo diode which generates a small current when light energy strikes it or more commonly a photo transistor or a photodarlington that allows current to flow if light strikes it.
Some of the manufacturers of proximity sensors include:

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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.
Programming Languages of PLC
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

Functions of each component:


CPU – This the unit that contains microprocessors
Input and 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 device – 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.
The Operation of a PLC
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

Disadvantages:
  • 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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