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Signals - A look at 4 - 20 Milliamp Signals - Arthur Holland, Holland Technical Skills - A good description of 4-20Ma technology.
Isolators, Converters and Interfaces: The “Ins” and “Outs” -
whether you call them signal isolators, signal converters or signal interfaces,
these useful process instruments solve important ground loop and signal
conversion challenges everyday. Just as important, they are called upon to do a
whole lot more. They can be used to share, split, boost, protect, step down,
linearize and even digitize process signals. This guide will tell you many of
the important ways signal isolators, converters and interfaces can be used, and
what to look for when specifying one - from our sponsor Moore
What is Signal Isolation? - In most processes there are pieces of electronic measurement and control equipment from many different manufacturers. The signals from these instruments are interconnected to each other and to sensors, transducers and output devices connected in the process loop. In any such measurement and control system there are several electrical interfacing problems that are likely to occur, all of which can be solved by incorporating the appropriate isolation between the signals - from APCS.
Signal Conditioning Engineers Guide - This guide contains a wealth of information on basic principles, applications, and functional safety. At over 23 Meg this is a massive download, but it is worth it - from Pepperl+Fuchs.
The following technical papers are from Acromag
Temperature Measurement Using RTDs - A RTD or Resistance Temperature Detector is a passive circuit element whose resistance increases with increasing temperature in a predictable manner. In choosing one you must consider a RTDs temperature coefficient of resistance (TCR), its relative sensitivity, its accuracy and repeatability, interchangeability, stability and drift characteristics, its insulation resistance, its response time, plus its packaging and the thermal transfer mechanism between the sensed material and the sensor element. You must also consider the negative effects of corrosion and contamination, shock and vibration, self-heating, meter loading, and in some cases, even thermoelectric effects.
Temperature Measurement Using Thermocouples - You are probably somewhat familiar with the thermocouple, but you must understand that choosing the type (J, K, T, etc.) is not as simple as just picking a compatible temperature span. You must give consideration to the sensor materials, the ambient temperature range, the sensor's sensitivity, and its reaction with the measurement environment. You must also be aware of the inherent limitations of the thermocouple and potential error sources. This white paper will help you to make an informed selection between sensor types and avoid potential problems in your application.
The following Signal Conditioner papers are from Omega.com
Introduction to Signal Conditioners - A signal conditioner is a device that converts one type of electronic signal into a another type of signal. Its primary use is to convert a signal that may be difficult to read by conventional instrumentation into a more easily read format. In performing this conversion a number of functions may take place.
Analogue I/O Functionality - Today, digital computers and other microprocessor-based devices have replaced analog recording and display technologies in all but the simplest data acquisition applications. And while computers have had an undeniably positive impact on the practice of data acquisition, they speak only a binary language of ones and zeroes. Manufacturing processes and natural phenomena, however, are still by their very nature analog. That is, natural processes tend to vary smoothly over time, not discontinuously changing states from black to white, from on to off. To be meaningfully recorded or manipulated by a computer then, analog measurements such as pressure, temperature, flow rate, and position must be translated into digital representations.
Digital I/O Functionality - In contrast to analog transducers that sense continuous variables such as pressure and temperature, many transducers provide an output that is one of two states: high or low, open or closed. A pressure might be too high or a temperature too low, triggering closure of a switch. Outputs, too, are not strictly analog-solenoid valves typically are opened or closed, many pumps and heaters are simply turned on or off. Pulse signals are another form of digital I/O, with one rotation of a turbine flowmeter or tachometer corresponding to a single, countable event. Digital I/O also can be used for parallel communications among plug-in expansion cards, and to generate clock and other timing signals.
Analogue Signal Transmission - Although the microprocessor and digital network technologies have fundamentally reinvented the ways in which today's data acquisition systems handle data, much laboratory and manufacturing information is still communicated the "old" way, via analog electrical signals. And a fundamental understanding of how analog signal transmission works must first begin with a discussion of electrical basics. To understand the ways in which an analog signal is transmitted over a circuit, it is first important to understand the relationships that make analog signal transmission possible. It is the fundamental relationship between voltage, current, and electrical resistance (Figure 3-1) that allow either a continuously varying current or voltage to represent a continuous process variable.
Digital Signal Transmission - Industrial networks that transmit data using digital signals often are an integral part of a data acquisition or process control solution. A basic understanding of the network technologies that are available for various applications is required to make the best implementation decisions-decisions that can have a profound effect on the ability to adapt to ever-changing technologies.
Data Acquisition Hardware - Previous chapters of this volume have acquainted you with the technology of how input/output (I/O) signals are generated, conditioned, linearized, and transmitted to a host computer or controlling system. In this chapter, we will look at the different kinds of data acquisition hardware that are available to help you perform all of these functions.
Acronyms at a Glance - A useful list
Signal Conditioning - This article reviews signal conditioning equipment features such as accuracy, adjustability, isolation, surge withstand capability, RFI/EMI protection, packaging, and repairability. Where possible, it also examines the relative importance of various features - from Acromag.
The following technical papers are from Acromag
Introduction to the Two-Wire Transmitter and the 4-20mA Current Loop - In two-wire 4-20mA control loops, we use 2-wire transmitters to convert various process signals representing flow, speed, position, level, temperature, pressure, strain, pH, etc., to 4-20mA DC for the purpose of transmitting the signal over some distance with little or no loss of signal. This paper reviews the operation of this transmission standard and its advantages, in particular as it relates to two-wire transmitters and the associated 4-20mA current loop.
White Paper: Why You Need USB Isolation for Industrial I/O - The USB port has become the most popular method for connecting virtually anything to a computer such as simple flash drives to complex industrial I/O equipment. Most computers built in the last 5 to 10 years have at least 1 USB port with newer computers having upwards of 6 or more. For most home users, having an isolated USB connection is not an issue, however for things such as industrial/remote I/O, data acquisition, IT or medical equipment, isolating a USB connection can be a necessity. This paper will look at the advantages of using USB, what isolation means, types of isolation, when and why USB connections should be isolated.
A Guide to Selecting the Right Isolator - A classical application isolates the control room equipment (computers, PLC, DCS etc.) from field devices which may have different ground potentials. In addition to breaking up ground loops, the isolators protect control room equipment from damaging transient spikes and noise generated in the field. Choosing the proper and most cost-effective isolator requires an understanding of the application and consideration of future expansion requirements.
Remote Emergency Shutdown Device Improves Safety and Performance at Oil Production Platform - Jim McConahay and Richard Conway -When Italian multinational oil and gas company ENI Petroleum needed help designing a real-time communications link to its Devil’s Tower oil well platform off the coast of Louisiana, they turned to Moore Industries for help. By integrating our NCS NET Concentrator System® into their communications system, ENI Petroleum has developed a reliable method for dealing with potential emergency situations that meets new federal regulations and reduces the possibility of false shutdowns - from Moore Industries.
Power to an Overloaded Loop
Compressor to PLC Interface
Convert Signal for DCS Analysis
Custom Linearization Handles Odd Shaped Tank
Cut Costs with Power Supply Sharing
Field-Configurable Converter Blocks RFI
HART Signal Interference
Isolator Gets Power from Input Side of Loop
Isolator "Passes" HART Digital Signal
Monitoring Battery Voltage
One Isolator Can Take the Place of Two
Protect Your Loop with Area Isolation
Put an End to "Bucking" Power Supplies
RFI Plagues Variable Frequency Drives
Share a Process Signal at Two Locations
Split Range Valve Control
Step Down Unsafe High Level Signals
Stop Ground Loops
Thanks to Moore Industries