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Signals - A look at 4 - 20 Milliamp Signals - Arthur Holland, Holland Technical Skills - A good description of 4-20Ma technology.
Signal
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
Industries.
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
2.12 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.
Stop
Ground Loops
Share
a Process Signal at Two Locations
Step
Down Unsafe High-Level Signals
Put
an End to "Bucking" Power Supplies
Boost
Power to an Overloaded Loop
Protect
Your Loop with Area Isolation
Convert
Signal for DCS Analysis
"Pass
Power" Saves Time and Money
Cut
Costs with Power Supply Sharing
Isolator
"Passes" HART Digital Signal
Field-Configurable
Converter Blocks RFI
Isolator
Gets Power from Input Side of Loop
One
Isolator Can Take the Place of Two
HART
Signal Interference
Monitoring
Battery Voltage
Split
Range Valve Control
RFI
Plagues Variable Frequency Driver
Custom
Linearization Handles Odd-Shaped Tank
Compressor
to PLC Interface
Thanks to Moore Industries
