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01 March 2003

Signal conditioning through time

The future sees the signal conditioner evolving into more of a network conditioner.

By Lee Sochay

Signal conditioners are the great melting pot of industrial communications.

A typical plant operates and manages many different pieces of equipment. This equipment needs to operate in specific manners and in specific sequences to achieve maximum production efficiency and profitability.

Pressure, level, temperature, speed, and flow signals need monitoring and controlling. Signal conditioning grew out of the need to incorporate the various types of signals generated from the different pieces of equipment to create a workable and interactive process.

Signal conditioning is as vital to the manufacturing process today as it ever was. From the early days of signal conditioning of the magnetic amplifier and analog circuitry to the digitally based technology of today, one can observe how changes in technology affect the role and design of signal conditioning products and applications.

In the future, without changing the basic role of incorporating various process signals in monitoring and control processes, we can see the signal conditioner evolve into more of a network conditioner.

PLC CAPABILITIES GROW UP

During the early days of process control, systems derived from hard-wired relay logic. Individual relays mounted in a panel and interconnected in a way to provide a sequentially logical system.

As the cost, size, and complexity of these relay panels increased, the programmable logic controller (PLC) surfaced as an alternative. At first, the PLC mimicked the larger relay panels and provided only for discrete I/O control.

Eventually, the capabilities of the PLC grew to include analog I/O signals as well. This is where signal conditioning became important. Originally, signal conditioning processed one signal at a time.

This process signal, whether a voltage or a current, would be transmitted to a controller so that it could influence the process in its logical sequence.

The main component of these signal con ditioning boxes contained the magnetic amplifier. This, along with other analog circuitry, would amplify and condition a signal either to boost its range or to convert it to a usable format.

These types of circuits would mount in a metal enclosure, which in turn could surface mount in a panel. The biggest advantages of the magnetic amplifier-based signal conditioners were that they were both reliable and rugged.

The disadvantages were that products utilizing the magnetic amplifier were labor intensive to manufacture. They required expensive magnetic cores, design, and manufacturing expertise and took up more panel real estate than the solid state designs of today.

BUNDLE SIGNALS TO RACK

In conforming to pressure to reduce factory floor footprint and facilitate the ease of use of signal conditioners on the plant floor, the next developmental step in the signal conditioning industry was the design of the rack-mounted system.

Instead of having single boxes mounted separately for each signal, signal conditioners could bundle and mount in one location. In many instances, you could have up to 10 channels of I/O in one rack-mountable box.

Depending on the design, you could have one bus for power and a terminal strip for the signal wiring. This more compact design helped reduce installation costs and create plant floor space. It also centralized and simplified maintenance on the signal conditioners.

The controls for configuring and calibrating the signal conditioning modules could be accessed from the front to prevent dismantling the single signal conditioner enclosure. This not only saved time but also added protection for the internal circuit board and wiring against the external environment.

The rack-mounted systems continue to be of use, but subtle improvements have helped them become even smaller and lighter. One such improvement was the use of plastic rather than metal enclosures.

With circuit board components continuing to get smaller in size, manufacturers used smaller plastic enclosures to house the signal conditioner electronics. The intention was to combine the smallest possible design and retain the ability to easily work on test points and terminal strips without losing functionality or performance.

Also, in an effort to increase adaptability and flexibility, enclosures came to market that fit field-mounted applications including signal conditioners in hazardous and explosive areas.

In certain applications, costs were lower because of the difference between installing thermocouple wire all the way back to a panel and using lower-cost signal cable in temperature monitoring and control systems.

AND THEN CAME THE MICROS

The next advancement came in the form of the octal socket plug-in module. Used in conjunction with field-mounted signal conditioners, these modules would take up less panel space than the rack mount or the metal-encased signal conditioners previously used.

Easy installation and maintenance continued. The socket, being a separate piece, could prewire and mount in a panel. The signal conditioner could then simply plug in to the socket. Adjustments could occur on the top of the signal conditioner or through enclosure openings.

An innovation in mounting the signal conditioning modules into control panels came from a European standard called DIN-rail mounting. This is a clip-on mounting that was to simplify the installation of the signal conditioners.

DIN rail

DIN rail mount

The advantages were that you could place the modules side by side and therefore save on panel space. On the marketing end, signal conditioners could sell in the European markets that were already standardizing on DIN rail.

All these developments and improvements that took place dealt with size issues, mounting capability, or physical characteristics. The analog circuit technology re mained foundationally the same. Then came the micros.

The microprocessor changed the way we view process monitoring and control. The incorporation of the microprocessor affects the hardware design as well as the system architecture.

It enhanced the information capability of process instrumentation such as field devices, I/O devices, and controllers. This enhancement means that fewer devices and smaller device packages are required to process the same amount of or more information, which results in decreased hardware and installation costs.

DUMPING JUMPERS' ANALOG

The use of digital signal processing improved the functionality of signal conditioners in many different ways. One way is that the configuration and calibration of signal conditioning modules could now take place using push buttons, dip switches, and toggle switches.

This capability makes the signal conditioning modules more user-friendly than their analog counterparts that used potentiometers and jumpers. During calibration, potentiometers needed to be adjusted, verified, ad justed again, reverified, and so on to reach the desired zero and span.

Also, the zero and span ranges in an analog circuit design were interactive. This means that by turning the zero potentiometer, the span point can also move and vice versa.

So, configuring and calibrating analog circuit-type signal conditioners could be a long and confusing process, monopolizing a service technician's valuable time, especially in a start-up or troubleshooting situation.

With a digitally based system, the zero and span settings are noninteractive. This means one could work on either the zero or span without affecting the other setting, simplifying the installation and commissioning of equipment.

Because the incoming process signals convert to a digital signal for processing, the types of incoming signals that are acceptable are more numerous.

For example, one temperature transmitter module can accept more than one type of thermocouple sensing element and also different types of temperature sensors such as resistance temperature detectors (RTDs).

Or a DC current input module can also accept a DC voltage input signal. These universal I/O ranges can cover a wide variety of applications, which can reduce costs in spare stock inventories.

PROTECTION AGAINST THREATS

Analog circuits by nature are susceptible to radio frequency interference, electromagnetic interference, and electrostatic discharge from other devices.

A signal conditioner's own radiated emissions can also affect its performance as well as that of other devices mounted nearby. To incorporate protection against such threats necessitated a higher cost and larger device.

With the development of digital circuit technology, coping with these threats is easier. Smaller circuit board components enable greater ability to incorporate filtering circuits to cut down on externally and internally generated noise.

Also, smaller circuit components allow for greater flexibility in circuit board layout to separate known generators of noise that may affect performance. The use of solid state circuitry also requires less power, which cuts down on emissions that affect itself and other devices as well.

And the multilevel circuit board and circuit component improvements allow for smaller and smaller packages, making installation less costly and saving on valuable plant space.

Included in the design changes with the digital technology were removable connectors. Like the plug-in sockets before, these eased installation and maintenance of the signal conditioning modules.

Octal socket

Surge signal detector

ENTERING THE REMOTE I/O WORLD

The next step in the age of the microprocessor was the ability to communicate to the signal conditioning modules via a serial link with a computer. Software programmability simplified and enhanced configuration, calibration, diagnostics, and overall functionality of signal conditioners.

Designs were based around typing in desired ranges, using pull-down menus to choose a sensor type, uploading and viewing the configuration of a particular module, downloading and testing the module and the desired configuration before field installation, saving current configurations for downloading to future required modules, characterizing different types of response curves, adding in alarm functions for added control and supervision, and even utilizing the capability of the microprocessor to perform different math functions.

Other advantages included compact module design, faster response times, and low power consumption. All these add up to a more flexible and rugged signal conditioner.

The implementation of the microprocessor also enables signal conditioning to enter the world of remote I/O through different communication buses and protocols. Among these protocols are Modbus-RTU, LonWorks, Profibus, AS-interface, DeviceNet, Foundation fieldbus, and others.

Modbus-RTU, for example, is a master/ slave technique communicating over RS-485, in which the master device initiates transactions or queries, and the slave devices respond to the master by supplying the requested information.

Signal conditioners can take the form of a slave device in delivering several channels of remote I/O in one module. Input and output signals include discrete signals, temperature signals (RTDs, thermocouples, or thermistors), DC currents and voltages, and even frequency signals.

RUGGED WAYS PLACE REMOTES

These incoming signals can convert to a digital message and become available in certain registers that the programming network assigns.

The host PC, PLC, or distributed control system utilizing the Modbus-RTU protocol then recognizes and converts the data messages into the process variables that are required to monitor and control the process.

This enables a faster and more flexible control system that uses less power and less plant floor space than those previously implemented.

Through the use of remote I/O signal conditioning devices, one can utilize the power of networking and the advantages of signal conditioners such as isolation between I/O, power, and network in which potential ground loops are eliminated.

Because these modules are monitoring devices only and interact with the host, purchasing separate remote control processors is not necessary. This saves in overall hardware costs, programming time, and installation and wiring costs.

Several signals can bundle, and only the RS-485 cable needs to route back to the host device. Communication may also be via the use of radio transceivers and phone modems.

These types of remote I/O products are incorporating into the signal conditioning model, providing low-cost, rugged ways to place remote I/O into the factory network.

PLACE I/O ON MIX OF NETWORKS

So, with advancements from analog circuitry from the early days of modern signal conditioning to the inclusion of the microprocessor and digital technology to incorporating software programmability and network communication, where does signal conditioning go from here?

With more and more competition in the marketplace and an increase in systems integration, manufacturers demand smaller size, greater flexibility, faster speed, and more function in their control systems.

Many different protocols exist today, and that will continue to be the case in the future. A kind of network conditioning or universal protocol I/O module may be required to compete in this future marketplace.

This would offer the ability to place I/O on any network or a mix of networks in a low-cost and efficient manner.

The advantages of evolving technologies in creating faster and smaller circuit components, where the physical characteristics of a module could also become smaller, would provide for increased flexibility and speed on the factory floor.

Even though we cannot predict exactly what the future may bring, the principles of greater speed and flexibility will certainly dominate the manufacturing process.

So although the classical applications of isolation, loop characteristics, and signal conversion seem to be diminishing, the networking applications seem to be on the rise.

The take-away from this evolution of signal conditioning is that we need to make use of the latest and greatest technologies to provide for innovative and useful forms of signal conditioning and network conditioning. IT

Behind the byline

Lee Sochay is an engineer of process products at Acromag, Inc. (www.acromag.com) in Wixom, Mich.

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