Sensors: Smart devices, smarter solutions

Sensors, today, can be found almost everywhere in industry, from conveyors and boilers to process lines and air handlers. Major sensor technologies are not new.
By Jeanine Katzel August 1, 1999

Sensors, today, can be found almost everywhere in industry, from conveyors and boilers to process lines and air handlers. Major sensor technologies are not new. Proximity and photoelectric sensing, liquid level control, and limit switches have been available for some time. However, thanks to advances and innovations in such areas as microprocessor technology, new, more intelligent products offer greater performance capabilities and more numerous features.

Essentially, sensors have gotten smarter. But what exactly does that mean? Definitions are as numerous as sensor types, but some common characteristics apply. Most simply, a sensor is a device that responds to a physical stimulus and transmits the resulting impulse in some way. Adding a microprocessor increases its ability to generate more accurate information. Regardless of sensor type, functionality can be increased with the integration of intelligence into the device.

The evolution of a standard sensor to an intelligent device can be described in five steps.

1. Initially, a basic signal conversion device converts a condition or image into a measurable signal. All sensors have this capability and many need nothing more to operate optimally.

2. The first level of intelligence adds some type of signal conditioning that translates that output signal into something more compatible with the system.

3. At the next level, a sensor is equipped with communicating capability. For example, a sensing device is connected to a compatible communications bus in a building. It broadcasts the values it is measuring and shares the data with other control systems.

4. More sophisticated devices make decisions. They perform diagnostics, and are self-calibrating or self-ranging.

5. At a higher level of intelligence, a control function is integrated with a sensor function. Such a mini-sensing system might check humidity and temperature, and control an actuator that activates a damper.

A smart sensor is not required in every application. Sometimes it is neither necessary, nor economically attractive. In every case, the user must look at the overall application, including the total installed cost of each option.

How smart can sensors get? In this new age of rapid technological advancements, the limits are truly unknown. However, the increasing abilities of these devices undoubtedly will continue to help plants gather, sort, and analyze information and bring untold benefits to the plant floor.

Plant Engineering magazine acknowledges with appreciation the special contributions made to this article by Eaton/Cutler-Hammer, Cleveland, OH; Honeywell, Inc., Minneapolis, MN; ISA, International Society for Measurement and Control, Research Triangle Park, NC; Omron Electronics, Schaumburg, IL; and Rockwell Automation/ Allen-Bradley, Milwaukee, WI.


Color recognition


Fiber optic

Flow monitor switches


Fluid/level control

Limit switches








Sensors and smart sensors: From simple to complex

A sensor is a device that responds to a physical stimulus and transmits the impulse that results. Without sensors, there are no data. Inputs such as heat and light are converted by the sensor into a variety of outputs, including resistance changes and color changes (A).

A smart sensor is typically defined as a sensing device that uses microprocessor technology to generate more accurate information than a regular sensor. For example, solid-state sensors operate with microprocessors that use digital pulses. Yet many of the sensors are analog. By converting analog signals to digital, the sensor can operate smarter, assuming some of the logic function that would otherwise reside in the microprocessor. More sophisticated capabilities can also be added to a sensor. These features include compatibility with multiplexed communication systems, advanced logic capabilities, and self-diagnostics.

In a basic sensor (B), a condition is converted to a measurable signal. Environmental compensation (C) lets a sensor correct for changes in its operating environment and may incorporate circuitry to protect itself. With a communications link (D), a sensor communicates with the system without using an intermediate interface device. It may also be able to interface with certain protocols. The smart sensor equipped with self-diagnostics (E) can let the system know when it is developing operating difficulties or generate an output that indicates it has failed. Adding logic and control switching (F) makes the sensor a mini-system, giving it the ability to decide what needs to be done and in what sequence.

This information is drawn from Smart Sensors by Paul W. Chapman. Illustrations are copyright ISA, reprinted with permission.

Sensors resource guide

A virtual treasure trove of information is available on smart sensors, sensor technologies, and sensor selection and application both in print and online. For information on the offerings of a specific manufacturer, check out the individual company web site. Use an internet search engine to locate information on various types of sensors in general. A few resources are noted here as a starting point. This web site for Eaton/Cutler-Hammer offers detailed information on proximity and photoelectric sensors. Select sensing solutions from the products and services listing on the home page. Choose North America from the efector, inc., home page, then select product overview to find data on sensors for industrial control applications. Included is information on sensors for position control and fluid and level control. A comprehensive Sensor Catalog is also available. For information about obtaining a copy, contact the company at 805 Springdale Dr., Exton, PA 19341; 610-524-2000; fax: 800-329-0436. The Honeywell, Inc., web site leads the user to volumes of information from numerous divisions, including sensing products and building controls. Catalogs, tutorials, PowerPoint presentations, comprehensive glossary, and more are offered. The web site of ISA, the International Society for Measurement and Control, features abundant information about sensors and related topics. Included are news items, information on the society, standards, publications, events, books, and much more. A web resources link takes users to a list of more sensors web sites. The Omron Electronics industry button takes the user to information about relays, sensors, and switches. A Sensor Guidebook (SGB2) featuring sensor basics and application information is also available. Contact the company at One E. Commerce Dr., Schaumburg, IL 60173; 847-843-7900; fax: 847-843-7787, for information on obtaining a copy. – Locate Watlow’s sensor offerings by clicking on Our Products in the main menu, then selecting sensors. Check out the Reference Data section for additional technical information.

Sensors magazine is a monthly publication devoted specifically to these devices. Contact the magazine at 174 Concord St., PO Box 874, Peterborough, NH 03458-0874; 603-924-9631; fax: 603-924-7408; or check out its web site at

Sensors Today from Rockwell Automation/Allen-Bradley features developments in presence sensing technology. For a free copy, contact the Rockwell Automation Response Center, Dept. 0942, 10701 Hampshire Ave. S., Bloomington, MN 55438; 800-223-5354, ext. 0942; fax: 800-500-0329. For online automation and sensors information, see the company web sites: Rockwell Automation at and

Smart Sensors by Paul Chapman, published by ISA, the International Society for Measurement and Control, is a self-study course on all facets of the application of sensors. Price: $48. Order number 1-55617-575-2 from ISA at P. O. Box 12277 Research Triangle Park, NC 27709; 919-549-8411; fax: 919-549-8288 or through the ISA web site.