Back to Basics: Sensor tips & techniques

Working with resistive sensor elements and rewiring proximity sensors are two recently discussed topics in the Ask Control Engineering blog.

07/16/2010


Working with resistive sensor elements and rewiring proximity sensors are two recently discussed topics in the blog, “Ask Control Engineering,” at www.controleng.com/blogs.

Regarding resistive sensor elements, a reader asked: “I’ve read that RTDs (resistance temperature detectors) are often the most precise temperature sensing devices. Given the relatively narrow range of resistance involved, how is it practical to get precision with two, three, or more decimal places on a Celsius scale?”

Sensors that measure a process variable using changes in resistance extend beyond RTDs. Thermistors also use resistance, as do many types of strain gages that are used in pressure and weight sensors. RTDs that use platinum wire can, at least in theory, measure temperature changes as small as 0.00001 °C. (Of course saying that one technology or another is the most accurate needs to be qualified in the context of specific types of application because few of those evaluations are true universally.) The kind of precision is only possible when coupled with highly sophisticated signal processing.

Modern electronics are capable of reading very small changes in resistance which makes this sort of thing possible. Interestingly enough, one of the basic elements of precise resistance measuring circuits dates back more than 150 years. The Wheatstone bridge is still the basic approach for quantifying very small changes in resistance that are characteristic of these sensing elements.

The traditional approach of four resistors arranged in a diamond formation is able to measure very small changes in resistance by looking at resistance differences. A Dataforth six-page application note, “Basic Bridge Circuits,” goes well beyond basic high-school physics and explains uses in industrial applications. Find it online at www.dataforth.com.

Sensor wiring

Separately, another reader of Ask Control Engineering wanted to know, “Is it practical to replace a 3wire proximity sensor with 2-wire type?”

Panasonic Electric Works says it can be done. When using inductive proximity sensors for a control application, it is common to choose a 3-wire dc proximity sensor with a dedicated NPN (ground switching) or PNP (positive switching) control output and bring the output into a PLC input. Choosing between polarities means determining how the common is wired and selecting accordingly. If your machines mix NPN and PNP sensors, there’s an alternative to stocking both types of spares to prevent a line-down situation.

Source: Panasonic Electric WorksPanasonic suggests that instead of having the control output circuit separated from the power circuit, the 2-wire design puts everything in parallel, which consolidates circuitry into one loop. In a typical 3-wire PNP circuit, the output wire is specific in its polarity so that it will only function on with a 0 V common. With the 2-wire variation, the output operation is along the two power wires in the form of a voltage drop, thus making the sensor free to work with either polarity on the common. With wiring as the diagram shows, a 2-wire sensor can replace NPN and PNP 3-wire models, just by following the flow of current. This greatly simplifies the usage and replacement of inductive proximity sensors across all applications, the company says. Find out more online at http://pewa.panasonic.com.

Mark Hoske is Content Manager of Control Engineering. Reach him at MHoske @CFEMedia.com.

 

Find other tips and answers at www.controleng.com/blogs

See also these links:  www.dataforth.com

http://pewa.panasonic.com

Ask Control Engineering blog

Control Engineering sensors channel

Cover story: Proximity Sensors: How to Choose, Use Them



No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
2017 Lubrication Guide; Software tools; Microgrids and energy strategies; Use robots effectively
Prescriptive maintenance; Hannover Messe 2017 recap; Reduce welding errors
Safety standards and electrical test instruments; Product of the Year winners; Easy and safe electrical design
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Mobility as the means to offshore innovation; Preventing another Deepwater Horizon; ROVs as subsea robots; SCADA and the radio spectrum
Future of oil and gas projects; Reservoir models; The importance of SCADA to oil and gas
Diagnostic functions for system safety; Specifying industrial enclosures; Effective decision support for a crisis
Transformers; Electrical system design; Selecting and sizing transformers; Grounded and ungrounded system design, Paralleling generator systems
Natural gas for tomorrow's fleets; Colleges and universities moving to CHP; Power and steam and frozen foods

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
The maintenance journey has been a long, slow trek for most manufacturers and has gone from preventive maintenance to predictive maintenance.
Featured articles highlight technologies that enable the Industrial Internet of Things, IIoT-related products and strategies to get data more easily to the user.
This digital report will explore several aspects of how IIoT will transform manufacturing in the coming years.
Maintenance Manager; California Oils Corp.
Associate, Electrical Engineering; Wood Harbinger
Control Systems Engineer; Robert Bosch Corp.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me