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 Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
Each year, a panel of Control Engineering editors and industry expert judges select the System Integrator of the Year Award winners.
Control Engineering Leaders Under 40 identifies and gives recognition to young engineers who...
Learn more about methods used to ensure that the integration between the safety system and the process control...
Adding industrial toughness and reliability to Ethernet eGuide
Technological advances like multiple-in-multiple-out (MIMO) transmitting and receiving
Virtualization advice: 4 ways splitting servers can help manufacturing; Efficient motion controls; Fill the brain drain; Learn from the HART Plant of the Year
Two sides to process safety: Combining human and technical factors in your program; Preparing HMI graphics for migrations; Mechatronics and safety; Engineers' Choice Awards
Detecting security breaches: Forensic invenstigations depend on knowing your networks inside and out; Wireless workers; Opening robotic control; Product exclusive: Robust encoders
The Ask Control Engineering blog covers all aspects of automation, including motors, drives, sensors, motion control, machine control, and embedded systems.
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
News and comments from Control Engineering process industries editor, Peter Welander.
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
This is a blog from the trenches – written by engineers who are implementing and upgrading control systems every day across every industry.
Anthony Baker is a fictitious aggregation of experts from Callisto Integration, providing manufacturing consulting and systems integration.
Integrator Guide

Integrator Guide

Search the online Automation Integrator Guide
 

Create New Listing

Visit the System Integrators page to view past winners of Control Engineering's System Integrator of the Year Award and learn how to enter the competition. You will also find more information on system integrators and Control System Integrators Association.

Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Control Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.