Back to Basics: Sensor tips & techniques
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.
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.
Panasonic 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
Ask Control Engineering blog