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Hardly a day passes by where we are not contacted by a desperate end-user or equipment manufacturer seeking assistance with a situation of sensors failing at an unacceptably high rate. Once we get down to the root cause of the failures, in almost every case it’s a situation where the specific sensors are being applied in a manner which all but guarantees premature failure.
Not all sensors are created equal. Some are intentionally designed for light-duty applications where the emphasis is more on economical cost rather than the ability to survive in rough service conditions. Other sensors are specifically designed to meet particular challenges of the application environment and as a result may carry a higher initial price.
Some things to think about when choosing a sensor for a new application:
I have been fascinated by all the media hoopla following the stunning announcement last month by scientists working at the OPERA project in Italy. Scientists there think that they may have discovered particles called neutrinos traveling at speeds faster than light. That is, the particles arrived at the detector earlier than light would have arrived when traveling in a vacuum.
Needless to say, if true that would appear to stand modern physics and the universe as we understand it upside down! So much of modern physics is based upon Einstein’s theory of relativity, which holds that nothing in the universe can travel faster than the speed of light.
There was an immediate spate of reporting and opinion declaring that “Everything we know about the universe is wrong”.
I thought to myself, “Really?”
Ever since the media hyping of “cold fusion” back in the 1980s, I have learned to be cautious when hearing reports of stunning new scientific discoveries. We all want to believe in amazing new revelations and the happy fact is that credible reports of fantastic advances in science and technology arrive on an almost daily basis. But when something is reported that seems “too good to be true”, or that doesn’t at all fit into our current understanding of the world…open-minded but healthy skepticism is warranted.
It’s another day at the plant, and the “Underside Clamp Retracted” sensor on Station 29, Op 30 is acting up again. Seems to be intermittently functioning…the operator says that the line is stopping due to “Error: Underside Clamp Not Retracted”.
You think to yourself, “Didn’t we just replace that prox last week?” A quick check of the maintenance log confirms it: that prox was indeed replaced last week. In fact, that particular prox has been replaced seven times in the last six months. Hmm….the frequency of replacement looks like it’s going up…four of the seven replacements were performed in the last two months.
What’s going on here? Is it really possible that seven defective proxes just all happened to end up at Station 29, Op 30, Underside Clamp Retract? Not likely!
When it comes to magnetic incremental linear encoders, sometimes the configurable performance parameters can be a tad obscure. One of the most puzzling is the feature called “Minimum Edge Separation”.
Just for review, a magnetic incremental linear encoder system consists of a precision-encoded magnetic tape and a precision magnetic reader head. It is a non-contact device and is an incremental measurement system. Incremental means the position is given as a series of pulses which must be counted by the controller. If the count is lost or corrupted for any reason (such as a power-down), the system must be re-zero’d to a known home position before controlled positioning/measuring can resume.
EUROMAP is an association of plastics and rubber machinery manufacturers based in Austria, France, Germany, Italy, Luxembourg, the Netherlands, Spain, Switzerland, Turkey and the United Kingdom. This group has produced a comprehensive set of Technical Recommendations for its members to follow.
The EUROMAP technical committee has recommended implementation of realtime Ethernet communications for peripheral devices. Document 75-2, for example, defines network architecture and specifications for line topology (daisy chain), ring topology, and star topology.
If you are thinking about network topology and communication protocols for your project, the EUROMAP Technical Recommendations can provide valuable examples of best practices as determined by a group of leading-edge industrial machinery companies.
Inductive proximity sensors in a welding environment face a variety of hazards. Hot metal particles – called weld spatter – are ejected from the welding process and can melt or burn their way through unprotected plastic sensor faces. Built-up weld spatter (often called weld slag) can eventually cause a sensor to trigger on falsely. If the slag can’t be removed, the sensor has to be replaced.
One solution to these issues is sensors made with tough ceramic faces. The ceramic face stands up to the hot weld spatter without melting, and doesn’t provide a good surface for slag adhesion. Even if slag does build up on a ceramic face, it can typically be removed during maintenance without the need for sensor replacement.
There are many excellent industrial trade magazines that offer equally valuable online content. If you are interested in the latest news and information about industrial sensor technology, be sure to subscribe or regularly follow these online publications:
In many cases, the mechanical components of an older machine can basically operate forever. Critical surfaces can be remachined, and bearings and gears can be replaced again and again to restore lost accuracy and repeatability.
But what about the control system? Sometimes older machines are retrofitted with a new controller to enhance its productivity and extend its useful life. Such refits should not stop with the controller alone. Many of the greatest improvements in machine performance can be obtained by upgrading the entire sensor package as well. Sensors are at the heart of today’s automation systems. They provide the critical information and feedback about what the system is doing, and the status and condition of products being handled and produced.
Peter Nachtwey of Delta Computer Systems has written an excellent primer on electro-hydraulic motion control. In addition to many design and component selection tips, he highlights the benefits of magnetostrictive linear displacement transducers (MLDT) for position feedback to the controller. Check out the article in the July edition of Design World online, called “A Second Look at Electro-Hydraulic Motion Control Systems.”
It’s a given that everything man-made can potentially fail at some point during its useful lifetime. Designers and users of equipment would very much like to predict how long something is likely to last, how frequently failures can be expected, and what application conditions can lead to excessive failure rates.
One traditional measure of reliability is MTTF or Mean Time To Failure. In the case of electronics, it’s a calculated number based on the failure rates of individual electronic components that make up the complete assembly.
There’s a growing trend toward determining reliability through physical testing. One such method is called HALT or Highly Accelerated Life Test. The goal of HALT is to subject electronic products to extreme conditions that will induce premature aging and stress to uncover weaknesses in the design or components. These weaknesses can then be designed out of the product during product development, before they ever reach the end-user application.
Scott Rosenberger
Mark Sippel
Jack Moermond
Henry Menke
Will Healy III
John Harmon
directionguy
Keith Dinwiddie
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