Pepperl+Fuchs Blog

Ask an Expert Industrial Sensors - Episode 3

Posted by John Appleson on Wed, Jul 16, 2014

This episode of Ask an Expert for industrial sensors examines and provides answers to interesting sensing questions we've received from customers just like you. We explore and answer these questions:

Background suppression photoelectric sensor VT18-8-H-120-M/40a/65b/118/128

  • Can this sensor slowly lose its range over time?
  • Can you also explain the units of time for a MTTF of 860 a?
  • What does a Mission Time of 20 a mean?

Ultrasonic sensors

I'm looking for an ultrasonic sensor to detect empty glass jars on a conveyor belt. What solutions do you have?Ask an Expert Industrial Sensors Episode 3Feel free to ask us your sensing questions, and we'll do our best to reply with the whys and hows of a particular solution.


John: Welcome to Ask an Expert! Hi, this is John Appleson, Marketing Manager with Pepperl+Fuchs. Today, I'm joined by Tracy Molnar. Tracy works as an Application Engineer here at Pepperl+Fuchs. So welcome, Tracy, and thanks for being here!

Tracy: Hi John, thanks for asking me.

John: Tracy, the first question asks about the photoelectric background suppression sensor in the VT18 Series (VT18-8-H-120-M/40a/65b/118/128). The environment seems to contain light airborne sand. The customer needs to know if this sensor will lose range over time. Tracy, can you shed some light on this?

Tracy: Sure. The optical components of the sensors themselves don't lose their effectiveness or capability over time, but there are some external conditions that could have an effect on the sensor's range or operation. A couple of those are the mounting or orientation might change, and you need to reposition the sensor, or maybe the target color changes, and you need to make sure that you readjust the sensitivity of the sensor. I'm thinking because of the airborne sand, it might be this third thing: The sensor has a plastic lens, and if there is any buildup of material on the lens over time, like dust or dirt,  that will affect both the range and the operation of the sensor. And, small scratches, even microscopic ones, on a plastic lens will also affect the sensor's ability to effectively detect the target. So if the environment the sensor is in has airborne sand that comes into contact with the sensor's lens, you might want to consider protecting the plastic lens with a piece of clear glass pressed up against it. That would be more resistant to scratches. And also, if it did become scratched, you could replace it. In an application where dust or dirt or sand is present, regular cleaning of the sensor's lens and inspecting it for scratches might also be necessary.

John: While we're on the subject of this VT18 Series sensor, the customer states that the MTTF, Mean-Time-To-Failure, is listed as 860 a on our datasheet. Can you explain to our audience what the 'a' stands for?

Tracy: The unit of measure for 'a' represents years. That's the unit of measure that you're going to find on our datasheets under functional safety related parameters.

John: The last question on this sensor, Tracy: Please explain what we're talking about in our brochures when we state Mission Time as 20 a?

Tracy: Well, we've already clarified that the 'a' stands for years. Mission Time is a term that is defined in ISO safety standard 13849-1. Basically, it's the maximum time a product is recommended to be used before it should be replaced, even if it's still working.

John: Ok, switching gears, this customer is looking for an ultrasonic sensor to detect empty glass jars from the side, running along a conveyor. They need an output when the jar is directly in front of the sensor, and not in between the jars. What are your recommendations? The customer also states that they are not looking for a photoelectric sensor due to the fact that butter splatter can find its way onto the sensor face.

Tracy: Well, based on the application description, there are several reasons why I think an ultrasonic sensor might be recommended over a photoelectric sensor. First of all, empty glass jars are clear, which can pose a problem for most standard photoelectric sensors. This is not an issue when using an ultrasonic sensor, which only depends on the target object's ability to reflect sound waves. Glass is a great target for an ultrasonic sensor. Ultrasonic sensors are also programmable, meaning you can teach-in any switching point or window within the sensor's range. In this application, you could strategically  select a distance that would trigger the sensor's output only when the side of the jar closest to the sensor was present. The output would then not be triggered in between jars, since the target distance at that point would be farther away. In this application, there is a potential for the sensor to be splattered with butter. While a single small drop splattered in the wrong spot on a photoelectric sensor lens would mean it could no longer properly detect the target, an ultrasonic transducer is much more forgiving and would continue to operate without interruption.

John: Well, that concludes this segment of Ask an Expert. I want to thank Tracy for joining me today, and thank our audience as well.

Questions about Industrial Sensors?  Ask an Expert!

Topics: Ultrasonic Sensors, Photoelectric Sensors, Ask an Expert

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