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:
1. Can the inductive sensor NJ6-F-N operate at -40 °C?
2. How far can a target reflector move sideways before a photoelectric sensor can't read it anymore?
3. Do you have distance sensors compatible with EtherNet/IP and ControlLogix Safety PLC?
4. What sensors could sense plastics and metals through a plastic tray?
Feel free to ask us your sensing questions, and we'll do our best to reply with the whys and hows of a particular solution.
Tracy: Hi John, thanks for asking me.
John: The first question asks about the temperature rating of the NJ6-F-N inductive sensor. The customer would like to know if this inductive sensor will operate at -40 °C. Tracy, is this the correct sensor to use for this application?
Tracy: Temperature ranges for storage and operation of our products can be found on the sensor data sheets, and these can usually be downloaded from our website. In the case of this inductive sensor, the listed range is -25 °C to +100 °C. Sensor temperature specifications are determined based on the tolerances of the components that make them up. Because we don't know which component will go out of tolerance or even fail if the sensor is operated outside the stated range, there's no way to predict how or when its response will deviate from its other specifications. For this customer's low temperature application, we do offer sensors with operating temperatures ranges down to -40 °C, but they have different housings from the flat, F-style housing he is currently using. An example with similar range and function is cylindrical model NJ5-18GK-SN.
John: This customer needs to sense a 1.6 in. by 2 in. piece of reflective tape and is using the Series 28 retroreflective sensor RL28-55/47/82b/105. The target is on a cart moving laterally from side to side, and it's about three feet away from the retroreflective sensor. How far can the cart move from side to side before the reflective tape can no longer be seen by the photoelectric sensor?
Tracy: To answer this customer's question, two factors come into play. The sensor's light spot size at the distance the target reflector is located, and the size of the reflector. As long as enough of the light spot hits the reflector and returns to the sensor, it's output will be activated. Once the reflector on the cart moves far enough in one direction or the other, not enough light will be returned, and the output will deactivate. Light spot size can be typically found on the sensor's data sheet, and in the case of this sensor, there's even a response curve graph that can be helpful. The graph shows five curves, each of which corresponds to a different target reflector. The reflector listed that is closest to the customer's is a larger four inch square piece of reflective tape. So, if we look at the three ft. distance for that curve, we can estimate that the offset distance to either side will be somewhat less with the smaller 1.6 in. by 2 in. reflector than is shown for the four inch square one. Ultimately, of course, the customer's question is best answered with testing.
John: Tracy, this next customer would like to detect the horizontal position of a hoist that operates in two planes, horizontal travel and vertical lift. The overall horizontal travel distance is 150 ft. He'd like to use EtherNet/IP interface to his ControlLogix Safety PLC. What would you recommend?
Tracy: We have several options for position measurement that might be used for this customer's application. In each case, either horizontal or vertical movement can be determined. One option is our VDM100 series of laser distance measurement sensors. These are retroreflective, which means they require a reflector, and are available with an EtherNet/IP interface built in. The sensor is mounted at one end of the travel path, and the reflector at the other end. As the two move toward and away from one another, the distance between them is measured using laser time-of-flight. Another option is our WCS system. This product uses an absolute value coded rail that is installed along the travel path. A read head travels over the rail and reads the absolute position. The reader has serial output, but we offer a separate EtherNet/IP interface for it. The third option, our PCV system, is similar in function to WCS, but the reader is camera-based, and the absolute position is read from an adhesive strip of DataMatrix codes. There is a PCV read head whose output matches that of the WCS read head, so that the same EtherNet/IP interface can be used with it.
John: In this next application, the customer would like to sense plastic and metal cans through a plastic tray that is approximately a half inch thick. Tracy, what are our options here?
Tracy: The customer can use an inductive sensor to detect metal through plastic. Which model to select depends on the size and material of the metal to be detected, and the distance he wants to mount the sensor away from the target. The plastic tray will take up space, but it won't affect the sensor's ability to detect the metal on the other side. So for detecting metal through a half inch thickness of plastic, the sensor will need to have more than a half inch range. For the detection of plastic through plastic, things get trickier. If the two materials have different dielectric properties, it might be possible to use a capacitive sensor for this, but only if the target's dielectric constant was significantly higher than the tray! Looking at things differently, if the plastic tray is clear, and the target can is not, a photoelectric sensor might be used for detection. In this case, the can material, whether metal or plastic, doesn't matter as long as it reflects light.
John: Well, that concludes this segment of Ask an Expert. I'd like to thank Tracy for joining me today, and thank our audience as well.