How to Troubleshoot Photoelectric Sensors

Applications for photoelectric sensors vary as do the design and function of the photoelectric sensors themselves, but there are certain things to look for if your set of photoelectric sensors has ceased to function.

Determine what types of photoelectric sensors are being used in the application. Is there a separate sender (transmitter) and receiver arranged in a through-beam configuration so that they are looking at one another? Is it a single photoelectric device using a reflector or bouncing a beam off the objects to be detected? Look for a part number and manufacturer name on the device. Try to find data sheets for the device on the company's website. Having the data sheets will reduce much of the guesswork involved in troubleshooting.

Determine the problem. Are the photo eyes registering an output with no object present? Are they ignoring objects that they should be detecting? Does the problem happen at a certain time of day? Are there environmental influences acting upon the photo eyes? If the photo eyes register an output when no object is present, first check the face of the eyes to be sure they are clean. If they are dirty, use a soft cloth and a non-abrasive, non-corrosive cleaner to wipe them clean. If the photoelectric system incorporates a reflector, be sure to clean it thoroughly too. Test the photo eyes now to see if they work.

If problems persist, try aligning the photo eyes. Using a length of string or wire have one person hold the line beside one photo eye and take your end across to the other. Pull the line tight so that it passes in parallel beside each eye, forming a straight line. It should be obvious that the photo eyes are out-of-alignment if one eye is parallel to the string and the other is not. Adjust both photo eyes as close as you can. Once they are pretty straight, make fine tuning adjustments to the sender eye only. If the sender photo eye is projecting a beam off to the side, then adjusting the receiver photo eye is worthless. This "string method" can also be used with an eye / reflector combo.

If the photoelectric system still is not functioning, check the data sheets for the sensors to determine what supply voltage is necessary to operate the eyes. Set your multi meter to the correct setting AC or DC and verify that enough power is present to operate the photo eyes. If the photo eyes use a visible beam, verify that you can see the beam coming from the sender. If the photo eyes use infrared light, it is possible to use a video camera equipped with the nighttime filming option (often referred to as "Night Shot") to see the beam projected from the infrared sender. Is the receiver photo eye positioned in direct sunlight? The light from the sun can have adverse affects o photoelectric systems. It is OK to mount the sender (transmitter) in direct sunlight as long as the receiver is shielded from it. Is the photoelectric system properly grounded? Be sure to use earth ground or at least a machine ground. Don't ground your photo eyes to a small screw or tiny DIN rail and think that will suffice.

If the photo eyes do not generate random signals, but instead ignore the objects that they are supposed to detect, then the problem is, more than likely, that the beam of light is not actually being broken. If there is a gain adjustment available on the photo eyes, adjust it to a lower setting. If this doesn't work, try positioning the photo eyes so that the object-to-be-detected passes closer to the receiver eye. If this is not possible, try narrowing the beam of light projected from the sender (transmitter) and/or narrow the field of view of the receiver. Both may be accomplished using a short, narrow piece of PVC pipe.

Check your splices. If the wires of the photo eyes have ever been spliced, check the splices and verify that they are clean. Don't just wrap them together and wind electric tape around the splice. Solder the connection and use a heat shrink tube to seal the connection. If all else fails, try calling the manufacturer of the photoelectric sensors or a distributor of the products and ask for troubleshooting assistance.

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How to Extract Harmful Fumes from Industrial Applications and Produce Clean Air

Many industrial machines, (ex. laser welding machines) produce dangerous fumes which, if not properly filtered, could cause serious health risks or even death for the employees who operate them. These machines and also smaller applications, like soldering stations should be interfaced with air extraction and filtration machines.

Contact a manufacturer of air extraction and filtration machines. Many air filtration company web sites offer tools to help you choose the correct machine and filter combination needed to filter the type of fumes or vapors present in your application. There are a number of harmful pollutants that may be filtered using the correct machine and filter combination including dust, gases, laser emissions, vapors, odors, etc. Invest enough time to make the correct decision as to which machine and filters will best work in your application.

Install the recommended machine, strictly adhering to the manufacturer's specifications, and be sure to use the correct filters. Keep several copies of the installation and maintenance instructions that were provided with the machine. If you have questions, do not hesitate to call the manufacturer. Improper installation could result in serious injury or death. Keep a supply of replacement filters in stock in order to avoid unnecessary down time or the temptation to operate without filtering the air.

Clean the air filtration machine regularly and replace the filters on schedule so that it continues to operate efficiently. Some air filtration machines include a warning system to alert the user when a filter is too saturated and needs replacement.


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How to Connect a Three-wire DC Sensor to a PLC

An application calls for a three-wire DC sensor to report to a PLC. How should they be connected together?  Check the input card of the PLC and find the voltage rating. Does it operate on DC voltage? Also determine whether the input needs to be sinking (NPN) or sourcing (PNP.) Look at the sensor and make sure the voltage rating and PNP / NPN type is the same. If there is a mismatch, refer to the manufacturer's instructions for both devices to determine compatibility.

If the PLC's input card is rated for DC voltage and it is a sinking (NPN) type input and also if the sensor operates on DC voltage in the same range and its output is sinking (NPN), then the two should be compatible. Connect the PLC to the power supply, but do not turn the power supply ON yet. Then connect the sensor's positive source voltage wire, which is normally brown, to the positive terminal of the power supply. Connect the negative or neutral sensor wire, normally blue, to the power supply as well, but on the negative terminal. Then connect the sensor's signal wire, normally black, to the PLC input card at the number address that you will assign as the sensor in the PLC program. When the sensor is activated, it will connect the PLC output to neutral or ground, which the PLC will interpret as a "true" condition and respond as programmed.

If the PLC's input card is rated for DC voltage and it is a sourcing (PNP) type input and also if the sensor operates on DC voltage in the same range and its output is sourcing (PNP), then the two should be compatible. Connect the PLC to the power supply, but do not turn the power supply ON yet. Then connect the sensor's positive source voltage wire, which is normally brown to the positive terminal of the power supply. Connect the negative or neutral sensor wire, normally blue, to the power supply as well, but on the negative terminal. Then connect the sensor's signal wire, normally black, to the PLC input card at the number address that you will assign as the sensor in the PLC program. When the sensor is activated, it will connect the PLC output to the positive terminal of the power supply, which the PLC will interpret as a "true" condition and respond as programmed.

How to Detect Visitors in a Driveway Using Infrared Photoelectric Sensors

Infrared photoelectric sensors may be used to detect objects in a number of ways. Mounting the sender and the receiver across from each other is known as a through-beam configuration and provides the strongest beam. Retro-reflective mounting places the sender and the receiver on the same side and bounces the infrared beam off an object when it is in close enough proximity.  Diffuse proximity mounting uses the same layout as retro reflective, but also uses a reflector on the side opposite the photo eyes. Any of these three configurations may be used to set up a driveway monitoring system depending on what type of structures are adjacent to the driveway.

Evaluate the best location to mount the sensors. This may be a mailbox, a fence post, a brick building, a bridge, or whatever standing structure happens to be within range of the driveway. If you mount the photoelectric sensors in a through-beam configuration they do not need to be very close to the driveway, whereas the other two options will need to be much closer, depending on the type of photoelectric sensors used, like ten to fifteen feet.

Trace a route back to your home for burying the sensor wires, but don't put the wires in the ground yet. Instead, extend the wires back to the house above-the-ground and connect the photo eyes to the appropriate power source or to an external amplifier if the photoelectric system includes one. Connect the output of the sensors to a buzzer or bell with a noise that is distinctly different from the sound that your doorbell or telephone makes so that you can easily distinguish them. Many photoelectric amplifiers include a relay output, so connecting them to a buzzer should be as easy as connecting a mechanical switch. See the manufacturer's website for wiring diagrams.

Test the system using different types of cars and trucks and also people. Adjust the sensitivity of the photoelectric system to the optimum level and mount the photo eyes at the right height from the ground to hit the midsection of most vehicles. When everything is set correctly, bury the conduit containing the wires and do your best to conceal the photo eyes from view. This system will provide a reliable means for detecting guests before they are standing on your doorstep and could also be interfaced with a security camera or lights. The opportunities are unlimited.

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How to Operate an Automatic Door Using Infrared Photoelectric Sensors

An automatic door is not complete without infrared photoelectric sensors to control it. Photo eyes provide a non-contact method to detect people or objects passing through the door and may be used in conjunction with a controller to open the door.

Choose a quality set of industrial infrared photoelectric sensors. It is imperative that sensors are used on an automatic door, not just to open the door, but to prevent the door from closing with a person or object underneath. Photoelectric sensors are ideal for this application because they do not require the door to make contact with the subject to detect their presence. Ultimately, the manufacturer should provide wiring diagrams and installation instructions to help interface sensors or other input devices with the door controller, but the following instructions should give you a general idea of how they should be used.

Mount the photoelectric sensors in a through-beam configuration. This will save you a lot of trouble later. Do not try to mount the photo eyes on one side and use a reflector on the other because you will find yourself constantly cleaning the reflector. Infrared photoelectric sensors provide a stronger and much more reliable beam when they are mounted across from each other. If the photoelectric system that you are using includes an external amplifier, compare the voltage ratings for the photoelectric system and the PLC. If they are a match, connect the output from the amplifier to the door's PLC input card.

Configure the photoelectric system to output all the time, but then take away that output when the photo eyes detect something. This is commonly listed by the manufacturer as a feature known as "Normally Closed", "Light Operate", or "Make Switching." Program the PLC to interpret an output from the photoelectric system as a "false" condition and the absence of that output as a "true" condition. When the PLC reads a "true" condition, it should open the door, and when the condition returns to "false" it should close the door. This does two things, it opens the door when an object is detected and also if the photoelectric system experiences a fault or loss of power.


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