The Glow of Success
Engineers don't often think of ultraviolet (UV) luminescence sensors for assembly applications, yet they are often the best choice. They are more sensitive and selective than photoelectric or infrared sensors, and they're more affordable than vision systems, which may cost six times as much.
Luminescence sensors detect things that glow under UV light. The sensor sends out a beam of UV light (generally at a wavelength of 350 to 380 nanometers), detects the resulting visible glow, and outputs an electrical signal that can be used to control equipment or trigger an indicator.
Luminescence sensors can be used in many applications that would be problematic for photoelectric sensors. For example, detecting clear objects is difficult with standard photo-eyes. But, if those objects glow under UV light, luminescence sensors can detect them reliably. Photo-eyes have difficulty detecting a mark against a background of similar color. Luminescence sensors do not have this problem, because the target will fluoresce in a different color when it's struck by UV light.
Luminescence sensors can replace vision systems in many applications. For example, detecting caps on bottles might require a vision system with a memory of the image of the cap in any orientation. If the cap glows under UV light, a luminescence sensor can detect it easily and dependably, and at much less cost.
Luminescence sensors can detect things that are inherently invisible. For example, they can tell if a bearing has been lubricated, because the grease fluoresces under UV light. Similarly, they can tell if adhesive has been applied to a part. Identification marks can be applied to products without regard to aesthetics, because the markings would be invisible under normal light.
Luminescence sensors are not the answer to every sensing need. For one, they're more expensive than photoelectric sensors. In addition, they only work with targets that fluoresce under UV light, or can be made to do so, which sometimes adds cost to the assembly.
Luminescence sensors can be more difficult to set up than photoelectric sensors, because the beam of UV light they emit is invisible, and the glowing spot it produces may be difficult to see with the naked eye. It's not always possible to dim the lights to see the glowing spot. Newer luminescence sensors have indicators that show signal strength, and some have numeric readouts and auto-teach functions to make setup easier.
Luminescence sensors are not vision systems. They cannot read a label or tell if it's crooked.
How It Works
A luminescence sensor consists of several components:
- A UV source, which is usually an LED, but sometimes a lamp.
- A beam splitter, which separates the UV and visible light.
- A photodetector, which detects visible light emitted by the target when UV light strikes it.
- An objective lens, which directs UV light toward the target and focuses the returning visible light onto the photodetector.
- A condensing lens, which focuses incoming visible light onto the photodetector.
- Electronics that control the sensor and produce the output.
Engineers may not consider luminescence sensors in their applications because they don't think of their targets as UV-sensitive. Yet, many ordinary materials-such as greases and UV-curable gaskets-fluoresce naturally under UV light. Others can be made to respond with UV-sensitive chemicals. UV pigments can be added to many materials, including clear and opaque plastics, paper and synthetic fibers. This is done by the material supplier, so it adds no additional steps to the assembly process.
Fluorescent coloring can also be applied to surfaces. Marks can be made manually, using chalk, crayons or stamp pads, or automatically. Automatic methods can be contact or noncontact. Contact methods-essentially mechanically operated felt-tip pens or stamps-provide clean, fast, accurate marks without overspray and spatter. They also economize on materials: 2 ounces of material can mark 20,000 to 40,000 parts.
Noncontact methods are for parts with irregular shapes, moving parts or hot parts. They typically take the form of a jet or spray. Jet applicators direct a short stream of material at the part. Since there is no atomization, overspray is avoided. This method is good for damp or oily parts, since the jet action helps displace contaminants. It is a good choice for color coding and quality control marking. Spray valves can be used to apply a stripe on a moving part or a band on a rotating part. Advantages include less need for aiming, and the ability to cover a large area with one application. The disadvantage is that the spray can get on other things.
Luminescence sensors are available with operating distances ranging from a few millimeters to 350 millimeters. Operating range can be set by installing the appropriate lens to focus the UV spot at the desired distance. Finer adjustments can be made by adjusting the intensity of the UV light, and by setting the detection threshold.
A longer operating distance is often an advantage because it allows the sensor to be mounted out of harm's way, where it can stay clean and not be struck by the target.
With the right combination of detector and lens, the size of the spot of UV light projected on the target can be adjusted from a wide area down to a dot a fraction of a millimeter across. A small spot makes it possible to check for both presence and positioning of a target, while a large spot gives a stronger return and can make alignment less critical.
UV sensors tolerate a variety of backgrounds. This is aided by the fact that the UV light (and thus the visible light that returns to the detector) is pulsed. The detector will respond to pulsed light and ignore steady light.
Sometimes, it's necessary to detect fluorescent marking on a surface that is itself fluorescent, such as invisible ink on paper or fabric that contains brighteners. Brighteners are actually UV-fluorescent chemicals that cause materials to look extra white, because they fluoresce under the UV rays in sunlight. Under these conditions, it may be necessary to choose a UV marking material that fluoresces in a markedly different color from that of the substrate. In rare cases, it may be necessary to mount a colored filter in front of the detector to filter out the interfering color.
UV sensors are available with response times ranging from 0.1 to 4 milliseconds. Most also have adjustable time delays. Faster switching speeds will prevent delays on the line.
As with most sensor technologies, the displays and controls on UV sensors vary. In general, it's important for the sensor to have as many adjustable parameters as possible, including UV output level, detection threshold, hysteresis, null offset and output pulse stretching. Some units have color-coded LEDs to indicate when certain levels have been reached, while others have a numerical display. The latter is useful in setup, because it tells how much visible light the sensor is picking up. When ambient illumination is high, it may be difficult to see the illuminated portion of the target, and the numerical signal-level indication makes aiming and adjustment easier.
An auto-teach function is also useful. It helps inexperienced users get the system up and running. Another useful feature is an analog output, which can be used as an input to a quality-control program. If average light levels go down over time, it is possible to take corrective action before targets start to be missed.
Luminescence Sensors in Assembly
Luminescence sensors are widely used in the automotive industry. One vehicle manufacturer uses a luminescence sensor to activate an automatic screwdriver. When the sensor sees a UV mark on a connecting rod, it signals the screwdriving spindle to actuate. Another automaker uses luminescence sensors to ensure that caps have been installed on connecting rod bearings in the right orientation.
Automotive suppliers use luminescence sensors to detect operator identification tape on ignition wire, copper fittings on muffler pipes, and UV-curable gaskets on various parts. A supplier of interior, exterior and under-the-hood components uses the sensors to detect adhesive tape on metal cylinders and to check for the presence of an adhesion promoter on parts. Another supplier uses the sensors to detect rivets marked with UV material.
Johnson Controls Inc. (Milwaukee) uses the sensors to detect a lubricant sprayed on foam cushions. The lubricant prevents the foam from squeaking against the seat frame. At first, the company added a dye to the lubricant, but that caused stains. Replacing the dye with an invisible UV-sensitive material solved the problem.
Other industries use luminescence sensors, too. A vacuum cleaner manufacturer uses the devices to detect glue on bags, and clear coatings on wired parts. In electronics assembly, luminescence sensors detect tape on wiring harnesses and conformal coatings on circuit boards. In furniture assembly, the sensors detect excess glue in wood joints.
A machine builder installed luminescence sensors in one of its automated assembly systems to detect a UV mark on air-conditioning units. An international food company uses the sensors to detect date codes and glue on packages, and pharmaceutical companies use them to detect tamper-proof seals on bottles.
UV luminescence sensors fill an important niche in the assembly process. Their greatest advantage is that they detect things that other sensors cannot, such as grease. In addition, they allow products to be clearly marked with information that will be invisible to the consumer. If you haven't considered luminescence sensors before, now might be a good time. New technologies have made these sensors better than before, with smaller spot sizes, longer sensing distances, easier setup and more dynamic range.
For more information, call 800-426-9912 or visit www.emxinc.com.
Luminescence, Fluorescence and Phosphorescence
Strictly speaking, luminescence is the emission of light without heat. A firefly's tail is luminescent.
Fluorescence is the immediate emission of visible light as a result of excitation by UV light. The emission ceases when the stimulus is removed. Signs that light up under black light are fluorescent. A fluorescent lamp consists of a glass tube coated on the inside with a material that glows when excited by UV light. The UV rays are produced when electricity passes through mercury vapor inside the tube.
Phosphorescence is the delayed emission of visible light as a result of excitation by UV light. Phosphorescent materials glow under UV light and continue to give off light after the stimulus is removed. So-called "glow-in-the-dark" materials are phosphorescent.