Sensing Technology Comparison
Photelectric sensing, as with many other sensors of different technology, can be applied in different modes of operation.
A very common mode is the proximity mode, where the light source from the sensor is reflected off the target and returned to the sensor’s receiver. This return of light to the sensor confirms the presence of the target. The proximity mode allows sensing from one side of the target, but the strength of the return light depends on distance to the target, color of the target, and incident angle to the target. Where those factors can be controlled, the proximity mode is easy to install, and can provide reliable sensing. Depending on physical spacing between sender and receiver in the proximity mode, will impact this mode’s sensitivity to accumulation of dust and dirt, and its survival in a that challenging environment.
Another popular mode for photoelectric sensors is the through-beam (or opposed) mode. In this mode, one way or another, the sensor’s sender and receiver are displaced, and the target passes in-between them. When the target passes between the sender and receiver, the light from the sender is blocked from the receiver, and this absence of light at the receiver confirms target presence.
The disadvantage of the through-beam mode is that one-half the sensor needs to be physically on the other side of the target, and where a fiber-optic through-beam is not used, both components of the through-beam system need to be powered, requiring wiring to each component. Transparent and some translucent targets may not provide sufficient attenuation of the light beam for the sensor to reliably detect their presence.
The third mode, Retro-Reflective, uses a reflector on one side of the target, and the sensor with its sender/receiver on the other side. When a target passes between the reflector and the sensor, the light beam path is blocked, and this absence of light at the receiver confirms the target presence. The advantage of this mode is independence of target color, incident angle to the target, and distance to the target.
The disadvantage of the Retro-Reflective mode is that the reflector may experience gradual accumulation of dust, dirt, residue, or moisture with a corresponding attenuation of light returned to the sensor, reduced sensing range, and subsequently, loss of effectiveness. Depending on the installation, the reflector may easily become misaligned or damaged. Some applications, because of the specifics, may not permit installation of a reflector. Transparent and some translucent targets may not provide sufficient attenuation of the light beam for the sensor to reliably detect their presence.
The terms light-operate or dark-operate merely describe whether the sensor’s output is in the ON state when light is returned to the sensor, or ON when light is not being returned to the sensor, respectively. Most sensors have this function as a selectable feature, so the user can switch to whatever logic state is needed for the application.
Some applications are space constrained, or simply need better control of the light beam. Applications of this nature are often best served by using fiber-optic light guides with the photoelectric sensor. The fiber-optics are a conduit for the light beam and are available in different apertures and geometric shapes. Fiber-optics, when chosen appropriately, may change the way the sensor responds to the target and performs. The geometric shape of the fiber, can be chosen to favor the sensing task. Fiber-optics can be selected to survive in either higher temperatures than the sensors, or in wash-down applications, where the sensor is not rated for that exposure.
In general, most applications for photoelectric sensing should be relatively clean environments. If the application is dusty or dirty, special considerations need to be addressed for the sensor to remain a reliable means of sensing, such as periodic sensor lens and/or reflector cleaning.
Compared to other sensing technologies, photoelectric sensing has some of the fastest response times, due in part to the fast speed that light. Ultrasonics, traveling at the slower speed of sound, have inherently slower response times.
The choice of LED light source for the photoelectric sensor is very important. IR (infrared) is non-visible, and can pass through some target materials, and can more easily pass through a reasonable accumulation of dust and dirt. Both of these properties could be used as an advantage for some applications. IR LED sources usually offer the longest sensing range, compared to other visible LED sources in the same product family from the same manufacturer. IR is the optimal LED source when attempting to sense darker colored targets in the proximity mode.
Since IR will typically pass-through transparent targets, and some translucent targets with minimal attenuation, it is not a recommended source for sensing such targets in either the retro-reflective or through-beam modes. IR is an excellent source for sensing these targets in the proximity mode.
Visible LED light sources have the advantage of being able to see where the light beam strikes the target in the proximity mode, but this mode and amount of light returned to the receiver are very dependent on the color of the target. Darker colors absorb more of a visible light source, so that the amount of reflected light and its subsequent sensing range is significantly reduced. The lesser light returned from darker targets could be an advantage, if trying to distinguish between lighter targets and darker targets.
Lasers are a subset of photoelectric sensing, with the distinction of having a well collimated light beam, compared to non-laser photoelectric sensors which have the beam spreading outward with increasing distance from the source. Most laser based photoelectric sensors are visible red LED light sources, and since the light is collimated in a single point, it is typically easy to see the where the projected beam strikes the target.
Lasers are available for all three modes of sensing, as covered in the Photoelectric Sensing segment, above. For clarification, this includes proximity mode, through-beam mode, and retro-reflective mode.
The advantages of laser-based photoelectric sensors include the ability to only see a small segment or spot on the target material, and thus ignore the rest of the surrounding product area. This is extremely helpful in many inspection tasks, where the inspection is confined to a given spot or location on the product. Another advantage of laser-based photo-electrics, is the long sensing range, and the ability to ignore other nearby, unintended targets.
Disadvantages of the laser is that it is typically a visible red wavelength, which is significantly absorbed by black or darker target materials, when used in the proximity mode, effectively reducing the sensing range. Another significant disadvantage of a laser is the precision alignment necessary for a highly collimated light beam to strike a small target. The longer the sensing distance, the more critical is alignment. Bracketry for mounting should be rigid, include some fine adjustment, and be able to lock in position to maintain that alignment. This is even more critical in the through-beam mode, where the receiver is physically just a small as the sender, and alignment between the two is challenging.
Incident angle with respect to the target, especially in the proximity mode, is critical, as the collimated beam may be easily reflected to some other location than the sensor.
Lasers come in different classifications, for human safety. Because the beam is collimated and represents a significant amount of energy in a relatively small area, users should always avoid looking directly into the source of the laser beam, to prevent possible damage to their eyes.
Traditionally, the laser may be a slightly higher price than its non-laser equivalent. The author’s opinion is that lasers have their place, but should not be considered as one-product for all applications, just because they sound high-tech.
Ultrasonic sensing, within the overall sensor market, is relatively new, or at least new to a lot of newer engineer and maintenance personnel. To that point, when being presented to a customer, there may be a need to educate the customer on its capabilities and trade-offs, and how to apply it effectively.
Ultrasonic sensors have the advantages of being able to operate in a dusty/dirty environment, well beyond the limit of a photoelectric. Ultrasonics essentially respond to all targets equally, independent of their color, as long as a return echo can be detected. Ultrasonics have an impressive sensing range, some up to 8 meters. Although their beam inherently spreads out with distance, their effective beam is more confined. Because of their broad beam, it is a good choice where the user does not want to respond to small variations in the surface of a target material.
One of the most important advantages of ultrasonics, is the ability to create not only a window of sensing in space, seeing only targets in that window, but even more importantly, to hold the near limit and far limit of that window quite precisely. Since this characteristic is independent of dust/dirt, or color of the object, it is an excellent choice for level sensing/monitoring, level control, roll diameter, and target presence applications where background suppression is essential.
Depending on the model selected, ultrasonic sensors are excellent for sensing the presence of all materials, but an advantage is that they will not see through the material. Thus in the proximity mode or through beam mode, it is an excellent choice for sensing translucent or transparent materials. This includes plastic bottles, clear films, and glass. In the proximity mode, the sensors beam should be perpendicular to the surface of the target, whereas through-beam mode will be independent of incident angle to the target. An additional advantage of ultrasonics is that they can be used in what may be called simply “using a background”. The analogy is photoelectric sensors used in the retro-reflective mode. The target passes between sensor and the background. The decision is made not on whether the beam is reflected from the target, but whether the beam is returned from the background. This is a powerful way to apply the sensor, when appropriate. The background can be any object that returns the ultrasound, e.g., the floor, the wall, the machinery itself, etc… Note that photoelectric sensors must use a standard photoelectric reflector.
Ultrasonic sensors are available in either discrete, analog, or a combination thereof these outputs. The analog models are available with either standard, 4-20 ma, or 0-10 v. The 4-20 ma analog output is often favored for its high electrical noise immunity, as well as its circuit break detection resulting in 0 current, instead of 4 ma. With a simple push of a button teach, the analog output is scaled within the near and far limits. Discrete models are available in proximity or through-beam modes, and analog models are proximity mode only.
Disadvantages of ultrasonics include the relatively slower response time, due to the sensor using media at the sound of speed. Fastest models may be approximately 2 ms.
Another factor to consider, is the dead-band of the selected model. All ultrasonics have a dead band, which is a distance measured outward from the sensing face in which the sensor’s response and respective output may be un-predictable. This dead-band is due to the dampened ringing of the emitter transducer, before the receiver can listen for the respective return echo. However, with the relatively long-range capability of ultrasonics, the dead band may not be a significant issue, because the recourse may be to simply back the sensor further away from the target, where applications permit. The longer the stated max range of the sensor, the larger the dead-band. Some tradeoffs are made by manufacturers, so dead bands may vary from manufacturer to manufacturer, for any given sensing range model.
Ultrasonic sensors typically have operating temperature specs at either the low or high end which need to be considered with respect to industrial applications. Indoor applications usually only affect the high end of the temperature limit, so they cannot be used near open flames or ovens. Low temperature limits restrict use outdoors in some climates, as use in cold storage areas.
Strong air currents will disrupt the emitted ultrasound waves, so use near fans or vents should be evaluated and if possible, used with partitions, or avoided altogether.
UV sensing is an interesting subset of photoelectric sensing, but with some significant distinctions. The UV sensor’s LED source emits light in the invisible UV spectrum. If the natural or engineered properties of the target material, in-turn, fluoresce in the visible spectrum when interrogated by a UV source, then the sensor sees this shift of UV to the visible spectrum as the target present. The sensor only responds to that UV shifted to the visible spectrum, so highly reflective targets that merely reflect visible or UV light, are totally ignored. One of observed benefits of proximity mode UV sensors is that UV appears to be significantly more tolerant of incident angle to the target, facilitating applications where space may be constrained, and the approach to the target is non-perpendicular.
The natural UV fluorescing materials include, as an example, some cooking oils or food grease. The engineered UV fluorescing materials are the commercially available UV fluorescing agents, that can be mixed into the target coloring dyes, painted on, or sprayed onto a substrate to give it UV fluorescing properties. The higher the concentration of UV fluorescing agent, the easier for the sensor to discriminate versus other existing UV fluorescing properties. Most white paper stock, and some printed graphics, or materials used in plastic containers already fluoresce to some degree when interrogated by a UV sensor. Therefore, higher concentrations of UV fluorescing agents can be applied to that same substate to distinguish the intentional UV that was applied, versus the existing UV fluorescing properties. Examples of products that often incorporate UV fluorescing agents so that they can be reliably sensed as present, are adhesives and tamper-evident seals. Without these products being present, the end produce may malfunction, and consequences are costly to the manufacturer.
A significant benefit of UV fluorescing agents is that they can be clear when applied, and invisible to the human eye. For example, UV Registration/Eye Marks are often totally invisible, but the UV sensor can easily detect them. This allows marking products in a manner that is aesthetically favored.
The primary disadvantage of adding a UV agent to an existing substrate is the additional process step to apply it, cost of the UV fluorescing agent, if the UV fluorescing agent is compatible/safe with the material on which it was applied, or will the UV fluorescing additive mixed into the product, alter the product’s performance, e.g. paints or wood stains. Application directly onto some products may need to address if it alters that product’s surface finish.
Inductive sensing has been used in the marketplace for a long time. It was the natural upgrade from mechanical limit switches. Whereas mechanical limit switches confirmed a product present through physical contact, the inductive proximity sensor accomplished some of the same function, but in a non-contact device. The technology is that a varying electromagnetic field as created in the inductive sensor, can be reduced when in the near presence of a target with ferrous (iron) content. This reduction in the established electromagnetic field, can be detected by the inductive sensor’s circuitry, and used to change its output state.
Inductive sensors are only for the proximity mode. Typical variations are discrete output in normally open or normally closed, and analog. Electrical output configurations include 2-wire ac, 3 wire, and 4-wire.
Mounting considerations are flush mount in metal, requiring a shielded sensor, and non-flush mount in metal, which can use either an un-shielded (typical) or shielded sensor.
Generally, sensing range of an inductive sensor is directly related to face diameter.
The advantage of inductive sensing is that it is often the optimum technology for the harshest environments, surviving in water/oil direct spray, and even some metal chips from machining/cutting operations. Unlike a mechanical limit switch, it has no moving parts, and will not get stuck in some position due to the accumulation or residue of these lubricants over time. For these applications, the inductive proximity sensor can go where the devil fears to tread. Since it is non-contact sensing, it cannot scratch or damage the target product.
The disadvantages of the inductive sensing are derating curves for sensing ranges various types of metals with less ferrous content, e.g., aluminum, copper, brass, stainless. Some manufacturers offer enhanced designs with higher prices, to sense these metals with significant deratings.
While increasing the sensor diameter will increase its sensing range, it also increases the size of the minimum target it needs to be presented with for detection. It becomes a tradeoff of physical size, sensing range, and minimum target considerations. The relatively short range of inductive proximity sensors often results in the moving target hitting and destroying the sensor face.
Unshielded inductive proximity sensors have longer sensed ranges than shielded inductive sensors, but they cannot be flush mounted.
Inductive proximity sensing has relatively slower response times than what a photoelectric sensor can accommodate.
Capacitive sensing is using the dielectric effect to detect the presence of target materials. This is specifically alteration of the E, or Electric field, that is emitted by the magnetic field of the capacitive sensor, by the target material. It is non-contact sensing, with a relatively short sensing range. The choices of product outputs are the typical normally open, and normally closed, as well as PNP or NPN.
The technology benefits from its ability to sense unusual targets such as liquids, and the human touch. Since it can sense liquids, yet ignore some plastics, it is often used to see through the plastic walls of a tank or reservoir and sense the (discrete) level of liquid in within. This can be an advantage since discrete or analog ultrasonics, which are excellent for tank level sensing, require an opening in the top of the tank, so they can be looking downward into the tank. Ultrasonics cannot see through a plastic or metal tank wall.
Because capacitive sensing can sense moisture or water, its reliability is somewhat humidity dependent.
Capacitive sensors may need to be purchased and evaluated for any given application, to determine suitability for both the target materials and incidental target materials that it will encounter. Interestingly, and maybe for this reason of uncertainty, some major manufacturers of various other industrial sensors, e.g., photoelectric, ultrasonic, inductive, etc…, appear to have discontinued their offerings of capacitive sensors.
The underlying message may be that if you were thinking of using a capacitive sensor, ask if some other technology sensor would perform the task as well?
MECHANICAL LIMIT SWITCH SENSING:
Mechanical limit switch sensing is likely the first, and therefore the oldest method, in industry, of sensing the presence of some object. Mechanical sensing is contact sensing. It was intuitive to have either an arm or plunger be activated by a moving target and use a set of electrical contacts mechanically attached to that arm or plunger. Mechanical limit switches are simple to install and understand for maintenance personnel.
Because of the target movement, single or multiple contacts could be activated, so normally open, normally closed, and combinations thereof could easily be integrated. A spring is used to return the arm or plunger to the resting state once the target has left the mechanical limit switch.
Unlike other sensor technologies that are active devices requiring power just to be able to sense and provide a usable output, mechanical limit switches are passive devices, requiring or consuming no power of themselves.
The robustness of their electrical contacts can directly control reasonable loads in the 10-20 amp or more range, and handle either ac or dc circuitry. Their electrical contacts are mechanically activated contacts, so mechanical limit switches are not solid-state devices, and their outputs are dry contacts. Even though they do not have npn or pnp outputs, they can provide the equivalent if so wired, and if necessary.
Variations of mechanical limit switches can even tell which way a product passes, for example, whether from right to left, or from left to right, simply by the mechanical activation used.
The disadvantage of mechanical limit switches is that they are contact sensing, and need initial relative movement between target and switch. This intervention of the mechanical limit switch may slow or impede the target movement, or accidently obstruct it; e.g, a mechanical (paddle) limit switch as used in a feeder bowl. It may even aesthetically deface the target.
In the harshest of environments, with lubricating oils being spray onto the targets, this accumulation of lubricants settling during periods of non-production, may cause the movements of mechanical limit switches to freeze up, so that maintenance personnel must intervene, and free-up the moving mechanical limit switch parts.
Because mechanical limit switches are contact sensing, their range is relatively short, compared to photoelectric, laser, or ultrasonic sensors.