Automation - Sensors
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Sensor technology plays a variety of essential roles in machine automation. Sensors provide information about products during manufacturing. They deliver updates about the condition of the equipment, to help guide maintenance and prevent downtime. Sensors also provide feedback on the motion of the motor to ensure accurate positioning. A range of sensor technologies have been developed to address different applications and environments. At Motion Solutions, our trained staff can help you determine the best sensor for your performance requirements, operating conditions, and budget.
Sensors can be used to determine the extrinsic properties of objects such as position, distance, and proximity. Sensors also can be used to evaluate intrinsic characteristics like temperature and color. Design engineers can choose among a wide variety of sensor systems. Here, we review just a few of the most common.
Presence/absence sensors use non-contact technology to detect the presence or absence of the object of interest and report the results to the controller or drive using an electrical signal. Because these sensors are noncontact, they minimize damage or wear to the device under test and to the sensor itself.
The environmental conditions of the application drive the specific technology choice. Key factors to consider include temperature, ambient light, moisture, airborne particulates, shock and vibration, and other contaminants. In addition, some sensor technologies require a metal target in order to operate. The Motion Solutions sales team is well-versed in our sensor portfolio. They can help you not only find the appropriate technology but a specific device that will achieve the accuracy, resolution, lifetime, and cost target that your application requires.
Time-of-flight measurements can be used to detect the distance to an object of interest or its position. Initially, optical time-of-flight measurements required laser-based sensors. More recently, manufacturers have developed LED-based systems that detect and process diffuse backscatter to return distance and position information. Time-of-flight measurements can also be taken using ultrasonic sensors.
In many products, color is just as important as function. Color sensors provide quantitative method for analyzing color in everything from labels to textiles to paints. Color sensors analyze the spectral content of the light reflected from a surface, comparing it to an internal reference to obtain a result. The color of light used for illumination is particularly important in these applications since the reflection spectrum is only a subset of the incident light.
Modern manufacturing puts the emphasis on maximizing operational equipment effectiveness (OEE) and increasing uptime. Condition monitoring is an essential tool for achieving this goal. Condition data provides insight into the health, operation, and performance of equipment, facilities, and even the products being manufactured. This typically encompasses factors like temperature, pressure, humidity, vibration, and current or voltage. An increase in the temperature of a motor might indicate lubricant breakdown, for example. The appearance of a spike in the vibration frequency spectrum of a pump might highlight erosion of the vanes. Increased current draw can indicate worn bearings or gears. With these types of insights, maintenance teams can troubleshoot faults more effectively and also help prevent them in the first place.
Some applications require information to be transferred about the products being manufactured. On an automobile assembly line, for example, data sensors can be used to read out the specifications of the order so that the chassis coming down the line gets the right options and the right color of upholstery. Data sensors can also present information about assets for tracking or maintenance purposes.
Data sensor types include barcodes, QR codes, and RFID tags. The data can be read out and saved to the controller, whereupon it can be displayed on the HMI, used for manufacturing operations, or correlated with sensor data for export to maintenance or management systems.
Mark sensors are specialty sensors designed to detect the registration marks put on materials typically to guide operations like printing and packaging. Also known as contrast sensors or eye sensors, the systems are distinct from standard machine-vision systems used in automation. These sensors evaluate the field of view based on contrast rather than on color or pattern. The system converts the image to grayscale. It then compares each region in the field of view to some set switching threshold. Depending on the algorithm, the software looks for a contrast above or below the switching threshold. Mark sensors typically use white-light LEDs for illumination and ultra-high-speed photo sensors to deliver response times on the order of several tens of microseconds.
No two applications or manufacturing environments are the same. Issues range from contamination to temperature extremes to high levels of electromagnetic interference. Equipment in some locations may be subject to extreme shock and vibration while elsewhere, humidity may be the problem. A number of sensor technologies have been developed to make it possible to build an effective system no matter what the conditions. At Motion Solutions, we carry a range of sensor types to provide you with the choices that will best serve your application.
Photoelectric sensors, or optical sensors, leverage the changes to an optical beam caused by interaction with the object under test. A photoelectric sensor consists of an emitter that generates the optical signal and a receiver (typically a photodiode) that detects it. Photoelectric sensors can be classified as through-beam or reflective; reflective photoelectric sensors can be further broken down into diffuse and retroreflective types.
In through-beam sensors, the emitter and receiver are placed on opposite sides of the objects of interest. The beam either passes through to the detector or is blocked. This type of configuration is good for detecting the presence or absence of components, or checking their position. Through-beam sensors tend to be more accurate in their detection. They use a very narrow beam and have a high signal-to-noise ratio. They can be used in multiple configurations, for example on a diagonal. On the downside, they are harder to mount and align. They require space for both transmitter and receiver, increasing footprint.
In reflectance mode, the emitter and receiver are on the same side of the object, either co-located or directly adjacent to one another. Light propagates from the emitter, reflects off of the object under test and returns to the detector. When light is reflected directly from the surface of the object under test, the system can capture an absorption spectrum that gives information about the material. Alternatively, the sensor can simply register the presence or absence of an object. More sophisticated versions can measure distance or even speed.
If the surface of interest is highly scattering, the signal may not be strong enough to reach the detector. In these cases, a retroreflective sensor might be a better solution. Retroreflectors are reflective components designed to reflect a bright optical signal directly back to the detector. When attached to the object of interest, a retro reflector simplifies detection.
Photoelectric sensors can incorporate LEDs, diffuse sources, or laser sources such as light-emitting diodes. The sensor technology can be used for distance measuring, object detection, and proximity sensing. Depending on the conditions of the application, specific colors or spectral bands should be used to optimize results.
Fiber sensors are optical sensors packaged in a rugged, economical, easy-to-deploy solution. As with conventional photoelectric sensors, they involve emitter and a detector. Unlike conventional systems, they do not use free-space optics. Instead, the light is confined to an optical fiber both for transmission from source to object and return of the captured signal to the detector. Fiber sensors are particularly useful in harsh environments, for example with high temperatures or contamination. The fiber enables the source, detector, and electronics to be located a safe distance away from adverse conditions. The compact size also makes it possible to apply sensors in areas that will not fit conventional sensor heads.
Capacitive sensors monitor the change in capacitance between the sensor plate and the object of interest. Capacitance varies as a function of the size and distance of the sensing object. These sensors are simple and solid-state. They can be used for metallic objects, resins, liquids, and powders. On the downside, they are strongly affected by factors like temperature, offset, surrounding objects, and EMI from power and signal cables.
Inductive sensors are based on the principle that eddy currents can change the impedance of a conductive material. The sensor applies an external magnetic field to induce eddy currents in the object of interest. A detector coil in the sensor generates an AC magnetic field for readout.
Eddy-current sensors can only be used with metallic objects. Various types of eddy-current sensors exist, including versions designed for use with aluminum. Inductive sensors are noncontact. They are extremely robust and generally impervious to contamination like oil and dust. The downside is that they are limited to metals, primarily ferrous.
A magnetic proximity sensor consists of a reed switch that is controlled by a magnet. When a magnet mounted on the object under test nears the reed switch, the switch closes. Magnetic proximity sensors are EMI immune. They also are not affected by contamination like oil and dust.
RFID sensor systems pass data from tagged objects to RFID readers. An RFID system consists of two parts: the RFID tag and the RFID reader or interrogator. Tags may be read only, write once read many, or read/write. Tags are classified as active, passive, or passive with battery assist. A passive tag needs to be interrogated by the RF signal from an active readout device. The readout device must be in proximity to the tag for a successful read. A passive tag with battery assist can send data over a greater distance. An active tag includes a power source so that the tag can broadcast its data to the reader.
Similarly, readout devices can be classed as active or passive. Typically, a passive tag is paired with an active reader and vice versa. For applications involving greater distances or interference, active tag-active sensor systems may be the best choice.
An ultrasonic sensor uses ultrasonic waves to calculate the distance to an object or to register its presence. Ultrasonic sensors can operate in transmission or reflectance mode. Transmission sensors, or through-beam sensors, consist of the detector on one side of the object under test and a receiver on the other side. The output signal is either blocked or attenuated by the objects.
In reflectance mode, the source and detector are located on the same side of the object under test. The object reflects the incident signal back to the detector. These types of sensors can be used to detect the presence or absence of the object of interest. Alternatively, they can measure distance using a time-of-flight calculation.
Ultrasonic sensors are very robust. They perform well in harsh conditions such as contamination, shock and vibration, airborne particulates, etc. Ultrasound is a good technology for most sensor applications, including distance measuring, liquid-level sensing, and presence/absence sensing. On the downside, they can be expensive.
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