In this article we will briefly mention the characteristics associated with other devices commonly are found in greater or lesser degree in industrial plants: weight, displacement and proximity.
WEIGHT MEASUREMENT
Weight is a variable needed to determine the level of solids in a silo, the transfer of solids through a conveyor belt or a download speed of feeder and logically the net weight of the product itself. Is defined as the force exerted on the object by gravity. The oldest known device for measuring bullet weight is the mechanical arm. Other devices are tilting the balance, the balance spring and a combination thereof.
Over time, there have been so-called load cells hydraulic, pneumatic and electric, the latter based mainly on the application of strain gages. The application type defines the shape and size of the load cells. Even now have wireless systems to send the weight information measured. For the laboratory case also, the technology has allowed the replacement of mechanical and electronic scales precision.
There are applications which can make measurements of weight in motion, as is the case of trucks. Unlike the majority of systems made for this purpose it is required that vehicle stops, this technology uses multiple microcells along the platform, which send information, electronically to a computer to perform the measurement.
When you need a noncontact method, especially for the removal of solids conveyors, used one based on Gamma-ray radiation, similar to measuring the level seen before. Here, rays are emitted from a source to the material. Depending on the amount of radiation absorbed by it and the speed of the belt, is determined the flow of material.
Figure 16 Balance using radioactive method (courtesy of Therrtio Measuretech)
DISPLACEMENT
It is often necessary to measure the linear displacement or position of a mechanical element. The methods differ depending on the type of application. What is common is that they all deliver an electrical signal proportional to displacement. We include devices such as linear variable differential transformer (LVDT), potentiometric transducers or linear encoders.
An LVDT is an electromechanical device with a movable magnetic core surrounded by three cylindrical windings. BC produces a signal proportional to the movement or its nucleus and is linear over a specified range. The primary winding is excited with an AC current, typically in the region of 1 to 10 kHz and 0.5 to 10 Vrms. The other two coils, the secondaries are wound in opposition, such that when the ferrite core is in its central position! the same voltage is induced in both. Depending on the displacement of the nucleus, the ac output is proportional to it, and the phase of the signal indicates the direction of motion with respect to its central position.
Figure 17 LVDT
The electronics associated with an LVDT, combines both information for the user to know the exact position of the shaft. This transducer is limited to relatively short distances due to its inherent construction. However, the resolution is virtually unlimited and rather is limited by external electronics.
From the point of view of their physical appearance, a potentiometric transducer is similar to an LVDT. But like a pot, its measurement is based on the change of resistance measured between the wiper and one end. Practically then is a variable resistor with linear displacement.
The encoders are commonly associated with the measurement of rotation or circular motion (such as RVDT or rotary variable differential transformer). However, there are linear encoders, including the optics can be mentioned, but also have the magnetic type, inductive and capacitive.
In basic terms, a linear optical encoder consists of a head scanner that moves with the engine and a glass or steel scale mounted on the stationary part of the system. The head contains a light source, photocells and electronics. When the head moves, the incident light is modulated on the scale of fine marks on the surface of it, producing sinusoidal output from the photocells. These outputs are outdated for two sinusoidal signals in quadrature. The electronics subsequently processed into digital signals.
Like the rotary encoders, these devices have two versions, a type that proveee ncremental the relative position and absolute type which provides a unique position. The resolution of these devices can be as thin as 0.001 mm, scan lengths up to 30 meters and speeds of up to 15 m / sec.
Figure 18 Linear Encoders
CLOSE
In many industrial processes require the presence of some object or mechanical device for control purposes. The simplest way is through the use of mechanical devices such as switches, limit switches (limit switches). But it is not always possible to use these methods involving contact with the element to be detected. In processes involving for example, sensing the passage of a product through a conveyor belt is necessary in cases rhayoría use a noncontact method. This is how the so-called solution apalrecen as proximity sensors or contactless. Among these are the photoelectric, inductive and capacitive's.
A photoelectric sensor uses light to detect the presence or absence of an object. There are several types: the transmitted beam (thru-Bejam), retro-reflective (reflex) and diffuse reflective. The transmitted beam sensor uses two devices (a transmitter and receiver) face to face. The detection occurs when an object blocks or cuts the beam of light between them.
Figure 19 Transmitted Beam Detector
The retro-reflective sensor emits a beam of light that returns back a share of a reflector. When an object blocks the beam between the sensor and reflector, the detection occurs.
Figure 20 retro-reflective sensor
A diffuse reflective sensor emits a beam to be reflected back to him by the same object to be detected.
Figure 21 Diffuse reflective sensor
There is a special type of detector reflectiyo, called diffuse background rejection (background rejection). It has two detectors and has a capacity for extreme sensing allows the device to detect objects regardless of color, reflectance, contrast or shape of the surface, ignoring objects that are outside the range of detection. An interesting example is illustrated below. This is required to detect the presence or absence of caps on the bottles. The detector must have the ability to sense different caps reflectivity and color to the same distance. It should also be able to ignore the edge of the bottle when it has no lid.
An interesting alternative is the use of fiber optic detector. This applies in situations where it becomes difficult to mount a traditional sensor for space reasons. The diameter of a fiber optic cable can be as small as 0.02 inches. It consists of a large number of fibers of glass, protected against shock or excessive flexing. Since it is light and not current that travels through the cable, the signal is not affected by electro magnetic interference (EMI) or vibration. They can withstand high temperatures and in some cases, chemicals. However its disadvantages are its limited range of detection.
There are two types of fiber optic sensors, the glass that transmit light efficiently through the visible spectrum and into the infrared spectrum of plastic and have a poor efficiency of transmission of infrared wavelengths. Consequently, the former can be used as light sources in both spectra while the latter only with sensors for the visible spectrum.
Figure 23 Comparison between optical fiber types:
Another alternative is the use of ultrasonic proximity sensors, similar to those used to measure level. They have, of the types of single or dual set point, retro-reflective and transmitted beam. Can accurately detect objects of glass or plastic transparent and translucent objects.
The high-frequency technology used in them, makes them virtually immune to interference from ambient noise and generally difficult conditions. They use special sonic transducer allows the transmission and sending sound waves. The ultrasonic transducer emits a number of sound waves that are reflected by an object back to the transducer. After the issuance of these, the ultrasonic sensor will change its status to the status of receiver. The time between the sending and reception is proportional to the distance of the object sensor.
The sensing is only possible within the detection area. This range can be adjusted by ei potentiometer sensor. If an object is detected within this area, the output changes state. The LED indicates the change internally built.
Figure 24 Ultrasonic Sensor with digital output (courtesy of Vydas)
The proximity of the inductive or capacitive electrical characteristic used to detect a nearby object, usually no more than an inch. The inductive only detect metallic objects while capacitive sensing can be both metallic and nonmetallic.
Inductiwo proximity detector produces a field of radio frequency oscillating invisible. When a metal object enters this area it affects. Each type and size of the sensor is sensing a specific range so that the detection has great accuracy and repeatability. With its ability to detect a small range, these sensors are very useful in precision measurement applications and inspection.
Among the advantages of these devices are its immunity to adverse environmental conditions, speed of response, detection of metallic objects through metallic barriers, time of life and solid construction to deliver signals to electronic equipment. Its disadvantages include its limited sensing range (up to 4 "), detect only metal objects and can be affected by accumulated metal filings in the face of the sensor.
A capacitive detector as an electrical capacitor consists of two plates separated by an insulator called, dielectric. On the device, one of the plates is part of the switch, the insulation is encapsulated in order to detect, the other plate. These sensors typically have a sensitivity adjustment and ueden detect any object whenever they have a higher dielectric constant than air. Liquids and metals have a high dielectric constant.
Among the advantages mentioned its ability to detect both metallic and nonmetallic objects and a senior 'than inductive, have rapid response, can detect liquids through metal barriers (glass, plastic) and long life and solid detested output . Its disadvantages are that are affected by changes in temperature and humidity are not as accurate as inductive.
