INDUSTRIAL SWITCHES
Limit switches
A limit switch is an electromechanical device that can be used to determine the physical position of equipment. Its primary purpose is to control the intermediate or end limits of a linear or rotary motion. For example, an extension on a valve shaft mechanically trips a limit switch as it moves from open to shut, or from shut to open. The limit switch is designed to give a signal to an industrial control system when a moving component such as an overhead door or piece of machinery has reached the limit (end point) of its travel, or just a specific point on its journey. It is often used as a safety device to protect against accidental damage to equipment.
(1) Operating principle
A linear limit switch is an electromechanical device that requires physical contact between an object and the activator of the switch to make the contacts change state. As an object (target) makes contact with the actuator of the switch, it moves the activator to the limit where the contacts change state.
Limit switches can be used in almost any industrial environment, because of their typically rugged design. However, the device uses mechanical parts that can wear over time and it is slow when compared to noncontact, electrical devices such as proximity or photoelectric sensors.
Rotary limit switches are similar to relays, in that they are used to allow or prevent current flow when in the closed or open position. The groupings within this family are usually defined by the manner in which the switch is actuated (e.g., rocker, foot, read, lever, etc.). Rotary limit switches can range from simple push-button devices, usually used to delineate between ON and OFF, to rotary and toggle devices for varying levels, through to multiple-entry keypads, for multiple control functions. In addition to maintaining or interrupting flow, and maintaining flow levels, switches are used in safety applications as security devices (locker switches) and as functionary actuators when controlled by sensors or computer systems.
Normally, the limit switch gives an ON/OFF output that corresponds to a valve position. Limit switches are used to provide full open or full shut indications as illustrated in Figure 4.1, which gives a typical linear limit switch operation. Many limit switches are of the push-button variety. In Figure 4.1, when the valve extension comes into contact with the limit switch, the switch depresses to complete, or turn on, an electrical circuit. As the valve extension moves away from the limit switches, spring pressure opens the switch, turning off the circuit. Limit switch failures are normally mechanical in nature. If the proper indication or control function is not achieved, the limit switch is probably faulty. In this case, local position indication should be used to verify equipment position.
(2) Basic types
The basic types of limit switches are: linear limit switches, where an object will move a lever (or simply depress a plunger) on the switch far enough for the contact in the switch to change state; rotary limit switches, where a shaft must turn a preset number of revolutions before the contact changes state, as used in cranes, overhead doors, etc.; and magnetic limit switches, or reed switches, where the object is not touched but sensed. Table 4.1 gives the details of the classes and types of limit switches.
Limit switches can differ in terms of their working and contact mechanisms. There are two types of switch working mechanisms; electromechanical and solid-state. Electromechanical limit switches have mechanical contacts, relays, or reeds. Solid-state limit switches are electronic and do not have moving parts. There are three choices for contacts; momentary contact, maintained contact, and positive opening. Momentary contact means that the switch is open or closed only during actuation. Maintained contact means that the limit switch contacts remain in the triggered position even after the actuator has been released, and are reset only by further mechanical action of the operating head. Positive opening means that the contact-point opens reliably and then remains open, in the activated position, even in the event of a mechanical failure.
Limit switches can also differ in terms of orientation and performance. The orientation of the limit switch actuator is critical when determining sizing requirements. Some limit switches have a top- mounted actuator, whilst others have the actuator on the side of the limit switch. In terms of performance, limit switches carry specifications for maximum current rating, maximum AC (alternating current) voltage rating, maximum DC (direct current) voltage rating, minimum mechanical life, minimum electrical life, and operating temperature. Some limit switches are designed for use in transistor-transistor logic (TTL) circuits.
Photoelectric switches
Photoelectric switches represent perhaps the largest variety of problem-solving choices in the field of industrial control systems. Today, photoelectric technology has advanced to the point where it is common to find a sensor or a switch that can detect a target less than 1 millimetre in diameter, while other units have a sensing range up to 60 meters.
A very familiar application of photoelectric switches is to turn a trap on at dark and off at daylight.
This enables the user to set the trap out earlier and retrieve it later in the morning while reducing wear on the battery. Quite often, a garage door opener has a through-beam photoelectric switch mounted near the floor, across the width of the door, which makes sure nothing is in the path of the door when it is closing. A more industrial application for a photoelectric device is the detection objects on a conveyor. An object will be detected any place on a conveyor running between the emitter and the receiver, as long as there is a gap between the objects and the switch’s light does not “burn through” the object. This term refers to an object that is thin or light in color, and so allows the light from the emitter to the target pass through, hence the receiver never detects the object.
Almost all photoelectric switches contain three components: an emitter, which is a light source such as a light-emitting diode or laser diode, a photodiode; a phototransistor receiver to detect the light source; as well as the supporting electronics designed to amplify the signal relayed from the receiver. Photoelectric sensing uses a beam of light to detect the presence or absence of an object; the emitter transmits a beam of light, either visible or infrared, which is directed to and detected by the receiver.
All photoelectric sensors and switches identify their output as “dark-on” and “light-on,” which refers to the output of the switch if the light source hits the receiver or not. If an output is present while no light is received, this would be called a “dark-on” output. In reverse, if the output is “on” while the receiver detects the light from the emitter, the sensor or switch would have a “light-on” output. The types of output needs to be selected prior to purchasing the sensor, unless it is user-adjustable, in which case it can be decided upon during installation, by either flipping a switch or wiring the sensor accordingly.
Photoelectric configurations are categorized by the method in which light is emitted and delivered to the receiver. There are three main categories; namely through beam, retroreflective, and proximity.
(1) Through beam photoelectric switches are configured by placing the emitter and detector opposite the path of the target and presence is sensed when the beam is broken.
(2) Retroreflective photoelectric switches are configured with the emitter and detector in the same housing and rely on a reflector to bounce the beam back across the path of the target. This type may be polarized to minimize false reflections.
(3) Proximity photoelectric switches have the emitter and detector in the same housing and rely upon reflection from the surface of the target. This mode can include presence sensing and distance measurement via analog output.
The proximity category can be further broken down into five submodes; diffuse, divergent, convergent, fixed-field, and adjustable-field. With a diffuse switch the presence of an object is detected when any portion of the diffusely reflected signal bounces back from the detected object. Divergent beam switches are short-range, diffuse-type switches without collimating lenses.
Convergent, fixed-focus, or fixed-distance optics (such as lenses) are used to focus the emitter beam at a fixed distance from the sensor or switch. Fixed-field switches are designed to have a distance limit beyond which they will not detect objects, no matter how reflective. Adjustable-field switches utilize a cut-off distance beyond which a target will not be detected, even if it is more reflective than the target. Some photoelectric switches can be set to different optical sensing modes. The reflective properties of the target and environment are important considerations in the choice and use of photoelectric switches.
Diffuse photoelectric switches operate under a somewhat different style to retroreflective and through-beams, although the operating principle remains the same. Diffuse photoelectric switches actually use the target as the reflector, such that detection occurs upon reflection of the light from the object back onto the receiver, as opposed to using an interruption of an emitted light beam. The emitter sends out a beam of light, often a pulsed infrared, visible red, or laser beam, which is reflected by the target when it enters the detectable area. Diffuse reflection bounces light off the target in all directions. Part of the beam will return to the receiver that it was emitted from inside the same housing which is locate. Detection occurs, and the output will either turn on or off (depending upon whether it is light-on or dark-on) when sufficient light is reflected back to the receiver. This can be commonly witnessed in airport washrooms, where a diffuse photo electric switch will detect your hands as they are placed under the faucet and the attending output will turn the water on. In this application, your hands act as the reflector.
To ensure repeatability and reliability, photoelectric switches are available with three different types of operating principles; fixed-field sensing, adjustable-field sensing, and background suppression through triangulation. In the simplest terms, these switches are focused on a specific point in the foreground, ignoring anything beyond that point.
(1) Standard fixed-field switches operate optimally at their preset “sweet spot”; the distance at which the foreground receiver will detect the target. As a result, these switches must be mounted within a certain fixed distance of the target. In fixed-field technology, when the emitter sends out a beam of light, two receivers sense the light on its return. The short-range receiver is focused on the target object’s location. The long-range receiver is focused on the background. If the long-range receiver detects a higher intensity of reflected light than the short-range receiver, the output will not turn on. If the short-range receiver detects a higher intensity of reflected light than the long-range receiver, an output occurs and the object is detected.
(2) Adjustable-field switches operate under the same principle as fixed-field switches, but the sensitivity of the receivers can be electrically adjusted, by using a potentiometer. By adjusting the level of light needed to trigger an output, the range and sensitivity of the switch can be altered to fit the application.
(3) Background suppression by triangulation also emits a beam of light that is reflected back to the switch. Unlike fixed- and adjustable-field switches, which rely on the intensity of the light reflected back to them, background suppression sensors rely completely on the angle at which the beam of light returns. Like fixed- and adjustable-field switches, background suppression switches feature short- range and long-range receivers in fixed positions. In addition, background suppression sensors or switches have a pair of lenses that are mechanically adjusted to focus the reflected beam precisely to the appropriate receiver, changing the angle of the light received. The long-range receiver is focused through the lens on the background. Deflected light returning along that focal plane will not trigger an output. The short-range receiver is focused, through a second lens, on the target. Any deflected light returning along that focal plane will trigger an output; an object will be detected.
Proximity switches
Proximity sensing is the technique of detecting the presence or absence of an object by using a critical distance. Proximity sensors detect the presence of an object without physical contact, and proximity switches execute the necessary responses when sensing the presence of the target. A position sensor determines an object’s coordinates (linear or angular) with respect to a reference; displacement means moving from one position to another for a specified distance (or angle). In effect, a proximity sensor is a threshold version of a position sensor.
Typical applications include the control, detection, position, inspection, and automation of machine tools and manufacturing systems. They are also used in packaging, production, printing, plastic merging, metal working, and food processing, etc. The measurement of proximity, position, and displacement of objects is essential in determining valve position, level detection, process control, machine control, security, etc. Special-purpose proximity sensors perform in extreme environments (exposure to high temperatures or harsh chemicals), and address specific needs in automotive and welding applications. Inductive proximity sensors are ideal for virtually all metal-sensing applications, including detecting all metals or, nonferrous metals selectively.
Proximity sensors and switches can have one of many physica and technology types. The physics types of proximity sensors and switches include capacitive, inductive, photoelectric, ultrasonic, and magnetic. Common terms for technology types of proximity sensors and switches include eddy current, air, capacitance, infrared, fiber optics, etc. Proximity sensors and switches can be contact or noncontact.
(1) Physics of different types of proximity sensors and switches
(a) Capacitive proximity sensors and switches
Capacitive sensing devices utilize the face or surface of the sensor as one plate of a capacitor, and the surface of a conductive or dielectric target object as the other. Capacitive proximity sensors can be a sensor element or chip, a sensor or transducer, an instrument or meter, a gauge or indicator, a recorder or totalizer, or a controller. Common body styles for capacitive proximity sensors are barrel, limit switch, rectangular, slot style, and ring. A barrel body style is cylindrical in shape, typically threaded. A limit switch body style is similar in appearance to a contact limit switch. The sensor is separated from the switching mechanism and provides a limit-of-travel detection signal. A rectangular or block body style is a one-piece rectangular or block-shaped sensor. A slot style body is designed to detect the presence of a vane or tab as it passes through a sensing slot, or U channel. A ring-shaped body style is a doughnut-shaped sensor, where the object passes through the center of the ring.
(b) Inductive proximity sensors and switches
Inductive proximity sensors are noncontact proximity devices that set up a radio frequency field by using an oscillator and a coil. The presence of an object alters this field, and the sensor is able to detect this alteration. The body style of inductive proximity sensors can be barrel, limit switch, rectangular, slot, or ring.
(c) Photoelectric proximity sensors and switches
These sensors utilize photoelectric emitters and receivers to detect distance, presence, or absence of target objects. Proximity photoelectric sensors have the emitter and detector in the same housing and rely upon reflection from the surface of the target. Some photoelectric sensors can be set for multiple different optical sensing modes, including presence sensing and distance measurement via analog output. The proximity category can be further broken down into five submodes; diffuse, divergent, convergent, fixed-field, and adjustable-field. A diffuse sensor senses presence when any portion of the diffuse reflected signal bounces back from the detected object. Divergent beam sensors are short- range, diffuse-type sensors without any collimating lenses. Convergent, fixed-focus, or fixed-distance optics (such as lenses) are used to focus the emitter beam at a fixed distance from the sensor. Fixed- field sensors are designed to have a distance limit beyond which they will not detect objects, no matter how reflective. Adjustable-field sensors utilize a cut-off distance beyond which a target will not be detected, even if it is more reflective than the target.
(d) Ultrasonic proximity sensors and switches
Ultrasonic proximity sensing can use a sensor element or chip, a sensor or transducer, an instrument or meter, a gauge or indicator, a recorder or totalizer, or a controller. The body style of the ultrasonic proximity sensors can be barrel, limit switch, rectangular, slot, or ring.
(e) Magnetic proximity sensors and switches
These noncontact proximity devices utilize inductance, Hall effect principles, variable reluctance, or magnetoresistive technology. Magnetic proximity sensors are characterized by the possibility of large switching distances available from sensors with small dimensions. Magnetic objects (usually permanent magnets) are used to trigger the switching process. As the magnetic fields are able to pass through many nonmagnetic materials, the switching process can be triggered without the need for direct exposure to the target object. By using magnetic conductors (e.g., iron), the magnetic field can be transmitted over greater distances so that, for example, the signal can be carried away from high- temperature areas.
(2) Technical types of proximity sensors and switches
(a) Eddy current proximity sensor or switch
In an eddy current proximity sensor, electrical currents are generated in a conductive material by an induced magnetic field. Interruptions in the flow of these eddy currents, which are caused by imperfections or changes in a material’s conductive properties, will cause changes in the induced magnetic field. These changes, when detected, indicate the presence of change in the test object. Eddy current proximity sensors and switches detect the proximity or presence of a target by sensing the magnetic fields generated by a reference coil. They also measure variations in the field due to the presence of nearby conductive objects. Field generation and detection information is provided in the kilohertz to the megahertz range. They can be used as proximity sensors to detect presence of a target, or they can be configured to measure the position or displacement of a target.
(b) Air proximity sensor or switch
The air proximity sensor is a noncontact, no-moving-part sensor. In the absence of an object, air flows freely from the sensor, resulting in a near zero output signal. The presence of an object within the sensing range deflects the normal air flow and results in a positive output signal. At low supply pressure, flow from the sensor exerts only minute forces on the object being sensed and is consequently appropriate for use where the target is light-weight or easily marred by mechanical sensors.
(c) Capacitance proximity sensor or switch
Many industrial capacitance sensors or switches work by means of the physics of capacitance. This states that the capacitance varies inversely with the distance between capacitor plates. In this arrangement a certain value can be set to trigger target detection. Note that the capacitance is proportional to the plate area, but is inversely proportional to the distance between the plates. When the plates are close to each other, even a small change in distance between them can result in a sizeable change in capacitance. Some of the capacitance proximity switches are tiny one-inch cube electronic modules that operate using a capacitance change technique. These sensors or switches contain two switch outputs a latched output, which toggles the output ON and OFF with each cap input activation, and a momentary output, which will remain activated as long as the sensor input capacitance is higher than the level set by the module’s adjustment screw.
(d) Infrared proximity sensor or switch
Infrared light is beyond the light range that is visible to the human eye, and falls between the visible light and microwave regions (the wavelength is longer than visible light). The longest wavelengths are red, which is where infrared got its name (“beyond red”). Infrared waves are electromagnetic waves, which is also present in heat; heat from campfires, sunlight, etc., is actually infrared radiation. Infrared proximity sensors work by sending out a beam of infrared light, and then computing the distance to any nearby objects by employing the characteristics of the returned signal.
(e) Fiber-optic proximity sensor or switch
Fiber-optic proximity sensors are used to detect the proximity of target objects. Light is supplied and returned via glass fiber-optic cables. These can fit in small spaces, are not susceptible to electrical noise, and have no risk of sparking or shorting. Glass fiber has very good optical qualities and good high-temperature ratings. Plastic fiber can be cut to length in the field and can be flexible enough to accommodate various routing configurations.