Monday, May 28, 2018

IAM Search

INDUSTRIAL CONTROL HANDBOOK - 1.2 SWITCHES AND TRANSDUCERS

Some simple sensors can distinguish between only two different states of the measured variable. Such sensors are called switches. Other sensors, called transducers, provide output signals (usually electrical) that vary in strength with the condition being sensed. Figure 1.5 shows the difference in outputs of a switch and a transducer to the same sensed condition.

Fig.1. 4 Non-linearity in a pressure sensorFig.1. 4 Non-linearity in a pressure sensor

 

1.2.1 Switches

The most commonly-used sensor in industry is still the simple, inexpensive limit switch, shown in Figure

1.6. These switches are intended to be used as presence sensors. When an object pushes against them, lever action forces internal connections to be changed.

Most switches can be wired as either normally open (NO) or normally closed (NC). If a force is required to hold them at the other state, then they are momentary contact switches. Switches that hold their most recent state after the force is removed are called detent switches.

Most switches are single throw (ST) switches, with only two positions. Switches that have a center position, but can be forced in either direction, to either of two sets of contacts, are called double throw

(DT). Most double throw switches do not close any circuit when in the center (normal) position, so the letters "co," for center off, may appear on the spec sheet.

Fig. 1.5 Switch output versus transducer output (switch without hysteresis)Fig. 1.5 Switch output versus transducer output (switch without hysteresis)

Switches that change more than one set of contacts ("poles") with a single "throw" are also available. These switches are called double pole (DP), triple pole (TP), etc., instead of the more common single pole (SP).

In switches designed for high current applications, the contacts are made crude but robust, so that arcing does not destroy them. For small current applications, typical in computer controlled applications, it is advisable to use sealed switches with plated contacts to prevent even a slight corrosion layer or oil film that may radically affect current flow. Small limit switches are often called microswitches.

Any switch with sprung contacts will allow the contacts to bounce when they change position. A controller monitoring the switch would detect the switch opening and closing rapidly for a short time. Arcing of current across contacts that are not yet quite touching looks like contact bouncing, too. Where the control system is sensitive to this condition, two solutions are possible without abandoning limit switches.

Fig.1.6 Limit switches. (Photograph by permission, Allen-Bradley Canada Ltd., A Rockwell International Company)

Fig.1.6 Limit switches. (Photograph by permission, Allen-Bradley Canada Ltd., A Rockwell International Company)

One solution, used in computer keyboards to prevent single keystrokes from being mistaken as multiple keystrokes, is to include a keystroke-recognition program that refuses to recognize two sequential "on" states from a single key unless there is a significant time delay between them. This method of debouncing a switch can be written into machine-language control programs.

Another solution to the bounce problem, and to the contact conductivity problem, is to select one of the growing number of non-contact limit switches. These limit switches are not actually limit switches but are supplied in the same casings as traditional limit switches and can be used interchangeably with old style limit switches. Objects must still press against the lever to change the state of these switches, but what happens inside is different.

The most common non-contact limit switch, shown in Figure 1.7, is the Hall effect switch. Inside this switch, the lever moves a magnet toward a Hall effect sensor. An electric current continuously passes lengthwise through the Hall effect sensor. As the magnet approaches the sensor, this current is forced toward one side of the sensor. Contacts at the sides of the Hall effect sensor detect that the current is now concentrated at one side; there is now a voltage across the contacts. This voltage opens or closes a semiconductor switch. Although the switch operation appears complex, the integrated circuit is inexpensive.

 

1.2.2 Non-Contact Presence Sensors (Proximity Sensors)

The limit switches discussed in the previous section are "contact" presence sensors, in that they have to be touched by an object for that object's presence to be sensed. Contact sensors are often avoided in automated systems because wherever parts touch there is wear and a potential for eventual failure of the sensor. Automated systems are increasingly being designed with non-contact sensors. The three most common types of non-contact sensors in use today are the inductive proximity sensor, the capacitive proximity sensor, and the optical proximity sensor. All of these sensors are actually transducers, but they include control circuitry that allows them to be used as switches. The circuitry changes an internal switch when the transducer output reaches a certain value.

Fig.1. 7 Hall effect limit switchesFig.1. 7 Hall effect limit switches

Fig. 1.8 Inductive proximity sensors. (Photograph by permission, Balluff Inc., Florence, Kentucky.)

Fig. 1.8 Inductive proximity sensors. (Photograph by permission, Balluff Inc., Florence, Kentucky.)

Fig. 1.8 Inductive proximity sensors. (Photograph by permission, Balluff Inc., Florence, Kentucky.)

The inductive proximity sensor is the most widely used non-contact sensor due to its small size, robustness, and low cost. This type of sensor can detect only the presence of electrically conductive materials. Figure 1.8 demonstrates its operating principle.

The supply DC is used to generate AC in an internal coil, which in turn causes an alternating magnetic field. If no conductive materials are near the face of the sensor, the only impedance to the internal AC is due to the inductance of the coil. If, however, a conductive material enters the changing magnetic field, eddy currents are generated in that conductive material, and there is a resultant increase in the impedance to the AC in the proximity sensor. A current sensor, also built into the proximity sensor, detects when there is a drop in the internal AC current due to increased impedance. The current sensor controls a switch providing the output.

Capacitive proximity sensors sense "target" objects due to the target's ability to be electrically charged. Since even non-conductors can hold charges, this means that just about any object can be detected with this type of sensor. Figure 1.9 demonstrates the principle of capacitive proximity sensing.

Inside the sensor is a circuit that uses the supplied DC power to generate AC, to measure the current in the internal AC circuit, and to switch the output circuit when the amount of AC current changes. Unlike the inductive sensor, however, the AC does not drive a coil, but instead tries to charge a capacitor. Remember that capacitors can hold a charge because, when one plate is charged positively, negative charges are attracted into the other plate, thus allowing even more positive charges to be introduced into the first plate. Unless both plates are present and close to each other, it is very difficult to cause either plate to take on very much charge. Only one of the required two capacitor plates is actually built into the capacitive sensor! The AC can move current into and out of this plate only if there is another plate nearby that can hold the opposite charge. The target being sensed acts as the other plate. If this object is near enough to the face of the capacitive sensor to be affected by the charge in the sensor's internal capacitor plate, it will respond by becoming oppositely charged near the sensor, and the sensor will then be able to move significant current into and out of its internal plate.

Optical proximity sensors generally cost more than inductive proximity sensors, and about the same as capacitive sensors. They are widely used in automated systems because they have been available longer and because some can fit into small locations. These sensors are more commonly known as light beam sensors of the thru-beam type or of the retroreflective type. Both sensor types are shown in Figure 1.10.

A complete optical proximity sensor includes a light source, and a sensor that detects the light.

Fig.1. 9 Capacitive proximity sensors.Fig.1. 9 Capacitive proximity sensors.

The light source is supplied because it is usually critical that the light be "tailored" for the light sensor system. The light source generates light of a frequency that the light sensor is best able to detect, and that is not likely to be generated by other nearby sources. Infra-red light is used in most optical sensors. To make the light sensing system more foolproof, most optical proximity sensor light sources pulse the infrared light on and off at a fixed frequency. The light sensor circuit is designed so that light that is not pulsing at this frequency is rejected.

The light sensor in the optical proximity sensor is typically a semiconductor device such as a photodiode, which generates a small current when light energy strikes it, or more commonly a phototransistor or a photodarlington that allows current to flow if light strikes it. Early light sensors used photoconductive materials that became better conductors, and thus allowed current to pass, when light energy struck them.

Fig. 1.10 Optical proximity sensors.

Fig. 1.10 Optical proximity sensors.Fig. 1.10 Optical proximity sensors.

 

Sensor control circuitry is also required. The control circuitry may have to match the pulsing frequency of the transmitter with the light sensor. Control circuitry is also often used to switch the output circuit at a certain light level. Light beam sensors that output voltage or current proportional to the received light level are also available.

 

Through beam type sensors are usually used to signal the presence of an object that blocks light. If they have adjustable switching levels, they can be used, for example, to detect whether or not bottles are filled by the amount of light that passes through the bottle.

 

RetroHective type light sensors have the transmitter and receiver in the same package. They detect targets that reflect light back to the sensor. Retroreflective sensors that are focused to recognize targets within only a limited distance range are also available.

 

1.2.3 Temperature Transducers

In the previous section, we discussed several transducer-type sensors that were used as switches. Temperature transducers are almost always used as transducers. Figure 1.11 shows the four types of temperature sensors we will examine.

Probably the most common temperature sensor is the metal RTD, or Resistive Temperature Detector, which responds to heat by increasing its resistance to electric current. The thermistor type of temperature sensor is similar, except that its resistance decreases as it is heated. In either case, there is only a tiny variation in current flow due to temperature change. Current through an RTD or thermistor must be compared to current through another circuit containing identical devices at a reference temperature to detect the change. The freezing temperature of water is used as the reference temperature.

Semiconductor integrated circuit temperature detectors respond to temperature increases by increasing reverse-bias current across P-N junctions, generating a small but detectable current or voltage proportional to temperature. The integrated circuit may contain its own amplifier.

 

Thermocouple type temperature sensors generate a small voltage proportional to the temperature at the location where dissimilar metals are joined. The reason a voltage is generated is still a source of debate. One possible reason may be that heat causes electrons in metals to migrate away from the heated portion of the conductor, and that this tendency is greater in one of the metals than in the other.

 

1.1.3 Force and Pressure Transducers

Pressure is a measure of force as exerted by some elastic medium. Compressed air exerts a force as it tries to return to its original volume. Hydraulic fluids like oil, although considered "incompressible" in comparison to gases, do in fact reduce in volume when pressurized, and exert force as they attempt to return to their original volume. Pressure sensors measure this force.

All pressure sensors measure the difference in pressure between two regions by allowing the pressure from one region to exert its force against a surface sealing it from a second region. In most pressure sensors, this second region is the ambient pressure. Pressure gauges measure how far the pressure from the measured region moves a load that is held back by room air pressure. Some pressure sensors have two controlled pressure inlets so that the secondary pressure inlet can be connected from a second pressure source. These pressure sensors are called differential pressure sensors.

Quite a few force sensors are spring-type devices, where a spring is compressed by the force. A position sensor detects how far the spring compresses, and therefore how much force caused that compression.

Figure 1.12 shows how this type of force sensor might be built. Position sensors are discussed later in this chapter.

 

Strain gauges are sensors that measure deformation due to pressure. Figure 1.13 shows that a strain gauge is essentially a long thin conductor, often printed onto a plastic backing in such a way that it occupies very little space. When the strain gauge is stretched, the conductor reduces its cross-sectional area and thus can carry less current. The change in resistance is small and so requires a reference resistance and compensating circuitry to compensate for other sources of resistance changes (such as temperature!). Strain gauges are often glued to critical machine components to measure the deformation of those components under loaded conditions.

Fig. 1. 11 Temperature sensorsFig. 1. 11 Temperature sensors

Fig. 1.12 A spring-type pressure sensorFig. 1.12 A spring-type pressure sensor

The piezoelectric strain sensor includes a crystalline material that develops a voltage across the crystal when the crystal is deformed. The small voltage requires amplification. Piezoelectric crystals are often used in accelerometers to measure vibration. Accelerometers will be discussed in a later section.

 

1.2.5 Flow Transducers

Flow sensors measure the volume of material that passes the sensor in a given time. Such sensors are widely used in process control industries. Figure 1.14, demonstrates the principles used in several types of flow sensors.

Fig. 1.13 Strain gauges and compensation circuitryFig. 1.13 Strain gauges and compensation circuitry

 

Pressure sensors are often used to measure fluid flow. The faster the fluid is flowing, the more pressure it will create in the open end of the pitot type flow meter. Pressure upstream from a restricted orifice in a pipe is always higher than pressure downstream from that restriction. The greater the flow rate, the greater the pressure difference, so if a differential pressure sensor compares pressures before and after the restriction, then flow rate can be determined. The restriction orifice required by such a sensor reduces the flow.

Temperature sensors are used to measure the flow rate of cool liquids. Quickly flowing fluids can cool a sensor in the stream more than slowly moving fluids can. A heat source keeps the sensor within its operational range. Temperature sensors can be small, so flow restriction is minimal.


GO TO NEXT PAGE: INDUSTRIAL CONTROL HANDBOOK - 1.3 POSITION SENSORS

GO BACK TO PREVIOUS PAGE: INDUSTRIAL CONTROL HANDBOOK - 1.1 QUALITY OF SENSORS

GO TO MAIN PAGE: INDUSTRIAL CONTROL HANDBOOK - TABLE OF CONTENTS

Comments (0)Add Comment

Write comment

security code
Write the displayed characters


busy

Related Articles

Promotions

  • ...more

Disclaimer

Important: All images are copyrighted to their respective owners. All content cited is derived from their respective sources.

Contact us for information and your inquiries. IAMechatronics is open to link exchanges.

IAMechatronics Login