Many types of transducers are beneficiaries of IC technology. The benefits are twofold. There is the advantage of the chips themselves -- enabling manufacturers to build more intelligence and better noise immunity into their transducers. However, it has become apparent that spin-offs from ICs have an equally important effect. For example, the same basic technology used to make the miniature circuits is helping to improve the resolution of optical encoders.
Especially impressive is how IC technology has helped reduce the cost of some devices. Just a decade ago, many pressure transducers cost $300 or more. Today, with the advent of micromachining, devices with similar capabilities may only cost a few dollars.
Temperature sensors: Temperature sensors include thermocouples, resistive temperature detectors (RTDs), and thermistors. Each device is limited in terms of measurement accuracy. The amount of error associated with a given measurement approach must also be considered when choosing a sensor.
Thermocouples are widely used to measure temperature despite low sensitivity and moderate accuracy. They consist of two dissimilar metal wires joined at one end called the sensing junction. The voltage produced across this junction is proportional to its temperature. Because of a wide variety of usable wire materials, thermocouples collectively span an operating temperature range from -200 to 2,000°C. Typical accuracies are from 1 to 3%, depending on material type and manufacturing variations. Sensitivities range from 40 to 80 ∝V/°C. Also, the response time for most thermocouples is on the order of a few seconds.
Connecting a thermocouple to a measuring device creates a second dissimilar-metal junction called the reference junction. In a type-J thermocouple, for example, the reference junction is formed where the iron lead connects to a copper terminal or wire. This junction is in series with the sensing junction. Voltage generated at the reference junction must be subtracted from the overall measured voltage to determine the temperature of the sensing junction.
To simplify temperature calculations, reference-junction temperature is maintained at a level that produces a known constant voltage. The most common reference is 0°C, which is the temperature of an ice bath. A 0°C reference insures repeatability and accuracy because the ice point of water is a constant. The National Institute of Standards and Technology accepts the ice point of water as the standard reference temperature.
An ice bath may not always be convenient, however. In that case, the most common alternate method of determining reference voltage is with an integrated-circuit temperature sensor. The IC sensor is placed near the reference junction and measures local temperature. From its temperature reading, reference-junction voltage may be calculated.
Thermocouples age or drift when used at high temperatures. Aging is usually a much greater problem in the low-cost base-metal thermocouples than in those using platinum-based noble metals.
The upper temperature limit of base-metal thermocouples in air is generally determined by their oxidation resistance. This limit varies with wire size. The type-K thermocouple is the most widely used due to its oxidation resistance and high melting point. Even so, decalibration drifts of 10°C have been recorded with as little as 50 hr of exposure at 1,250°C.
A new nickel-base thermocouple called nicrosil/nisil has higher stability in air and better air-oxidation resistance at high temperatures than base-metal types. Its high-temperature drift is three times less than that of a type-K device. Estimates are that maintenance costs for the new thermocouple are 12 times lower than those of type K.
Most materials change resistance with temperature variations. Two types of temperature transducers use this phenomenon: RTDs and thermistors. Depending on the measured temperature, these devices are increasingly used in lieu of thermocouples in many applications. The reason is simplicity. Thermocouples need special wires and cold-junction compensation; RTDs and thermistors do not. Thermistors find use up to about 100°C, while RTDs are practical up to about 400°C.
The primary difference between RTDs and thermistors is that the former use a metal-sensing element while the latter use a semiconductor. As a result, RTDs offer better linearity and are more stable at high temperatures. But RTDs usually cost more and require more complex measuring circuits.
RTDs typically consist of a wound platinum wire inside a glass or stainless-steel package. Platinum is preferred for several reasons. It is malleable, linear, has a higher resistivity than more common metals, and resists corrosion. While some low-cost RTDs use nickel alloy or copper wire, these sensors are usually less sensitive and have lower temperature ranges.
Wire-type industrial RTDs wrap the sensing wire around a bobbin. But for high-precision or laboratory measurements, the wire is left unsupported, a type of construction called a birdcage element. Because birdcage elements are free to expand or contract with temperature changes, they experience less strain. This, in turn, minimizes strain-induced resistance changes but makes the devices susceptible to failures caused by vibration.
While wire-wound bobbin RTDs are strong enough for most applications, a more rugged version is the thin-film type. Here, manufacturers screen a metal film (slurry) onto a ceramic substrate. The cured film is laser trimmed and sealed.
One problem with RTDs not made of platinum is limited temperature range. For example, copper types can measure only up to about 120°C. To increase range, some manufacturers use permalloy (Ni-Fe), which has a resistivity several times greater than that of many other metals.
In picking temperature sensors, it is important to realize that packaging considerations limit RTDs to below about 400°C unless specifically designed for higher temperatures. The basic problem is iron poisoning of the platinum sensing element. At elevated temperatures, small amounts of iron impurities can leach out of the stainless-steel package and corrode the platinum. Two methods can prevent this. One is to use higher quality stainless-steel packages (324 or 327 alloys). The other solution is to place a quartz sheath between the detector and stainless-steel package.
Acquiring data from RTDs involves resistance measurements. Platinum RTDs start at 100Ω at 0°C and vary by 0.0385Ω for every 0.1°C change in temperature. Unfortunately, package leads made from copper wire also have a measurable temperature dependence. As temperatures increase, the resistance of the copper leads also increases. A lead resistance variation of as little as 1Ω may cause measurement errors of a few degrees.
To avoid errors caused by lead-resistance variations, several measurement schemes use more than two lead wires. In multiple-lead RTDs, a constant-current source excites the sensor through one pair of leads, while the voltage drop is read from a separate lead pair. Because extremely small currents are in the voltage measurement loop, resistive drops are negligible.
One type of multiple-lead RTD has three wires and is commonly interfaced to measurement systems through a bridge-completion circuit. However, several weaknesses are associated with this technique. In order to provide repeatable measurements, lead resistances must closely match and the bridge must remain balanced. Although a potentiometer may be used to balance the bridge, it also causes major inconveniences. For one, the electromechanical device is unstable over time and requires frequent adjustment. Also, an adjustment is necessary every time the RTD is replaced.
A four-wire ohms-measurement circuit is a better way to cancel the effect of lead resistance. A stable power supply forces a known current through the RTD and the primary lead resistances. Because the input impedance of the measuring circuit is usually greater than 10MΩ, virtually no current flows and no voltage is dropped across voltage sense leads. The resistance of the RTD is found by dividing measured voltage by source current.
Thermistors, ike RTDs, are temperature-sensitive resistors. As temperature increases, thermistor resistance decreases, but at a much faster rate than that of RTDs. As a result, thermistors can sense minute changes in temperature that are otherwise undetected by RTDs and thermocouples.
Temperature coefficients for thermistors are typically as large as -2 to -6%/°C, compared to about 0.4%/°C for RTDs. Most thermistors have a resistance of 5,000Ω at 25°C and sensitivities between 100 and 300Ω/°C, much larger than the 0.4-Ω/°C sensitivity for 100-Ω RTDs. Bridge-completion and ohms-measurement circuits are not needed when connecting thermistors to acquisition equipment because lead resistances are negligible with respect to sensor variations.
An average thermistor with a temperature coefficient of 4%/°C for example, changes 200Ω/°C. At this level of sensitivity, a measurement lead with a resistance of 10Ω only contributes a 0.05°C error. The same lead causes an error of 25°C in a 100-Ω RTD, 500 times worse.
Compared to other sensors, thermistors have a limited measuring range, typically from -80 to 150°C. Also, because they are often made from semiconductors or sintered mixtures of metal oxides, they can sustain permanent damage at temperatures above their specified operating range.
Another problem is called self-heating error. As thermistors dissipate power they may warm slightly. Because they are so small, typical power dissipation constants are about 1 mW/°C. When measuring cold temperatures, for example, it is possible for the device to be warmer than the temperature being measured. Thermistors also are more nonlinear and curve-fitting polynomial equations or resistance-temperature lookup tables are often used to approximate temperature.
A significant advantage of thermistors, however, is that they easily interface with PCs, requiring very little circuitry. In many cases a simple voltage-divider network is sufficient. A dc supply produces a voltage Vs that is shared between the thermistor and the fixed resistor R. The voltage Vo across the fixed resistor is measured to calculate thermistor resistance RT.
The best value for the fixed resistor is determined from several temperature curves. The resistor value is chosen based on the specified thermistor, resistance ratio, temperature range, and required linearity. Most thermistor manufacturers provide literature that simplifies the complicated process.
Manufacturers sometimes place thermistors in two groups determined by lead attachment. The first classification is the bead type. These have platinum wires sintered into a ceramic body (bead). Beads are sometimes left bare or have an organic coating (epoxy), but those sealed in glass have the best stability (temperature drift over time). Stability is directly proportional to glass thickness because thicker glass helps prevent changes caused by oxidation. For instance, a bare bead drifts ten times more than a glass-coated one.
Metallized surface-contact thermistors form the second group. These are called chips or flakes. In contrast to bead types, leads are not sintered directly into the ceramic. Instead, the sintered ceramic is coated with a metallic contact. Either the chip manufacturer or user attaches leads to this contact.
One advantage of chip thermistors over bead types is that the chips are easily trimmed by cutting or grinding. Thus, they are easy to match and, therefore, are interchangeable. While matched bead thermistors are available, they cost more than interchangeable chips.
Interchangeability is one of the major advantages of thermistors. The 2,252Ω elements have a unit-to-unit variation of less than ±0.2°C from 0° to 70°C. Decalibration drifts are on the order of less than 1°C per year if used constantly at 150°C. Drifts of less than 0.3°C can be expected at temperatures below 100°C
These devices typically comprise sensing elements and switching contacts housed in a single mechanical assembly. The sensing element measures temperature and actuates the contacts in response to thermal variations. Switches may open or close on temperature rise depending on their internal construction.
In most applications, temperature switches provide one of two functions: control or cutoff. Temperature switches operating in the control mode are called thermostats. They are used to maintain the temperature of a system within a specified range. Their contacts either pass or block control currents as a function of system temperature.
In the cutoff mode, temperature switches protect equipment against over or undertemperature conditions. In many cases, temperature cutoffs are a backup for a primary temperature controller. Once temperatures return to normal, some cutoffs automatically reset, while others must be manually reset. One type of cutoff, called thermal fuse, must be replaced following actuation.
Electromechanical temperature-actuated switches are available in several different forms. Designers can choose from reed switches, liquid-filled thermostats, mercury-in-glass tubes, and a family of differential-expansion devices.
Differential-expansion thermostats are the most common ones used today. They operate on various switching mechanisms including bimetallic discs, fused-bimetal elements, and mechanically linked assemblies. Some switch with a snapping action, while others, called creep-type switches, change states gradually.
Differential-expansion thermostats make use of an interaction between two metals with widely varying coefficients of thermal expansion. The interaction causes a movement that actuates a set of contacts, opening or closing them.
One type of differential-expansion switch is the fused bimetal thermostat. In bimetal thermostats, the sensor is made of two strips of dissimilar metal bonded into one element. When the temperature changes, unequal expansion of the two metals causes the strip to bend into an arc. The movement either makes or breaks an electrical contact.
In some cases, the bimetal element is the current-carrying conductor, while in others it only pushes the conductors together. In either case, the movement of the bimetal strip is gradual, and the amount of movement is proportional to the temperature. The make temperature is calibrated with an adjusting screw that varies the position of the fixed contact.
Fused-bimetal thermostats are typically used in electric blankets. Normally closed contacts pass current, allowing the blanket to heat. Electric blankets remain at a safe operating temperature with full heating current applied. If an overtemperature occurs, the bimetal strip deflects and breaks the circuit. When the cause of overheating is removed, the switch returns to a normal state.
A potential problem with fused-bimetal thermostats is contact arcing. Because the contacts slowly open and close, arcing may occur during intermediate states where contacts are close together or only lightly touching. Arcing wears out the mating surfaces of the contacts and raises contact resistance. As a result, cycle-life limits of creep-type fused-bimetal thermostats should be closely observed, especially in products where many actuations are expected.
In contrast, snap-acting bimetal thermostats open and close extremely fast with greatly reduced arcing. Switching times are as low as 0.1 msec. Rapid-contact separation insures long contact life as well as calibration stability. It also reduces the amount of radio-frequency interference that moving contacts often generate.
Most snap-acting elements are concave bimetal discs. When the temperature changes, expansion on one side is much greater than that on the other. The stress created by the unequal expansion increases until it overcomes the biasing stress. At that point, the disc inverts with a snap into a convex shape. When normal temperatures return, the process is reversed.
Bimetal-disc thermostats usually have a fixed (tamperproof) temperature setting and a relatively large operating differential. The large differential makes them useful when a substantial dead zone (10 to 20°F) is desirable. Some bimetallic switch makers recommend a minimum differential of 15°F, and 25°F for systems operating above 250°F. This optimizes the trade-off between the operating differential and the crispness of the snap action. Switching elements with small operating differentials have reduced snap action.
Bimetal-disc thermostats respond to temperature changes down to about 3 or 4°F. They can sense airstream or surface temperatures and radiant heat. Their operating range is from -20 to 1,490°F. They can switch dry circuit loads of 400 Vac at up to 50 A.
Another type of differential-expansion thermostat is a mechanically linked strut and shell. Unlike bimetal switches where two metals are bonded together into a single element, the active metals in mechanically linked switches remain separate. The more thermally active metal, usually brass or stainless steel, is the switch housing or shell. The less active metal, usually a high-nickel alloy, is a strut assembly on which the contacts are mounted.
As temperatures vary, the shell and strut expand or contract unequally, opening or closing the contacts. Actuation temperatures are adjustable with a screw that either compresses or stretches the strut assembly. Because their outer shells are active sensing elements, strut-and-shell switches respond extremely fast to temperature changes.
One advantage of strut-and-shell thermostats is that they typically have high-resolution sensitivity, down to 0.1°F. Because the switching mechanism makes or breaks slowly (creep action), almost any temperature change causes a corresponding change in contact spacing. That is, contact action can be produced by very small temperature variations. In contrast, snap-acting bimetal elements often have resolution sensitivities of several degrees because a finite amount of energy must be absorbed to overcome the restraining forces holding the contacts in place.
Strut-and-shell thermostats also have high vibration and shock resistance. Because the strut assembly is constantly under tension or compression, it resists mechanical shock and vibration. This allows it to operate reliably and accurately under physically harsh conditions. Such hazards can cause problems in other types of differential-expansion switches.
Another advantage of the strut-and-shell switch is that it "anticipates" rapid temperature changes as a result of an inherent time lag between the shell and internal struts. The lag allows the shell to lead the struts by a time interval proportional to the rate of temperature change. In the case of rapid temperature change, the shell exerts a larger net force on the struts and actuates the switch sooner than for a gradual change. This produces several degrees or more of anticipation, leading to a tighter control band.
Another mechanically linked type operates on a stainless-steel sensing tube. The tube contains a two-section inner rod whose active section expands at a much lower rate than the tube. The expansion difference is multiplied by a lever that actuates a snap switch or pilot valve. Sensing-tube thermostats operate at up to 2,000°F.
Liquid-filled thermostats, although slower than differential-expansion types, typically handle more current because their switches are not part of the sensing system. As a result, a wide variety of interchangeable switches can provide various types of service including 20 A at 120 or 240 Vac, narrow differential, high inrush, and manual reset.
Liquid-filled thermostats employ temperature-sensing bulbs that contain an incompressible liquid. The type of liquid determines the operating range. As the temperature varies, the volume of the liquid expands or contracts. This displacement is transmitted hydraulically through a bellows or diaphragm. Bellows motion, in turn, can be transmitted through a push rod or other mechanical linkage to actuate the contacts of a dedicated electrical switch.
Both local and remote temperature control is possible with liquid-filled switches. Local-bulb types enclose the sensing liquid, bellows, and push rod in a single shell. The electrical control switch is attached to the top of the shell.
Remote temperature control is provided by a bulb-and-capillary arrangement. The metal bulb, filled with sensing fluid, is connected to the bellows and switch mechanism through a thin capillary tube about 6 to 10 ft long. The capillary transmits fluid expansion from the metal bulb to the bellows.
In contrast to differential-expansion thermostats, liquid-filled versions are less sensitive and considerably slower. Because of this, they respond to average temperature rather than instantaneous. Average temperature measurement is appropriate for applications with a large thermal time constant. For example, liquid-filled thermostats are suited for controlling temperatures in waterbed mattresses, commercial cooking ovens, and environmental chambers.
Bulb-and-capillary thermostats control remote switches, removed from the environment or process under control. Control switches, therefore, do not have to operate under the severe temperature conditions where the sensors are located. Also, because switches are not incorporated in the sensing element, liquid-filled bulbs can have a smooth tubular shape. Such a profile is mechanically strong and easy to design in a product. Under a waterbed mattress, for instance, the bulb withstands high forces and is not a puncture threat.
Mercury-in-glass thermostats are the most accurate of the electromechanical temperature switches. They provide resolution sensitivity and repeatability equal to that of RTDs and thermistors. Resolutions to ±0.05°F are possible. Temperature ranges extend from -40 to 550°F.
Their construction consists of a mercury-filled bulb with an attached glass capillary tube. Two wires penetrate the capillary wall and act as a normally open contact. As temperature increases, the expanding mercury travels up the capillary tube until the two wires are shorted. The switching temperature is determined by the distance between the wires.
Advantages of mercury temperature switches include low cost, long lifetimes with a minimum of 1 million cycles, and limited drift less than 0.09°F. Also, they can switch with reduced arcing by the addition of an arc-suppressing gas. The most notable disadvantage, however, is a phenomenon called separation. Separation is a break in the mercury column caused by a sudden shock or extreme overtemperature. Although the condition is correctable, it typically requires human intervention.
Current ratings for mercury thermostats are typically between 10 and 25 mA at 115 Vac. As a result, additional control circuits are often needed to handle higher loads. Integrated solid-state relays provide up to 5 A of switching capability, with 1A being standard. A solid-state interface, such as a transistor or SCR, allows mercury thermostats to switch inductive or capacitive loads.
Mercury-in-glass thermostat makers, now offer miniaturized packages, some no larger than a penny. Also available are mercury thermostats with multiple set points, adjustable set points, and built-in solid-state relays.
Reed-switch temperature sensors work according to a simple principle of magnetics: at a critical temperature, called the Curie point, certain materials undergo a transition from being ferromagnetic to paramagnetic. That is, they go from a state of low reluctance to high reluctance (low permeability). This transformation actuates a set of contacts.
A temperature-sensitive reed switch is constructed with two toroidal magnets separated by a ferrite collar. Operating temperature is determined by the Curie point of the ferrite collar. The ferrite is a magnetic material produced by substituting iron atoms in a crystalline structure with nickel zinc, manganese zinc, and other materials. By varying the composition of the ferrite, the Curie temperature can be adjusted over some range. One reed-switch maker offers sensors that operate from 14 to 482°C.
At temperatures below its Curie point, the collar shunts magnetic flux from the south pole of one magnet to the north pole of the other. Here, the stack behaves like a continuous magnetic tube. Flux lines are coupled from one end of the stack to the other through the reeds, holding them closed.
Above the Curie point, the permeability of the ferrite collar decreases, causing it to resist the passage of flux lines. At this point, the two magnets are no longer coupled and flux lines remain localized near the individual magnets. Because there is no magnetic field at the center of the stack, the reed contacts open.
When designing with reed-switch thermostats, be aware of the magnetic-field environment of the application. Testing has shown that subjecting a reed switch to strong magnetic fields or enclosing one within a magnetic shunt -- such as a ferrous-metal clamp -- causes the apparent operating temperature to vary. However, the effects of common fields, such as those near motors or solenoids and mounts on sheet-metal enclosures and I-beams, usually are not severe.
Ferrous metal within the vicinity of a reed switch provides a low-reluctance shunt path that diverts some of the flux lines outside the package. This reduces the amount of flux within the package that otherwise holds the reeds together. For example, mounting reed switches directly on a …in. steel plate can cause trip temperature to drop by 4°C (7°F). Test data reveal that environmental effects can be minimized when sensors are mounted 1/8 in. or more away from flat ferromagnetic surfaces.
Innovations in product packaging are chiefly responsible for improvements in the mature temperature switch market. For example, one temperature switch maker recently announced a -in. snap-acting bimetallic disc with an internal heating element that provides remote control of the operating temperature set point.
A biasing heater allows the single device to activate at any number of set points within a given range. By changing the voltage applied across the heater, varying amounts of supplemental heat is delivered to the bimetal element. Adding heat to the sensing element allows the device to switch at lower ambient temperatures.
Primary applications for the device include gas and electric clothes dryers. Here, the thermostat provides control over the temperature of the drum. One device allows dryers to have multiple setpoint temperatures corresponding to various fabric types.
In conventional designs with multiple temperature settings, dryers typically contain three thermostats with nominal calibrations of 135, 145, and 155 ±5°F. Because the set points are so close, it is possible for two adjacent settings to operate at very near the same temperature.
Internally heated thermostats called Anticipators, instead of controlling operating temperature, anticipate temperature changes. This forecasting is possible when a fraction of the control current is diverted through the internal heating element. The advantages are reduced thermal overshoot in heating applications, and a tighter operating differential without sacrificing crisp snap action.
Other new thermostats have built-in tilt switches and pilot lights for indicating unsafe or dangerously high setpoints. Tilt switches protect against fire in the event electrical appliances or heaters are knocked over. Typical causes of upset include misuse, children, animals, and earthquakes.
Other improvements include miniature thermostats available in a wide variety of packaging styles. Bimetallic snap-acting temperature switches are now available in standard TO-220, and 8-pin DIP packages. Standard packaging allows the thermostats to be mounted on PC boards with automatic pick-and-place equipment. Nickel-plated copper mounting brackets provide good thermal contact to heat sinks or other thermally conductive surfaces.
Temperature-control ICs are basically thermal switches. They monitor high and low set points, changing states when temperature drifts outside the prescribed band. Their logic-compatible output turns motors, valves, heaters, fans, and power supplies on and off via relays and semiconductor devices. They also directly drive LEDs and talk to microcontrollers. Some even produce a voltage proportional to temperature with high accuracy
Set points are programmed with external resistors, or they can be fixed on-chip at the factory. Switch-selectable resistive networks and potentiometers provide multiple settings, letting users adjust temperature range as in heaters, dryers, and ovens.
Thermostatic chips measure temperature in different ways. Some use on-chip sensors, others connect to remote thermistors. Remote sensing is handy for tough jobs involving hostile or cramped environments and where fast response is vital. Typical pricing for the programmable ICs is around $2 in large quantities.
Packaging options run the gamut from low-cost plastic DIPs to industrial and military-grade cases. Other options include 8-pin ceramic DIPs, small outlines, TO-220, and TO-99 metal cans. Chips are also available in die form for hybrids and special mounting.
Silicon thermostats have all the advantages inherent to solid-state construction. Free of moving parts, they are immune to shock and vibration which often trouble mechanical devices. Life cycles are virtually unlimited because there are no contacts to open and close. And unlike bimetal switches, with typical cycle lives under 100,000, performance does not degrade with time.
Logic-compatible I/O simplifies communication networks that will be the trademark of smart products. These networks not only will link internal components, but also be a window into appliances, cars, and other machines for diagnostics and teleservicing. Interfacing mechanical switches to digital networks is difficult at best.
Another plus for thermostatic ICs is their accuracy. Silicon thermostats have accuracies of a few degrees or better compared to 15 to 25°F for their bimetal counterparts. Temperature ICs also are programmable, so manufacturers can stock one component for applications covering a range of set points and hysteresis.
Standard IC packaging is convenient, particularly when thermostats mount on printed-circuit boards. These devices work well with high-speed production equipment such as pick-and-place machines and tape-and-reel feeders.
Solid-state thermostats are not without disadvantages, however. They are more expensive, require power to operate, and cannot switch more than 25 mA at a few volts. External power switches are required to control all but the smallest loads, making products more expensive and complex. Mechanical thermostats, on the other hand, can switch 20 to 50 A at over 120 V.
Other drawbacks include self-heating and susceptibility to electrostatic discharge. ESD spikes over 2 kV, for example, can damage on-chip logic gates. And unless compensated, self-heating effects can change temperature settings. Most product literature comes with charts and tables to help determine cooling requirements.
Strain gages convert mechanical compression and elongation into electric signals. The most widely used strain gage for stress analysis is the bonded-resistive type. It is often chosen because of low cost, small size, insensitivity of temperature change, and high sensitivity to strain. Bonded-resistive strain gages can be as small as 16 mm2 and have a strain sensitivity or "gage factor" of 2.
Bonded-resistive strain gages produce a change in electrical resistance proportional to variations in strain. The sensor is made from a grid of very fine wire or a thin metallic foil bonded to a thin insulating backing called a carrier matrix. The carrier matrix attaches to test specimens with an adhesive. When the specimen is mechanically stressed (loaded), the strain on the surface is transmitted to the resistive grid through the adhesive and carrier layers.
The most widely used technique for reading strain gages is the Wheatstone bridge method because of its high sensitivity. The basic configuration, where the strain gage is one leg of the bridge, is called a quarter bridge. Other configurations include half and full-bridge networks.
When the specimen is strained or loaded, a change in sensor resistance causes a measurable variation in voltage. The amount of adjustment needed to rebalance the bridge is equal to the stress-induced variation in strain-gage resistance. The bridge is rebalanced when the output is nulled.
Most strain gages are supplied with a calibration factor typically expressed as a ratio of signal to excitation per full-scale pressure, mV/V/psi. For example, a strain gage may have a calibration factor of 30 mV/V/100 psi. The voltage difference measured by the Wheatstone bridge is applied to the calibration factor through software to determine the strain or load on the sensor and test specimen.
Full-scale load is normally specified within the calibration factor. Full-scale represents the maximum load that the sensor can withstand. Going beyond this value may cause inaccurate readings, and in some cases, damage the strain gage.
Pressure sensors are undergoing rapid change. Many foil or wire-bonded devices are giving way to silicon-based sensors. One reason is that silicon has excellent mechanical qualities that make it a good sensing element. For example, silicon has a higher elastic limit than steel in both tension and compression. And a single-crystal silicon diaphragm retains its original shape and dimensions even after being subjected to many pressure cycles. Steel, being a polycrystalline, tends to fatigue and break because of stress accumulation at intercrystal boundaries. Silicon also has a much higher gage factor than foil or wire-based devices.
Many types of pressure sensors are available from hundreds of different manufacturers. Not all meet the same service demands. There are vast differences between a bona fide hydraulic transducer and a volume-produced, low-cost sensor in the under $10 price range. Sorting through the possibilities to select the best transducer for a given application can be a formidable task.
Motion transducers use a bellows or Bourdon tube to convert pressure to an output. In one common type, the LVDT, an inductive member is driven into or out of a coil. It contains numerous pivots and linkages, making it nonlinear and susceptible to wear and vibration, but it has the advantage of inherently high output.
Pressure potentiometers have characteristics similar to those of LVDTs. In this case, a wiper is driven across a resistive coil, with output determined by wiper position. Compared to an LVDT, it has the added disadvantage of coil wear. If continuously operated in about the same pressure range, it may suddenly short out or produce severely nonlinear output. These sensors are rather inexpensive.
Silicon or "chip" transducers are widely used in high-volume applications. There are two types of silicon pressure sensors, capacitive and piezoresistive. Capacitive devices are much more stable, sensitive, and temperature resistant. But piezoresistive types are easier to make. And because they cost less, they predominate the market. They also have better linearity than capacitive devices.
Capacitance transducers use a flexing diaphragm to produce capacitance changes proportional to applied pressure. Such devices run the full gamut of price and performance. Because of their low price, a common application of these devices is in automobiles. One problem is that operation at normal hydraulic pressure dictates a large diaphragm. Therefore, while generally not used in the fluid-power arena, they are well suited to low-pressure systems.
Both gage and absolute versions of piezoresistive sensors are available. The sensor typically consists of a Wheatstone bridge etched on a silicon diaphragm. The diaphragm is housed in standard TO-8 and TO-5 packages. When powered with a constant current source, the output voltage is proportional to pressure.
Some silicon sensors use special packaging to handle high pressures. In one such arrangement, a stainless-steel case and diaphragm are used. An oil column couples the piezoresistive sensor to the diaphragm.
Pressure transducer packaging is a fundamental part of pressure sensor design. Manufacturers often house the same silicon chip in several different packages in both gage and absolute versions.
Electropneumatic transducers: These have long been used to provide regulated air pressures for the control of process systems. Typically, the accuracy required of such systems has not been critical; however, industry today is demanding far greater accuracy of its pneumatic control systems. This has spurred the development of a new generation of transducers that can control pressures with accuracies of ±0.25%.
These transducers can operate on electrical inputs of 1 to 5 mA, 4 to 20 mA, 10 to 50 mA, 1 to 5 Vdc, or 1 to 9 Vdc. These ratings correspond to a wide variety of industrial controllers and computers. Typical output of a transducer is a 3 to 15 psig pressure signal, which is nearly linear in relation to input current.
Transducer output flow is about 0.1 scfm at 15 psig, a rate that is too slow to actuate many components in a reasonable time. To overcome this problem, flow rate can be increased with the use of a volume booster. Volume boosters can be built into the transducer to boost output flow by any ratio required.
Practically all electropneumatic transducers operate on instrument-quality air, at 20 ±2 psig. This is essentially clean, oil-free air and is recommended to ensure the reliable operation of the nozzle/orifice combination. Some transducers, however, can operate on reasonably clean shop air at supply pressures to 150 psig. Modifications made to allow operation on such air include the use of a larger nozzle and a 40-∝m air filter.
Electropneumatic transducers typically are of three basic types: voice-coil beam, voice-coil beam dampened by an oil dashpot, and torque motor.
Voice-coil beam transducers use a nozzle/flapper arrangement to convert a small mechanical motion into a proportional pneumatic signal. The flapper arm is attached to the transducer base by a flexure strip. Various nozzle/flapper arrangements are used but the basic operation of each is similar.
In operation, current passes through the coil, and the resultant magnetic field reacts with that of a permanent magnet to move the coil assembly up or down. The resultant thrust on the coil is transmitted to the flexure strip to vary the position of the flapper, controlling air flow through the nozzle and varying output pressure in proportion to the current passing through the coil.
Damped transducers operate in a similar manner except that the arm controlling flapper position is attached to a float suspended in silicone oil. The oil dashpot effectively isolates the nozzle/flapper from vibration. However, it has the added effect of slowing response time. Such transducers also are sensitive to mounting angle because the oil can leak out. Typically, mounting angle is limited to about ±10°.
Torque-motor transducers also have similar operating principles, except that a conventional torque motor replaces the voice-coil beam arrangement to position the flapper. An advantage of such transducers is that the torque motor maintains its position even under fairly severe vibration.
Of the three types, voice-coil beam transducers are the most widely used. In this group are units that range from low-cost devices used in some HVAC systems, which do not require great accuracy, to higher-cost precision units that provide accuracies of ±0.25% or better on linearity and hysteresis combined. Voice-coil units also are typically smaller and lighter than oil-damped or torque-motor types.
Oil-damped transducers also are more costly than simple voice-coil devices, but they provide more stable operation when subjected to vibrations of up to 100 Hz (and sometimes as high as 200 Hz). However, absolute distinctions are difficult to make in this regard because the industry is only now beginning to characterize the performance of electropneumatic transducers under vibration loads. Preliminary test results indicate that damped transducers may provide an advantage over new lightweight undamped units only at frequencies below about 10 Hz.
Torque-motor transducers also provide accuracies as high as ±0.5% and are extremely stable. However, they can weigh three to four times as much as voice-coil units and cost three to five times as much.