af-fected by its temperature.
T R A N S D U C E R T E C H N O L O G Y P A R T 3
versible; by inputting electrical en-ergy, the junction can be cooled. Com-mercial use of the Peltier effect is lim-ited to small, non-moving-part refrigerators that operate from a DC source. If the aim is to create a refrig-erator, it is important that the amount of input current not be excessive. That is, there is a current level for the ther-mocouple where the amount of cool-ing is greater than the heatcool-ing because of electrical current in the junction re-sistance. If the current is excessive, heating overpowers the cooling.
Mechanical reaction
All solid materials are elastic, a phe-nomenon indicated by their modulus of elasticity. When these materials are subjected to external stress, they de-form. This well-known physical princi-ple has been put to work in transducers.
The most common implementation in fluid power is the Bourdon tube pres-sure gage, Figure 39. The instrument is made by bending a piece of tubing into semicircular form. When pressure is applied at the input port, the internal pressure causes the tube to straighten, deflecting the tip. Calibrating tip move-ment to known pressures, the device becomes a useful pressure transducer.
Note that input is fluid pressure while output is mechanical displacement.
Other types of transducers use the material modulus of elasticity principle as well. To measure pressure, di-aphragms, bellows, and simple hollow tubes are used. Pressurized fluid, intro-duced into the fluid cavity of a simpli-fied diaphragm pressure transducer, Figure 40, acts on the diaphragm. The diaphragm deflects because of the force generated by pressure fluid and the de-flection is imparted to a cantilever beam through a small push pin. Deflec-tion of the cantilever can be calibrated against known pressures to create a useful pressure transducer.
The commercial version of this con-cept converts cantilever deflection to an electrical output by mounting strain gages on the beam. Note in Figure 40 that the top of the raised cantilever beam is in compression while the bot-tom of the beam is in tension. With two strain gages bonded to the top and two bonded to the bottom of the cantilever, the gages can be interconnected electri-cally to form a bridge circuit. The two tensioning gages are diagonally oppo-site one another in the bridge while the two compressing gages occupy the complimentary diagonals. Thus, all four gages are active and all are af-fected by input pressure, Pin. This con-struction offers maximum sensitivity.
Force transducers are called load cells. One type has a proving ring that carries and transducers the unknown force, Figure 41. Applied external load force fLcauses the ring to become ellip-tical. Cross-center distance x is a func-tion of the amount of force applied and can be calibrated; x may be converted to an electrical signal with an LVDT or
with strain gages mounted on the inside and outside surfaces of the ring and on the axis that is transverse to the direc-tion of applied force fL.
Torque transducers, also called torque shafts, are used to measure the torque transmitted in a rotating shaft.
The metal of the shaft twists in the presence of the transmitted torque, causing an angular displacement be-tween the input and output ends of the shaft, Figure 42. During the calibration process, the amount of angular twist or wind-up, is measured for each known torque and can then be used in a mea-surement situation to infer the amount of an unknown torque. In commercial versions of torque tubes, the angular deflection has been transduced using strain gages or LVDTs.
Thermal expansion — When mate-rials undergo a change in temperature,
T R A N S D U C E R S P A R T 3
Pin
x
, ,,, ,,
Cantilever beam
Diaphram Push
pin
Fluid medium Pin
Ti
To
O Fig. 39. When internal pressure in a
Bour-don tube pressure gage straightens the tube, the tip moves in direct proportion to the amount of pressure.
Fig. 40. In one version of a diaphragm pressure transducer, the pressure-de-flected diaphragm imparts motion through a push-pin to a cantilever beam. Deflection of the cantilever is calibrated against inlet pressure.
Fig. 42. Applied torque causes the shaft to twist, resulting in an angular difference between the input and output ends of the shaft. The amount of twist can be cali-brated against known torques.
fL
Fig. 41. The proving ring is a metal ring that deforms on application of external load source fL. Once deflection x has been calibrated against known load forces, other x deflections are transduceable re-flections on unknown loads.
a change in the physical dimensions of the body accompanies that change. It is usually an expansion with a tempera-ture increase, and occurs in liquids, gases, and solids. This principle has been used in temperature transducers;
such bi-metal thermometers consist of two dissimilar metals bonded together, Figure 43. Their major dissimilarity is their respective temperature coeffi-cients of expansion. When the bonded pair is exposed to a change in tempera-ture, one expands more than the other to bend the assembly. The degree of bending can be calibrated for use as a
temperature indicator.
The advantage of bonding two mate-rials is that the amount of bending is greater than the simple expansion or contraction of either material alone.
Thus the effect of the temperature change is apparent more readily in the bending of the assembly than it would be with only one material. Mounting strain gages on either side of the bi-metal strip and then forming them into a four active-gage bridge circuit con-verts mechanical instrument output to an electrical signal. An LVDT or other position-sensing transducer might be connected to the tip.
Liquid expansion is used in the famil-iar household liquid-in-glass thermome-ters. Alcohol-based solutions and mer-cury are liquids of choice because they have very low freezing temperatures;
this gives the thermometer a large tem-perature range. The expansion liquid is contained in a relatively large bulb at the bottom of the transducer. The bulb is at-tached to a thin capillary tube that has been evacuated and sealed. The amount of liquid used in the manufacture of the instrument exceeds the volume of the bulb by a slight amount at room temper-ature so the liquid reaches up into the capillary at that temperature. When the bulb is exposed to an elevated tempera-ture, the expanding liquid rises farther in the evacuated capillary tube, and with calibration, becomes a useful tempera-ture transducer, Figure 44.
Although it is difficult to convert this output to an electrical signal, it has been done by immersing a resistive ele-ment into the capillary. Then the liquid shorts out a portion of the resistor so that resistance changes with tempera-ture. Gases also expand in the face of rising temperature and this principle has been put to work to make tempera-ture transducers. Construction consists of filling an evacuated bulb with a gas (usually nitrogen because of its low freezing temperature), then connecting the bulb to a pressure transducer through a capillary tube. As the temper-ature of the gas changes, the internal pressure changes, reflecting the tem-perature of the bulb.
Electromechanical energy conversion In electromechanical energy conver-sion, electrical energy is first stored in
an electromagnetic or electrostatic field. Then, movement of an output member extracts some of the field en-ergy to do useful mechanical work. The electromagnetic field surrounds a cur-rent-carrying conductor and is com-monly called a magnetic field. In the electromechanical energy conversion process, this is the most commercially predominant method of energy conver-sion. Note also that the energy con-tained in the magnetic field is directly proportional to the amount of magnetic flux in the field, and that the amount of flux can be increased by forming the current-carrying conductor into a coil so that the flux produced by one wrap of the coil adds to that of the next wrap, and to the next, and so on. The flux can be further increased by providing it with a path that has low reluctance (re-sistance) with the most common path material being iron. The resulting de-vice is an iron-core inductor, but some-what complex in that at least one part of its iron core is able to move.
Electrostatic fields for the most part are invisible to non- technical ob-servers and are mostly misunderstood even when their effects can be seen.
These fields exist because a positive electrical charge is separated from a negative charge — a charged capacitor.
The electrostatic field exists in the di-electric between the capacitor plates and is substantially confined to that re-gion. One plate is positively charged and the other is negatively charged, creating the necessary and sufficient condition for an electrostatic field.
This kind of field also exists after walking across a carpet when the hu-midity is low. The arc and shock that occur when a metal object is touched is palpable evidence of the energy that is stored in the electrostatic field. The shock arises because of the current that results from the electrical charges be-ing redistributed between the charged body and the neutral metal. The heat of the arc dissipates the energy that was stored because of charge separation.
Passing a comb through your hair also picks up electric charges and the comb is able to attract small pieces of paper. Lifting the paper against gravity constitutes an electromechanical en-ergy conversion because work is done on the paper. It is possible to harness
1998/1999 Fluid Power Handbook & Directory
A/183 T R A N S D U C E R S P A R T 3
Fig. 43. Two materials with different tem-perature coefficients of expansion form the bi-metal thermometer assembly. Dur-ing manufacture, (a), the assembly is straight but when exposed to elevated temperature, (b), greater expansion of material 1 causes the assembly to bend.
Tip movement x is a measure of tempera-ture change.
Fig. 44. When the thermally expandable liquid in the liquid-in- glass thermometer, (a), is exposed to an elevated tempera-ture, the liquid rises in the sealed-and-evacuated capillary tube giving an indica-tion of temperature change, (b).
the energy of the electrostatic field to make workable motors but they have not met with commercial success be-cause the ability to:
• store sufficient energy requires a large number of capacitor plates, and
• generate sufficient force and torque requires extremely high voltages.
When making the calculations needed to build machines of a practical speed, power, and torque capacity, the machine either becomes very large, the voltage becomes excessive, or both.
Electromagnetic energy conversion, therefore, has become much more pop-ular than electrostatic energy conver-sion and is certainly more important to practitioners of electrohydraulics.
Quartz crystals generate an electrical
charge when subjected to an external force. The process is reversible. That is, when subjected to an external voltage, the crystal will change its shape or gen-erate a force if its motion is con-strained. To apply this phenomenon, the quartz is formed into helical coil that produces a useable motion of the tip and provides a force on the order of a few pounds. Quartz motors have been used as valve operators, but are not commercially popular.
Of the electromagnetic energy con-verters, there are many in regular use in today’s industrial environment. Mo-tor types include induction, syn-chronous, DC, stepper, brushless DC, and torque. Linear machinery includes AC, DC, and proportional solenoids,
linear-force and linear-induction mo-tors. All have found use in fluid power applications, but the few that are sig-nificant in the electronic control of hy-draulic systems are torque motors, pro-portional solenoids, and linear force motors. These three are the links be-tween the electronic world and the hy-draulic world.
Torque motors, with only 1° or 2° of angular rotation, are popular on servo-valves and also can be found on some proportional valves. Proportional solenoids got their name from their ap-plication to that class of continuously variable, electrically modulated control valves commonly called proportional valves. Linear force motors are used on both servo and proportional valves.