Tài liệu Cảm biến trong sản xuất P8 ppt

Sensors with melt contact can be divided into types for thecontrol of the chemical composition, types for the control of the temperature, andtypes for the control of the dosage or the le

With modern automated casting methods and the increasing use of integrated manufacturing (CIM) systems in foundries, inaccuracy of the parame-ters must be avoided to guarantee a high quality of the casting products and toavoid a cost intensive interruption of production The aims of perfect productionand total quality management (TQM) require sensors which also control the moldfilling and the solidification processes and thereby permit efficient process controland process control engineering [3, 4].

computer-This demanding process control can only be realized with sensors which are justed to the severe conditions in a foundry such as high temperatures, difficultaccessibility of the measuring point and the chemically aggressive effect of themelts Because of the operating conditions, the sensors for casting process con-trol, shown in Figure 4.1-1, can be divided into ‘sensors without melt contact’ and

ad-143

4

Sensors for Process Monitoring

Sensors in Manufacturing Edited by H K Tönshoff, I Inasaki

Copyright © 2001 Wiley-VCH Verlag GmbH ISBNs: 3-527-29558-5 (Hardcover); 3-527-60002-7 (Electronic)

4 Sensors for Process Monitoring

‘sensors with melt contact’ Further subgroups are distinguished by the particularcontrol task, the control of the alloy composition, the temperature, the dosage andcurrent of the melt and solidification This division deliberately does not distin-guish with regard to the separate casting processes as many of them do not allow

a general summary without double naming of the sensors and also lack clarity

In this classification, the physical measuring principle will be a final istic The control and regulation of these casting parameters determine the quality

character-of the casting products and the productivity character-of the foundries

4.1.1.2 Sensors with Melt Contact

The functional groups of this type of sensor come directly into contact with themelt or the mold or are separated from the melt by protecting tubes Normallythe protecting tubes consist of thermodynamic permanent ceramics with hightemperature stability as aluminium melts, for example, have a corrosive effect onthe sensor material Sensors with melt contact can be divided into types for thecontrol of the chemical composition, types for the control of the temperature, andtypes for the control of the dosage or the level

4.1.1.2.1 Sensors for Controlling Chemical Characteristics

The gas content, the chemical composition, and the purity of the melt are of sive significance for the quality of the component The chemical composition ofthe melt determines, in addition to the solidification characteristics of the castingwhich are influenced by the grain refining agent above all through the elementcontent of the alloy, the mechanical properties of the component The solventpower of metal melts for gases decreases with decrease in temperatures Because

deci-of this, evolution deci-of gaseous hydrogen and oxygen which are absorbed from theatmosphere and dissolved in the metal melts takes place and pores are formed inthe casting To guarantee a perfect component, the gas content must be controlledfrequently before and during serial casting [5–7]

Partial Pressure Measurement

As hydrogen is the only gas which dissolves in aluminium melts, the hydrogencontent can be simply controlled with the Chapel (continuous hydrogen analysis

by pressure evaluation in liquids) and the Telegas or Alscan process With the pel process a porous graphite punch which is connected through a gas-tight cera-mic tube to a pressure gage will be immersed in the melt and evacuated for ashort time The graphite punch reacts like a bubble into which the hydrogen dif-fuses out of the melt until the pressure in the probe and the hydrogen partialpressure in the melt are the same If the state of equilibrium is reached the hy-drogen content of the melt at a constant temperature can be calculated by usingthe Sievert laws [6–9]:

4.1 Casting and Powder Metallurgy 145

where CH= concentration of hydrogen dissolved in aluminium, pH2= partial

pres-sure of segregated hydrogen, T = temperature, and A, B = Sievert constants,

de-pending on the alloy composition

The chapel process is easy to handle, reliable, fast and has been proved cially in Europe

espe-Thermal Conductivity Measurement

The Telegas or Alscan method has a ceramic probe below the melt level, out ofwhich pure inert gas or nitrogen flows continuously into the melt and is then col-lected in a hood While the blowholes are rising the dissolved hydrogen diffusesout of the melt until equilibrium of the gas circulation is reached The hydrogenpartial pressure is measured with a thermal conductivity-measuring cell [10–12].The telegas or Al scan method is especially used in the USA In contrast to thechapel process, the measurements must be carried out over a longer period, atleast 15 min

Electromotive Force Measurement

In the steel and copper industry, an electrochemical cell made of ceramic (Figure4.1-2) is used for determining the oxygen content in the melt [13–16] The gageheads contain a thermoelectric couple (see next section) and a voltaic cell whichhas a mixture of a metal and an oxide, eg, Cr/CrO, inside with a known oxygenpartial pressure as a reference material

4 Sensors for Process Monitoring

146

Fig 4.1-2 Electromotive force cell

On immersing the gage head in the melt, an electromotive force between thereference material and the melt arises because of the oxygen ion conductivity ofthe partially stabilized ZrO2 The relationship obeys the Nernst law:

E ˆ …RT=4F† ln …pO2=p0

where E = energy, R = gas constant, T = temperature, F = Faraday constant, and pO2,

p0O2= partial pressure of oxygen at the two electrodes

The potential difference as a measure to calculate the oxygen activity of themelt can be used here The temperature of the cell is an important factor in themeasurement A voltaic cell can be used at higher temperatures for the measure-ment of the oxygen content of solid or liquid metals, slag, and mattes With thissensor the hydrogen, magnesium, and sodium contents can be determined whenaluminium is melted [17, 18]

Resistance Measurement

The Liquid Metal Cleanliness Analyzer (LiMCA) is used to control the purity ofthe melt continuously The measuring principle is mainly based on the registra-tion of very small resistance modifications in the microohm range in liquid alumi-nium or magnesium caused by non-metallic inclusions The robust and safe LiM-

CA sensor is used in light metal foundries and consists of a heat-resistant tubefor sampling and two electrodes, one in a test-tube and the other in the surround-ing melt [5, 19–21]

4.1.1.2.2 Sensors for Controlling Temperature

The temperature of the melt and the mold is of decisive significance for the correctmold filling and the cycle time of the serial casting, which implies the productivity ofthe company Temperature sensors with melt contact are based on the principle ofconduction, in contrast to the temperature sensors without melt contact These sen-sors are also separated by protecting tubes or layers of aggressive melts There is adivision between thermoelectric couples and resistance pyrometers

Thermoelectric Couple Measurement

Thermoelectric couples (Figure 4.1-3) are based on the thermoelectric effect beck effect) They consist of two wires of different metals with the ends soldered

(See-4.1 Casting and Powder Metallurgy 147

Fig 4.1-3 Structure of a thermoelectric couple

or welded A voltage arises when the two ends have different temperatures Thisthermoelectric voltage depends on the metals used and on the temperature differ-ence between the junction point and the connecting point (summing point) of themeasuring instrument The measurement of the thermoelectric voltage is carriedout using high-resistance voltage measuring instruments If necessary, possibledisturbing secondary thermal effects at supplying parts must be eliminatedthrough calibration lines The measuring range is between –200 and 25008C de-pending on the metals The following metal pairs are used: platinum/platinumrhodium, nickel/chrome nickel, iron/constantan, and copper/constantan [9, 22,23].

Thermoelectric couples can also be produced without a protecting tube in verysmall sizes with a minimum diameter up to 0.5 mm and a free choice of thelength These so-called sheath thermoelectric couples are the most commonlyused temperature sensors in light metal foundries because of their flexibility andreasonable price

Resistance Pyrometer Measurement

The resistance pyrometer is based on the principle of a change in the electrical sistance with variation in the temperature of a conductor or semiconductor De-pending on the predominant electrical conducting mechanism, a difference ismade between pyrometers with a positive (metals) and a negative (high-tempera-ture conductors, negative temperature coefficient resistors, thermistors) resistance-temperature characteristic curve Resistance pyrometers require analog or digitalelectrical connections for measurement and for higher demands measuringbridges and compensators are used Similar to the thermoelectric couple, the ad-vantages of these sensors are the reasonable price, the robustness, the flexibility,and the simple handling

re-4.1.1.2.3 Sensors for Controlling the Dosage/Level

A correct dosage is decisive for quasi-stationary thermal economy of the mold andtherefore significant for the quality of the casting By reducing the cycle materialthe economy of the foundry is favored [24]

Contact Electrode Measurement

The easiest and most common way to control the dosage is realized with a tact electrode When the melt touches the contact electrode a signal will be sent tothe installation control which controls the dosage process [25, 26]

con-Inductive Sensing

In light metal furnaces, inductive level sensors which are protected from the melt

by suitable austenitic or ceramic protecting tubes are used to control the level tinuously This principle is based on an induced voltage in a conducting loop inthe sensor This voltage causes an electric current which forms a magnetic fieldaround the sensor A signal is originated by the variation of the magnetic field by

con-4 Sensors for Process Monitoring

148

the melt [27] This type of level sensor is expensive, susceptible to wear and costly

in maintenance

4.1.1.3 Sensors without Melt Contact

The physical measuring methods and the technical realization of this kind of sors are relatively complicated and complex, although necessary in order to guar-antee continuous production and quality assurance Since these sensors do nottouch the melt, which is often chemically aggressive, and since they are not ex-posed to high thermal stresses, it is unlikely that they will fail Sensors withoutmelt contact can be divided into different types: sensors for controlling the cur-rent and solidification, for controlling the temperature, for controlling the dosage,pressure, level, and route

sen-4.1.1.3.1 Sensors for Controlling Current and Solidification

Precise knowledge of the melt current, the solidification and the thermal

econo-my of the mold is an important factor in the design of casting dies With thisknowledge it is possible to attain perfect heating and cooling circuits, cycle times,and temperature distribution for directional solidification For the continuous cast-ing process the control of the position of the solidification contour is of great im-portance since the continuous cast velocity and the charging depend on this posi-tion If the meniscus is not respected, liquid metal may flow over or run out [28–30]

X-ray Imaging

X-rays from a radioactive source, typically a rod-type emitter (eg, Co-60) in a leadprotector, radiograph the mold Since solid metals absorb X-rays better than meltsowing to their higher density, the position of the solidification contour can be de-tected by a scintillation meter The sprue, ie, the melt current during die casting,can be supervised and the position of the solidification contour can be directed in

a continuous casting mold Figure 4.1-4 shows a schematic diagram of this vising method for continuous casting [30–35]

super-The complex protection of the workplace against radioactive radiation reducesthe number of applications of this supervising method X-ray processes and com-puted tomography (CT) are additionally used for nondestructive component test-ing and for the quality testing of safety components Defects in casting, eg, inclu-sions, sink-holes, pores, cracks, etc., can be detected [37, 38]

4.1 Casting and Powder Metallurgy 149

Thermal Imaging

Cameras for thermal imaging visualize infrared radiation, ie, thermal radiationfrom object surfaces Since the atmosphere is not transparent to thermal radiationover the whole radiation spectrum, these cameras are divided into near-, medium-and far-infrared cameras according to the sensitivity of their sensors [29, 30] Theflow of metal melts is examined for model molds consisting of a solid mold withdie sinking and even face and which is closed by a moveable, transparent moldhalf of solid foam (aerogel) (Figure 4.1-5) Owing to its transparency to visiblelight and thermal radiation in the near-infrared range, the flow and solidification

of steel, lead, aluminium, and magnesium melts, etc., can be observed [29, 39].Since the assembly is complex and the use of the aerogel slab is difficult, ther-mal imaging for the examination of the flow of melts is only used in research orfor the design of molds

4.1.1.3.2 Sensors for Controlling Temperature

If the metallurgical melt flow is correct, up to 100% of rejects in die casting canoccur owing to the wrong temperature of the mold Non-contact temperature sen-sors permit a correct mold design and effective continuous control of the melttemperature at positions difficult to access or at temperatures that destroy contactsensors [40]

4 Sensors for Process Monitoring

150

Fig 4.1-4 Principle of X-ray imaging

4.1 Casting and Powder Metallurgy 151

Thermal Imaging

For thermal imaging of the mold temperature, as shown in Figure 4.1-6, mainlyfar-infrared cameras are used due to the emission spectrum [29, 41] With theseexaminations a relationship between the die casting temperature, the flow tem-perature of the cooling system, and the cast cavity could be found [40] Further,thermal imaging is used for the verification of simulation results and mold de-signs [41, 42]

Another application of this type of camera is the supervision of the cast perature for continuous casting Additionally, conventional cameras are used forthe observation of the billet surface, the billet orientation, etc [43]

tem-Pyrometry

Pyrometry is based on the same physical rules of thermal radiation and thermalimaging In contrast to thermal imaging cameras, pyrometers detect the tempera-ture only at intervals, but they are more economical, easier to use, and they have

an excellent accuracy of up to ±1% In general, foundries use total radiation rometers for low temperatures and ratio or two-color pyrometers for higher tem-peratures as emitted by iron and steel melts Total radiation pyrometers can easy

py-be tested as their signals are directly subject to the Stefan-Boltzmann law For thetwo-color pyrometer two partial radiations in different wave ranges are considered.Ratio pyrometers measure the temperature of the object by the ratio of the radia-tion density of two different spectral regions The advantage is that the transmis-sion distance does not influence the measuring results [31, 44, 45]

In the steel industry, pyrometers have been used since the 1950s for the vision of melt temperatures [31, 46] Additionally, they are used for continuouscasting for the control of the billet temperature, ie, for the control of the coolingsystem [47]

super-4 Sensors for Process Monitoring

152

Fig 4.1-6 Thermogram of a model mold

Magnetic Field Measurement

Magnetic field measurement is used for the high-precision heating of thixo billets.For this die casting process the cast material, the so-called thixo billet, is in arange between the solidus and liquidus temperatures, ie, the material is partly sol-

id and partly liquid This thixotropic state causes a change in the magnetic fieldwhich can be measured by a field-measuring sensor to an accuracy of down to0.5% [48, 49] This complex measuring process, which has to be calibrated forevery alloy composition, is used over the whole cross-section of the thixo billet be-cause of the highly required even distribution and measurement of the tempera-ture

4.1.1.3.3 Sensors for Controlling Dosage, Pressure, Level, and Route

The physical measuring method of this type of sensor is often the same so that itseems reasonable to combine these process parameters into one control group.Sensors in this group are mainly used for the supervision of die casting which, inspite of a more frequent use of sensors, is still called ‘black box technology’.Further applications are continuous casting and break-mold casting [50, 51]

Pneumatic Sensing

Important machine parameters of die casting are the injection shot velocity andthe pressure The pressure is supervised by pneumatic sensors in the hydraulicsystem of the die casting machine Pneumatic sensors are also used in the fur-nace gas chamber of dosage furnaces which have shown a high degree of reliability

in the aluminium industry (Figure 4.1-7) [41, 52, 53]

Another application of pneumatic sensing is level measurement in dosage orblast furnaces Figure 4.1-8 shows the functional principle of this sensor, whichmeasures the pressure necessary for the exhaust of nitrogen bubbles from a cera-mic tube on the bottom of the melting pot [54]

4.1 Casting and Powder Metallurgy 153

Fig 4.1-7 Dosage furnace

For these control types, conventional pressure gages are used which are subject

to the pneumatic or hydrostatic principle

Displacement Transducer

The control of the injection shot velocity in die casting is the essential criterionfor turbulence-free filling and therefore for components with only a few pores.The injection shot velocity is controlled in three phases depending on the pistondisplacement Magnetic displacement transducers measure the piston position.The principle of this type of sensor is based on the influence of magnetic effects(eg, the Hall effect) which depend on the displacement [55] The sensors aremaintenance-free and extremely robust

Acceleration Meter

In order to avoid the adhesion of the billet to the mold in continuous casting and

to assure a clean billet surface, the continuous cast mold is set in an oscillatingmotion, vertical to the billet This oscillation is supervised by seismic accelerationmeters which represent a mass-spring damping system The system consists of

an inert seismic plate, a spring with a force proportional to the displacement and

a damping component proportional to the velocity [22, 56, 57]

4.1.1.3.4 Eddy Current Sensing

Eddy current measurements represent another solution for the supervision of thelevel in a mold in the continuous casting process (Figure 4.1-9) According to Qui[58], the detection of sullage which must not enter the mold is another applica-tion when liquid steel is filled from the ladle into the tundish [59–61] The chang-ing level of the steel bath influences the number and course of the eddy current

in liquid steel and the surrounding conductive objects The resulting change inthe electromagnetic field is measured [62]

4 Sensors for Process Monitoring

154

Fig 4.1-8 Level measurement in a blast furnace [54]

Force Sensing

The most conventional way to measure the level continuously is furnace weighingwith maintenance-free electronic load cells In general, the level is indicated di-rectly at the furnace by means of a signal lamp (see Figure 4.1-7) or it is indicated

to the master computer The load cell is cheap, maintenance-free and can be usedfor general purposes Charging appliances are equipped with the same systemsfor balancing the material [63–66]

These load cells are based on the physical principle of piezoelectric force ing technology (Figure 4.1.-10) When force is exerted on a piezoelectric crystal(eg, quartz, barium titanate (BaTiO3)), negative crystal lattice points are offsetagainst positive ones so that a difference in charge can be measured at the crystalsurfaces as a function of force [22]

sens-The function of sensors for the measurement of the internal pressure in ing chambers is subject to the same physical principle With the measurement ofthe pressure development, important knowledge about the melt flow, mold fillingand solidification during the filling process is achieved [67– 69]

cast-4.1 Casting and Powder Metallurgy 155 Fig 4.1-9 Principle of

eddy current sensing

Fig 4.1-10 Schematic

diagram of a

piezo-electric force gauge

Laser Level Measurement

Laser sensors are used for the measurement of the meniscus in the continuouscast process and for level control of the launder and the sprue in automatic break-mold casting methods of aluminium and steel (Figure 4.1-11) [4, 70–73]

In laser level measurement, an emitter gives short light impulses at a high quency (approximately 10 Hz) in the direction of the metal bath surface Fromthere a small proportion is reflected and sensed by a receiver The transit time is

fre-a mefre-asure of the level [51]

Camera Level Measurement

Another system for level measurement in molding boxes works with a cameraand secondary image processing so that the stopper control can keep the menis-cus in the sprue at a constant level (Figure 4.1-12) [2, 74]

4 Sensors for Process Monitoring

156

Fig 4.1-11 Principle of laser level measurement

Fig 4.1-12 Principle of camera level measurement