AWS WHC1 10-2001 Monitoring and Control of Welding and Joining Processes.pdf

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1、AWS Resourcesfor EngineersMonitoringand Controlof Weldingand JoiningProcesseswelding know-how for engineersii 2001 by American Welding SocietyAll rights reservedNo portion of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by anymeans, including mechanical, p

2、hotocopying, recording, or otherwise, without the prior written permission of thecopyright owner.Authorization to photocopy items for internal, personal, or educational classroom use only, or the internal,personal, or educational classroom use only of specific clients, is granted by the American Wel

3、ding Society (AWS)provided the appropriate fee is paid to the Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA01923; telephone: (978) 750-8400; Internet: .The Welding Handbook is the result of the collective effort of many volunteer technical specialists who provideinformation to assist w

4、ith the design and application of welding and allied processes.The information and data presented in the Welding Handbook, and this chapter, are intended for informationalpurposes only. Reasonable care is exercised in the compilation and publication of the Welding Handbook to ensurethe authenticity

5、of the contents. However, no representation is made as to the accuracy, reliability, or completenessof this information, and an independent, substantiating investigation of the information should be undertaken bythe user.The information contained in the Welding Handbook shall not be construed as a g

6、rant of any right of manufac-ture, sale, use, or reproduction in connection with any method, process, apparatus, product, composition, or sys-tem, which is covered by patent, copyright, or trademark. Also, it shall not be construed as a defense against anyliability for such infringement. Whether the

7、 use of any information in the Welding Handbook would result in aninfringement of any patent, copyright, or trademark is a determination to be made by the user.Printed in the United States of AmericaiiiACKNOWLEDGMENTSThis chapter from the Welding Handbook, Ninth Edition, Volumn 1, “Welding Science a

8、nd Technology,” has beenselected by the AWS Product Development Committee as a service to industry professionals.The Welding Handbook Committee and the editors recognize the contributions of the volunteers who have cre-ated, developed, and documented the technology of welding and shared it in the pa

9、st editions of the WeldingHandbook. The same enthusiasm, dedication, and willingness to share that they made a tradition continue withthis ninth edition of the Welding Handbook.The Welding Handbook Committee and the editors extend appreciation to the AWS technical committees whodeveloped the current

10、 consensus standards that pertain to this volume. They are also grateful to L. P. Connor,editor of Volume 1, eighth edition, and the members of the AWS technical staff for the engineering assistance theygenerously contributed.ivCONTRIBUTORSWELDING HANDBOOK COMMITTEEH. R. Castner, Chair Edison Weldin

11、g InstituteB. J. Bastian, First Vice-Chair Benmar AssociatesR. S. Funderburk The Lincoln Electric CompanyJ. M. Gerken, Sr. ConsultantI. D. Harris Edison Welding InstituteL. C. Heckendorn Intech R irregularitiesin sheet thickness, particularly in resistance spot weld-ing; and variations in weld joint

12、 size and location in arcwelding or joint gap in brazing.PROCESS RESPONSE VARIABLESThe process response variables are the products ofthe welding processthe actual current, voltage, travelspeed, and cooling rate, for examplethat produce thedesired weld properties, such as weld size, shape, micro-stru

13、cture, and soundness. For typical welding processes,it is impossible to sense most process response variablesdirectly because no sensors exist for the in-situ measure-ment of mechanical properties or weld microstructure.Indirect measurement is therefore often performed ofmore practical control varia

14、bles such as temperature,weld profile, weld size, penetration, and radiation.SENSING DEVICESIn its most basic form, a sensor is a transducer thatconverts a property from one physical form to another,often to an electrical signal. Welding process sensorsobtain information about the welding process by

15、 con-verting physical phenomena from the input and processresponse variables into signals that can be utilized bymonitoring or control equipment. Signal conditioning,amplification, and isolation are often required beforesensor output can be fed into a monitor or controller.Sensors can be simple or c

16、omplex in design. Exam-ples of simple sensors include current shunts, whichconvert the current flowing in the welding circuit to aproportional voltage, and thermocouples, which con-vert temperature into a voltage signal. Complex sensorsare composed of several individual sensors that operatetogether.

17、 An example is the machine vision sensor,which is used in robotic arc welding. This sensor is anentire subsystem, including a video camera (a trans-ducer that converts light intensity into a video signal), avideo signal digitizer, and a microprocessor, whichextracts information from the video image.

18、 Machinevision sensors, which are described in more detail laterin this chapter, provide information on the location andgeometry of weld joints and the size and shape of weldpools.Figure 1Flow Diagram of the Welding Process4 MONITORING AND CONTROL OF WELDING AND JOINING PROCESSESThe physical and ope

19、rational characteristics of somesensors make them advantageous for use with specificwelding processes as compared to other sensors. Forinstance, sensors that provide a direct measurement ofthe process variables are more desirable than those thatprovide indirect measurements. Sensing devices thatrequ

20、ire contact with the process or weldment are lessdesirable than those requiring no direct contact. In addi-tion, sensors that can be applied from the face of theweld are more desirable than those requiring access tothe interior or back of the weld. Overall, sensors thatcan be applied to multiple pro

21、cesses are highly desirable.Table 1 presents an overview of the typical physicalproperties associated with the welding processes alongwith their corresponding sensors and measured units.These physical properties and their sensing devices aredescribed in more detail below.TIMETime, the measurable dur

22、ation of an event, is com-monly measured in units of minutes or seconds. Arcwelding may be partitioned into several eventsupslopetime, weld time, downslope time, and total arc time. Theupslope time is the time interval during which the cur-rent changes continuously from the initial current to thewel

23、ding current. The weld time is the time interval fromthe end of the upslope time to the beginning of thedownslope time. The downslope time is the time duringwhich the current is changed continuously from thewelding current to the final current. The total arc time isthe time interval for which a weld

24、ing arc is sustained.Other welding processes such as resistance weldingare also partitioned into discrete intervals that can bemeasured in units of time. In resistance welding, timeperiods are often measured by the number of 60-hertz(Hz) cycles of alternating current.TEMPERATUREMany different types

25、of transducers are available fortemperature measurement. These include thermocou-ples, thermistors, resistive-temperature devices, opticalpyrometers, photon detectors, and thermal imagingTable 1Common Sensors and Units of MeasurePhysical Property Sensor(s) UnitsTime Timer, counter Second, cycle*Temp

26、erature Thermocouple, resistive-temperature device, thermistor, pyrometerFahrenheit (Celsius)Force Load cell, piezoelectric, linear variable differential transformer, and capacitive force sensorsNewton, pound (kilogram)Pressure Displacement- and diaphragm-type pressure sensors Pounds force per squar

27、e inch (kilopascal)Flow rate Differential pressure flow meter, mechanical flow meter, mass flow meterCubic feet per minute (liters per minute)Electric current Current shunt, Hall-effect sensor, Rogowski coil AmperesElectric potential Voltmeter VoltDisplacement Potentiometer, voltage differential tra

28、nsformer, inductiveand capacitive sensors; synchro; resolver; encoder; ultrasonic sensor; machine vision sensorInch (millimeter)Velocity Potentiometer; voltage differential transformer; inductiveand capacitive sensors; synchro; resolver; encoder; ultrasonic sensor; machine vision sensorInches per mi

29、nute (millimeters per second)Acceleration Potentiometer; voltage differential transformer; inductiveand capacitive sensors; synchro; resolver; encoder; ultrasonic sensor; machine vision sensorInches per minute squared (millimeters per second squared)Radiation (visible light, infrared, ultraviolet)Ph

30、otodiode, phototransitor, solar cell sensor Lumen (candela)Acoustic (sound) energy Microphone Decibel*Cycle = Interval of time based on frequency.MONITORING AND CONTROL OF WELDING AND JOINING PROCESSES 5cameras. Thermocouples, thermistors, and resistive-temperature devices can be used simply as cont

31、act sen-sors or as part of more complex noncontact sensors.Examples of the latter include thermopiles, bolometers,and radiometers, which measure radiant thermalenergy. The temperature sensors most commonly usedfor welding applications typically measure the tempera-ture range between room temperature

32、 and the meltingtemperature of the material being welded.ThermocoupleThermocouples are simple temperature sensors thatconsist of two dissimilar materials in thermal contact.The operation of the thermocouple is based on theSeebeck, or thermoelectric, effect. This effect producesa voltage between two

33、thermocouple junctions that areat different temperatures. The voltage, normally in mil-livolts (mV), is proportional to the temperature differ-ence between the junctions. Thermocouples areavailable in a variety of configurations and types as wellas for application in a variety of temperature ranges.

34、 Acommon thermocouple, the K-type, is made withnickel-chromium and nickel-aluminum wires and isapplicable to environments ranging from room temper-ature to 2280F (1250C).ThermistorThermistors are temperature-sensitive resistors fabri-cated from semiconducting materials whose resistancevaries inverse

35、ly with temperature. These devices aretypically used when high sensitivity is required. Theresistance of a 5000 ohm () thermistor may decreaseby approximately 11.1 for each degree Fahrenheit(20 /C) increase in temperature. The usable range ofa thermistor is typically from room temperature to620F (32

36、7C).Resistive-Temperature DeviceResistive-temperature devices (RTDs) consist ofresistive elements that are formed from semiconductingmaterials that exhibit a positive coefficient of resistivitywith a change in temperature. These sensors are stableand provide a reproducible response to temperaturecha

37、nges. Resistive-temperature devices are available forapplication in a variety of temperature ranges betweenroom temperature and 1832F (1000C).Optical PyrometerOptical pyrometers are noncontact devices that mea-sure the thermal radiation emitted by a source. Theradiation may be in the infrared or vis

38、ible light range,depending on the temperature. Infrared pyrometersemploy the infrared portion of the spectrum by using athermal detector to measure the temperature of the sur-face of the body emitting the infrared waves.Photon Detector and Thermal Imaging CameraPhoton detectors measure temperature b

39、y generatinga voltage that is proportional to the density of the pho-ton flux impinging on the sensor. Thermal imagingcameras are sensitive to infrared radiation. They areused for temperature sensing and more complex mea-surements of temperature distributions.FORCEForce sensors are based upon the pr

40、inciple that abody will deform in proportion to an applied force. Theforce is indirectly determined by measuring the physicaldeformation.Load Cell SensorsThe load cell sensor consists of a structure (a cantile-ver beam, shear beam, diaphragm, proving ring, orcolumn) that deforms when subjected to a

41、force and anetwork of strain gauges that produce an electrical sig-nal proportional to this deformation. The choice ofstructural element is based primarily upon the physicalsize constraints of the sensor and the maximum force tobe applied.Other Force SensorsOther force sensors include the piezoelect

42、ric, linearvariable differential transformer (LVDT), and capaci-tive force sensors.PRESSUREPressure sensors measure the distortion produced bypressure acting upon a deformable member. They con-vert this distortion into an electrical signal through ameasurement of displacement, strain, or piezoelectr

43、icresponse.Displacement- and Diaphragm-Type Pressure SensorsDisplacement-type pressure sensors employ a Bourdontube as the elastic element and a LVDT as the sensor.The C-shaped Bourdon tube has an oval cross sectionthat tends to straighten as internal pressure is applied.6 MONITORING AND CONTROL OF

44、WELDING AND JOINING PROCESSESOne end of the tube is fixed, while the other is free tomove. As pressure is applied to the Bourdon tube, thedisplacement of the free end is converted to a voltage bythe LVDT.Diaphragm-type pressure sensors utilize either aclamped circular plate (diaphragm) or a hollow c

45、ylinderas the elastic element and electrical resistance straingauges as the sensors. As pressure increases, the dia-phragm deforms, causing the strain gauges to changeresistance.Piezoelectric-Type Pressure SensorPiezoelectric-type pressure sensors use a piezoelectriccrystal as both the elastic eleme

46、nt and the sensor. Thecrystal is enclosed in a cylindrical shell that has a thinpressure-transmitting diaphragm on one end and a rigidsupport base for the crystal on the other end. As pres-sure is applied to the face of the crystal in contact withthe diaphragm, an electrostatic charge is generated.

47、Themagnitude of the charge depends on the pressure, thesize of the crystal, and the orientation of the crystalsaxis.FLOW RATEThe objective of flow rate measurement is to deter-mine the quantity of flow of a liquid or gas (e.g., cool-ing water or shielding gas). In some instances, a flowmeter returns

48、 this information directly, but in mostcases, the signal is derived from some property of theflow, such as volume, heat transfer rate, or momentumflux. In most cases, the flow meter signal requires cor-rection for pressure, temperature, or viscosity beforethe flow rate can be determined. The measure

49、ment offlow can be accomplished using many different physicalprinciples. However, the majority can be classified asvolume or mass flow rate measurements.Differential Pressure Flow MetersDifferential pressure flow meters are the devicesmost commonly used for the measure of flow volume.A constriction in the flow path results in a correspond-ing change in the velocity and pressure of the fluid. Thepressure differential can be measured using pressuregauges. Differential pressure flow meters include theVenturi meter, the flow nozzle meter, the orifice meter,and the