Cook, G.E., Anderson, K, Barnett, R.J., Wallace, A.K., Spee, R, Sznaier, M, Sanchez Pena, Rs."Industrial Systems The electrical Engineering Handbook Ed. Richard C. Dorf Boca Raton: CRc Press llc. 2000
Cook, G.E., Anderson, K., Barnett, R.J., Wallace, A.K., Spée, R., Sznaier, M., Sánchez Peña, R.S. “Industrial Systems” The Electrical Engineering Handbook Ed. Richard C. Dorf Boca Raton: CRC Press LLC, 2000
George E. Cook Kristin anderser 104 Marel Corporation Robert joel Barnett Industrial Systems Alan k. wallace Oregon State University Rene spee 104.1 Welding and Bonding Control System Requirements. System Parameters. Welding Mario sznaier System. Sensing. Modeling .Control. Conclusions 104.2 Large Drives University Park Configurations. Selection and Compatibility. Principles and Features of Operation. Control Aspects. Future Trends Ricardo s. sanchez pena 104.3 Robust Syster iversity of Buenos A Robustness and Feedback. Robust Stability and Performance F Control . Structured Uncertainty. Robust Identification 104.1 Welding and Bonding George E. Cook, Kristinn Andersen, and Robert Joel Barnett Most welding processes require the application of heat or pressure, or both, to produce a bond between the parts being joined. The welding control system must include means for controlling the applied heat, pressure, and filler material, if used, to achieve the desired weld microstructure and mechanical properties. Welding usually involves the application or development of localized heat near the intended joint Welding processes that use an electric arc are the most widely used in industry. Other externally applied heat sources of importance include electron beams, lasers, and exothermic reactions(oxyfuel gas and thermit). For fusion welding pI a high energy density heat source is normally applied to the prepared edges or surfaces of the members to be joined and is moved along the path of the intended joint. The power and energy density of the heat source must be sufficient to accomplish local melting Control System Requir Insight into the control system requirements of the different welding processes can be obtained by consideration of the power density of the heat source, interaction time of the heat source on the material, and effective spot size of the heat source A heat source power density of approximately 10 W/cm2 is required to melt most metals [Eagar, 1986 Below this power density the solid metal can be expected to conduct away the heat as fast as it is being introduced. On the other hand, a heat source power density of 106 or 107 W/cm will cause vaporization of most metals within a few microseconds, so for higher power densities no fusion welding can occur. Thus, it can be concluded that the heat sources for all fusion welding processes lie between approximately 10and 10 W/cm heat intensity. Examples of welding processes that are characteristic of the low end of this range include oxyacetylene welding electroslag welding, and thermit welding. The high end of the power density range of welding is occupied by laser beam welding and electron beam welding. The midrange of heat source power densities is filled in by the various arc welding processes. For pulsed welding, the interaction time of the heat source on the material is determined by the pulse duration, whereas for continuous welding the interaction time is proportional to the spot diameter divided by the travel speed. The minimum interaction time required to produce melting can be estimated from the relation for a planar heat source given by[ Eagar, 1986] c 2000 by CRC Press LLC
© 2000 by CRC Press LLC 104 Industrial Systems 104.1 Welding and Bonding Control System Requirements • System Parameters • Welding System • Sensing • Modeling • Control • Conclusions 104.2 Large Drives Configurations • Selection and Compatibility • Principles and Features of Operation • Control Aspects • Future Trends 104.3 Robust Systems Robustness and Feedback • Robust Stability and Performance • H∞ Control • Structured Uncertainty • Robust Identification 104.1 Welding and Bonding George E. Cook, Kristinn Andersen, and Robert Joel Barnett Most welding processes require the application of heat or pressure, or both, to produce a bond between the parts being joined. The welding control system must include means for controlling the applied heat, pressure, and filler material, if used, to achieve the desired weld microstructure and mechanical properties. Welding usually involves the application or development of localized heat near the intended joint. Welding processes that use an electric arc are the most widely used in industry. Other externally applied heat sources of importance include electron beams, lasers, and exothermic reactions (oxyfuel gas and thermit). For fusion welding processes, a high energy density heat source is normally applied to the prepared edges or surfaces of the members to be joined and is moved along the path of the intended joint. The power and energy density of the heat source must be sufficient to accomplish local melting. Control System Requirements Insight into the control system requirements of the different welding processes can be obtained by consideration of the power density of the heat source, interaction time of the heat source on the material, and effective spot size of the heat source. A heat source power density of approximately 103 W/cm2 is required to melt most metals [Eagar, 1986]. Below this power density the solid metal can be expected to conduct away the heat as fast as it is being introduced. On the other hand, a heat source power density of 106 or 107 W/cm2 will cause vaporization of most metals within a few microseconds, so for higher power densities no fusion welding can occur. Thus, it can be concluded that the heat sources for all fusion welding processes lie between approximately 103 and 106 W/cm2 heat intensity. Examples of welding processes that are characteristic of the low end of this range include oxyacetylene welding, electroslag welding, and thermit welding. The high end of the power density range of welding is occupied by laser beam welding and electron beam welding. The midrange of heat source power densities is filled in by the various arc welding processes. For pulsed welding, the interaction time of the heat source on the material is determined by the pulse duration, whereas for continuous welding the interaction time is proportional to the spot diameter divided by the travel speed. The minimum interaction time required to produce melting can be estimated from the relation for a planar heat source given by [Eagar, 1986] George E. Cook Vanderbilt University Kristinn Andersen Marel Corporation Robert Joel Barnett Vanderbilt University Alan K. Wallace Oregon State University René Spée Oregon State University Mario Sznaier Pennsylvania State University — University Park Ricardo S. Sánchez Peña University of Buenos Aires Argentina
ADVANCED WELDING TORCH concept of variable polarity plasma arc(VPPA)welding employs a variable current waveform that ables the welding system to operate for preset time increments in either of two polarity modes for nost effective joining of troublesome light alloys such as aluminum and magnesium. Although the VPPA concept dates back to 1947, it was never fully developed. ding techniques were inadequate for the job of joining the nuge alun. recog In the late 1970s, when the Space Shuttle was in early development, NASA recognized that the gments of the Shuttle External Tank. Marshall Space Flight Center(MSFC) initiated the development of VPPA welding The B&B Precision Machine Variable Polarity Plasma Arc welding torch. Photo courtesy of National Aeronautics and Space Admin m =[pIpal where Pa is the heat source density(watts per square centimeter)and K is a function of the thermal conductivity and thermal diffusivity of the material. For steel, Eagar gives K equal to 5000 W/cm2/s. Using this value for K, one sees that the minimum interaction time to produce melting for the low power density processes, such as beam processes, such as laser beam welding with a power density on the order of 10 W/cm,, is 25 us Interaction times for arc welding processes lie somewhere between these extremes An example of practical process parameters for a continuous gas tungsten arc weld(GTAW) are 100 A, 12 V, gas, 2.4-mm diameter electrode, and 50-degree tip angle has been found to be approximately 8 x 10 W aa and travel speed 10 ipm(4.2 mm/s). The peak power density of a 100-A, 12-v gas tungsten arc with argon shieldi e 2000 by CRC Press LLC
© 2000 by CRC Press LLC tm = [K/pd]2 where pd is the heat source density (watts per square centimeter) and K is a function of the thermal conductivity and thermal diffusivity of the material. For steel, Eagar gives K equal to 5000 W/cm2 /s. Using this value for K, one sees that the minimum interaction time to produce melting for the low power density processes, such as oxyacetylene welding with a power density on the order of 103 W/cm2 , is 25 s, while for the high energy density beam processes, such as laser beam welding with a power density on the order of 106 W/cm2 , is 25 ms. Interaction times for arc welding processes lie somewhere between these extremes. An example of practical process parameters for a continuous gas tungsten arc weld (GTAW) are 100 A, 12 V, and travel speed 10 ipm (4.2 mm/s). The peak power density of a 100-A, 12-V gas tungsten arc with argon shielding gas, 2.4-mm diameter electrode, and 50-degree tip angle has been found to be approximately 8 3 103 W/cm2 . ADVANCED WELDING TORCH he concept of variable polarity plasma arc (VPPA) welding employs a variable current waveform that enables the welding system to operate for preset time increments in either of two polarity modes for most effective joining of troublesome light alloys such as aluminum and magnesium. Although the VPPA concept dates back to 1947, it was never fully developed. In the late 1970s, when the Space Shuttle was in early development, NASA recognized that the thenexisting welding techniques were inadequate for the job of joining the huge aluminum segments of the Space Shuttle External Tank. Marshall Space Flight Center (MSFC) initiated the development of VPPA welding. The B&B Precision Machine Variable Polarity Plasma Arc welding torch. (Photo courtesy of National Aeronautics and Space Administration.) T
In the course of its development, it became apparent that the technique had broad potential for improving weld reliability and lowering costs. Since there were no suitable commercially available tools for VPPA C expanded its development effort to include a welding torch that would have dual utility, as nent of NASAs external tank welding system and as a component of derivative systems for com applications. The torch contract was awarded to B&B Precision Machine, Owens Cross Road, Alabama. B&B, working in cooperation with MSFC's Materials and Processing Laboratory, developed and patented a shuttle-use torch and continued development of VPPA. A major step in the late 1980s was a program to fully automate the system and eliminate the hand of the welder on the controls. In 1989, a NASA decision to change the material of the external tank triggered a B&B development. The new alloy in some cases required"tack"welds prior to robotic seam welding Since tack welds are performed by hand, B&B was assigned to develop a smaller version of the torch that would be easier to manipulate and would meet the needs of applications where access was limited. B&B delivered a prototype small torch in 1992 The small torch has the same features and advantages as the original torch, but it fits in approximately half the space. The VPPA welding system and the B&b torch continue to make all the welds in the external ank of the Space Shuttle and they have been selected as the preferred welding approach for the International pace Station.( Courtesy of National Aeronautics and Space Administration. A small version of the B&B torch is used in commercial sheet metal welding. (Photo courtesy of National Aeronautics and Space Administration.) Assuming an estimated spot diameter of 4 mm, the interaction time(taken here as the spot diameter by the travel speed)is 0.95 s. At the other extreme, 0.2-mm(0.008-in )material has been laser welded in /min(1270 mm/s)at 6 kW average power. Assuming a spot diameter of 0.5 mm, the interaction time is 3.94 104s. Spot diameters for the high density processes vary typically from 0.2 mm to 1 mm, while the spot diameters for arc welding processes vary from roughly 3 mm to 10 mm or more. Assuming a rule of thumb of 1/10 the spot diameter for positioning accuracy, we conclude that typical positioning accuracy requirements for the high power density processes is on the order of 0.1 mm and for the arc welding processes is on the order of l mm. The required control system response time should be on the order of the interaction time and, hence, may vary from seconds to microseconds, depending on the process chosen. With these requirements it can be conclude that the required accuracy and response speed of control systems designed for welding increases as the power c2000 by CRC Press LLC
© 2000 by CRC Press LLC Assuming an estimated spot diameter of 4 mm, the interaction time (taken here as the spot diameter divided by the travel speed) is 0.95 s. At the other extreme, 0.2-mm (0.008-in.) material has been laser welded at 3000 in./min (1270 mm/s) at 6 kW average power. Assuming a spot diameter of 0.5 mm, the interaction time is 3.94 3 10-4 s. Spot diameters for the high density processes vary typically from 0.2 mm to 1 mm, while the spot diameters for arc welding processes vary from roughly 3 mm to 10 mm or more. Assuming a rule of thumb of 1/10 the spot diameter for positioning accuracy, we conclude that typical positioning accuracy requirements for the high power density processes is on the order of 0.1 mm and for the arc welding processes is on the order of 1 mm. The required control system response time should be on the order of the interaction time and, hence, may vary from seconds to microseconds, depending on the process chosen. With these requirements it can be concluded that the required accuracy and response speed of control systems designed for welding increases as the power In the course of its development, it became apparent that the technique had broad potential for improving weld reliability and lowering costs. Since there were no suitable commercially available tools for VPPA welding, MSFC expanded its development effort to include a welding torch that would have dual utility, as a component of NASA’s external tank welding system and as a component of derivative systems for commercial applications. The torch contract was awarded to B&B Precision Machine, Owens Cross Road, Alabama. B&B, working in cooperation with MSFC’s Materials and Processing Laboratory, developed and patented a shuttle-use torch and continued development of VPPA. A major step in the late 1980s was a program to fully automate the system and eliminate the hand of the welder on the controls. In 1989, a NASA decision to change the material of the external tank triggered a new B&B development. The new alloy in some cases required “tack” welds prior to robotic seam welding. Since tack welds are performed by hand, B&B was assigned to develop a smaller version of the torch that would be easier to manipulate and would meet the needs of applications where access was limited. B&B delivered a prototype small torch in 1992. The small torch has the same features and advantages as the original torch, but it fits in approximately half the space. The VPPA welding system and the B&B torch continue to make all the welds in the external tank of the Space Shuttle and they have been selected as the preferred welding approach for the International Space Station. (Courtesy of National Aeronautics and Space Administration.) A small version of the B&B torch is used in commercial sheet metal welding. (Photo courtesy of National Aeronautics and Space Administration.)
INDIRECIWELD PARAMEIERS_(IWP) PARAMETERS_(DWP) VOLTAGE MATERIAL BEAD WIDT PARAMATERS NETRATION ELECTRODE FEED RATE MECHANICAL ELECTRODE EXTENSION MICROSTRUCTURE RAVEL ANGLE :GRAINSIZE FOCUSED SPOT SIZE EPTH OF FOCUS PULSE DURATION DISCONTINUITIES: EPETITION RATE POROSITY BEAM POWER INCOMPLETE FUSION FIGURE 104.1 Input and output variables of welding process. density of the process increases. Furthermore, it is clear that the high power density processes must be automated because of the humans inability to react quickly and accurately enough System Parameters The variables of the welding process are separated here into direct weld parameters(DwP)and indirect weld parameters(IWP)[Cook, 1981]. The DWP are those pertaining to the weld reinforcement and fusion zone ies of the completed weld, weld microstructure, and discontinuities. The IWP are those inpu collectively control the DWP. The IWP are the welding equipment setpoint variables, e.g travel speed, electrode feed rate, travel angle, electrode extension, focused spot size,and beam power Welding System The various DWP, or process variables, that we would like to control and the many possible IWP, or equipment variables, that we may set to achieve the desired output are shown in Fig. 104.1. From the standpoint of feedback control, the welding process depicted in Fig. 104.1 presents two principal problems: (1)in most cases the relationships between the IWP and DwP are nonlinear, and(2)the variables are generally highly coupled With most production welding today, the designer of the welded part specifies the desired weld characteristics he DwP), including acceptable tolerance windows. The job of the welding engineer then is to determine a set of IwP that will produce the desired DwP. Most automated welding systems today may be expected to have ood control over the IWP, including joint tracking for heat source positioning. Therefore, if production floor onditions do not differ too much from the laboratory conditions under which the weld procedures were developed, then the welding operation can be expected to satisfy quality inspection and control procedures. If not, human operators must be depended upon to provide the necessary feedback to make corrective actions in the welding equipment settings The human involvement in this scenario can be reduced or eliminated by sensing selected DWP, comparing he sensed variables with desired values, and implementing a multivariable controller that will reduce auto- matically the error between the desired and sensed DwP to zero or an acceptably low difference Dynamic and steady-state process models are required for both design and stable operation of the multivariable feedback control system. However, the models do not need to be as globally accurate as the models required for open loop control. In exchange for accuracy, the models used for control system purposes must be computable in real time, and generally, it is important that they provide both steady-state and dynamic information of the nterrelationships between the coupled variables of the system. It is generally important that these relationships e 2000 by CRC Press LLC
© 2000 by CRC Press LLC density of the process increases. Furthermore, it is clear that the high power density processes must be automated because of the human’s inability to react quickly and accurately enough. System Parameters The variables of the welding process are separated here into direct weld parameters (DWP) and indirect weld parameters (IWP) [Cook, 1981]. The DWP are those pertaining to the weld reinforcement and fusion zone geometry, mechanical properties of the completed weld, weld microstructure, and discontinuities. The IWP are those input variables that collectively control the DWP. The IWP are the welding equipment setpoint variables, e.g., voltage, current, travel speed, electrode feed rate, travel angle, electrode extension, focused spot size, and beam power. Welding System The various DWP, or process variables, that we would like to control and the many possible IWP, or equipment variables, that we may set to achieve the desired output are shown in Fig. 104.1. From the standpoint of feedback control, the welding process depicted in Fig. 104.1 presents two principal problems: (1) in most cases the relationships between the IWP and DWP are nonlinear, and (2) the variables are generally highly coupled. With most production welding today, the designer of the welded part specifies the desired weld characteristics (the DWP), including acceptable tolerance windows. The job of the welding engineer then is to determine a set of IWP that will produce the desired DWP. Most automated welding systems today may be expected to have good control over the IWP, including joint tracking for heat source positioning. Therefore, if production floor conditions do not differ too much from the laboratory conditions under which the weld procedures were developed, then the welding operation can be expected to satisfy quality inspection and control procedures. If not, human operators must be depended upon to provide the necessary feedback to make corrective actions in the welding equipment settings. The human involvement in this scenario can be reduced or eliminated by sensing selected DWP, comparing the sensed variables with desired values, and implementing a multivariable controller that will reduce automatically the error between the desired and sensed DWP to zero or an acceptably low difference. Dynamic and steady-state process models are required for both design and stable operation of the multivariable feedback control system. However, the models do not need to be as globally accurate as the models required for openloop control. In exchange for accuracy, the models used for control system purposes must be computable in real time, and generally, it is important that they provide both steady-state and dynamic information of the interrelationships between the coupled variables of the system. It is generally important that these relationships FIGURE 104.1 Input and output variables of welding process