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The development of sensors for manufacturing automation
Jbzsef HaWany Memorial
123
Manufacturing Control and Monitoring
The Development of Sensors for Manufacturing Automation R i c h ar d L. Kegg Cincinnati Milacron, 4701 Marburg Ave, Cincinnati, 01145209, U.S.A. This paper is a brief history of the development of sensors for metal cutting machines. It summarizes the main problems of industry, the main areas of sensor research and the resolution of problems in industry during the last years. Mass production and batch manufacturing are treated separately, since the problems in these two areas appear to be quite different at any given time. It seems from this study that there is not a good correlation between the major research efforts and the main problems of industry. No major industrial applications have come from the popular research topics, and the successful, high-impact sensors have come from elsewhere. It appears that the communication between manufacturers and researchers could be improved to the benefit of both sides.
Keywords: Sensors, Automation, Research effectiveness, Mass production, Batch manufacturing.
1.~edu~on This paper has three objectives: to trace the history of manufacturing problems, research activity and sensors applications in the modern metal cutting industry; to look for relationships among these three threads; to suggest future needs of industry for sensors development.
2. Before 1950 In the late 1940's, batch manufacturing technology was static. The machines and processes were mature. Production was based on operator skills and these skills were available in adequate supply. Mass production, in contrast, was growing rapidly. Here industry had many problems. 2.1. Problems of Mass Production
L. K q g received BS (1958), MS (1963), and PhD (1965) degrees in Mechanical Engineering from the University of Cincinnati. He was trained in machine tool operation and construction at Cincinnati Milacron. For twenty-eight years he performed a variety of R & D tasks at Milaeron, including published work on metalworking machines, processes and systems. In 1986 he left R & D to those w h o are younger and better at solving differential equations, and moved into the commercial area. His present job is Vice-President, Engineering, Aerospace and Special Machines, for Milacron. Elsevier Computers in Industry 14 (1990) 123-129 0166-3615/90/$3.50 © 1990 - Elsevier Science Publishers B.V.
The rapid growth of automation created the first wave of unmanned machining. One man was responsible for ten or more machines. This created problems in two areas: process management and accuracy. Before automation the operator could select any feed or speed he wanted. In this way, he could compensate for a harder batch of raw material or a weaker batch of cutting tools. He could also observe "how the cut was going" and decide to change tools early if needed. When this degree of control was taken away, there were frequent problems: premature tool failure, scrapped parts and machine wrecks. The second problem that came with unattended machining was loss of accuracy. Before automation, any time there was a process change such as a new cutter or heavier stock, the operator would back off his final machine setting and approach
124
Jbzsef Hatoany Memorial: Manufacturing Control and Monitoring 1
Computers in lndtcvtm,
piece" and often several other pieces. Inevitably some of these "setup pieces" found their way in with the good parts. The loss of production and degradation of quality were recognized by shop floor people but were accepted as a necessary offsetting disadvantage to the benefits of automation.
t I i
2.2. Research on Machining Processes
I I 1
I 1
Fig. 1. Mechanical metal cutting dynamometer.
size in two or three steps. This could not be done with automation which had to try for final size on the first cut. This resulted in "scrapping the setup
Fig. 2. Strain gage dynamometer.
Researchers at this time concentrated on the fundamentals of machining processes. The basic mechanism of chip formation was studied and the geometries of various machining processes were also explored. The goals of this work were to identify maximum safe feeds and speeds for various materials and processes. This would give higher productivity without the need for operator attention to tool management. For this research many sensors were developed. The mechanical dynamometer (Fig. 1) and later the strain gage dynamometer (Fig. 2) allowed measurement of cutting forces.
Computers in Industry
R.L. Kegg / Development of Sensors
~MACHINEO KIqW((OC~
i wo~pt(c( ~
I
(XT(14~'~1~ OAR
PoTgNTO~Vf.I'TRI
125
insulated from electrical grounding, then the remote junction of a wire from the workpiece and a wire from the cutting tool became the cold junction of a thermocouple pair and the tool-workpiece contact became the hot junction. The current flowing in the circuit was then proportional to the temperature difference between the two junctions. Although these force and temperature sensors yielded useful scientific information, they could not be applied successfully in production. They were extremely fragile and could not be shophardened.
M-2HS$TOOL -~.
Fig. 3. Thermo-electric measurement of cutting temperature.
Temperature measurements were frequently made in the lab in order to obtain some indication of the rate of cutting tool wear. The most popular sensing method was the tool-work thermocouple technique (Fig. 3). If the workpiece could be
Fig. 4. Diameter size control gage for plunge grinding.
3. The 1950's and 60's
Batch manufacturing continued to have few problems during this period. Numerical control had not yet made much economic impact, and the increased use of tungsten carbide was under the watchful eye of the skilled machinist.
126
Jbzsef Hatvany Memorial." Manufacturing Control and Monitoring
Computers in lndust~"
3.1. Maturing of Mass Production
The growth of new automation applications declined in this period. Industry's attention swung toward optimizing the use of high production equipment. The problem of tool life management was solved by installing cycle counters and changing tools well in advance of the number of cycles which previously wore them out. This problem ceased to exist. Size gages were developed to automatically assure dimensional accuracy. Bore gages could measure the size of a hole and send a compensation signal to an automatically adjustable boring bar. The greatest new application of sensors was the use of in-process gages for plunge grinding. These gages (Fig. 4) measured the workpiece diameter continuously during the grinding operation. When correct size was attained, the gage signalled the machine to stop grinding. This technique was so successful it became a standard procedure in mass production industries. Major transfer line users began to study the productivity of their lines. Each found that the actual number of parts produced per year was about half of the theoretical maximum. This result was widely discussed and publicized. However the causes of the production loss (tooling problems, sticking automation mechanisms, machine hydraulics and relay failures) were kept secret. This led many people to the incorrect conclusion that machine failures, such as spindle and slideway bearing wear, were responsible and that sensors were needed in these areas. In fact, major machine builders had long designed for indefinite bearing life under normal duty cycles, and this was not a significant problem.
Fig. 5. Torque and deflection sensors for adaptive control.
cause skilled machinists sometimes reacted to these sounds. Based on this belief, a team of researchers developed a laboratory apparatus which, for one set of cutting conditions, could d i s t i n ~ i s h between a sharp and a dull lathe tool from the sound of cutting. This was only a demonstration of feasibility, requiring an array of electronics bigger than the machine tool itself. However it was the first major milestone in the use of electronics to extract information from sensors in manufacturing. The focus was not on the sensor itself (an accelerometer or microphone) but on the use of sophisticated electronics and computer-like devices to analyze the sensor data.
3.2. Research on Adaptioe Control
Adaptive control researchers of the 60's wanted to measure tool wear rate and optimize cutting conditions. These measurements were not possible so they settled on power, torque, force and deflection sensing (Fig. 5). Early adaptive controllers did not have a significant impact on production, but they did focus attention on the problem of developing "shop-hardened" sensors. One remarkable research effort deserves special mention [6]. It had always been believed that machining noise contained useful hidden information be-
4. The 1970's 4.1. Better Control in Mass Production
Programmable Logic Controllers r e p l ~ the huge relay cabinets which controlled the automation of the 60's. The controIlers themselves were far more reliable. Furthermore their logic capability allowed the development of diagnostic programs. These programs could help direct technicians to problems involving external devices which
Computers in Industry
failed to provide expected input signals to the PLC. 4.2. Growth of N C
In this decade, numerical controls applications grew rapidly. As machines became able to automatically carry out sequences of complicated motions and even change their own cutting tools, a single operator could control several machine tools. This presented batch manufacturing with the same sensing problems which mass production had experienced earlier. The measure-and-compensate techniques of skilled machinists were no longer possible. Management of tooling was out of the question when the operator had to move from machine to machine and might not be present to hear unexpected noises. Toward the end of this time period, studies of N C machine productivity appeared. In spite of their great productivity, N C machines were typi-
Fig. 6. Measuring probe on a machining center.
R.L Kegg / Developmentof Sensors
127
cally in cycle as little as 25% of the work day. The rest of the time was spent waiting for humans to de-bug N C programs, manage tooling, load parts, troubleshoot failures and other activities. The need for improved techniques in these three areas became apparent. 4.3. Research on Tool Condition Sensing
In the 70's a great deal of research was aimed at the development of sensors for cutting tool condition. Cutting forces and torques, vibrations and sounds were researched as possible indicators of tool wear rate. None was found to be useful under the required range of cutting conditions. A very innovative technique was developed [1] which involved the implantation of low-level radioactive material at the tip of the cutting tool. The radioactivity level of the chips provided a good indicator of tool wear rate. However, it was never applied in industry, possibly because of the fear of radioactivity.
128
Jbzsef Hatvany Memorial." Manufacturing Control and Monitoring
5. The 1980's The main trend in mass production in the 80's was a move to more flexibility. CNC machines began to appear in lower-volume production lines. Some automotive manufacturers even tried computer controlled FMS. This created no new sensor requirements. There was little development of sensing in mass production during this time. 5.1. Unattended Cells and Systems The need to improve the output of machining centers led to growth in the 80's of applications of FMS and machining cells. These and even single CNC machines were sometimes run completely unattended at night [4]. This further increased the problems of tool management and accuracy control. Fortunately sensors became available to help with these problems. The introduction of the probe (Fig. 6) made it possible for machines to manage their own accuracy by measuring and responding to each individual workpiece. These probes were applied to turning and machining centers and quickly became standard throughout industry. Cycle counting, which had been used to minimize tool life problems in mass production, extended to variable CNC cycles by accumulating time in the cut for each tool. This allowed each tool to be flagged as dull, well before it might be expected to wear out. This technique saw a widespread usage and substantially solved the problem of premature tool failure.
Computers in lndust(v
An innovative force sensor, which has since become commercially available, uses strain gages on the machine tool bearings to sense the magnitude of the force pulses as rolling elements go past the strain gage [5]. The signal peaks are used to indicate the relative magnitude of forces acting on the bearing. Force measurements were also used in research aimed at sensing tool fracture and chipping in the turning operation [3]. By recording the force pattern during a fraction of a revolution, any changes in force can be compared to a series of force variation patterns which are associated with tool chipping and tool breakage. This approach appears to be sufficiently general to have broad application.
6. Summary Only grinder gages and probes have found widespread usage in industry. These devices did not evolve from popular research fields. Conversely popular research topics have failed to produce significant relief from industrial problems. Apparently we are not doing a good job of communicating industry's needs to the research community. As a result of this study, the author recommends the following targets, in order of priority, for future sensors R& D: accurate, wide-range bore and diameter measuring devices; more hardened versions of todays CNC measuring devices; anticipation of machine wreck condition; anticipation of device failure; finer focused diagnostics; tool wear and breakage sensors.
5.2. Innovative Uses of Electronics and Computers Early in the 80's a vibration sensor was used in a pilot operation to distinguish between sharp and dull drill bits [7]. This sensing scheme was successful in anticipating drill failure, but it was never made general enough to be broadly applied in manufacturing industry. Considerable research was done in acoustic emission [2]. In this technology, the sonic and ultra-sonic vibrations caused by plastic deformation are analyzed to determine the differences between dull tool and sharp tool frequency patterns.
References [1] N.H. Cook, S.A. Basile, K. Subramanian and W.H. Grace, Tool Wear Sensors, Final Report to National Science Foundation, MIT, Cambridge, MA, August 1976. [2] D. Dornfeld and E. Kannatey-Asibth "Quantitative relationships for acoustic emission from orthogonal:~tal cutring", Trans. ASME, Ser. B, August 1981, pp. 330-340. [3] J.W. PoweR, et al., "Cutting tool sensors, Carbide Toot J., U.S.A., June 1985, pp. 12-17. [4] R. Seifried, "Unattended Machining", SME Paper MR85468, Society of Manufacturing Engineers, Detroit, MI, 1985. [5] L. Stockline, "New developments in tool condition monitoring", in: Proc. 8th Int. Conf. on Automated Inspection
Computers in Industry and Product Control, IFS Publ. Ltd., Bedford, England, 1987, pp. 69-75. [6] EJ. Weller, H.M. Schrier and B. Weichbrodt, "What sound can be expected from a worn tool", Trans. ASME, Ser. B, August 1969, pp. 525-534.
R.L Kegg / Development of Sensors
129
[7] K. Yee and D. Blomquist, "An on-line method of determining tool wear by time domain analysis", SME Paper No. MR82-901, Society of Manufacturing Engineers, Detroit, MI 1982.
123
Manufacturing Control and Monitoring
The Development of Sensors for Manufacturing Automation R i c h ar d L. Kegg Cincinnati Milacron, 4701 Marburg Ave, Cincinnati, 01145209, U.S.A. This paper is a brief history of the development of sensors for metal cutting machines. It summarizes the main problems of industry, the main areas of sensor research and the resolution of problems in industry during the last years. Mass production and batch manufacturing are treated separately, since the problems in these two areas appear to be quite different at any given time. It seems from this study that there is not a good correlation between the major research efforts and the main problems of industry. No major industrial applications have come from the popular research topics, and the successful, high-impact sensors have come from elsewhere. It appears that the communication between manufacturers and researchers could be improved to the benefit of both sides.
Keywords: Sensors, Automation, Research effectiveness, Mass production, Batch manufacturing.
1.~edu~on This paper has three objectives: to trace the history of manufacturing problems, research activity and sensors applications in the modern metal cutting industry; to look for relationships among these three threads; to suggest future needs of industry for sensors development.
2. Before 1950 In the late 1940's, batch manufacturing technology was static. The machines and processes were mature. Production was based on operator skills and these skills were available in adequate supply. Mass production, in contrast, was growing rapidly. Here industry had many problems. 2.1. Problems of Mass Production
L. K q g received BS (1958), MS (1963), and PhD (1965) degrees in Mechanical Engineering from the University of Cincinnati. He was trained in machine tool operation and construction at Cincinnati Milacron. For twenty-eight years he performed a variety of R & D tasks at Milaeron, including published work on metalworking machines, processes and systems. In 1986 he left R & D to those w h o are younger and better at solving differential equations, and moved into the commercial area. His present job is Vice-President, Engineering, Aerospace and Special Machines, for Milacron. Elsevier Computers in Industry 14 (1990) 123-129 0166-3615/90/$3.50 © 1990 - Elsevier Science Publishers B.V.
The rapid growth of automation created the first wave of unmanned machining. One man was responsible for ten or more machines. This created problems in two areas: process management and accuracy. Before automation the operator could select any feed or speed he wanted. In this way, he could compensate for a harder batch of raw material or a weaker batch of cutting tools. He could also observe "how the cut was going" and decide to change tools early if needed. When this degree of control was taken away, there were frequent problems: premature tool failure, scrapped parts and machine wrecks. The second problem that came with unattended machining was loss of accuracy. Before automation, any time there was a process change such as a new cutter or heavier stock, the operator would back off his final machine setting and approach
124
Jbzsef Hatoany Memorial: Manufacturing Control and Monitoring 1
Computers in lndtcvtm,
piece" and often several other pieces. Inevitably some of these "setup pieces" found their way in with the good parts. The loss of production and degradation of quality were recognized by shop floor people but were accepted as a necessary offsetting disadvantage to the benefits of automation.
t I i
2.2. Research on Machining Processes
I I 1
I 1
Fig. 1. Mechanical metal cutting dynamometer.
size in two or three steps. This could not be done with automation which had to try for final size on the first cut. This resulted in "scrapping the setup
Fig. 2. Strain gage dynamometer.
Researchers at this time concentrated on the fundamentals of machining processes. The basic mechanism of chip formation was studied and the geometries of various machining processes were also explored. The goals of this work were to identify maximum safe feeds and speeds for various materials and processes. This would give higher productivity without the need for operator attention to tool management. For this research many sensors were developed. The mechanical dynamometer (Fig. 1) and later the strain gage dynamometer (Fig. 2) allowed measurement of cutting forces.
Computers in Industry
R.L. Kegg / Development of Sensors
~MACHINEO KIqW((OC~
i wo~pt(c( ~
I
(XT(14~'~1~ OAR
PoTgNTO~Vf.I'TRI
125
insulated from electrical grounding, then the remote junction of a wire from the workpiece and a wire from the cutting tool became the cold junction of a thermocouple pair and the tool-workpiece contact became the hot junction. The current flowing in the circuit was then proportional to the temperature difference between the two junctions. Although these force and temperature sensors yielded useful scientific information, they could not be applied successfully in production. They were extremely fragile and could not be shophardened.
M-2HS$TOOL -~.
Fig. 3. Thermo-electric measurement of cutting temperature.
Temperature measurements were frequently made in the lab in order to obtain some indication of the rate of cutting tool wear. The most popular sensing method was the tool-work thermocouple technique (Fig. 3). If the workpiece could be
Fig. 4. Diameter size control gage for plunge grinding.
3. The 1950's and 60's
Batch manufacturing continued to have few problems during this period. Numerical control had not yet made much economic impact, and the increased use of tungsten carbide was under the watchful eye of the skilled machinist.
126
Jbzsef Hatvany Memorial." Manufacturing Control and Monitoring
Computers in lndust~"
3.1. Maturing of Mass Production
The growth of new automation applications declined in this period. Industry's attention swung toward optimizing the use of high production equipment. The problem of tool life management was solved by installing cycle counters and changing tools well in advance of the number of cycles which previously wore them out. This problem ceased to exist. Size gages were developed to automatically assure dimensional accuracy. Bore gages could measure the size of a hole and send a compensation signal to an automatically adjustable boring bar. The greatest new application of sensors was the use of in-process gages for plunge grinding. These gages (Fig. 4) measured the workpiece diameter continuously during the grinding operation. When correct size was attained, the gage signalled the machine to stop grinding. This technique was so successful it became a standard procedure in mass production industries. Major transfer line users began to study the productivity of their lines. Each found that the actual number of parts produced per year was about half of the theoretical maximum. This result was widely discussed and publicized. However the causes of the production loss (tooling problems, sticking automation mechanisms, machine hydraulics and relay failures) were kept secret. This led many people to the incorrect conclusion that machine failures, such as spindle and slideway bearing wear, were responsible and that sensors were needed in these areas. In fact, major machine builders had long designed for indefinite bearing life under normal duty cycles, and this was not a significant problem.
Fig. 5. Torque and deflection sensors for adaptive control.
cause skilled machinists sometimes reacted to these sounds. Based on this belief, a team of researchers developed a laboratory apparatus which, for one set of cutting conditions, could d i s t i n ~ i s h between a sharp and a dull lathe tool from the sound of cutting. This was only a demonstration of feasibility, requiring an array of electronics bigger than the machine tool itself. However it was the first major milestone in the use of electronics to extract information from sensors in manufacturing. The focus was not on the sensor itself (an accelerometer or microphone) but on the use of sophisticated electronics and computer-like devices to analyze the sensor data.
3.2. Research on Adaptioe Control
Adaptive control researchers of the 60's wanted to measure tool wear rate and optimize cutting conditions. These measurements were not possible so they settled on power, torque, force and deflection sensing (Fig. 5). Early adaptive controllers did not have a significant impact on production, but they did focus attention on the problem of developing "shop-hardened" sensors. One remarkable research effort deserves special mention [6]. It had always been believed that machining noise contained useful hidden information be-
4. The 1970's 4.1. Better Control in Mass Production
Programmable Logic Controllers r e p l ~ the huge relay cabinets which controlled the automation of the 60's. The controIlers themselves were far more reliable. Furthermore their logic capability allowed the development of diagnostic programs. These programs could help direct technicians to problems involving external devices which
Computers in Industry
failed to provide expected input signals to the PLC. 4.2. Growth of N C
In this decade, numerical controls applications grew rapidly. As machines became able to automatically carry out sequences of complicated motions and even change their own cutting tools, a single operator could control several machine tools. This presented batch manufacturing with the same sensing problems which mass production had experienced earlier. The measure-and-compensate techniques of skilled machinists were no longer possible. Management of tooling was out of the question when the operator had to move from machine to machine and might not be present to hear unexpected noises. Toward the end of this time period, studies of N C machine productivity appeared. In spite of their great productivity, N C machines were typi-
Fig. 6. Measuring probe on a machining center.
R.L Kegg / Developmentof Sensors
127
cally in cycle as little as 25% of the work day. The rest of the time was spent waiting for humans to de-bug N C programs, manage tooling, load parts, troubleshoot failures and other activities. The need for improved techniques in these three areas became apparent. 4.3. Research on Tool Condition Sensing
In the 70's a great deal of research was aimed at the development of sensors for cutting tool condition. Cutting forces and torques, vibrations and sounds were researched as possible indicators of tool wear rate. None was found to be useful under the required range of cutting conditions. A very innovative technique was developed [1] which involved the implantation of low-level radioactive material at the tip of the cutting tool. The radioactivity level of the chips provided a good indicator of tool wear rate. However, it was never applied in industry, possibly because of the fear of radioactivity.
128
Jbzsef Hatvany Memorial." Manufacturing Control and Monitoring
5. The 1980's The main trend in mass production in the 80's was a move to more flexibility. CNC machines began to appear in lower-volume production lines. Some automotive manufacturers even tried computer controlled FMS. This created no new sensor requirements. There was little development of sensing in mass production during this time. 5.1. Unattended Cells and Systems The need to improve the output of machining centers led to growth in the 80's of applications of FMS and machining cells. These and even single CNC machines were sometimes run completely unattended at night [4]. This further increased the problems of tool management and accuracy control. Fortunately sensors became available to help with these problems. The introduction of the probe (Fig. 6) made it possible for machines to manage their own accuracy by measuring and responding to each individual workpiece. These probes were applied to turning and machining centers and quickly became standard throughout industry. Cycle counting, which had been used to minimize tool life problems in mass production, extended to variable CNC cycles by accumulating time in the cut for each tool. This allowed each tool to be flagged as dull, well before it might be expected to wear out. This technique saw a widespread usage and substantially solved the problem of premature tool failure.
Computers in lndust(v
An innovative force sensor, which has since become commercially available, uses strain gages on the machine tool bearings to sense the magnitude of the force pulses as rolling elements go past the strain gage [5]. The signal peaks are used to indicate the relative magnitude of forces acting on the bearing. Force measurements were also used in research aimed at sensing tool fracture and chipping in the turning operation [3]. By recording the force pattern during a fraction of a revolution, any changes in force can be compared to a series of force variation patterns which are associated with tool chipping and tool breakage. This approach appears to be sufficiently general to have broad application.
6. Summary Only grinder gages and probes have found widespread usage in industry. These devices did not evolve from popular research fields. Conversely popular research topics have failed to produce significant relief from industrial problems. Apparently we are not doing a good job of communicating industry's needs to the research community. As a result of this study, the author recommends the following targets, in order of priority, for future sensors R& D: accurate, wide-range bore and diameter measuring devices; more hardened versions of todays CNC measuring devices; anticipation of machine wreck condition; anticipation of device failure; finer focused diagnostics; tool wear and breakage sensors.
5.2. Innovative Uses of Electronics and Computers Early in the 80's a vibration sensor was used in a pilot operation to distinguish between sharp and dull drill bits [7]. This sensing scheme was successful in anticipating drill failure, but it was never made general enough to be broadly applied in manufacturing industry. Considerable research was done in acoustic emission [2]. In this technology, the sonic and ultra-sonic vibrations caused by plastic deformation are analyzed to determine the differences between dull tool and sharp tool frequency patterns.
References [1] N.H. Cook, S.A. Basile, K. Subramanian and W.H. Grace, Tool Wear Sensors, Final Report to National Science Foundation, MIT, Cambridge, MA, August 1976. [2] D. Dornfeld and E. Kannatey-Asibth "Quantitative relationships for acoustic emission from orthogonal:~tal cutring", Trans. ASME, Ser. B, August 1981, pp. 330-340. [3] J.W. PoweR, et al., "Cutting tool sensors, Carbide Toot J., U.S.A., June 1985, pp. 12-17. [4] R. Seifried, "Unattended Machining", SME Paper MR85468, Society of Manufacturing Engineers, Detroit, MI, 1985. [5] L. Stockline, "New developments in tool condition monitoring", in: Proc. 8th Int. Conf. on Automated Inspection
Computers in Industry and Product Control, IFS Publ. Ltd., Bedford, England, 1987, pp. 69-75. [6] EJ. Weller, H.M. Schrier and B. Weichbrodt, "What sound can be expected from a worn tool", Trans. ASME, Ser. B, August 1969, pp. 525-534.
R.L Kegg / Development of Sensors
129
[7] K. Yee and D. Blomquist, "An on-line method of determining tool wear by time domain analysis", SME Paper No. MR82-901, Society of Manufacturing Engineers, Detroit, MI 1982.