available tool condition monitoring systems and to compare it to reported research achievements. Keywords: Industrial application; Tool monitoring. 1.
Int J Adv Manuf Technol (1999) 15:711–721 1999 Springer-Verlag London Limited
Commercial Tool Condition Monitoring Systems K. Jemielniak Warsaw University of Technology, Institute of Manufacturing Engineering, Warsaw, Poland
One of the most significant developments in the manufacturing environment is the increasing use of tool and process monitoring systems. Many different sensor types, coupled with signal processing technologies are now available, and many sophisticated signal and information processing techniques have been invented and presented in research papers. However, only a few have found their way to industrial application. The aim of this paper is to present the state of the art in commercially available tool condition monitoring systems and to compare it to reported research achievements. Keywords: Industrial application; Tool monitoring
1.
Introduction
The quest for process automation is driven by the growing costs of human labour and quality demands and makes the monitoring of manufacturing systems inevitable. Therefore, extensive research work is taking place world-wide in the area of tool and process condition monitoring, which has been one of the most important focuses of research effort for more than twenty years. Numerous papers have been published (see eg. [1–3]) presenting many ideas, and many approaches have been proposed to accomplish tool condition monitoring. In the nature of scientific research, there is a gap between science and technology. There is a time lag between research and commercialisation, and any new technology takes time to mature [8]. Some work in this field can be used in commercially available systems. Other work is still in the exploratory stages. There may be several ideas and possibilities for studying certain phenomena, which all show the potential to become new and useful commercially profitable techniques. However, many of them will never be applied, as they will prove to be unreliable or not economic. As actual use in a commercial system is the final justification of the applicability of any research work
Correspondence and offprint requests to: Professor K. Jemielniak, Warsaw University of Technology, Institute of Manufacturing Engineering, Narbutta 86, 02–524 Warsaw, Poland. E-mail: k.jemielniak얀wip.pw.edu.p1
concerning tool condition monitoring, reviewing such systems is worthwhile. Each tool condition monitoring (TCM) system consists of (Fig. 1): sensors, signal conditioners/amplifiers and a monitor. The monitor uses a strategy to analyse the signals from the sensors and to provide reliable detection of tool and process failures. It can be equipped with some signal visualisation system and is connected to the machine control. All these elements will be discussed here. Although there are many firms offering various tool condition monitoring systems and equipment, six leading suppliers are considered in this survey: Artis; Brankamp; Kistler; Montronix; Nordmann and Prometec.
2. Sensors The sensor is a key element of any tool/process monitoring system. Although numerous different sensor types have been invented and applied in laboratories [5], only a few are now
Fig. 1. Example of tool condition monitoring system configuration [34].
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in commercial use. A summary of sensors applied by different suppliers is presented in Table 1. 2.1
Power
A power sensor measures the spindle or axis drive power for a.c., d.c., or variable frequency motors (frequency range 0 ⫼ 200 kHz) [6]. In all suppliers’ versions (see Table 1) it measures power directly, using a voltage and current measurement. This sensor is easy to install on both new and existing machines. The power sensor is installed directly in the electrical cabinet. The current is measured by 1–3 Hall effect sensors [7]. Use of three balanced Hall effect sensors eliminates large phase shift errors [6]. When two sensors in two phases are used, the third current quantity results from the two that are measured, making it possible to take the usual grid fluctuation between three phases into account [8]. Effective power measurement has the advantage over simple current measurement [9] in that the idle current, which provides no information about the motor load on the tool, is not measured [10]. Power is linear, so a change in motor load is a change in power. Current is not a sensitive indicator of power at low loads in three-phase motors [6]. The sensor can provide an indication of a missing tool or tool wear in certain applications [11]. Using an additional logarithmic signal amplifier, small tools can also be monitored [8,10]. If the power-measuring curve is ‘wavy’, or displays ‘ripples’ or peaks, or even brief (⬍1 s) collapses, the power measuring value can be smoothed [10]. The quality of effective power monitoring depends largely on the relationship between the cutting power and the nominal drive power of the motor. This means that small tools (such as drills with a diameter of less than 1 mm) which are individually driven by powerful motors may be monitored only on good spindle drives – see the example given in Fig. 2 referring to spiral drills in steel or aluminium under normal cutting conditions [10]. Owing to the inert masses, the output signal has a low-pass filter characteristic. Therefore, tool breakage is not detected directly, but only after consequential damage has occurred. A
Fig. 2. Power sensor application range [10].
measurement of the main spindle power often fails to produce better results, since the spindle power is proportional to the main cutting force – the least wear sensitive parameter [1]. 2.2 Torque
Much more accurate tool and process monitoring can be achieved by measuring mechanical torque directly instead of measuring the power consumed by the spindle motor. It is especially useful for tapping and multi-spindle applications where power monitoring is often not sufficient, as only a small percentage of the available motor load is used in each cut. The sensor can monitor tool wear, tool breakage, did-not-cut condition, thread depth, oversized or undersized pre-drilled holes, and damaged or missing threads on taps. A torque tool sensor (TTS) is built especially for tapping in (Fig. 3) [8,12]. It uses strain gauges for torque measurement and non-contact signal transmission, which consists of a rotor integrated in the tool shaft and a stator mounted firmly on the machine frame. The rotor forms a tight ring in the upper part of the tool shaft or of the clamping chuck and contains the full bridge strain gauges and the electronics for acquiring and transmitting the values measured. The stator is installed approximately 5 mm
Table 1. Sensors applied in commercial tool condition monitoring systems. Physical quantity (sensor type)
Force related quantities
AE
Others
Suppliers Artis
Brankamp
Power
•
•
Torque Strain Distance/displacement 1 ⫼ 3 axis force sensor Measuring plate
• •
Acoustic emission AE fluid sensor Rotating AE sensor
•
Vibration and ultrasound Camera Laser
Kistler
Montronix
Nordmann
Prometec
•
•
•
• •
• •
•
• •
• •
•
•
•
•
•
•
• • •
•
•
• • •
• • • • • • • •
•
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Fig. 4. (a) Strain sensor and (b, c) measuring pins [15].
Fig. 3. Torque tool sensor [8].
away from the rotor and serves both as the transmitter of the power supply and as the receiver of the measuring signal. TTS can be used for multi-spindle machines because each tool is monitored separately. Montronix has introduced a torque sensor based on magnetoelastic sensing methods [13]. Accu-Torque has magnetic properties that are affected by mechanical torque. The sensor includes a small torque-sensing ring integrated onto the rotating spindle shaft, and non-rotating pick-up. The ring converts mechanical shaft torque into a linearly proportional magnetic field. The pick-up converts this field into a linearly proportional electrical signal and acts as a non-contact means of gathering shaft torque information. Excellent rotating quartz 4-component (Fx, Fy, Fz and Mz) dynamometers for measuring cutting forces and torque on a rotating tool spindle are also available [14]. However, because of their high cost (some $30 000) they can be considered only as laboratory tools for scientific study. 2.3
Strain and Distance
It is relatively easy to retrofit various strain and displacement sensors. Their principal application is indirect force measurement. A quartz strain transducer (Fig. 4(a), [15]) can be mounted on a part of the machine whose mechanical stressing is large enough, and as undisturbed as possible. Transverse (Fig. 4(b) ) and longitudinal measuring pins (Fig. 4(c) ) measure quasi-static and dynamic strains in the structure of machine components or fixtures using the force bypass principle [15]. The sensors are installed in 5–10 mm holes. Owing to the larger measuring range (length of pressure bar) the axial measuring pin creates a larger measuring signal than the radial pin [16]. Both sensors are insensitive to force acting transverse to the chosen axis.
Fig. 5. RetroBolt [17].
In view of the unavoidable interference and the fact that these sensors generally possess a low level of sensitivity, they are normally suitable only for breakage identification during rough machining [1] and for press force monitoring [15]. The RetroBolt ([17], Fig. 5) is a retrofit force sensor typically used on turret and tool block lathes. This single-axis sensor is installed as a washer under a bolt head. The bolt selected must be in the load path. The sensor measures the small changes in bolt tension resulting from cutting forces exerted on the tool. Typical monitoring performance using the RetroBolt includes overload detection, machine protection, and detection of catastrophic tool breakage in semi-roughing and roughing applications. All force transmitting parts in machines and fixtures, such as housings, spindles, carriages, holders, rods, etc. are deformed elastically by the forces acting on them, i.e. they are upset, stretched, bent or twisted. As a result of this deformation, a detectable displacement occurs between 2 points (reference point and measuring point, see Fig. 6) on the surface of the force transmitting parts. Because of high machine rigidity, the elastic deformations are extremely small. Induction-based distance sensors are capable of resolving displacements of as little as 5 nm resulting from forces acting on machine parts [10,18]. 2.4 Force Transducers
One-, two- and three-axis quartz force transducers (Fig. 7(a) ) are load cells with integral cables for use in a hostile machining environment [15,19]. Design features include rugged, rustproof
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Fig. 6. Example of distance sensor installation [10]. Fig. 8. Wireless AE sensor [25].
direct mounting into the machine structure without requiring a bolted joint. Although several producers provide such force transducers, the largest variety (shapes, forms) is offered by Kistler [15]. In fact, some other suppliers also use Kistler’s sensors and measuring pins. Bearings and bushings equipped with strain gauges for indirect force measurement [20,21] are no longer in use. 2.5 Acoustic Emission Fig. 7. Three-axis force transducer [15,19].
housing, electrically ground-isolated mounting faces, and an armoured cable. They are typically applied in machine tools in a force shunt (parallel) configuration as opposed to a direct (serial) configuration. This results in a significant portion of the applied load (typically 90% or more) being carried by the machine structure with only a small percentage (the remaining 10% or less) being carried by the load cell. Thus, effective load capacities and stiffnesses are ten times, or more, greater than for the direct configuration. Because of force shunting, it is extremely unlikely that the load cells will be damaged by an overload. The load cell installation affects neither the machine stiffness nor its integrity. The sensitivity of the transducer depends on the point of force application. Linearity and the hysteresis of the system are influenced by installation conditions. Three main configurations are recommended: 1. Transducer plates (Fig. 7(b) ) are configured as precision shims to be mounted in a bolted joint in the machine structure. The plates include one, or two load cells and must be installed in the cutting force path, typically located between the turret and the cross-side, or between the Hirth ring and the turret housing. Transducer plates offer the most design control and the best performance but are sometimes difficult to accommodate in existing structures. 2. Pockets provide a less intrusive alternative to plates (Fig. 7(c) ). A load cell is mounted in a precision pocket in a bolted joint within the machine structure. It is preloaded via precision fitting of an adjustment shim during installation. 3. This alternative is similar to the pocket concept above, but uses an adjustable wedge to preload the load cell (Fig. 7(d ) ). Special fitting requirements are avoided at the expense of a slightly larger package. Wedges allow the possibility of
An acoustic emission (AE) sensor measures the high-frequency energy signals produced by the cutting process. When a tool breaks, the sensor also measures the AE energy resulting from the fracture. An AE sensor is best suited to applications where the level of background AE signal is low compared to the sound of tool breakage. This makes the AE sensor ideal for breakage detection of small drills and taps. This sensor is easy to install on both new and existing machines. It detects forceproportional monitoring signals even in machining operations, which generate very small cutting forces. In combination with true power, it increases the reliability of breakage monitoring [8]. It is used especially with solid carbide tools, or very small tools on large machines and multispindles. Most of the sensors have to be attached to the machine tool surface [8,22,23], sometimes with different mounting variants, e.g. side, top, or bottom connection, and spring disk fixing [24]. However, there are alternative methods of AE wave transmitting. A rotating, wireless AE sensor consists of a rotating sensor and a fixed receiver [8,10,25] (Fig. 8). It is suitable for applications where signals from rotating shafts or translatory components such as slides, pallets, spindles, etc. have to be passed to a fixed receiver for analysis and monitoring. When applied in grinding, the sensor enables precise detection of any sparking, which enables optimisation of depth setting [8]. In another approach, an AE sensor receives the acoustic waves via a jet of cooling lubricant, which can be connected directly to the tool or workpiece (Fig. 9, [10,26]). Its advantage over the conventional AE sensor is that it measures close to the tool. It should be used instead of a conventional AE sensor when an uneven acoustic transmission caused by variable joints or interference with the acoustic emissions, could result from other tools. The fluid AE sensor allows detection of highfrequency stress waves from moving or rotating workpieces or
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Fig. 10. Ultrasound and vibration sensor and its amplifier [51].
2.7 Camera
Fig. 9. Fluid sound sensor [26].
A camera can be used in the case of multiple spindle heads, accommodating 20 or more different spindles, when monitoring each individual spindle is too expensive [8]. The camera monitors the missing or breakage of individual drills in multiple spindle heads. During the learning cycle, the camera records the multispindle head and stores the image. Afterwards, the head is monitored by a comparison of contours. It can also be used for all kinds of processes where optical recognition outperforms a system based on other sensors, e.g. for checking assembly, tool, location, completeness, spacing or transfer [30]. 2.8 Laser
components, or from materials with very rough surfaces. In some cases the acoustic waves in the coolant stream are damped so much by the air bubbles that the resulting measurement is too low. Particles of metal and dirt, on the other hand, do not cause problems. An objective control of the transmission of the AE via a measuring test is possible using the so-called lead test. Consistent acoustic impulses can be generated by breaking the leads of a pencil on the connection surface of the measuring jet (i.e. on the tool or workpiece). Although announced a few years ago [19,27], a dual-mode sensor for the simultaneous measurement of acoustic emission and one to three orthogonal force components, never reached the production line. The advantage of this dual-mode sensor was the backing-up of the force measurement by another process variable, the acoustic emission, necessitating only one installation point. However, it appeared that the best place for the cutting force measurement is usually not the same as the best for AE measurement.
2.6
A light barrier offers a reliable tool breakage and tool missing monitoring system if tools are too small to be monitored by force, true power, or if there is no suitable place available to mount an AE sensor [8,10,31]. Cutting edges at a milling cutter can also be monitored in this manner (Fig. 11). The edge of the tool is examined with a laser beam with resolution in the m range. This also allows the measurement of thermal deformations within machine tools, e.g. the lengthening of the tool spindles in a machining centre. The high measurement sensitivity is achieved by a quantitative analysis of the strength of the laser beam shadowing. The intensity of the laser beam shadowing is displayed on the tool monitor as a measurement curve and is monitored within the tolerance ranges. An
Vibration and Ultrasound
A piezoelectric vibration sensor measures the mechanical vibration of the machine structure resulting from the cutting process, typically up to 10 kHz [11,28]. It can be used to detect missing tools, broken tools, out-of-tolerance parts, machine collision and severe process faults [29]. It is also possible to monitor excessive vibration on bearings or spindles. The vibration sensor is easy to install on new or existing machines. An ultrasound and vibration sensor (Fig. 10) is suitable for measuring vibration-induced oscillations up to ultrasonic range (100 –: 80 000 Hz) in machine components [18].
Fig. 11. Artis’ laser used for detection of cutting-edge breakage at a milling cutter.
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additional application for the laser sensor is the monitoring of the roughness of workpieces, chatter marks, or feed marks by analysing laser light reflected from the workpiece. The laser optics are protected from the spatter of cooling lubricant by an air barrier. 2.9
Signal Transmission and Conditioning
A common solution in every supplier’s system is a separate signal conditioner/amplifier, designed for a specific type of sensor (e.g. Figure 10). They are robust, vibration resistant, electrically isolated, and closed in a sealed case. Connecting cables are shielded with steel braid. So, sensors, connections, cables and amplifiers are very well suited to the harsh environment in the machine tool in terms of splash protection, moisture proofing, and resistance to aggressive media and to flying chips. Because force, AE and vibration sensors are piezoelectric transducers, industrial pre-amplifiers convert charge signals from quartz sensor into proportional voltage signals (amplification with selectable gain). An acoustic emission amplifier takes the raw signal from the sensor and provides signal conditioning for enhanced AE monitoring. By using amplification, filtering, and r.m.s. averaging, the raw AE signal is converted into an AE energy signal that can be visualised and monitored at lower frequencies. Raw signal output is applied for diagnostic/service use only. Also vibration amplifiers can be equipped with filters, and r.m.s. converters [29].
3.
The System Configuration
Fig. 12. Monitors offered by Montronix.
2. System Cx/Cz: a monitor for transfer machine application. TS50: a modular, network-capable two-sensor monitor, uses limit strategy for collision and tool breakage and tool overload detection. 3. TS100: a multisensor monitor for multistation applications. Uses limit strategy for collision, tool breakage, wear and overload detection. 4. TS200, TS220: a comprehensive monitor for two- and fouraxis lathes, offered in three configurations:
There are two basic configurations of tool and process condition monitoring systems: compact and modular. Montronix, Brankamp, Nordmann, and Kistler produce the former. In such a system, the core element is the monitor. These monitors are universal, i.e. they can be fed with signals from different types of sensor. Signals generated by the sensors and conditioned by amplifiers are send to the monitor, which is directly connected to the machine control (PLC/CNC). Only Kistler, which is basically a producer of excellent force, stress, vibration and AE sensors (used also by some other TCM systems suppliers) offers one (CoMo II) universal monitor [15,32]. Other producers offer at least two solutions – simple, economical and more advanced and expensive. For example Nordman’s SEMB1 is a one-channel monitor based on a limit set manually or by the NC-program (only 16 possible levels). It is equipped with a 16 LED display of the measured value generated by one of the sensors (AE, active power, force, strain, pressure, distance) [33]. An alternative is SEM-68000 (see Fig. 1) – which provides up to 16 measuring points or channels, graphic display, and different monitoring strategies based on limits (tolerance range, mean height, trend, evaluation of static, smoothed and dynamic values) [34]. Montronix offers a larger variety of monitors [35–39] (Fig. 12):
5. TS300: a plug-in monitor for open architecture controls occupies a single ISA board slot. The ISA bus interface eliminates the need for either a serial or a parallel I/O interface. It is based on a 40 MHz RISC processor, which minimises signal processing and interrupt load on the control CPU. Any industrial PC can be used as the human– machine interface. 6. PDT: (portable diagnostic tool) a flexible monitor for testing performance on new applications.
1. System E: an easy, user-installed monitor for simple transfer machine applications.
A graphic display for Montronix monitors is available only via the integrated process monitor (IPM) software which
i Collision only (limit strategy). ii Collision and breakage (limit ⫹ pattern recognition). iii Collision, breakage and wear (limit ⫹ pattern recognition ⫹ wear estimator).
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Fig. 13. Examples of monitors offered by Brankamp.
Fig. 15. Promos modules (examples).
Fig. 14. Modular tool and process monitoring system produced by Artis.
enables the use of a laptop computer. The IPM graphically displays the normal machining process and automatically compares it with any abnormal condition. In addition, the IPM can be used to optimise the monitor’s protection parameters. The IPM for open architecture machine controls replaces hardwarebased front panels on monitoring products. It can be configured so that the IPM display runs in the background, but will automatically become visible when the monitor generates an alarm. Brankamp also produces several monitors, from simple CMS process control for collision detection, to a 4-channel multisensor, 4 floating limits C 8060 monitoring system [40–42] (Fig. 13). Brankamp has designed this universal process terminal for stamping, cold forming and cutting machines. It is equipped with a touch screen which is both a colour graphic display and a keyboard [43]. Modular assembly of tool/process condition monitoring systems is another concept of systems configuration. It is represented by the MTC system by Artis [8] and PROMOS offered by Prometec [44]. A modular tool and process monitoring system (MTC, Fig. 14) consists of a number of sensor dedicated amplifiers and monitoring modules (a separate module for each type of sensor). They can be operated and their data made visible by a separate control panel. Another special module enables the operation of monitoring modules and data visualisation via the control panel of the machine control. It is also possible to represent and save machining patterns on a PC with the ViDi software. MTC can be installed centrally or be decentralised. With a centralised module arrangement, all the modules are
located together in one subrack. This arrangement is used, for example, for machining centres and lathes because, in these cases, the points of measurement and the modules are not far away from one another. With a decentralised arrangement, the modules are mounted individually near the machines, either on the station or in the electrical cabinet. The display is arranged within the operator’s field of vision. PROMOS (the modular process monitor system of Prometec; Fig. 15) consists of sensors, dedicated sensor modules, universal monitor modules, machine interface modules, and operator panel modules. One- or two-channel monitoring modules perform their monitoring tasks independently of one another, in accordance with the specific requirements of the respective machining stations to which they are assigned. The messages (breakage, wear, etc.) are transmitted from the monitor module to the machine’s programmable controller (PLC) via the machine interface module. All machine interface modules can be installed in a switch cabinet. The system can be operated either via a simple operator panel module OPM12 or via a much more sophisticated module, equipped with a graphic display (the OPM20 operator panel module). Other options are: a PC/notebook; an appropriate human interface panel module; and the installation of Prometec’s operation and visualisation software in an open NC control system. A summary of the visualisation methods applied in TCM systems is presented in Table 2.
4. Monitoring Strategy An analogue (electrical) signal from the sensor is usually (after basic signal conditioning, e.g. primary filtering) converted to a digital form. The time series obtained is then processed to extract signal features that are sensitive to the parameters of interest in the monitored process. The detection of process irregularities is achieved by the implementation of some sensing methodology, called a monitoring strategy. Monitoring strategies applied by different suppliers can be grouped into six approaches, summarised in Table 3. Most of them are based on static (remaining fixed during the processing of the current workpiece) limits.
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Table 2. Visualisation methods applied in TCM systems. Visualisation method
Simple digital/bar display Graphic display Via open CNC control system Via PC/notebook
Supplier Artis
Brankamp
Kistler
• • • •
• •
•
•
•
Montronix
Nordmann
Prometec
•
• • •
• • • •
• •
Table 3. Strategies applied in TCM systems. Strategy Static limits
Simple fixed limits Time defined limits Part signature
Artis
•
Brankamp
Kistler
•
• •
Simple Fixed Limits
The simplest strategy is based on fixed limits, which apply to the raw, filtered, averaged signals or the area under the curve. During machining of the first workpiece, the monitor signal is calibrated automatically (normalised to 100%) cycle-by-cycle for each cut or each tool by means of the teach-in process (or via manual calibration) and the positions of limits relative to this standard are defined. A typical example of it is the strategy implemented in monitors TS50 and TS100 [36,38]. Limits are (Fig. 16): 1. L1: Fast acting upper limit typically set at the maximum allowable range for the machine. Stops machine immediately for a machine-threatening event (collision).
Nordmann
Prometec
•
• • •
• •
• • •
Pattern recognition Wear estimator Dynamic limits
4.1
Montronix
•
2. L2: Fast acting upper limit set a multiple (e.g. 250%) of the maximum learned signal for each tool. Stops machine immediately for a major tool fracture. 3. L3: (TS100 only) Slow acting upper limit set at a multiple (e.g. 150%) of the maximum learned signal for each tool. Stops machine, or inhibits the next cycle, when the process is out of the allowable range owing to worn tool. 4. L4: Lower limit set at a multiple (e.g. 50%) of the maximum learned signal for each tool. Used to detect a missing tool and inhibit future cycles. Instead of fixed limits L1 and L4, floating limits can be applied. They track cycle-to-cycle trends by using information from the current cycle to adjust the limits used for the next cycle automatically [45]. Missing-tool detection, for example in multispindle applications, may be improved using this technique. Floating limits are also more sensitive to short-term rapid events than fixed limits. Similar strategies, based on static fixed limits, are used in PROMOS [44] and SEM-B1 [33].
4.2 Time Defined limits
Fixed limits can be time-displaced, and thus can be better adjusted to real monitored processes than simple fixed limits. They monitor one or more subareas of a signal progression. The widest variety of time-defined limits is used in Nordmann’s strategy [34]. Examples are given in Fig. 17: 1(a) 1(b) 2 Fig. 16. (a) Fixed and (b) floating limits.
For sudden heavy tool wear detection. Triggers an alarm when the workpiece is cut too early. Stops the machine immediately when triggered owing to tool fracture, run out of machined part or wrong adjustment.
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Timing between the stepped limits and the current cycle is critical for maintaining performance and avoiding missed or false alarms [45]. 4.4 Pattern Recognition
Fig. 17. Some of time defined limits used in Nordmann’s strategy [34].
3
As 2, however, stops the machine only when it is crossed, but not when measured curve runs completely undemeath it. 4(a)–4(c) Must be crossed at least once to detect the cutting of a workpiece, to test tool presence at the end of a cut, and for self-checking of the correct acoustic coupling of the lubricant. 5(a) Triggers an alarm at the end of a cut when the mean height of the curve, calculated during a chosen period of time, reaches it owing to tool wear when cutting force sensor is used. 6 Triggers an alarm at the end of a cut when the mean height drops down to its value owing to tool wear, as some tools become quieter with increasing wear. The “rising through” and “falling through” limits used in Prometec strategy are similar to 4a and 4b described above [44]. Kistler’s box function monitors whether the signal progression enters or leaves through the prescribed side of the box [32]. The remaining sides must not be touched. It can also be used to control the stop position of a process. 4.3
Montronix developed a tool-breakage detection strategy based on pattern recognition, available only on the TS200 series (for turning) [45,46]. The system stores a number of reference force shapes or patterns that are indicative of tool breakage (Fig. 19), e.g. when carbide breakage cutting force suddenly rises for a while, then drops to zero. When a ceramic tool breaks, the cutting force drops to zero. Patterns are also stored for chipping and interrupted cuts. The system continuously monitors the signal for one of the break patterns. If one or more patterns are identified, a break is declared within 10 ms of the breakage. Break detection through pattern recognition has the advantage of being independent of the magnitude of the process signal value. The monitor can, therefore, be optimised to ignore process changes that are not related to tool condition, such as material dimension, feedrate, interrupted cuts, and part hardness. 4.5 Wear Estimator
The wear estimator is another proprietary technique developed by Montronix used for turning tool flank wear estimation [45,46]. The method uses the relationship between all three cutting force components, and, therefore requires a three-axis force sensor. It enables normal tool wear, which primarily affects passive and feed forces, to be distinguished from process variations such as workpiece runout and variations in workpiece hardness, which can adversely affect systems using simple threshold or signature techniques. Generally, a wom tool is not a catastrophic event and when detected it is usually possible to continue machining to the end of the current operation.
Part Signature
The cutting process is divided into segments using stepped limits based on time or position, with a particular set of limits for each individual segment. It creates a part “signature”, which can more closely track a complex cutting cycle than a single set of fixed limits [45]. Stepped limits can be used as timedefined limits described above, or can create floating reference limits surrounding measurement curves like a tight tube (Fig. 18). All suppliers but Prometec use this strategy at least in some of their monitors.
Fig. 18. Part signature visible on Brankamp’s monitor screen [41].
4.6 Dynamic Limits
Prometec’s PROMOS system incorporates dynamic limits for tool-breakage detection (Fig. 20) [44]. The two dynamic limits
Fig. 19. Pattern recognition for breakage detection [45,46].
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is optimised to enhance breakage-specific signal components. The signal is also automatically used to cope with the wide difference in force or signal values produced, for example, by large roughing tools as compared to small finishing tools. Signal adaptation automatically keeps signals at an optimum level for analysis. The combination of feature conditioning, automatic signal adaptation and dynamic limits means that monitoring functions are fully automated over a wide range of force or sensor signals in completely different machining situations, without manual adjustments or a teach-in phase. Tool breakages are detected practically at the instant of breakage (typically 5 ms), by means of typical changes in the sensor signal.
5. Conclusions
Fig. 20. Dynamic limit strategy of Prometec.
above and below the monitor signal follow the monitor signal continuously for every load level at a limited adoption speed (not to be confused with a signal pattern or a signal tube). In the case of an extremely fast crossing of one of two dynamic limits, they are frozen (rendered static) and total breakage, breakage, chipping, workpiece cavity, hard cut interruption, etc. are distinguished from one another via visual comparison with the monitor signal. Slow but large load changes due to variations in cutting depth (hardness, oversize, out-of-roundness of workpiece), such as occur during initial cuts, in particular when machining cast and forged parts, are tolerated at a ratio up to 1:4. Another version of Prometec’s dynamic limit is presented in Fig. 21. Instead of the direct force signal, a special feature signal is processed from the sensor signal [47,48]. This feature
Most commonly used, in tool condition monitoring systems, are sensors measuring cutting force components or quantities related to cutting force (power, torque, distance/displacement and strain). They are relatively easy to install in existing or new machines, and do not influence machine integrity and stiffness. All systems suppliers also use acoustic emission sensors, especially for monitoring small tools and for grinding. Some applications use vibration sensors and optical systems (laser, camera). All sensors used in TCM systems are well adjusted to harsh machine tool environments. Despite much research work concerning advanced signal analysis (e.g. FFT, cepstrum, ARMA, statistical analysis), the dominant signal processing technique is filtering (low-pass, high-pass and band-pass), averaging and r.m.s. Since constant (at least during processing current workpiece) limits only work when all restrictions (depth of cut, workpiece material, etc.) remain constant, the use of more elastic, selfadjusting limits is more appropriate in most cases [1,20]. However, only Prometec in “dynamic limits” and Montronix in “wear estimator” and “pattern recognition” strategies use more sophisticated signal processing. Monitoring systems developed in laboratories, are often multisensor systems embodying complex AI-based strategies to integrate information, extract features and make more reliable
Fig. 21. Prometec’s dynamic limit strategy for milling.
Commercial Tool Condition Monitoring Systems
decisions on the state of the tool and process [1,3,49]. In commercially available systems, the one sensor – one tool/process approach dominates. Multisensor here means providing the best sensor for each application. Sometimes using two different sensors for one process is recommended for the multiple detection capability of the system. However even then the signal processing and detection techniques applied to each sensor are primarily independent [50]. Only the Montronix “wear estimator” strategy uses more than one signal for monitoring the wear of one tool (exclusively for turning).
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23. “Acoustic emission sensor”, brochure of Montronix company, Ann Arbor, USA, 1994. 24. “Wide band acoustic emission sensor WAE 100”, brochure of Prometec company, Aachen, Germany. 25. “Wireless AE sensor AEL 200”, brochure of Prometec company, Aachen, Germany. 26. “Fluid sound sensor WAE 100”, brochure of Prometec company, Aachen, Germany. 27. C. Cavalloni and A. Kirchheim, “New acoustic emission sensors for in-process monitoring”, Kistler Instrumente AG Winterhung, 1994. 28. “Acceleration instrumentation for a dynamic world”, brochure of Kistler company, Winterhur, Switzerland, 1993. 29. “Vibraton sensor”, brochure of Montronix company, Ann Arbor, USA, 1996. 30. “Bankamp optical protection system”, brochure of Brankamp company, Erkrath, Germany, 1997. 31. “Laser system for tool setting and breakage detection”, brochure of Blum company, Ravensburg, Germany, 1997. 32. “Control monitor CoMo II”, brochure of Kistler company, Winterhur, Switzerland, 1997. 33. “Tool monitoring unit SEM-B1”, brochure of Nordmann company, Ko¨ln, Germany, 1997. 34. “Tool monitoring for the protection and unsupervised operation of machine tools”, brochure of Nordmann company, Ko¨ln, Germany, 1997. 35. “TS200 Series tool monitors”, brochure of Montronix company, Ann Arbor, USA, 1991. 36. “TS100 Series tool monitors”, brochure of Montronix company, Ann Arbor, USA, 1996. 37. “TS300 Series tool monitors”, brochure of Montronix company, Ann Arbor, USA, 1997. 38. “TS50 Series tool monitors”, brochure of Montronix company, Ann Arbor, USA, 1997. 39. “A word of monitoring solutions”, brochure of Montronix company, Ann Arbor, USA, 1996. 40. “Bankamp CMS”, brochure of Brankamp company, Erkrath, Germany, 1994. 41. “Bankamp C 8060”, brochure of Brankamp company, Erkrath, Germany, 1997. 42. “Process monitoring for metal processing”, brochure of Brankamp company, Erkrath, Germany, 1997. 43. “Bankamp GT”, brochure of Brankamp company, Erkrath, Germany, 1997. 44. “Modular process monitor system PROMOS”, brochure of Prometec company, Aachen, Germany. 45. “Signal processing techniques”, brochure of Montronix company, Ann Arbor, USA, 1995. 46. “TS Series tool monitors”, brochure of Montronix company, Ann Arbor, USA, 1991. 47. W. Kluft and G. Schneider, “Methoden zur Werkzeugbrucherkennung beim Fra¨sen sowie beim Drehen und Bohren in der Einzelteil – und Kleinserienfertigung”, Proceedings of the International CIRP/VDI Conference, “Monitoring of Machining and Forming Processes”, VDI Berichte, 1179, pp. 259–269. 48. “Automatic tool monitor MA 300 and MA 302 for machining centres and milling machines”, brochure of Prometec company, Aachen, Germany. 49. D. Dornfeld, “Monitoring technologies for intelligent machining”, Proceedings of the International CIRP/VDI Conference, Monitoring of Machining and Forming Processes, VDI Berichte, 1179, pp. 71–90, 1995. 50. A. Varma and W.A. Kline, “Multi sensor tool monitoring – applications experience and future trends in industry and education”, Proceedings of the International CIRP/VDI Conference Monitoring of Machining and Forming Processes, VDI Berichte, 1179, pp. 223–233, 1995. 51. “Ultra sound and vibration sensor AE 30”, brochure of Prometec company, Aachen, Germany. 52. “Process listening system PLS20”, brochure of Prometec company, Aachen, Germany.
