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An industrial application for on-line detection of instability and wire breakage in wire EDM
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 5 ( 2 0 0 8 ) 101–109
journal homepage: www.elsevier.com/locate/jmatprotec
An industrial application for on-line detection of instability and wire breakage in wire EDM b ´ I. Cabanes a,∗ , E. Portillo a , M. Marcos a , J.A. Sanchez a
Department of Automatic Control and System Engineering, E.T.S.I. of Bilbao, University of the Basque Country, Alameda Urquijo s/n, 48013, Bilbao, Spain b Department of Mechanical Engineering, E.T.S.I. of Bilbao, University of the Basque Country, Alameda Urquijo s/n, 48013, Bilbao, Spain
a r t i c l e
i n f o
a b s t r a c t
Article history:
One of the main challenges in wire electrical discharge machining (WEDM) is avoiding wire
Received 30 August 2006
breakage and unstable situations as both phenomena reduce process performance and can
Received in revised form
cause low quality components. This work proposes a methodology that guarantees an early
15 December 2006
detection of instability that can be used to avoid the detrimental effects associated to both
Accepted 19 April 2007
unstable machining and wire breakage. The proposed methodology establishes the procedures to follow in order to understand the causes of wire breakage and instability. In order to quantify the trend to instability of a given machining situation, a set of indicators related to
Keywords:
discharge energy, ignition delay time, and peak current has been defined. Wire breakage risk
WEDM
associated to each situation is evaluated comparing the evolution of those indicators with
Analysis
some previously defined threshold values. The results of this work will be used to develop
Diagnosis
a real-time control strategy for increasing the performance of the WEDM process.
Instability
© 2007 Elsevier B.V. All rights reserved.
Wire breakage Detection
1.
Introduction
WEDM is one of the most extended non-conventional machining processes. It is widely used to machine dies and moulds aimed at producing components for many industries. The main advantage of WEDM is its capability for the production of high complexity shapes with a high degree of accuracy, independently of the mechanical properties of the material (especially, hardness, brittleness and resistance). WEDM is based on material removal through a series of electrical discharges applied between two electrically conductive electrodes—workpiece and wire. Dielectric fluid is injected into the gap, which is the space between electrodes.
∗
Corresponding author. Tel.: +34 94 601 39 51; fax: +34 94 601 41 87. E-mail address: [email protected] (I. Cabanes). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.04.125
Thus, there is no contact between tool and workpiece during the process. Each discharge is generated as follows: the WEDM machine power supply applies a voltage between workpiece and wire. It starts the ionisation period of the dielectric fluid, which is known as ignition delay time. Dielectric ionisation induces the discharge that vaporises all the material around. Before applying the voltage for the next discharge, the dielectric cools the gap and removes the erosion debris during a period of time known as off-time. A servo control is used to control the gap size. Most of servo control systems take the discharge voltage as the feedback signal. On the other hand, the wire is continuously running at a constant speed. Fig. 1 shows a schematic diagram of the WEDM process.
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Nomenclature A1 A2 A3 BV Ei Eref Ipi Ip ref M N pN (Eref ) pN (Ip ref ) pN (td ref ) tdi td ref TR 2
low level alarm medium level alarm high level alarm basic variable energy of the discharge i energy reference value peak current of the discharge i peak current reference value basic window sliding window energy indicator peak current indicator ignition delay time indicator ignition delay time of the discharge i ignition delay time reference value time period for pN (td ref )
Wire breakage and unstable machining are two of the most important aspects of the WEDM process due to their detrimental effects, such as reduction of machining performance and surface damage (Wang et al., 1992). Different factors can lead to wire breakage, such as the generation of shortcircuits, inefficient removal of erosion debris as well as other types of stochastic phenomena that appear during the cutting process. Usually, if an abnormal situation is detected, the machine operator manually adjusts the parameters of the machine using his/her own experience. Moreover, WEDM machines work most of the time without operator assistance. Therefore, it would be desirable to predict wire breakage and unstable machining in order to on-line readjust the machine parameters. In accordance with this objective, this paper proposes a methodology for on-line detection of instability and wire breakage. The methodology is based on acquiring knowledge of the process through an exhaustive experimental database containing data collected from both stable and unstable machining situations. The layout of the paper is as follows: Section 2 presents a brief review of previous work. Section 3 describes the acquisition system designed to capture the required signals. Section 4 explains the generation of the experimental database. Section 5 is dedicated to the definition of wire breakage indicators and to the establishment of heuristic rules for the setting of different alarm levels. Section 6 presents some industrial application examples. Section 7 provides a summary and some concluding remarks.
Fig. 1 – Wire electrical discharge machining process.
2.
Literature review
The study of different aspects that have influence on the performance of electrical discharge machining (EDM) process has been carried out from the beginning of this type of machining technology (Cooke and Crookall, 1973). In fact, it is still difficult to understand all the aspects concerning EDM due to its strong stochastic nature and the multiple parameters of the process. One of the great challenges in WEDM is to increase the grade of stability by avoiding wire breakage. This aim should be achieved for any workpiece thickness, workpiece material (such as ceramics, steel and other metals), machine power supply, etc. In this respect, this paper proposes to acquire online knowledge of the behaviour of the discharges in the gap in order to optimise the control strategy of the process. Some research lines can be clearly identified in the literature survey. In most of these works the capture of voltage and/or current signals is carried out because these signals provide worthy information related to the gap stability. Different authors have employed those signals for different aims, such as discharges classification, discharges location and the estimation of the thermal load on the wire. One of the most interesting lines is based on detecting the discharge position on the wire. The concentration of successive discharges at one point of the wire can lead to wire breakage. In this way, many authors construct an equivalent circuit of a standard EDM machine in order to develop monitoring systems for discharge location (Obara et al., 1999; Shoda et al., 1995; Lauwers et al., 1999; Obara et al., 1990; Kunieda et al., 2001). Some of the authors have considered the discharge position in the design of mathematical thermal models. The wire breakage is detected when a temperature threshold is exceeded (Lauwers et al., 1999). Obara et al. (1990) studied three gap monitoring signals depending on discharge position (ignition delay time, discharge voltage and a radio signal) in order to predict wire breakage. All the signals from a transistor controlled WEDM machine were elongated by a sample-andhold circuit and recorded in a digital oscilloscope. This work concluded that a decrease in discharge voltage was always observed before wire breakage. Another interesting research line considers discharge classification in order to ensure machining stability. Dekeyser et al. (1988) developed an electronic device (pulse discriminating system) to distinguish 13 different types of discharges. In order to study the behaviour of current and voltage before wire breakage, the authors carried out tests for provoking wire breakage by increasing the pulse frequency. One of the main contributions of this work was that the succession of pulses of some specific type has influence on wire breakage. Afterwards, Watanabe et al. (1990) reduce the discharge classification to four basic types: normal, arcing, shortcircuits and open-circuit discharges. This classification is the most commonly used nowadays. Each type of discharge is defined by establishing threshold values for discharge current and/or discharge voltage. In this respect, another pulse discriminating system was developed by Yan and Liao (1995). Discharge classification was established only using discharge voltage. This system presents some restrictions. First, the sampling time or sliding window
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 5 ( 2 0 0 8 ) 101–109
parameter defined to count each type of discharges was limited to a lowest value of 0.55 ms. In addition, it had to be selected before the on-line acquisition of the experiment. Second, the recording space was restricted to 4 MB. Later, the same working group observed two types of wire breaking phenomena: the first type is the sudden rise of the sparking frequency and the second type is related to the slight increase in sparking frequency and to the increase in the ratio of arcs and shortcircuits (Liao et al., 1997; Liao and Woo, 1997). Recently, Wu and Li (2001) have utilized the discharges classification to establish a new servo control strategy in a new prototype of EDM machine (High Speed WEDM). In this case, the selected sliding window of the pulse discriminating system was 10 ms and the data recorded continuously was between 6000 and 9000 samples, which correspond to 60–90 s. In other works, discharge current and/or discharge voltage have been acquired and analysed to detect wire breakage without discharge categorising. An example of this trend is the research carried out by Wang and Rajurkar (Wang et al., 1992). In this work, a sparking frequency monitor was developed. It was used to count discharges along a sliding window, which can be set in the range of 10 s–655.35 ms. In this case, the recording space is limited to 4096 points. The authors established two indicators related to the temperature of the wire. The first of them is the average sparking frequency, since a higher average sparking frequency increases the thermal load on the wire. Therefore, the wire breaks when the average sparking frequency exceeds an established level. The second indicator is based on the variation of the sparking frequency along a specific time period obtained empirically. In this case, wire breakage is caused by the high thermal energy density per unit length of the wire. As shown in the related works, the HW detecting systems developed for the analysis of wire breakage presents the commented limitations related to the storing capacity and sliding window tuning. In fact, the off-line selection of the sliding window in the literature review has been normally defined by the try-and-error method. This implies repeating the experiment until the proper sliding window is found. Moreover, the time invested in the design and development of proprietary HW systems is significant. On the other hand, most of the systems have been designed for specific conditions, such as workpiece material, machine power supply and machining parameters. Therefore, the system should be re-designed in order to analyse other operational conditions. Thus, this paper proposes a methodology that allows to configure a SW system that uses commercial hardware in order to develop flexible industrial applications. It has been applied to the on-line detection of instabilities and wire breakage in WEDM. The stochastic nature of this process has led to the definition of different levels of alarms triggered by the application. Subsequently, control algorithms could use the levels of the alarms in order to perform the control strategy. Recent technological advances in commercial off the shelf (COTS) HW and SW components, allow to employ more powerful systems that provide better performance. Hence, the developed application uses an acquisition system based on a commercial data acquisition PC Board. It provides continuous sampling of the complete current and voltage signals at a very
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high sample rate. The acquisition system can be integrated into a real-time processing system. Among its advantages, a higher flexibility stands out due to many factors. First, the system can be easily employed under different experimental conditions (power supply, workpiece material . . .). Second, due to the high rate of acquisition it allows to record the complete signals of the process. Therefore, a preliminary analysis of the different variables proposed by the different authors has been performed aimed at the selection of the most appropriate to anticipate wire breakage (Portillo et al., 2004a,b). Finally it is remarkable the flexibility of the system in the selection of the sliding window since it is selected after storing the tests. This avoids repeating the same type of test with different sliding windows. The analysis of WEDM process is based on an exhaustive experimental database that contains information about both stable and controlled unstable operation. This extensive database has been found as a key factor in the definition of global and quality rules to detect wire breakage. In fact, the results of the empirical analysis of the experimental database have showed new situations of wire breakage. Thus, the methodology involves the definition of an acquisition system, the design and the obtaining of an experimental database, a preliminary analysis for the selection of appropriate variables, the establishment of wire breakage indicators, the definition of different wire breakage alarm levels from a set of heuristic rules and, finally, the development of the industrial application and the subsequent validation.
3.
Acquisition system
An acquisition system has been designed to capture discharge current and discharge voltage in order to obtain an extensive experimental database. The evolution of these two signals for stable and unstable tests will allow to characterise both cutting regimes. Since the acquisition system captures the complete signals, it can be utilized when new conditions of workpiece material, machine power supply or machining parameters are applied. Considering current and voltage signals characteristics, the requirements of the acquisition system are the following: continuous-time acquisition (without loss of information), minimum sample and storage rate of 5 M samples/s per channel, two analogue input channels with independent resolution, and finally, minimum input range of +1 V for current signal and +10 V for voltage signal. The high acquisition frequency and simultaneously data storage without loss of information reduce drastically the commercially available hardware and software for the acquisition system. After comparison among different alternatives, the NI 6115 PC Board with PCI bus has been selected as it meets all the requirements. Fig. 2 shows the block diagram of the acquisition system. Discharge voltage and discharge current signals are transferred to the BNC adaptor, which is attached to NI 6115 data acquisition board through PCI bus. LabviewTM high speed libraries (Lab View, 2000) have been used for the development of two software applications required for exploiting the acquisition system. One of them
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establish reference values for a set of discharge parameters. When the reference values are not followed, a transition to unstable machining can be detected. Brass wire of 0.25 mm diameter (DIN 160) and 50 mm height (AISI D2) tool steel work parts have been used. The tests have been performed on an ONA E-250 wire EDM machine. Stable cutting tests correspond to straight rough cutting using the parameters obtained from machine table look-up, as Fig. 3 illustrates. The definition of unstable cutting tests considers three different causes of instability. The general purpose of these tests is to provoke cutting instabilities and wire breakage (see Fig. 3):
Fig. 2 – Acquisition system. • Poor flushing: poor flushing can produce an insufficient cooling of the gap as well as a greater quantity of debris. These factors can
performs the real-time acquisition and storage at the required sample rate (5 M samples/s per channel). The second application displays off-line graphs of the acquired signals. It can show any selected 200 s frame of the test file. A typical continuously stored test can last from 4 to 60 min. The application also records test header and parameters.
4.
increase instabilities and wire breakage risk. Thus, in order to provoke cutting instabilities and wire breakage due to lack of dielectric, stair-shaped pieces have been designed. • Complex geometries: these tests provide unstable cutting behaviour due to a change in the direction of movement of the wire. The cut angles range from 15◦ to 120◦ . • Variation of sparking frequency: the increase in sparking frequency is applied by decreasing the value of the off-time parameter. Therefore, the time between discharges is reduced. This means that the time used for cleaning and cooling the gap also decreases while the density of discharges increases. All these factors converge on
Experimental database
In the present work, the obtaining of an exhaustive experimental database is a key factor in order to define global and quality indicators. Thus, a battery of tests that provides a wide variety of sources of instabilities has been designed. Both types of regimes, stable and unstable, have been considered. The stable cutting tests are used to
a higher risk of wire breakage. These tests have been performed on straight and corners cutting.
Fig. 4 shows some workpieces resulting from the stable and unstable tests.
Fig. 3 – Experimental database.
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Fig. 4 – Workpieces resulting from stable and unstable tests.
5.1. Establishment of indicators for wire breakage monitoring
5. Instability and wire breakage on-line detection As mentioned in the literature review, the authors have used different variables (such as energy, peak current, discharge voltage, ignition delay time, discharges frequency, . . .) aimed at different objectives: discharges classification, wire breakage analysis, establishment of servo control strategies, etc. According to the authors contributions, the first step proposed is to select among those different variables the most suitable to anticipate wire breakage in an industrial application. Since the acquisition system captures the complete information of the signals, it has been possible to join all the variables contributed by each author in a unique analysis application. This preliminary analysis has been carried out on a set of tests defined in Section 4. Definition 1. Analysis time interval is the period of time in which one point of the wire can be exposed to discharges during the time required to pass through the workpiece. It depends on the wire feed and on the workpiece thickness. As the workpiece thickness is known (50 mm) and the wire feed is one of the adjustable machining parameters, the time interval to be analysed before wire breakage is calculated. Its value is 333 ms. This means that the time interval in which wire breakage should be avoided is significantly restrictive. Thus, the indicators will be obtained by an increasing calculation method in order to deal with the time constraints. During this preliminary analysis, two different tendencies towards wire breakage have been observed. The first tendency shows a sharp increment of discharges energy. The second one consists of a sharp fluctuation and increment of discharges current, as well as a considerable increment of ignition delay time. According to these results, the selected variables for the definition of wire breakage indicators are the following:
In this section, the wire breakage indicators are established based on the above selected variables. Taking into account the two tendencies observed in the preliminary analysis, it is deduced that high values of peak current, ignition delay time and energy are clearly relevant before wire breakage. Therefore, reference values must be set in order to discriminate high values of the variables (Ip ref , td ref and Eref ). Definition 2. Each reference value, Ip ref , td ref and Eref , is defined as the class when the relative cumulative frequency achieves 99% in stable cutting. Fig. 5 shows an example of the frequency distribution employed to define td ref . Definition 3. Wire breakage indicators, pN (Ip ref ), pN (td ref ) and pN (Eref ), are defined as a succession of percentages of discharges that exceed, respectively, the pre-defined reference values in a time period N. The percentages are continuously calculated along time. Peak current indicator is designated as pn (Ip ref ), ignition delay time indicator as pN (td ref ) and energy indicator as pN (Eref ). Definition 4. Sliding window N is the time period in which each percentage value is computed.
• Peak current (Ipi ) of the discharge i • Ignition delay time (tdi ) of the discharge i • Energy (Ei ) of the discharge i:
Ei =
tei
Ipi Ui dt
(1)
0
where tei is the duration of the discharge i and Ui is the discharge voltage.
Fig. 5 – Frequency distribution of ignition delay time in stable cutting.
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Fig. 7 – Calculation of pN (Eref ) with and without the basic window M.
uous line delimits the analysis time interval (333 ms). Clearly, a value of N equal to 5 ms is discriminatory. Definition 5. N is divided into sub-windows, called basic windows M. When the newest basic window M fills up, it is appended to the sliding window, the oldest basic window is removed, and statistics over the sliding window are recomputed.
Fig. 6 – Evolution of pN (Ip
ref )
with different values of N.
The sliding window N has been established using a heuristic method. On one hand, the criteria applied is that if N is too small, the successive percentages fluctuate excessively avoiding to distinguish relevant from irrelevant increases. On the other hand, the longer N, the less sensitive or the more smoothed the successive percentages will be. Consequently, relevant increases would not be easily detected. Moreover, N should be small enough to reduce the size of the employed register to store data. Therefore, all these compromises have been considered for the tuning of N. To illustrate this, Fig. 6 shows an example of the evolution of one of the indicators 2.5 s before wire breakage for different values of N. The vertical discontin-
In order to make the calculation of the wire breakage indicators independent from the starting time of the signal processing, as well as to reduce the delay for detection of wire breakage, the indicators are obtained each basic window M. Thus, it should be short enough. Otherwise, M should be high enough to obtain a representative sample to calculate the successive percentages (more than one discharge). In order to achieve this compromise M is set at N/5. Fig. 7 gives a schematic example of the calculation of the energy indicator with and without M. It shows that if M is not used it is possible to carry out an erroneous detection of wire breakage. The generic algorithm that defines the evolution of each wire breakage indicator is shown in the appendix section.
5.2. Heuristic rules for the definition of different wire breakage alarms In the establishment of a set of heuristic rules to anticipate wire breakage, is essential to employ the complete experimental database. In this way, the industrial application developed in this work will enable to increase the number of correctly detected wire breakage cases. The heuristic rules have been empirically deduced through the analysis of the behaviour of the wire breakage indicators.
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• RULES 1 refers to the set of rules that anticipate a sharp increment of discharges energy. RULES 2 is related to the detection of a sharp fluctuation and an increment of peak current, as well as a considerable increment of ignition delay time. Three levels of alarm have been defined for each case RULES 1 and RULES 2: - A1 : low level alarm. - A2 : medium level alarm. - A3 : high level alarm. Low level alarms (A1 ) refers to an alerting but not very dangerous situation. If a low level alarm is activated, other tasks related to the medium level alarms are executed. Medium level alarms (A2 ) are related to unstable machining. Finally, high level alarms (A3 ) warn about the probable wire breakage. • RULES 1: the analysis of the wire breakage tests related to this case has showed that the rise time of the energy indicator from 30 to 70% ranges from 2 to 3 ms. Thus, the detection rules are defined as follows: - A1 is triggered when the energy indicator exceeds 30%. - A2 is triggered when the energy indicator exceeds 50%. - A3 is triggered when the energy indicator exceeds 70% and the rise time from 30% (A1 ) to 70% is lower than a time period equal to 3 ms. • RULES 2: in this case, the analysis has revealed that the level of ignition delay time indicator is high 150 ms before the beginning of the instabilities prior to wire breakage. Moreover, the peak current indicator exceeds 20% more than once during a maximum time interval of 100 ms. Both time intervals have been set by the analysis of the corresponding wire breakage tests. As the high level of ignition delay time indicator is concerned, it is detected by the cumulative sum of the ignition delay time indicator:
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Fig. 8 – Signals behaviour in stable cutting.
First, stable cutting tests have been analysed by the developed application. Fig. 8 depicts the behaviour of the above specified indicators during 500 ms in stable cutting. Although it is difficult to distinguish each evolution of pn (Ip ref ), pN (td ref ) and pN (Eref ), the Fig. 8 shows that the wire breakage indicators fluctuate maintaining below 10%. Thus, the alarms are not triggered, which is obviously expected during stable cutting. Fig. 9 shows the application results obtained from one of the tests defined for the variation of sparking frequency in straight cutting. In this example, the alarms related to RULES 1 have been triggered. The time depicted is 500 ms before wire breakage. The vertical discontinuous line delimits the analysis time interval. The black circles represent the moments in which A1 , A2 and A3 are activated. The time elapsed since the
i=TR 2 +nM
g(i) ≥ 2
(2)
i=nM
where g(i) are the successive values of the ignition delay time indicator and TR 2 is the above commented time period of 150 ms. The detection rules are defined as follows: - A1 is triggered when the condition expressed in Eq. (2) is true. - A2 is triggered when A1 has been triggered and when the peak current indicator exceeds 20%. - A3 is triggered when A2 has been triggered and the peak current indicator exceeds 20% again. Applying the propositions above, a specific application has been developed employing LabviewTM Software. This application does not take into account the specific type of test that it is being analysed. It means that all the defined indicators are evaluated at any case. Therefore, theoretically more than one alarm can be activated at the same time.
6.
Applications examples
In this section, industrial application examples are performed in order to validate the methodology proposed in this paper.
Fig. 9 – Signals behaviour in unstable cutting (RULES 1).
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Acknowledgements It is gratefully acknowledged the financial support of the University of the Basque Country (Project 1/UPV00146.363-T15319/2003) and of the Department of Education, Universities and Research of the Basque Country Government (research fellowship BFI04.383).
Appendix A The generic algorithm that defines the evolution of each wire breakage indicator is the following:
Fig. 10 – Signals behaviour in unstable cutting (RULES 2).
application triggers A1 until the wire breaks is around 200 ms. This result is considerably higher than the period of time of 20–50 ms considered in previous researches (Obara et al., 1990). Next example shows the results obtained during another test in which wire breakage is caused by poor flushing in straight cutting. During this test, the alarms related to RULES 2 have been triggered. Fig. 10 depicts also 500 ms before wire breakage. In this case, the time elapsed since the application triggers A1 until the wire breaks is shorter than the one in RULES 1 case (approximately 120 ms).
(1) For each basic window of size M, the total number of discharges MT and the number of discharges whose basic variables BV (Ipi , Ei or tdi ) exceeds the respective reference value M(BV) are computed:
M(BV) =
M
pj , where p
j=0
1, if BV ≥ RV 0, if BV < RV
(3)
(2) For the first sliding window of size N, compute the total number of discharges NT and the number of discharges whose basic variables BV (Ipi , Ei or tdi ) exceeds the respective reference value NBV :
NBV =
N
M(BV)j
(4)
j=1
7.
Conclusions and future work NT =
This paper proposes a methodology that allows to configure a SW system that uses commercial hardware in order to develop flexible industrial applications. The methodology has been followed as applied to process instability and wire breakage detection in WEDM. First, an acquisition system has been developed aimed at storing an extensive experimental database based on stable and unstable tests. The results of a preliminary analysis of a set of tests have revealed the influence on wire breakage of discharge variables, such as peak current, discharge energy and ignition delay time. Related to these discharge variables, wire breakage indicators have been defined. Based on the analysis of the indicators, two set of heuristic rules have been deduced. Each set of rules consists of three levels of alarm which warn about the increasing risk of wire breakage. The results of the empirical analysis have been implemented in an industrial application for on-line detection of instabilities and wire breakage. Future work will involve the development of a real-time control strategy so as different parameters of the machine are automatically readjusted. In this respect, the number and thresholds of alarms can be extended in order to assure recovering of stable operation. Therefore, the final aim is to increase cutting process performance while avoiding wire breakage.
N
MTj
(5)
j=1
where j = 1, 2, . . . N (3) For the following sliding windows of size N, compute the sliding calculation: (3.1) NBVj+1 = NBVj + M(BV)j+1 − M(BV)j−N
(6)
(3.2) NTj+1 = NTj + MTj+1 − MT
(7)
pN(reference value)j+1 =
NBVj+1 NTj+1
(8)
where j = 1, 2, . . ..
references
Cooke, R.F., Crookall, J.R., 1973. An investigation of some statistical aspects of electro-discharge machining. Int. J. Machine Tool Des. Res. 13, 271–286. Dekeyser, W., Snoeys, R., Jennes, M., 1988. Expert system for WEDM, based on pulse classification and thermal modelling. Rob. Comput.-Integr. Manuf. 4, 219–224. Kunieda, M., Saga, S., Yoshino, H., Ohta, T., Kobayashi, M., 2001. Control of discharge locations in EDM with locally imposed high electric field. Proc. XIII ISEM 2, 485–495.
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Lab View Measurements Manual, July 2000. Lauwers, B., Kruth, J.P., Bleys, P.H., Van Coppenolle, B., Stevens, L., Derighetti, R., 1999. Wire rupture prevention using on-line pulse localisation in WEDMB. VDI Berichte 1405, 203–213. Liao, Y.S., Woo, J.C., 1997. The effects of machining settings on the behaviour of pulse trains in the WEDM process. J. Mater. Process. Technol. 71, 433–439. Liao, Y.S., Chu, Y.Y., Yan, M.T., 1997. Study of wire breaking process and monitoring of WEDM. Int. J. Machine Tools Manuf. 37 (4), 555–567. Obara, H., Abe, M., Ohsumi, T., 1999. Control of wire breakage during wire EDM. Int. J. Electr. Machining 4, 53–58. Obara, H., Okuyama, Y., Komeya, M., Ioka, T., 1990. Study on detection of EDM discharging position. JSEME 24 (47), 12–22. ´ Portillo, E., Cabanes, I., Marcos, M., Orive, D., Sanchez, J.A., 2004a. Study of pulse instability in wire-electro discharge machining process. Ann. DAAAM 2004 & Proc., 371–372.
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´ Portillo, E., Cabanes, I., Marcos, M., Orive, D., Sanchez, J.A., 2004b. ´ ´ del proceso de corte por Analisis de la inestabilizacion ´ por hilo. In: XV Congreso Maquinas´ electroerosion ´ 1, pp. 95–111, in Herramienta y Tecnolog´ıas de Fabrication Spanish. Shoda, K., Kaneko, Y., Nishimura, H., Kunieda, M., Fan, M.X., 1995. Development of adaptive control system to prevent EDM wire breakage. EDM Technol. 3, 17–22. Wang, W.M., Rajurkar, K.P., 1992. Monitoring sparking frequency and predicting wire breakage in WEDM. Sensors and Signal Processing for Manufacturing, vol. 55. ASME, pp. 49–64. Watanabe, H., Sato, T., Suzuki, I., Kinoshita, N., 1990. WEDM monitoring with a statistical pulse-classification method. Ann. CIRP 39/1, 175–178. Wu, J., Li, M.H., 2001. The identification of the servo control state in wire electrical discharge machining process. ISEM XIII, 423–433. Yan, M.T., Liao, Y.S., 1995. Adaptive control of WEDM process using the fuzzy control strategy. ISEM XI, 343–352.
journal homepage: www.elsevier.com/locate/jmatprotec
An industrial application for on-line detection of instability and wire breakage in wire EDM b ´ I. Cabanes a,∗ , E. Portillo a , M. Marcos a , J.A. Sanchez a
Department of Automatic Control and System Engineering, E.T.S.I. of Bilbao, University of the Basque Country, Alameda Urquijo s/n, 48013, Bilbao, Spain b Department of Mechanical Engineering, E.T.S.I. of Bilbao, University of the Basque Country, Alameda Urquijo s/n, 48013, Bilbao, Spain
a r t i c l e
i n f o
a b s t r a c t
Article history:
One of the main challenges in wire electrical discharge machining (WEDM) is avoiding wire
Received 30 August 2006
breakage and unstable situations as both phenomena reduce process performance and can
Received in revised form
cause low quality components. This work proposes a methodology that guarantees an early
15 December 2006
detection of instability that can be used to avoid the detrimental effects associated to both
Accepted 19 April 2007
unstable machining and wire breakage. The proposed methodology establishes the procedures to follow in order to understand the causes of wire breakage and instability. In order to quantify the trend to instability of a given machining situation, a set of indicators related to
Keywords:
discharge energy, ignition delay time, and peak current has been defined. Wire breakage risk
WEDM
associated to each situation is evaluated comparing the evolution of those indicators with
Analysis
some previously defined threshold values. The results of this work will be used to develop
Diagnosis
a real-time control strategy for increasing the performance of the WEDM process.
Instability
© 2007 Elsevier B.V. All rights reserved.
Wire breakage Detection
1.
Introduction
WEDM is one of the most extended non-conventional machining processes. It is widely used to machine dies and moulds aimed at producing components for many industries. The main advantage of WEDM is its capability for the production of high complexity shapes with a high degree of accuracy, independently of the mechanical properties of the material (especially, hardness, brittleness and resistance). WEDM is based on material removal through a series of electrical discharges applied between two electrically conductive electrodes—workpiece and wire. Dielectric fluid is injected into the gap, which is the space between electrodes.
∗
Corresponding author. Tel.: +34 94 601 39 51; fax: +34 94 601 41 87. E-mail address: [email protected] (I. Cabanes). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.04.125
Thus, there is no contact between tool and workpiece during the process. Each discharge is generated as follows: the WEDM machine power supply applies a voltage between workpiece and wire. It starts the ionisation period of the dielectric fluid, which is known as ignition delay time. Dielectric ionisation induces the discharge that vaporises all the material around. Before applying the voltage for the next discharge, the dielectric cools the gap and removes the erosion debris during a period of time known as off-time. A servo control is used to control the gap size. Most of servo control systems take the discharge voltage as the feedback signal. On the other hand, the wire is continuously running at a constant speed. Fig. 1 shows a schematic diagram of the WEDM process.
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Nomenclature A1 A2 A3 BV Ei Eref Ipi Ip ref M N pN (Eref ) pN (Ip ref ) pN (td ref ) tdi td ref TR 2
low level alarm medium level alarm high level alarm basic variable energy of the discharge i energy reference value peak current of the discharge i peak current reference value basic window sliding window energy indicator peak current indicator ignition delay time indicator ignition delay time of the discharge i ignition delay time reference value time period for pN (td ref )
Wire breakage and unstable machining are two of the most important aspects of the WEDM process due to their detrimental effects, such as reduction of machining performance and surface damage (Wang et al., 1992). Different factors can lead to wire breakage, such as the generation of shortcircuits, inefficient removal of erosion debris as well as other types of stochastic phenomena that appear during the cutting process. Usually, if an abnormal situation is detected, the machine operator manually adjusts the parameters of the machine using his/her own experience. Moreover, WEDM machines work most of the time without operator assistance. Therefore, it would be desirable to predict wire breakage and unstable machining in order to on-line readjust the machine parameters. In accordance with this objective, this paper proposes a methodology for on-line detection of instability and wire breakage. The methodology is based on acquiring knowledge of the process through an exhaustive experimental database containing data collected from both stable and unstable machining situations. The layout of the paper is as follows: Section 2 presents a brief review of previous work. Section 3 describes the acquisition system designed to capture the required signals. Section 4 explains the generation of the experimental database. Section 5 is dedicated to the definition of wire breakage indicators and to the establishment of heuristic rules for the setting of different alarm levels. Section 6 presents some industrial application examples. Section 7 provides a summary and some concluding remarks.
Fig. 1 – Wire electrical discharge machining process.
2.
Literature review
The study of different aspects that have influence on the performance of electrical discharge machining (EDM) process has been carried out from the beginning of this type of machining technology (Cooke and Crookall, 1973). In fact, it is still difficult to understand all the aspects concerning EDM due to its strong stochastic nature and the multiple parameters of the process. One of the great challenges in WEDM is to increase the grade of stability by avoiding wire breakage. This aim should be achieved for any workpiece thickness, workpiece material (such as ceramics, steel and other metals), machine power supply, etc. In this respect, this paper proposes to acquire online knowledge of the behaviour of the discharges in the gap in order to optimise the control strategy of the process. Some research lines can be clearly identified in the literature survey. In most of these works the capture of voltage and/or current signals is carried out because these signals provide worthy information related to the gap stability. Different authors have employed those signals for different aims, such as discharges classification, discharges location and the estimation of the thermal load on the wire. One of the most interesting lines is based on detecting the discharge position on the wire. The concentration of successive discharges at one point of the wire can lead to wire breakage. In this way, many authors construct an equivalent circuit of a standard EDM machine in order to develop monitoring systems for discharge location (Obara et al., 1999; Shoda et al., 1995; Lauwers et al., 1999; Obara et al., 1990; Kunieda et al., 2001). Some of the authors have considered the discharge position in the design of mathematical thermal models. The wire breakage is detected when a temperature threshold is exceeded (Lauwers et al., 1999). Obara et al. (1990) studied three gap monitoring signals depending on discharge position (ignition delay time, discharge voltage and a radio signal) in order to predict wire breakage. All the signals from a transistor controlled WEDM machine were elongated by a sample-andhold circuit and recorded in a digital oscilloscope. This work concluded that a decrease in discharge voltage was always observed before wire breakage. Another interesting research line considers discharge classification in order to ensure machining stability. Dekeyser et al. (1988) developed an electronic device (pulse discriminating system) to distinguish 13 different types of discharges. In order to study the behaviour of current and voltage before wire breakage, the authors carried out tests for provoking wire breakage by increasing the pulse frequency. One of the main contributions of this work was that the succession of pulses of some specific type has influence on wire breakage. Afterwards, Watanabe et al. (1990) reduce the discharge classification to four basic types: normal, arcing, shortcircuits and open-circuit discharges. This classification is the most commonly used nowadays. Each type of discharge is defined by establishing threshold values for discharge current and/or discharge voltage. In this respect, another pulse discriminating system was developed by Yan and Liao (1995). Discharge classification was established only using discharge voltage. This system presents some restrictions. First, the sampling time or sliding window
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parameter defined to count each type of discharges was limited to a lowest value of 0.55 ms. In addition, it had to be selected before the on-line acquisition of the experiment. Second, the recording space was restricted to 4 MB. Later, the same working group observed two types of wire breaking phenomena: the first type is the sudden rise of the sparking frequency and the second type is related to the slight increase in sparking frequency and to the increase in the ratio of arcs and shortcircuits (Liao et al., 1997; Liao and Woo, 1997). Recently, Wu and Li (2001) have utilized the discharges classification to establish a new servo control strategy in a new prototype of EDM machine (High Speed WEDM). In this case, the selected sliding window of the pulse discriminating system was 10 ms and the data recorded continuously was between 6000 and 9000 samples, which correspond to 60–90 s. In other works, discharge current and/or discharge voltage have been acquired and analysed to detect wire breakage without discharge categorising. An example of this trend is the research carried out by Wang and Rajurkar (Wang et al., 1992). In this work, a sparking frequency monitor was developed. It was used to count discharges along a sliding window, which can be set in the range of 10 s–655.35 ms. In this case, the recording space is limited to 4096 points. The authors established two indicators related to the temperature of the wire. The first of them is the average sparking frequency, since a higher average sparking frequency increases the thermal load on the wire. Therefore, the wire breaks when the average sparking frequency exceeds an established level. The second indicator is based on the variation of the sparking frequency along a specific time period obtained empirically. In this case, wire breakage is caused by the high thermal energy density per unit length of the wire. As shown in the related works, the HW detecting systems developed for the analysis of wire breakage presents the commented limitations related to the storing capacity and sliding window tuning. In fact, the off-line selection of the sliding window in the literature review has been normally defined by the try-and-error method. This implies repeating the experiment until the proper sliding window is found. Moreover, the time invested in the design and development of proprietary HW systems is significant. On the other hand, most of the systems have been designed for specific conditions, such as workpiece material, machine power supply and machining parameters. Therefore, the system should be re-designed in order to analyse other operational conditions. Thus, this paper proposes a methodology that allows to configure a SW system that uses commercial hardware in order to develop flexible industrial applications. It has been applied to the on-line detection of instabilities and wire breakage in WEDM. The stochastic nature of this process has led to the definition of different levels of alarms triggered by the application. Subsequently, control algorithms could use the levels of the alarms in order to perform the control strategy. Recent technological advances in commercial off the shelf (COTS) HW and SW components, allow to employ more powerful systems that provide better performance. Hence, the developed application uses an acquisition system based on a commercial data acquisition PC Board. It provides continuous sampling of the complete current and voltage signals at a very
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high sample rate. The acquisition system can be integrated into a real-time processing system. Among its advantages, a higher flexibility stands out due to many factors. First, the system can be easily employed under different experimental conditions (power supply, workpiece material . . .). Second, due to the high rate of acquisition it allows to record the complete signals of the process. Therefore, a preliminary analysis of the different variables proposed by the different authors has been performed aimed at the selection of the most appropriate to anticipate wire breakage (Portillo et al., 2004a,b). Finally it is remarkable the flexibility of the system in the selection of the sliding window since it is selected after storing the tests. This avoids repeating the same type of test with different sliding windows. The analysis of WEDM process is based on an exhaustive experimental database that contains information about both stable and controlled unstable operation. This extensive database has been found as a key factor in the definition of global and quality rules to detect wire breakage. In fact, the results of the empirical analysis of the experimental database have showed new situations of wire breakage. Thus, the methodology involves the definition of an acquisition system, the design and the obtaining of an experimental database, a preliminary analysis for the selection of appropriate variables, the establishment of wire breakage indicators, the definition of different wire breakage alarm levels from a set of heuristic rules and, finally, the development of the industrial application and the subsequent validation.
3.
Acquisition system
An acquisition system has been designed to capture discharge current and discharge voltage in order to obtain an extensive experimental database. The evolution of these two signals for stable and unstable tests will allow to characterise both cutting regimes. Since the acquisition system captures the complete signals, it can be utilized when new conditions of workpiece material, machine power supply or machining parameters are applied. Considering current and voltage signals characteristics, the requirements of the acquisition system are the following: continuous-time acquisition (without loss of information), minimum sample and storage rate of 5 M samples/s per channel, two analogue input channels with independent resolution, and finally, minimum input range of +1 V for current signal and +10 V for voltage signal. The high acquisition frequency and simultaneously data storage without loss of information reduce drastically the commercially available hardware and software for the acquisition system. After comparison among different alternatives, the NI 6115 PC Board with PCI bus has been selected as it meets all the requirements. Fig. 2 shows the block diagram of the acquisition system. Discharge voltage and discharge current signals are transferred to the BNC adaptor, which is attached to NI 6115 data acquisition board through PCI bus. LabviewTM high speed libraries (Lab View, 2000) have been used for the development of two software applications required for exploiting the acquisition system. One of them
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establish reference values for a set of discharge parameters. When the reference values are not followed, a transition to unstable machining can be detected. Brass wire of 0.25 mm diameter (DIN 160) and 50 mm height (AISI D2) tool steel work parts have been used. The tests have been performed on an ONA E-250 wire EDM machine. Stable cutting tests correspond to straight rough cutting using the parameters obtained from machine table look-up, as Fig. 3 illustrates. The definition of unstable cutting tests considers three different causes of instability. The general purpose of these tests is to provoke cutting instabilities and wire breakage (see Fig. 3):
Fig. 2 – Acquisition system. • Poor flushing: poor flushing can produce an insufficient cooling of the gap as well as a greater quantity of debris. These factors can
performs the real-time acquisition and storage at the required sample rate (5 M samples/s per channel). The second application displays off-line graphs of the acquired signals. It can show any selected 200 s frame of the test file. A typical continuously stored test can last from 4 to 60 min. The application also records test header and parameters.
4.
increase instabilities and wire breakage risk. Thus, in order to provoke cutting instabilities and wire breakage due to lack of dielectric, stair-shaped pieces have been designed. • Complex geometries: these tests provide unstable cutting behaviour due to a change in the direction of movement of the wire. The cut angles range from 15◦ to 120◦ . • Variation of sparking frequency: the increase in sparking frequency is applied by decreasing the value of the off-time parameter. Therefore, the time between discharges is reduced. This means that the time used for cleaning and cooling the gap also decreases while the density of discharges increases. All these factors converge on
Experimental database
In the present work, the obtaining of an exhaustive experimental database is a key factor in order to define global and quality indicators. Thus, a battery of tests that provides a wide variety of sources of instabilities has been designed. Both types of regimes, stable and unstable, have been considered. The stable cutting tests are used to
a higher risk of wire breakage. These tests have been performed on straight and corners cutting.
Fig. 4 shows some workpieces resulting from the stable and unstable tests.
Fig. 3 – Experimental database.
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Fig. 4 – Workpieces resulting from stable and unstable tests.
5.1. Establishment of indicators for wire breakage monitoring
5. Instability and wire breakage on-line detection As mentioned in the literature review, the authors have used different variables (such as energy, peak current, discharge voltage, ignition delay time, discharges frequency, . . .) aimed at different objectives: discharges classification, wire breakage analysis, establishment of servo control strategies, etc. According to the authors contributions, the first step proposed is to select among those different variables the most suitable to anticipate wire breakage in an industrial application. Since the acquisition system captures the complete information of the signals, it has been possible to join all the variables contributed by each author in a unique analysis application. This preliminary analysis has been carried out on a set of tests defined in Section 4. Definition 1. Analysis time interval is the period of time in which one point of the wire can be exposed to discharges during the time required to pass through the workpiece. It depends on the wire feed and on the workpiece thickness. As the workpiece thickness is known (50 mm) and the wire feed is one of the adjustable machining parameters, the time interval to be analysed before wire breakage is calculated. Its value is 333 ms. This means that the time interval in which wire breakage should be avoided is significantly restrictive. Thus, the indicators will be obtained by an increasing calculation method in order to deal with the time constraints. During this preliminary analysis, two different tendencies towards wire breakage have been observed. The first tendency shows a sharp increment of discharges energy. The second one consists of a sharp fluctuation and increment of discharges current, as well as a considerable increment of ignition delay time. According to these results, the selected variables for the definition of wire breakage indicators are the following:
In this section, the wire breakage indicators are established based on the above selected variables. Taking into account the two tendencies observed in the preliminary analysis, it is deduced that high values of peak current, ignition delay time and energy are clearly relevant before wire breakage. Therefore, reference values must be set in order to discriminate high values of the variables (Ip ref , td ref and Eref ). Definition 2. Each reference value, Ip ref , td ref and Eref , is defined as the class when the relative cumulative frequency achieves 99% in stable cutting. Fig. 5 shows an example of the frequency distribution employed to define td ref . Definition 3. Wire breakage indicators, pN (Ip ref ), pN (td ref ) and pN (Eref ), are defined as a succession of percentages of discharges that exceed, respectively, the pre-defined reference values in a time period N. The percentages are continuously calculated along time. Peak current indicator is designated as pn (Ip ref ), ignition delay time indicator as pN (td ref ) and energy indicator as pN (Eref ). Definition 4. Sliding window N is the time period in which each percentage value is computed.
• Peak current (Ipi ) of the discharge i • Ignition delay time (tdi ) of the discharge i • Energy (Ei ) of the discharge i:
Ei =
tei
Ipi Ui dt
(1)
0
where tei is the duration of the discharge i and Ui is the discharge voltage.
Fig. 5 – Frequency distribution of ignition delay time in stable cutting.
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Fig. 7 – Calculation of pN (Eref ) with and without the basic window M.
uous line delimits the analysis time interval (333 ms). Clearly, a value of N equal to 5 ms is discriminatory. Definition 5. N is divided into sub-windows, called basic windows M. When the newest basic window M fills up, it is appended to the sliding window, the oldest basic window is removed, and statistics over the sliding window are recomputed.
Fig. 6 – Evolution of pN (Ip
ref )
with different values of N.
The sliding window N has been established using a heuristic method. On one hand, the criteria applied is that if N is too small, the successive percentages fluctuate excessively avoiding to distinguish relevant from irrelevant increases. On the other hand, the longer N, the less sensitive or the more smoothed the successive percentages will be. Consequently, relevant increases would not be easily detected. Moreover, N should be small enough to reduce the size of the employed register to store data. Therefore, all these compromises have been considered for the tuning of N. To illustrate this, Fig. 6 shows an example of the evolution of one of the indicators 2.5 s before wire breakage for different values of N. The vertical discontin-
In order to make the calculation of the wire breakage indicators independent from the starting time of the signal processing, as well as to reduce the delay for detection of wire breakage, the indicators are obtained each basic window M. Thus, it should be short enough. Otherwise, M should be high enough to obtain a representative sample to calculate the successive percentages (more than one discharge). In order to achieve this compromise M is set at N/5. Fig. 7 gives a schematic example of the calculation of the energy indicator with and without M. It shows that if M is not used it is possible to carry out an erroneous detection of wire breakage. The generic algorithm that defines the evolution of each wire breakage indicator is shown in the appendix section.
5.2. Heuristic rules for the definition of different wire breakage alarms In the establishment of a set of heuristic rules to anticipate wire breakage, is essential to employ the complete experimental database. In this way, the industrial application developed in this work will enable to increase the number of correctly detected wire breakage cases. The heuristic rules have been empirically deduced through the analysis of the behaviour of the wire breakage indicators.
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• RULES 1 refers to the set of rules that anticipate a sharp increment of discharges energy. RULES 2 is related to the detection of a sharp fluctuation and an increment of peak current, as well as a considerable increment of ignition delay time. Three levels of alarm have been defined for each case RULES 1 and RULES 2: - A1 : low level alarm. - A2 : medium level alarm. - A3 : high level alarm. Low level alarms (A1 ) refers to an alerting but not very dangerous situation. If a low level alarm is activated, other tasks related to the medium level alarms are executed. Medium level alarms (A2 ) are related to unstable machining. Finally, high level alarms (A3 ) warn about the probable wire breakage. • RULES 1: the analysis of the wire breakage tests related to this case has showed that the rise time of the energy indicator from 30 to 70% ranges from 2 to 3 ms. Thus, the detection rules are defined as follows: - A1 is triggered when the energy indicator exceeds 30%. - A2 is triggered when the energy indicator exceeds 50%. - A3 is triggered when the energy indicator exceeds 70% and the rise time from 30% (A1 ) to 70% is lower than a time period equal to 3 ms. • RULES 2: in this case, the analysis has revealed that the level of ignition delay time indicator is high 150 ms before the beginning of the instabilities prior to wire breakage. Moreover, the peak current indicator exceeds 20% more than once during a maximum time interval of 100 ms. Both time intervals have been set by the analysis of the corresponding wire breakage tests. As the high level of ignition delay time indicator is concerned, it is detected by the cumulative sum of the ignition delay time indicator:
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Fig. 8 – Signals behaviour in stable cutting.
First, stable cutting tests have been analysed by the developed application. Fig. 8 depicts the behaviour of the above specified indicators during 500 ms in stable cutting. Although it is difficult to distinguish each evolution of pn (Ip ref ), pN (td ref ) and pN (Eref ), the Fig. 8 shows that the wire breakage indicators fluctuate maintaining below 10%. Thus, the alarms are not triggered, which is obviously expected during stable cutting. Fig. 9 shows the application results obtained from one of the tests defined for the variation of sparking frequency in straight cutting. In this example, the alarms related to RULES 1 have been triggered. The time depicted is 500 ms before wire breakage. The vertical discontinuous line delimits the analysis time interval. The black circles represent the moments in which A1 , A2 and A3 are activated. The time elapsed since the
i=TR 2 +nM
g(i) ≥ 2
(2)
i=nM
where g(i) are the successive values of the ignition delay time indicator and TR 2 is the above commented time period of 150 ms. The detection rules are defined as follows: - A1 is triggered when the condition expressed in Eq. (2) is true. - A2 is triggered when A1 has been triggered and when the peak current indicator exceeds 20%. - A3 is triggered when A2 has been triggered and the peak current indicator exceeds 20% again. Applying the propositions above, a specific application has been developed employing LabviewTM Software. This application does not take into account the specific type of test that it is being analysed. It means that all the defined indicators are evaluated at any case. Therefore, theoretically more than one alarm can be activated at the same time.
6.
Applications examples
In this section, industrial application examples are performed in order to validate the methodology proposed in this paper.
Fig. 9 – Signals behaviour in unstable cutting (RULES 1).
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Acknowledgements It is gratefully acknowledged the financial support of the University of the Basque Country (Project 1/UPV00146.363-T15319/2003) and of the Department of Education, Universities and Research of the Basque Country Government (research fellowship BFI04.383).
Appendix A The generic algorithm that defines the evolution of each wire breakage indicator is the following:
Fig. 10 – Signals behaviour in unstable cutting (RULES 2).
application triggers A1 until the wire breaks is around 200 ms. This result is considerably higher than the period of time of 20–50 ms considered in previous researches (Obara et al., 1990). Next example shows the results obtained during another test in which wire breakage is caused by poor flushing in straight cutting. During this test, the alarms related to RULES 2 have been triggered. Fig. 10 depicts also 500 ms before wire breakage. In this case, the time elapsed since the application triggers A1 until the wire breaks is shorter than the one in RULES 1 case (approximately 120 ms).
(1) For each basic window of size M, the total number of discharges MT and the number of discharges whose basic variables BV (Ipi , Ei or tdi ) exceeds the respective reference value M(BV) are computed:
M(BV) =
M
pj , where p
j=0
1, if BV ≥ RV 0, if BV < RV
(3)
(2) For the first sliding window of size N, compute the total number of discharges NT and the number of discharges whose basic variables BV (Ipi , Ei or tdi ) exceeds the respective reference value NBV :
NBV =
N
M(BV)j
(4)
j=1
7.
Conclusions and future work NT =
This paper proposes a methodology that allows to configure a SW system that uses commercial hardware in order to develop flexible industrial applications. The methodology has been followed as applied to process instability and wire breakage detection in WEDM. First, an acquisition system has been developed aimed at storing an extensive experimental database based on stable and unstable tests. The results of a preliminary analysis of a set of tests have revealed the influence on wire breakage of discharge variables, such as peak current, discharge energy and ignition delay time. Related to these discharge variables, wire breakage indicators have been defined. Based on the analysis of the indicators, two set of heuristic rules have been deduced. Each set of rules consists of three levels of alarm which warn about the increasing risk of wire breakage. The results of the empirical analysis have been implemented in an industrial application for on-line detection of instabilities and wire breakage. Future work will involve the development of a real-time control strategy so as different parameters of the machine are automatically readjusted. In this respect, the number and thresholds of alarms can be extended in order to assure recovering of stable operation. Therefore, the final aim is to increase cutting process performance while avoiding wire breakage.
N
MTj
(5)
j=1
where j = 1, 2, . . . N (3) For the following sliding windows of size N, compute the sliding calculation: (3.1) NBVj+1 = NBVj + M(BV)j+1 − M(BV)j−N
(6)
(3.2) NTj+1 = NTj + MTj+1 − MT
(7)
pN(reference value)j+1 =
NBVj+1 NTj+1
(8)
where j = 1, 2, . . ..
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