Tài liệu Cảm biến trong sản xuất P9 pdf

4.2 Metal Forming E. Doege, F. Meiners, T. Mende, W. Strache, J. W. Yun, Institute for Metal Forming and Metal Forming Machine Tools, University of Hannover, Germany The profitable use of advanced monitoring systems is more and more integrated in modern mass manufacturing processes since reliable equipment is available. The idea is to improve the metal forming process due to the high availability of tools and machines by decreasing machine setup and failure times. Therefore, it is important to employ new sensor technologies in metal forming systems for the observation of process signals. 4.2.1 Sensors for the Punching Process In the last 30 years, enormous improvements have been achieved in the stamping process concerning economic production, accuracy and possible shape of the parts [1]. Today’s tools are more sophisticated and more expensive. The costs of a mod- ern multi-stage tool can be more than $ US 100000 and requires constant process monitoring to achieve high availability of the tool. This aspect is very important for the trend of just-in-time production. Also, customer requirements for 100% quality control can be fulfilled with indirect quality control by the process signals. Therefore, the demand for tool safety devices and process control units is increas- ing constantly [2]. Traditional limit switches [3] are not sufficient. The manufac- turers’ expectations for modern process control systems are as follows: · complete quality assurance and documentation (100% indirect product quality control); · protection of expensive and complex multi-stage tools against breakage and subsequent damage; · machine overload protection; · detection of feeding faults; · extended production time with no supervision (ghost shifts); · decrease of setup times and support with stored parameters; · fewer production stoppages by premature recognition of process disturbances; · permanent process monitoring to support the user with process information to permit optimal process setting; · higher press speeds to increase productivity; · control of existing tools; · no sensor handling in the tooling room. To fulfil all these requirements, the process control system should have sensors which are sensitive enough to recognize the disturbances and they must guaran- tee easy handling in daily production (no cables in the tool room). The signal pro- cessing must also be very sophisticated to detect breakage, wear, and process trends. 4 Sensors for Process Monitoring172 Sensors in Manufacturing. Edited by H.K. Tönshoff, I. Inasaki Copyright © 2001 Wiley-VCH Verlag GmbH ISBNs: 3-527-29558-5 (Hardcover); 3-527-60002-7 (Electronic) 4.2.1.1 Sensors and Process Signals The most common process signals for the monitoring of the punching process and the press load [5] are forces and acoustic emissions. Both signals include pro- cess information, which can be controlled or analyzed by process monitoring de- vices. In addition, these devices need one of the following signals as a reference basis for the monitoring or analysis: · time; · crankshaft angle; · slide path. Normally the time signal is used as the reference base for process monitoring/ control. However, the time base depends on the press speed. If the press changes speed and if the signal is controlled by the window or the tolerance band tech- nique [6, 7], the time-based signal will vary and could cause a press stop. In this case a crankshaft resolver with a high resolution at the lower dead center will be used. A linear distance sensor at the press slide can alternatively be used as a ba- sis for the process signals. The disadvantage of the linear distance sensor is the low resolution at the lower dead center, because of the sine shape of the slide path. In Figure 4.2-1 the typical process signals of a punching process are shown. Acoustic Emission Sensors Short-term disturbances (tool breakage or cracks in the product material) can be easily detected by acoustic emission sensors [6–8]. This sudden change in the press load produces an acoustic emission signal up to 150 kHz, traveling through the tool and the machine. Most importantly, acoustic emission sensors should be placed as close as possible to the metal forming process to avoid disturbances (the 4.2 Metal Forming 173 Fig. 4.2-1 Typical signals of the punching process machine’s vibrations). Each mechanical contact (gap) between the forming pro- cess and the acoustic emission sensors filters the acoustic emission spectrum as a low pass. Therefore, the acoustic emission sensors should be placed into the tool [6] or next to the tool. In Figure 4.2-2 a piezoelectric sensor is shown with a very wide transmission band, which enables the sensor to measure acoustic emission signals in the 100 kHz range, because the piezoelectric element is mounted in a damping mass with no seismic mass (no resonance). Force Sensors The most important process signals are the signals of the force sensors (see Fig- ure 4.2-3), which are placed in the structure or on the surface. Piezoelectric force sensors or piezoelectric transverse measuring pins are mostly used in the struc- ture. On the surface the common devices are piezoelectric or resistive strain gages. A later calibration of all these sensors is necessary, because the strain and the sensitivity of the sensors depend on the surrounded structure of the machine or tool. Existing monitoring systems are mostly based on simple force monitor- ing. The force signal is mainly used for process monitoring. When the adjusted force limit is exceeded, the machine will automatically be stopped by the emer- gency stop. 4.2.1.2 Sensor Locations In Figure 4.2-4 the most common sensor locations for the punching process con- trol are shown. Acoustic emission sensors must be placed very close to the pro- cess. Typical locations for the acoustic emission sensors are the upper and the lower tool or the slide and the table. A greater distance to the process will in- crease the noise signal by the press. The force signal is normally measured by sensors which are placed in the press frame, the connecting rods, the slide or directly in the tool [9]. Some presses are equipped by the press manufacturer with sensors to protect the press against 4 Sensors for Process Monitoring174 Fig. 4.2-2 Acoustic emission sensor (Kistler Instrumente AG) force overload. The distance between these sensors, which are placed in/on the press frame or the connecting rod, and the forming process is too large to detect more than the force overload. The signals of these force sensors and the acoustic emission sensor underlie many disturbances, eg, the press drive and vibrations. The best sensor signals can be obtained when the force sensors are directly placed in the tool (see Figure 4.2-5). The second best solution for the signal quality is to place the force sensors directly above or under the tool. See the sensor plate and table locations in Figure 4.2-4. 4.2 Metal Forming 175 Fig. 4.2-3 Force and strain sensors for process control (Kistler Instrumente AG) Fig. 4.2-4 Possible sensor locations at a forming press [9] 4.2.1.3 Sensor Applications In this section, sensor applications, which are close to the forming process, will be described in detail. The integration into the top plate of the upper tool is shown in Figure 4.2-5. With this application a single forming operation can be perfectly monitored. The influence of a neighboring forming operation on the measured force signal is very low. Typical sensors for this application are piezo- electric transverse measuring pins or force rings, because the sensors are placed in the structure. The total or a part of the forming operation force is transmitted and measured by the sensors. A disadvantage is the large number of expensive sensors in a tool and the bad tool handling in daily production. The very rough environment in the tool shop also complicates the handling of the tools with sen- sitive sensor cables. Better tool handling and lower sensor costs can be achieved when the sensors are integrated into machine parts or remain at the press structure. One solution, which was presented by Terzyk et al. [6], is the integration of force sensors into the slots of the press table. In Figure 4.2-6, two slot force sensors are shown, 4 Sensors for Process Monitoring176 Fig. 4.2-5 Force sensors integrated into the upper tool [6] Fig. 4.2-6 Table slot force sensors [6] which are placed under the lower tool. The advantage of this solution is the high flexibility and the integration into existing processes, because the shape of the ta- ble slots is standardized. On the other hand, the slots must be cleaned and must have straight surfaces. These sensors cannot be placed in the center of the tool, because there are holes in the table in this area for scrap transportation. A good combination of process-sensitive signals and good handling is achieved by a multi-sensor plate, which is placed between the slide and the upper tool. In Figure 4.2-7 the scheme of the multi-sensor plate is presented. The multi-sensor plate consists of a frame plate, which has the same shape as the slide, and several sensor cassettes, which contain force and acoustic emission sensors. The follow- ing requirements are the basis for the development of the system: · easy handling in the production workshop; · short distance to the process; · integration of several force sensors for detailed process monitoring; · connection devices for additional sensors; · improved process control by a combined force/acoustic emission monitoring; · modular design for high flexibility; · integration into existing tool-press systems. Easy handling is solved by using a modular cassette system, which is fixed by a frame plate and two guiding rails to the press slide (Figure 4.2-7). During a tool change the sensors will remain at the slide. All cables between the cassettes and the docking station are integrated in the frame plate. Because of the modular de- sign, the multi-sensor plate can easily be adapted to the requirements of the user. The number and the locations of the standardized sensor cassettes can be changed. The docking station houses the charge amplifier and the connectors for additional sensors and is mounted on the frame plate. The frame plate has a height of 25 mm and the same shape as the slide, so that the tools can be fixed to the slide in the usual way. 4.2 Metal Forming 177 Fig. 4.2-7 Scheme of the multi-sensor plate [9] A multi-sensor plate with four cassettes and the docking station for a 500 kN press is shown in Figure 4.2-8. Some typical process signals measured with the multi-sensor plate are pre- sented below. A production tool with 11 forming operations (cutting, deep draw- ing, stamping) separated into four modules will be analyzed by means of the mul- ti-sensor plate. The workpiece, the tool setup, the force and acoustic emission sig- nals are shown in Figure 4.2-9. The two force cassettes of the multi-sensor plate are placed above the first and above the last (fourth) module to demonstrate the local resolution of the system. The acoustic emission cassette is placed in the mid- dle of the tool. The measured signals contain information on the cutting/forming process, on the blank holder and on the tool stop reaction. The contact of the blank holder oc- curs at point A in the force diagram and at point 1 in the acoustic emission dia- gram. Characteristic cutting operations can be identified at B/2 and C/3. The re- sulting cutting impact is very significant in the acoustic emission signal (peaks 2 and 3). Owing to an incorrect slide height (too tight), the upper tool is running on the stops of the lower tool (impact at D/4). The tight tool mounting causes an increase in the force signals up to point E. The force signal above the first mod- ule is higher than that above the last module, because the stops are in the first two modules of the tool (four modules). The lower dead center is reached at point E (highest force signal). The lift-off of the stops and of the blank holder occurs at the moments F/5 and G/6. The force curve is evidence for the incorrect adjusted slide height (too tight). The correlation between the force signals and the acoustic emissions in the dia- grams is significant and the combination of the two signals permits the identifica- tion of different cutting/forming operations. 4 Sensors for Process Monitoring178 Fig. 4.2-8 Multi-sensor plate for a 500 kN press 4.2 Metal Forming 179 Fig. 4.2-9 Force and acoustic emission signals of a modular metal forming tool measured with the multi-sensor plate for a 500 kN press The sensor signals should be significant so that the user can ‘see and under- stand’ the complex forming operation. Especially for the tool setup the stored sig- nals of previous setups can be very helpful by using the same setup and therefore saving time and achieving the same product quality. A tight slide position causes unnecessary high press forces in the lower dead center and product defects. This load decreases the tool and the machine lifetime and increases the energy consumption. The signals in Figure 4.2-10 were mea- sured in a press shop with a production tool. At the normal slide height a force signal of a cutting operation is measured before the lower dead center. At the tight setup of the tool (0.6 mm lower) a significant second peak occurs at the low- er dead center. The stored signals of the force cassettes enable the user to setup the tool properly with less load for the machine and the tool. Another important aspect for the monitoring of the punching process is the de- tection of tool breakage. For this detection acoustic emission sensors should be used, because the reaction of the tool on overload and breakage is more signifi- cant in the acoustic emission signal than in the force signal. The force com- presses the punch and energy will be stored in the punch. After the breakage (overload), the stored energy is released as acoustic emissions to the environment like a compressed spring. These acoustic emissions have significant amplitudes and can easily be detected in a ‘silent moment’ of the process. In Figure 4.2-11 the signals of force and acoustic emissions of a normal punch and of a breaking punch are shown. There are only slight differences in the force signals. Especially the ‘small valley’ around 60 ms cannot be found in the signal 4 Sensors for Process Monitoring180 Fig. 4.2-10 Force signals of normal and incorrect tool setup of the breaking punch. This is the moment when the punch moves upwards in the mold. The acoustic emission signal is more significant. The punch causes a second peak at the moment of breakage. This event can easily be detected with a narrow tolerance band [6] around the ‘normal’ curve. 4.2.2 Sensors for the Sheet Metal Forming Process Sheet metal forming is a complex process which is affected by a manifold of in- fluences. The high demands to quality and cost efficiency at the production of sheet metal components are increasing continuously. These high requirements can only be met with optimum designed and faultless manufacturing processes. Hence it is necessary to have fundamental knowledge about the behavior of the used materials and machines as well as the possibilities for the control of the ac- tual process parameters. Furthermore it is of great importance to control the course of events during the forming operation because the process affecting pa- rameters cannot be kept constant for any space of time. Material and tool proper- ties as well as machine parameters are subjected to variations which are affecting the process stability adversely. Improvements can be achieved by the on-line mea- surement of indirect and direct process describing parameters and their transfer to a process monitoring system. 4.2 Metal Forming 181 Fig. 4.2-11 Force and acoustic emission signals of a breaking punch