Design of a Machine Tool Monitoring System

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DESIGN OF A MACHINE TOOL MONITORING SYSTEM Gilberto Herrera Rulz·, Claudia Guth~rrez Mazzotti*, George Kovacs Computer and Automation Institute, Hungarian Academy of Sciences Kende ut 13-17, Budapest, Hungary

Keywords: monitoring, tool wear and tool breakage detection, machine tools, adaptive control Abstract Automation is one of the main trends in modern manufacturing production. Related to this concept, the monitoring systems and each of their components also play an important role in the solution of the problems of producing parts of high quality, flexibility, and economic advantages. The design and development of a monitoring system for machine tools is shown in this article. It is based on broken tool detection and monitoring of temperature and tool wear. The general hardware structure is described, and the experimental results are given as part of the whole development work. Introduction Nowadays the supervisory systems applied to the machining processes play a very important role in manufacturing. In order to solve the problems of producing parts with high quality, flexibility, and economic advantages, the machine tools need to be automatic, and the automation process requires monitoring tasks for its purposes. A monitoring system can be seen as the eyes and ears in a manufacturing system, which send information to the main controller about what is going on in the process itself. When there are no eyes or no ears or none of them, or simply they are not able to send information to the main controller with a certain degree of veracity, no suitabiiity, neither reliability can be expected from the controller response signals. Disturbances inevitably occur during any machining process in every automated manufacturing system. These disturbances bring the possibility of damage, workpiece rejects and equipment downtime. For instance, Un and Mathew [1] mention in their research work that in machining systems tool failure contributes on average up to 6.8% downtime of machining centers. In machining systems most tools fail by fracture or by gradual wear. Tool change strategies are now based on the most conservative estimates of tool life from past tool wear data. This approach does not allow for tool fracture or chipping of the cutting edge as these are generally catastrophic processes.

*On leave from the ITESM Universitv. Mexico

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Here we describe the monitoring system which is based on the tqol breakage detection, tool wear and templ3rature monitoring. The elements chosen for the sensing tasks are: proximity switches, for checking the geometry of the tools in an off-line process; instrumentation resistors, for the motion torque monitoring, based on the current measurement of the feed motors, and temperature sensors to detect any abnormality related to the cuttin~J process. The system was instalied in a conventional vertical milling machine tool model TOSS FNK25A, which was retrofitted for research purposes [2]. Physical structure of the monitoring system

In Fig. 1 the block diagram of the physical components for the monitoring system is depicted. The first block corresponds to the milling machine tool itself where the monitoring system performs its supervisory activities. Beside the tool holder we can see a solid device where two proximity switches are mounted. They are located on the table following the X axis, for the tool in order to be approached and measured sometimes for the ~Ieometry ct-Iecking in an off-line process. In this same block we can see the instrumentation resistor connected to the feed X axis DC motor. Optional instrumentation resistors can also be connected to the Y and Z axes DC motors in order to measure the current there.

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Fig. 1 Block diagram of the physical components for the monitoring system

The temperature sensors are located on designated points of the most important elements and units of the machine tool, like the motor armature for instance, or the upper side of the tool holder.

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In this same Fig. 1 we can see the blocks that correspond to the hardware for signal conditioning and isolation of the information coming from the sensing elements; also we can see the data acquisition card which collects that information and sends to the computer system. A 16 I/O programmable logic controller (PLC) unit drives several signals of the CNC control/er like spindle on, spindle off, coolant and limit switches.

Description of the components of the monitoring system As mentioned, before, inductive proximity switches were used in the system to measure the geometry of the tools. These elements can be activated in a range from 10 to 30 volts where they respond at the output with the same value of the input voltage when the presence of a metallic material is as far as 2 millimeters from them. The signals for these switches are transformed to a 5 volts level, then they are opto-coupled, and finally a schmitt trigger circuit sends the TIL levels to the data acquisition card. The prinCiple for learning the geometry of the tools for wear and breakage detection is based on the frequency signals reading by the computer system. The instrumentation resistors have a voltage-current relation of 150 millivolts-15 amperes. These resistors, normally called "shunt conductors" are made of a copper wire. Their resistance does not exceed 0.035 ohm. They are connected through low pass filters and isolation amplifiers to the data acquisition card. The cutoff frequency of the filter was chosen according to the time constant of the DC motor [3]. The isolation amplifier is used to eliminate noise signals and to handle two different electrical environments. This amplifier isolates the measuring system from the data acquisition card. The data acquisition card was designed according to the requirements of the monitoring system [4]. It is based on a 12 bits analog-to-digital converter with 35 microseconds maximum conversion time. It has 16 single ended or 8 differential input channels and can handle 0 to 20 or 0 to 10 volts unipolar range, and 10 or 5 volts bipolar range for the input signals. The card can be calibrated by software before any reading is made to the sensors; this calibration process eliminates gain, offset, and linearity variations accumulated through the whole card circuit.

Functional description of the monitoring system The controller system of the maclline tool has the capability of multiprocessing. It can handle several tasks designated to a specific processor in the same time. For instance, while processor 1 can fetch and decode numerical control (NC) instructions, lead NC programs, edit functions, manage the bus, etc., processor 2 is dedicated to the monitoring task during the machining process, including software limit switch supervision and data processing for optimal adaptive control functions. In the same way, the management of workcycles and subroutines, interpolation, travel to reference point, feed generation, and override , are handled by processor 3. Finally

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processor 4 realizes the functions of a PLC, which reads all inputs byte by byte at intervals of 10 milliseconds, performs the needed operations, and generates the output signals byte by byte to the output cards. When the machining process starts, the tool is approached to the proximity sensors, then rotated and moved in front of the sensor (Fig. 2). It is assumed that tools are appropriate before the very first use, then the system can IE~arn the geometric features of such a tool. During the checking process, the current frequency signal is compared with the previous stored one. When the geometry has been recognized by the system, an error value is established by the user in order to give all alarm signal of excessive tool wear or breakage. If this signal is sent to the controller, the machining process can not start unless the tool is replaced by a new one.

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Fig. 2 Different ways of measuring cutting tools by the proximity switches

The motor current signals are sampled at a frequency which safes the lectures of the 'aliasing' problem. As these signals have only low frequency components of interest for this measuring technique, the sampling frequency is chosen according to the cutoff frequency of the filter, around 2500 Hz. The motor current signals were measured in the workshop vvhen milling was performed in C-45 work material, with an inserted end mill HSS, 2 teeth, 20 millimeters diameter tool. The experiment was repeated under different conditions for revolution speed in the spindle, for different feedrate values, and for different clepths of cut. Fig. 3 shows a typical current signal obtained during one of the experiments with a revolution speed of 450 RPM, a feedrate of 25 millimeters/minute, and 3 milLmeters depth of cut. From this figure is interesting to note the cyclic changes in the current which reflect the milling process where the teeth enter and leave the workpiece developing a cutting force related to these current ~hanges. The signals from the temperature detectors are electronically conditioned in order to get 1 millivolt for each temperature degree of change. The deviant states related to

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Cyclic curren t chang es / tooth

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Typica l curren t signal obtain ed with an inserte d end mm HSS, 2 teeth, 20 mm diame ter tool. Experi ment perform ed in a milling proces s for tile monito ring task.

signals. They warn the temperature can be recognized on the basis of these sensor for rising temperature control proce ssor of abnormal condition. There are four limits to classi fy signais. (threshold values) and a limit for the rate of change in temperature ones obtained from the The signals obtained from the motor 's current togeth er with the al adaptive control different sensing elements, can be processed and fed to an option this AC routine is very (AC) routine which :s handled by proce ssor 2. The strategy of ss with pre-established simple because it comp ares every vaiue read from the proce to the CNC syst em. threshold values in order to send any correction comm and signal t signal, the system tried When this routine was tested for the case of the motor 's curren to a pre- estab lished to maintain the cutting load constant, reducing the feed rate also value during the milling process. system that was Figure 4 shows the block diagram of the multiprocessor based implemented. This desig ned in this project, in which the different proce ssor tasks were system can be achiev ed figure shows how the desired flexibility and performance of the and software struct ures, by means of multip roces sor systems by the modula.r hardware ronizing th e several where the operating system function s are defined for synch also defined. proce ssor tasks . The parall el bus and the proce ssor tasks are ecture, this gives us the The monitoring system is designed under the VME bus archit nts and let us to add flexibi !ity to use comm ercial cards that can fill our requ ireme additional cards in the future. be transp orted to any The software was devel oped in "e" language, so it can the monit ori ng task micro proce ssor board . This software includes the CNC language, age in ord er not to and the AC routine. It was avoided to progr am in assembl er langu depend from any specific microprocessor.

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CONCLUSIONS

A multiprocessor based monitoring system was developed and tested in a retrofitted CNC vertical milling machine. From the several methods that can be used in monitoring systems, measuring of cutting tool geometry and monitoring of the spindle and feed motor current were chosen. The system provides two functions complementing each other: off- line tool checking , and in-process load monitoring. Because the checking of tools with complex edge geometry is complicated, these roughing tools can be effectively monitored by the measuring of the spindle motor current, also through an instrumentation resistor as in the case of the feed motor monitoring. From the feed motO!' current measurement it is possible to observe a cyclic change of this current, which varies according to the change in depth of cut and also with the change in feed rate values of spindle speed during machining. Along the set of experiments realized in the workshop, an approximate change of 1 ampere could be observed, when the depth of cut was varied from 3 to 5 millimeters, and a feedrate of 25 to 50 millimeters/minute. The signals from the measuring process can be conditioned and fed to an optional AC routine which sends control commands to the CNC unit in order to modify the feedrate, or the revolution speed in the case of the spindle motor current signal. The system is made of several processing devices which raise the efficiency of the multitasking feature. Because, of this, every function can be separately programmed.

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9. I , Cselle, 1984, "Modular measuring and control system for machine tool monitoring", IF AC 9th. Triennial World Congress, pp 2373-2378, Budapest, Hungary.

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