Research on tool breakage monitoring system in the

Research on tool breakage monitoring system in automatic machining

in recent years, due to the rapid development of industrial automation, the requirements for real-time monitoring of industrial production process are becoming higher and higher At the same time, the requirements for fault monitoring and real-time monitoring and control of production quality of FMS, CIMS and other major expensive equipment are more urgent At present, among the many methods for monitoring the cutting state of tools, acoustic emission (AE) method can continuously monitor the whole process of tool damage. It avoids the low-frequency area with serious vibration and audio noise pollution in the production process until the sample is damaged, and is almost free from the limitation of materials However, at present, contact acoustic emission sensors [1, 2] are mostly used at home and abroad, which are not only inconvenient to use, but also easy to be disturbed by the working environment; Scholars at home and abroad have used advanced technologies and algorithms such as fuzzy pattern recognition and artificial neural networks to improve the detection rate, but reduce the real-time performance, make the system complex and reduce the reliability

the author has studied an intelligent acoustic emission sensor monitoring system suitable for monitoring tool damage in automatic machining process. It integrates the sensor, preamplifier, signal processing system and control system into one, and has small volume and compact structure. It can be directly installed next to the main shaft of the machine tool, with simple operation, convenient installation, high sensitivity and strong anti-interference ability

1 principle and structure of monitoring system

single or multiple burst acoustic emission signals will appear when metal or non-metallic materials are broken or crack propagation. The frequency of these acoustic emission signals is greater than 100kHz, which is 4 ~ 5 times greater than the AE signal of normal cutting, and the duration is 0.2 ~ 1.0ms. Therefore, as long as the frequency, amplitude and duration of acoustic emission signals can be identified, the tool damage signal can be identified Due to the high frequency of the acoustic emission signal of tool breakage, only the high-speed signal collector can extract the original acoustic emission signal. If the envelope signal is collected and processed, the requirements for the subsequent identification and processing circuit can be reduced The principle of signal recognition is shown in Figure 1 The size of AE signal is identified by amplitude discrimination of detection signal, and then the duration of AE signal is identified by pulse width discrimination, that is, the acoustic emission signal of tool damage can be identified by using the double threshold discrimination method of amplitude and pulse width The reason for using double threshold judgment is that only using amplitude threshold judgment, there are often some interference pulse signals that are not tool damage, causing false alarms. The experiment shows that the duration of AE signal of tool damage is longer than the interference pulse. Therefore, only the AE signal that exceeds the amplitude threshold and has a duration of more than 0.2ms can be judged as tool damage

Figure 1 principle of acoustic emission signal recognition

instead of using the traditional contact AE sensor, the monitor uses the new AE sensor (integrated in the monitor) developed by the author to transmit signals through fluid. See Figure 2 for the installation diagram of the monitor When in use, it is installed at the installation place of the original lubricating coolant pipe of the machine tool. When the tool is damaged, the AE signal is transmitted to the monitor through the lubricating coolant in the direction opposite to the flow direction of the lubricating coolant with the professionally designed data collection and amplification system and control software These liquids are good media for AE signals The propagation frequency range is large, from a few Hz to dozens of MHz, and because only longitudinal waves can be transmitted in the liquid, the interference loss and signal distortion in the signal propagation process are small. Under certain boundary conditions, it can also ensure that the attenuation of the signal varying with the propagation distance is small [3, 4] This provides a guarantee for the use of fluid as the transmission medium of acoustic emission signals Because this installation method has short signal transmission path and no other contact surface in the transmission path, it has high sensitivity and strong anti-interference ability Because the monitor adopts a special adaptive filter to eliminate vibration and noise, although the instrument is installed near the spindle of the machine tool with great impact of vibration and noise, it will not cause false alarm The monitoring system block diagram is shown in Figure 3

Figure 2 installation diagram of the monitor

Figure 3 acoustic emission monitoring and processing method

the interference signals such as vibration and noise transmitted from the spindle are mainly transmitted through the sensor housing. The special adaptive filter used in this system can filter out all kinds of interference signals from the housing and retain the AE signal of tool damage transmitted from the fluid medium The signal after adaptive filtering enters the high pass filter (the cut-off frequency is 100kHz), and the AE signal after high pass filtering has a certain attenuation. After being amplified again, it enters the detector, and the detected AE signal enters the comparator for waveform identification First, compare the amplitude of the envelope of the AE signal. When it reaches the preset amplitude threshold, the comparator 1 triggers turnover and starts to output the signal, and then enters the corrugated tension fixture: width 60mm, clamping height 50mm, 1 set/2 pieces into the pulse width comparison, that is, the signal enters the integrator and starts to integrate. The integrator works according to the following calculation formula:

where uo (T) is the output of the integrator; K is the constant determined by the circuit parameters; UI (T) is the input of integrator, that is, the output of comparator 1; Uo (0) is the output of integrator at the beginning of integration, which is adjusted to 0 by circuit parameters

the flip signal of comparator 1 accumulates for a certain time. When the output of integrator is higher than the preset pulse width threshold, comparator 2 triggers flip (when the output of integrator is lower than the pulse width threshold, it is considered as impulse interference) The output signal of comparator 2 triggers the alarm circuit and gives the machine tool shutdown signal Such monitoring and processing method is completely realized by hardware circuit in real time, which is a better practical method to monitor tool damage

2 structure of tool breakage monitoring system

the structure of the monitor is shown in Figure 4. It is composed of a sensing cavity and an electronic circuit cavity The sensing chamber receives signals from the sensing elements The electronic circuit cavity includes power module, pre-processing module, post-processing module and control module In addition to the above acoustic emission monitoring and processing, they can also give the control signal of the machine tool after the alarm to stop the machine tool At the same time, the user is prompted with audible and visual alarm, and the relay alarm output interface is provided for the user to choose The circuit boards in the sensor cavity, electronic circuit cavity and electronic circuit cavity adopt modular design, and plug-in connection is adopted between them, which is convenient for maintenance and replacement Because the industrial electromagnetic noise makes the working environment of the monitor worse, the anti-interference design of the monitor is carried out At the same time, according to the electromagnetic compatibility design principle, a point grounding and shell shielding structure are adopted As a promising environmental protection material, the electromagnetic and power interference experiments of electric drill and machine tool start and stop have proved that a series of measures taken are effective

Figure 4 Schematic diagram of the structure of the monitor

3 experiment

the drill bit damage experiment was carried out on the CA6140 ordinary lathe, and the cooling and lubricating fluid system on the machine tool was directly used. The monitor was connected in series at the root of the cooling and lubricating fluid flexible pipe, and the cooling and lubricating fluid was 20 # engine oil High speed steel fried dough twist drills with different diameters (0.5 ~ 2.1mm) and different cutting parameters (cutting speed: 400 ~ 1400r/min, feed rate: 0.028 ~ 0.036mm/r) were used to drill 45# steel and duralumin. 56 experiments were carried out, and the success rate was 98%

the damage experiment of high speed steel fried dough twist bit was carried out on re3020 machining center. The working material was 45 × steel and the cooling lubricant was 20 × oil The experimental results are shown in the table below

experimental data sheet of drill bit damage on the machining center

during the experiment, artificial knocking on the sensor shell and the spindle of the machining center, as well as the start and stop and speed change of the machining center did not cause false alarms, which proved that the instrument has strong anti-interference ability

this intelligent monitor is mainly used for real-time automatic monitoring of tool damage, and can also be used for other metal and non-metal dynamic crack monitoring