Research on on-line monitoring system for LCM proc

Research on composite liquid forming (LCM) process monitoring system

Abstract: in the composite liquid forming (LCM) process, it is very important to understand its flow front and curing process for the quality and reliability of products. In this paper, a new monitoring method is proposed, and on this basis, a set of LCM process monitoring system is developed, which realizes the real-time monitoring of the flow front and curing process, and shows good results in the monitoring experiment

key words: LCM monitoring composite flow front curing

liquid composites molding (LCM) process is widely used in aviation industry, automobile industry and some non-traditional composite industries because of its advantages of flexible operation, strong designability and low cost. The quality and performance of LCM process components largely depend on the flow filling process of resin and its chemical curing reaction [1]. In order to reduce defects such as incomplete mold filling or dry spots, reduce the scrap rate and improve product quality; It is difficult for non automatic control professionals to solve the problem of unstable product quality in the production of lipid based composites. Real time monitoring of LCM process will be of great significance to obtain high-quality composite products

over the past decade, many monitoring elements [2,3] and technologies [4,5] have been developed for LCM processes whose sales account for more than 8% of the total adhesive sales. The existing monitoring technologies can be divided into non embedded and embedded. Non embedded monitoring methods, such as ultrasonic monitoring, thermal spectrum and camera, require quite expensive monitoring equipment; Embedded monitoring technologies such as thermal monitoring with thermocouple as monitoring element, pressure monitoring with pressure sensor as medium, optical fiber monitoring technology, DC monitoring and dielectric monitoring technology. Domestic LCM process monitoring started late, and many scientific researchers have done a lot of research in the field of optical fiber curing monitoring [6,7], but little progress has been made in the monitoring technology of resin flow process. Therefore, this paper independently developed the LCM process monitoring system based on the resistance monitoring principle, and monitored the resin flow process and curing process at the same time

1 principle of LCM process monitoring system

in order to investigate the state of the whole component in the process, it is necessary to cross discharge excitation wires and induction wires on the upper and lower surfaces of the preform as shown in Figure 1 to determine the distribution of multiple monitoring points. All wires are exposed near the intersection and insulated at other positions

Figure 1 monitoring wiring diagram in the mold

the monitoring system actually utilizes the weak conductivity of the resin. When the resin infiltrates the fiber preform near the monitoring point driven by the pressure gradient, the conductor at both ends of the monitoring point changes from non-conductive to conductive, which can be regarded as a sudden change in resistance; During the curing process, the conductivity of the resin near the monitoring point will continue to decrease and the resistance will continue to increase. The purpose of monitoring the curing process is achieved by scanning the change of resistance in real time. The principle of monitoring resistance change is shown in Figure 2

Figure 2 Schematic diagram of signal conversion

where Rx is the resistance at the monitoring point of the preform, RC is the reference resistance, add DC voltage UD at both ends of the two series resistors RC and Rx, uout is the voltage divided by the reference resistance, UIN is the voltage amplified by the amplification circuit, and ad represents the value after a/d conversion

it can be seen from Figure 2 that the resistance of the preform at the monitoring point and the reference resistance form a voltage dividing circuit. According to the basic principle of the voltage dividing circuit:

where n is the amplification factor, 256 is the accuracy of the a/d conversion chip used in the system, UreF is the a/d conversion reference voltage. Combined with equations (1), (2) and (3), the resistance value at the monitoring point at any time is obtained:

from equation (4), the ad value is inversely proportional to the resistance RX at the monitoring point, As long as the ad values of all monitoring points are scanned circularly, the change of resistance can be monitored. During resin filling, the ad value will change from small to large before and after the resin infiltration monitoring point. A preset threshold value is used to judge the infiltration state of the monitoring point (the monitoring signal is less than the threshold value, and the monitoring system thinks that the resin has not reached the monitoring point, otherwise it makes a positive judgment), so as to investigate the position of the resin flow front in the mold cavity. After the mold filling, with the continuous curing of the resin, the resistance increases and the ad value decreases. By monitoring the ad value, we can also know the curing condition of the resin in real time

2 system hardware design

the hardware composition block diagram of the monitoring system is shown in Figure 3. The design of its lower MCU adopts 89C52 chip, adc0809a/d conversion chip, 8255 parallel i/o expansion chip, RS-232 serial port communication module, 0~12v drive circuit, 16 independent amplification circuits, 5 independent keyboards and 8-bit seven segment code display module. Excitation wire and induction wire, i.e. voltage output line and electrical signal input line, adopt special copper wire, which supports up to 16 channels respectively, and can be realized by cooperating with the lower computer software × Signal scanning and acquisition of 16 monitoring points

Figure 3 hardware composition block diagram of LCM monitoring system

the upper computer is an ordinary PC (there must be bridges and sometimes sightseeing boats passing through with serial ports). Through the supporting developed upper computer software, the functions of real-time analysis, display, saving data and simulating the process are realized

3 system software design

3.1 lower computer software

the lower computer software uses C51 language, and the most important part of the program is to realize data scanning and collection and serial port transmission. Among them, data scanning and acquisition includes output voltage scanning and a/d conversion scanning, with 16 channels respectively. Therefore, 8255i/o expansion chip and two 8-channel a/d conversion chips (ADC0809) are used. The main scanning and acquisition procedures are as follows:

for (i=0; I