Design of wireless sensor network monitoring system for PCB electroplating current

Using computers, sensor technology and wireless communication, this paper designs a wireless sensor network monitoring system for PCB electroplating current detection. The monitoring system uses NRF9E5 microcontroller as the core of the wireless sensor node, uses the design of Hall current sensor and detection circuit, and realizes wireless communication between the wireless sensor node and the computer.

The main function of PCB is to connect various electronic components to form a predetermined circuit. Its quality will directly affect the performance of the entire electromechanical product. PCB plating current is one of the important factors affecting the quality of PCB. At present, the detection of PCB plating current is performed by operators holding handheld monitoring equipment at certain intervals; therefore, there are many disadvantages, such as non-real-time detection, easy to miss and misdetect, and long-term work in the plating environment will damage human health. In view of the fact that manual detection can no longer meet the needs of the development of today's manufacturing technology, this paper proposes a wireless transmission solution for PCB plating current, that is, to place sensor nodes where the plating current needs to be detected, and the monitoring center will monitor and control the collected current data in a unified and real-time manner.

1 System Framework

The architecture of the PCB electroplating current monitoring system based on wireless sensor networks is shown in Figure 1, which can be divided into three layers: data acquisition layer, system communication layer and management layer.

The data acquisition layer is the bottom layer of the entire system, which consists of sensor nodes and aggregation nodes placed in the working environment. Through the built-in data acquisition unit, the sensor node can collect the current signal flowing through the PCB board, sort the data, and send it to the aggregation node by wireless communication; the aggregation node aggregates, analyzes and stores the data sent by the sensor node, and waits for the computer's command to prepare for communication with the computer.

The system communication layer refers to the communication between the aggregation node and the computer, using the RS232 serial communication method. By setting P0_ALT. 1=1, P0_DIR. 1=l, select the second function of the P01 and P02 pins of NRF9E5 as the RXD and TXD of the serial port, and use the chip MAX232 to convert the level and connect to the serial port of the computer to process the data collected from each node, and send control instructions and valid data through the main module to complete the setting of each node and realize the control of the field equipment. The management layer is the process of the computer analyzing and processing the collected electroplating current data. The computer can collect the current data of the node where the Hall sensor is installed according to the needs of the manager, and complete various management functions such as data processing, chart display, control and storage; when the current flowing through the PCB board exceeds or falls below the given value range, the system gives an audible and visual alarm.

2 System Hardware and Design

The wireless sensor node is the basic component of the entire wireless network. Its main task is to collect PCB electroplating current data, pre-process the data, respond to commands from the host computer, and then send the collected current data to the host computer. The wireless sensor node consists of an NRF9E5 single-chip microcomputer system, a wireless communication unit, a sensor unit, and an indicator alarm circuit.

2.1 NRF9E5 MCU System

NRF9E5 wireless microcontroller is a wireless microcontroller chip launched by NordicVLSI in 2004. It is a true system-level chip. The built-in 433MHz, 868MHz, and 915MHz transceivers are the same as those of the NRF905 chip. The embedded micro 8051 controller has a 4-input 10-bit 80kS/s A/D converter and a wide operating voltage (1.9 to 3.6V). It can work in ShockBurst mode, that is, automatically handle prefixes, addresses, and CRC modes; the CPU also extends 2 data pointers to facilitate reading data from off-chip RAM. The NRF9E5 microprocessor has 256-bit data RAM and 512-bit ROM. After power-on reset or software reset, the processor automatically executes the code in the ROM boot area. User programs are usually loaded from EEROM to a 4kbit RAM under the guidance of the boot area. This 4kbit RAM can also be used to store data. The establishment of the entire single-chip microcomputer system requires very few peripheral components, saving resources and being very beneficial to achieving low power consumption.

2.2 Wireless Communication Unit

Since the NRF9E5 wireless microcontroller has built-in NRF905's 433MHz, 868MHz, and 915MHz transceivers, the design of the entire system is more concise and reliable. Through software programming, the transceiver can automatically complete functions such as sending, receiving, and monitoring. From the perspective of system design, it is only necessary to interpret and control the data in the data packet with software.

2.3 Sensor unit

The sensor unit is mainly responsible for the collection of current information, which is the basis for ensuring the performance of the entire system. First, the current signal needs to be converted into a voltage signal. Here, the TBC-LTA series Hall current sensor is used, with a working power supply voltage of ±(12～15)V and an operating temperature of -40～85℃. In order to improve the measurement accuracy, combined with the actual measured current value, this article selects the TDC503LTA sensor from the TBC-LTA series DC current sensor.

NRF9E5 contains a 10-bit linear AD converter with a conversion rate of 80kS/s. The AD converter reference voltage can be selected by software from the Aref input or the internal bandgap reference voltage of 1.22V. The AD converter has 5 inputs that can be selected by software. Its typical application is the start/stop mode, and the sampling time is controlled by software. It is 10 bits by default and can be set to 6 bits, 8 bits or 12 bits by software if necessary. At the same time, the AD converter can also be used in differential mode. When AIN0 is used as the negative input terminal, AIN1~3 are used as the positive input terminal. [page]

2.4 Indicator light alarm circuit

When the current flowing through the circuit board is lower than the lower limit or higher than the upper limit, the indicator alarm circuit will generate an audible and visual alarm so that the management personnel can take immediate countermeasures. In this design, a red LED light and a speaker are used as the indicator alarm circuit.

3 System Software Design

3.1 Lower computer software design

In modern wireless communications, data is transmitted in the form of data packets. For wireless chip systems such as NRF9E5, each transmission and reception of data is also carried out in the form of data packets. The data packet format is an important part of the communication protocol. The wireless data packet formats of NRF9E5 are: Preamble, ADDR, PAYLOAD, and CRC. Among them, Preamble is the leading code, which is automatically added by the hardware; ADDR sends a 32-40-bit address code; PAYLOAD is the valid data (maximum 32 bits); CRC is the cyclic redundancy checksum, which can be automatically added by the built-in CRC error correction hardware circuit and can be set to 0 bits, 8 bits, or 16 bits. The most important task of the sensor node is to send the collected data. To realize the wireless transmission of data, the wireless transceiver in the processor must work in the transmission mode. The transceiver (NRF905) of NRF9E5 has three working modes: ShockBurst receiving (RX) mode, Shock Burst sending (TX) mode, and energy saving mode. Its data transmission process is shown in Figure 3.

3.2 Host computer software design

The host computer software design uses Visual C++ for programming, and designs a real-time current display panel and a database for data storage. After the wireless sensor network is deployed, the data of many sensor nodes are uploaded to the computer, and the computer program, i.e., the host computer program, stores these data to facilitate future data query and processing. Visual C++ provides users with three database access methods, namely ODBC, DAO, and OLEDB. ODBC provides an application programming interface (API), and any database can achieve the purpose of managing the database through these APIs. The general process of using the ODBC database provided by MFCAppWizard is:

1) Use database tools such as Access to create a database;

2) Define an ODBC data source in Windows for the database created in 1);

3) Select the data source defined above in the document application wizard that creates the database process;

4) Design the interface and associate controls with data table fields.

In this design, a database is first established, and several data tables are established according to the number of sensor nodes. Then, the ODBC classes provided by MFC: record set class, database class and visual record set class are used for programming to store the data uploaded by each wireless sensor node in each data table according to their node number.

4 Conclusion

The wireless sensor node of this system is based on the low-power NRF9E5 single-chip microcomputer, uses the TBC-LTA series Hall current sensor, and integrates computer technology and wireless sensor network technology to perform real-time data acquisition and A/D conversion of PCB electroplating current, and analyzes and stores the collected current data, providing a good operating environment for monitoring the production of PCB electroplating current, improving the quality of PCB boards, and enhancing the competitiveness of products. This system has the characteristics of low cost, low power consumption, high detection accuracy, simple operation and humanization, which can well solve the problem of PCB electroplating current monitoring and has good application and development prospects.