Abstract : This paper introduces the design scheme of a remote power monitoring system based on the AD7755 power metering chip and CAN bus. The functional structure, working principle, and basic characteristics of each component module of the system are described in detail, along with the software components of the host computer system. This system can not only manage and schedule the power consumption of each room in a building, but also monitor power consumption to achieve energy conservation.
Design of Intelligent system of electricity long-range control based on CAN Bus
0 Introduction
With the revision of my country's Electricity Law and the gradual formation of the electricity market, power suppliers must ensure power quality and reliability, which requires a highly automated management system. To this end, the author developed a remote power monitoring system based on the CAN bus. This system primarily monitors the power consumption of various rooms in an office building in real time. It can monitor the electrical parameters of each room in real time, such as voltage, current, power, energy, and three-phase imbalance, and upload data to the terminal database in real time, realizing the monitoring and control of the electrical load in each room.
1 System Design Scheme
The system mainly consists of two parts: an electricity parameter acquisition section and a real-time data publishing section. The electricity parameter acquisition section primarily uses the AD7755 electricity metering chip and the 80C51 microcontroller to collect electricity parameters. It divides a day into 48 time periods, each half hour, and sends the collected electricity data once per period. The data is then transmitted to the server via the SJA1000 CAN controller and a CAN transceiver. The server then publishes this data through a dynamic webpage. Users can access real-time and historical data via the network.
Figure 1 System Structure Diagram
Fig. 1 Diagram of System
2 System Hardware Design
2.1 Power Acquisition Circuit Design
This circuit uses the AD7755 energy metering chip as its core to measure the electricity consumption of each room in the office building and convert the energy signal into a standard pulse signal, which is then sent to the microcontroller processing circuit. Figure 2 shows the application circuit schematic of the AD7755. In the figure, the current and voltage signals are sent to each input channel through their respective transformers. The rated values on the voltage and current channels should be designed at half the scale of the maximum input voltage so that the meter can meet the overvoltage and overcurrent requirements. The microcontroller counts the input pulses, and the value of the count reflects the amount of energy consumed. The AD7755's no-load threshold and starting current characteristics will eliminate the leakage current effect in the meter. If the output frequency generated by the load is lower than the minimum output frequency specified by the AD7755, the AD7755 will not output any pulses.
Figure 2 Schematic diagram of AD7755 application circuit
Fig.2 Application Circuit Diagram of AD7755
This system utilizes the power processing function of the AD7755 to measure electrical energy with the assistance of a microcontroller. The method is simple, effective, and has high practical value.
2.2 CAN Bus Communication System Design
The CAN communication protocol is primarily implemented by the CAN controller. The CAN controller mainly consists of a section that implements the CAN bus protocol and a section that interfaces with the microprocessor. For different models of CAN bus communication controllers, the structure and function of the circuitry implementing the CAN protocol are mostly the same, while the structure and method of the microprocessor interface differ somewhat.
The intelligent nodes of this CAN bus system use the 89C51 as the node microprocessor. The CAN bus communication interface uses the PHILIPS SJA1000 and 82C250 chips. The SJA1000 is an independent CAN communication controller, and the 82C250 is a high-performance CAN bus transceiver.
The SJA1000 is a standalone CAN controller, a replacement for the PHILIPS PCA82C200 CAN controller. The SJA1000 operates in both BasicCAN and PeliCAN modes. The PeliCAN mode supports the CAN 2.0B protocol with many new features.
Figure 3 shows the hardware circuit schematic of the system's intelligent node. As can be seen from the figure, the circuit mainly consists of four parts: an 89C51 microcontroller, an independent CAN communication controller SJA1000, a CAN bus transceiver 82C250, and a high-speed optocoupler 6N137. The 89C51 microprocessor is responsible for initializing the SJA1000 and controls the SJA1000 to perform communication tasks such as data reception and transmission.
Figure 3 Schematic diagram of the hardware circuit of the system's intelligent node
Fig.3 Hardware Circuit Diagram of System Intelligent Nodes
3 System Software Design
3.1 Database Selection
The system contains a large volume of data across multiple categories. Managing complex data makes the design and management of the system's database crucial to its success. Considering the size of the system's data and compatibility with the operating system, SQL Server 2000 was chosen for this system. SQL Server is inexpensive, easy to use, and maintains even large databases, making it particularly suitable for enterprises that need to build medium to large databases but are not very proficient in database platforms. Table 1 lists the general principles for database selection.
3.2 Database table structure design
Based on the requirements analysis, the structural design of important tables in the system database is as follows:
The User_Infor table records relevant information about users, such as: employee ID, employee name, user permissions, user password, contact number, department ID, etc.
The Department Information table Depart_Infor records relevant information about the department, such as: department ID, department name, person in charge ID, department code, etc.
The Room_Infor table records relevant information for the room, such as: data collection node ID, room ID, department ID, etc.
The Time_Divide table records information about dividing a day into time periods, such as time periods and time period IDs.
The power consumption meter Elec_Power records information related to the room's power consumption and maximum power, such as: data collection node ID, date, time period ID, power consumption, and maximum power.
Figure 4. ER diagram of the power management system
Fig.4 the ER diagram of Electricity Management System
3.3 Login Module Design
In any database application system, users must first pass system authentication to access the system and use its functional modules; this process is called login. Generally, the login module needs to perform the following tasks: a) determine whether the user is allowed access to the system based on the username and password entered; b) determine the user's permissions based on the user type.
Below is a flowchart of the login module's workflow.
Figure 5. Login Module Flowchart
Fig.5 Flow Chart of Login Module
3.4 Query Module Design
After a user successfully logs in, they are taken to the query interface. Users can select a date and enter a time to query electricity consumption information for different time periods, thus understanding their electricity usage. For electricity management departments, this information can also serve as an important basis for limiting electricity consumption, charging appropriate fees, and formulating electricity consumption plans.
This module is primarily implemented through a perfect combination of VB and SQL database.
This section describes how to implement data source access operations in an application using the ADO object model, including common operations such as adding, deleting, querying, and modifying data. The key code in the application is as follows:
Private myConn As New ADODB.Connection
Private myRecord As New ADODB.Recordset
Dim mySQL As String
Set myConn = New ADODB.Connection
myConn.ConnectionString = "Server=localhost;Database=electricity;Trusted_Connection=yes"
myConn.Open
Set myRecord = New ADODB.Recordset
myRecord.Open “ electricity” , myConn, adOpenDynamic
mySQL = "SQL statement"
myConn.Execute mySQL
Adoc1.Refresh
Set DataGrid1.DataSource = Adodc1
4. Conclusion
This system utilizes the AD7755 power metering chip and CAN bus technology to implement a remote power monitoring system. Due to the high reliability, high baud rate, and multi-master node communication characteristics of the CAN bus, high-speed and reliable data communication is achieved. The perfect combination of SQL database and VB enables the real-time release of power information. Experiments have proven that this remote intelligent power monitoring system is feasible.
