Design of heating metering system based on CAN bus technology

1 Introduction
CAN bus technology was born in the field of automobile control. With the continuous improvement of its technology, its application field has also been continuously expanded. Since 2000, many domestic manufacturers have been conducting research and development on this bus technology after the introduction and use stage since CAN bus technology entered China, and have achieved certain results. At present, domestic CAN bus technology has been applied in many fields such as automobile control, CNC machine tools, medical equipment and building automation, and is one of the bus technologies that has received widespread attention [1].
CAN bus has the characteristics of a field bus and is suitable for the application of distributed systems. At present, China has a certain ability to develop CAN bus products, and its application cost has also been reduced. Therefore, CAN bus technology has been widely used in the transformation of traditional control systems. With the commercialization of heat energy and the reform of the heating billing system, the domestic residential heating system will also change. The most important and difficult part is the transformation of the heating metering system. In newly built residential quarters, different heating methods can be adopted according to the requirements of the current national heating system, and the heating system of the residential quarters can be designed according to the specific heating methods. However, how to transform the heating metering system of old residential buildings with a large stock is a difficult point, because this may involve the transformation of existing heating pipelines, and the engineering volume of heating pipeline transformation is relatively large, and the construction period is long. In particular, pipeline transformation has a great impact on residents, and implementation may encounter relatively large resistance [2, 3]. Therefore, according to the requirements of the transformation of old residential heating systems, it is necessary to design a simple and practical residential heating metering system. This paper completes the design of a heating metering system based on CAN bus technology. 2 Introduction to CAN bus technology

2.1 CAN bus structure
CAN (Controller Area Network) bus is a field bus standard launched by Bosch, Germany in 1990. It has the characteristics of strong real-time performance, high reliability, and cheap development tools. Motorola, Intel, Philips and other companies
provide hardware support for it.
The CAN bus system is composed of many CAN nodes, and the CAN bus connects each node. The maximum total length of the bus is 10 kilometers. The CAN bus can be connected to the upper network through a linking device. The upper network can be an information network or other standard bus.
The CAN bus protocol complies with the OSI model, and its data link layer and physical layer are described by the CAN2.0 protocol specification. The CAN2.0 protocol specification does not describe the application layer of the CAN bus protocol, so its application layer needs to be constructed separately [4].

2.2 CAN bus node
The CAN bus node consists of three parts: microprocessor system, CAN controller and CAN bus transceiver. The composition and connection diagram of the CAN bus node is shown in Figure 1.

The microprocessor system in the node is composed of a microprocessor and a local application circuit. The microprocessor is the control center of the entire node. The local application circuit is connected to the field equipment. The CAN transceiver in the node is connected to the CAN bus. The CAN bus node is the key to connecting the field equipment and the bus.
The CAN bus node has two important functions, namely: bus communication function and local control function. The bus communication function completes data transmission with other nodes on the bus; the local control function completes the control of the field equipment connected to this node. The node plus the field equipment constitutes a control system, and it is a control system with communication function. The CAN bus is an information channel that connects many such control systems together. The system built with CAN bus technology is a distributed system, which reflects the essence of field bus technology [5, 6]. 3 Overall design
3.1 Demand and technical analysis
The centralized heating method in China is mainly used in residential buildings. At present, although the heating methods in old houses are different, the basic structure of their heating pipelines is the so-called "series and trip" type. According to different heating areas in the house, each heating pipeline is distributed in different positions in the house, and each pipeline on the upper and lower floors is serially connected with a radiator. The schematic diagram of the single-layer single-pass heating pipeline structure is shown in Figure 2.

The equipment layer of the residence is equipped with a horizontal return water pipeline. For example, a unit with two bedrooms, two living rooms, one bathroom and one kitchen has at least 6 heating pipelines passing through it. Without changing the existing heating pipelines, the heat consumed by each pipeline of each household must be measured separately, that is, the heat value consumed from point A to point B in Figure 2, and the total heat consumed by the household is obtained by accumulating. Due to the spatial distribution, the heating metering system must adopt a fully distributed system. CAN bus technology meets this requirement. Moreover, CAN bus products are low in cost compared with other field bus products and are suitable for the transformation of existing systems. Therefore, CAN bus technology is used in the design. To measure the heat consumption in civil residences, the system is generally required to measure the flow and temperature of the corresponding heat transfer medium. Therefore, flow and temperature measurement points should be set at both ends of each household's single heating line, that is, points A and B in Figure 2. The heat consumed by the radiator of the household can be calculated based on the flow difference and temperature difference between the two points. In order to simplify the on-site equipment, the calculation of the heat value is completed in the host. Unit houses are characterized by overlapping layers. In such houses, one measurement point is set on each household's single heating line to meet the metering requirements. For example, in a residential building with 21 floors, the number of measurement points on each single heating line is 22. If each floor is calculated as 10 households and each household has 6 heating lines, the residential building has a total of 1320 measurement points.
3.2 Overall design
The master/slave structure is adopted in the system design, and the system structure block diagram is shown in Figure 3. The host is connected to the CAN bus through a CAN bus adapter, and the node adopts a unified power supply method. The terminator is required by the CAN bus and is installed at both ends of the bus. Node 1 to node n are CAN bus nodes, which mainly complete the collection of flow value and temperature value and communication with the host. The system has the function of automatic data collection, avoiding the process of manual table lookup.

The host accesses the node by patrol inspection. The accumulation of flow is completed in the node, and the temperature value is collected by the node for the host to read. The maximum length of the bus can reach 10 kilometers, which can fully meet the requirements of current high-rise residential buildings.
Under normal heating conditions, if there is no leakage, the flow on a single heating line should be equal. Since the horizontal return water pipeline is in the equipment layer, the flow sensor is only installed on the equipment layer. The flow values ​​at these points are equal to the flow values ​​in the residents' rooms, which can reduce the amount of engineering work for renovation. The disadvantage is that the leakage fault of the residents' heating facilities cannot be detected. The temperature sensor is installed at each measuring point. Taking a residential building with a floor height of 21 as an example, its equipment layer is between the top floor and the middle floor, so nodes are installed on the top floor, the bottom floor and the middle equipment layer.
The temperature sensor is installed at each measuring point. The temperature sensor cable and the CAN bus (using unshielded twisted pair) must pass through the upper and lower floors of the residents along the heating pipeline. If a wireless system is used, wiring can be avoided to reduce the impact on the residents, but considering the reliability and stability of the system, a wired system was finally selected. 4 Node hardware and software design
4.1 Node hardware design
In the CAN node hardware design, the 89C51 single-chip microcomputer is used as the processor, and the CAN controller and CAN transceiver are the widely used Philips SJA1000 and 82C250. The node hardware structure block diagram is shown in Figure 4. The node has the functions of flow value accumulation, temperature acquisition, power display and power-off protection, light alarm and bus communication.

The temperature sensor uses the digital temperature sensor DS18B20 produced by DALLAS Corporation in the United States. The sensor has a temperature measurement range of -55 to +125°C and a temperature measurement resolution of up to 0.0625°C. The measurement accuracy in the range of -10°C to +85°C is ±0.5°C. Since each DS18B20 has a unique continuous 64-bit product number, multiple sensors can be connected to one cable under the control of the microprocessor. The longest cable length can reach 150 meters, which can meet the requirements of system design [7].
4.2 Node software design
The node software consists of three parts: node communication program, temperature acquisition program and flow acquisition program. Here we will briefly introduce the flow acquisition program. In northern China, a heating season is about 150 days. If the accumulated flow value is an 8-bit decimal number, the maximum flow count can reach 9999999.9 tons, which can meet the requirements of most applications. The accumulated count of node traffic in the design adopts the form of uncompressed BCD code, which occupies a total of 8 units. The units starting from 60H-67H in the 89C51 microcontroller are used as the accumulated units of software counting, with 60H as the lowest 8 bits. Traffic collection needs to record the accumulated value of traffic, so the continuity of its work is very important. However, when the node itself fails, the accumulated value of the current traffic will be lost. At this time, only the host has the accumulated value of the traffic sent by the node last collected in the entire system. Therefore, the software design of the traffic collection node should consider how to restore the accumulated value of traffic when the node is put into use again after the fault is repaired. The software design adopts the method of measuring the incremental value of traffic in the field traffic collection, and the host reads it and accumulates it. The flow chart of the node traffic collection program is shown in Figure 5.

5 Conclusion
The heating metering system based on CAN bus technology designed in this paper is suitable for the renovation of the heating system of old residential buildings in China. The test system has been running for a heating season in a five-story office building with a construction area of ​​about 6,000 square meters. The reliability and metering accuracy of the system have met the design requirements. The renovation of the heating metering system of old residential buildings is the focus and difficulty of China's heating system reform. It is hoped that the work of this paper can provide some experience for the renovation of the current civil residential heating system.
References
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