This combination greatly promotes the development of sensor technology and the process of informationization. The application of fieldbus technology has promoted the development of sensors in the direction of intelligence and networking. At the measurement control level of the automation process, a large number of intelligent sensors are connected together via a field bus to form a distributed networked measurement and control system. However, due to historical reasons, there is no unified fieldbus standard in the world. The existing protocols such as Profibus, FF, Lonworks, HART and CAN are incompatible with each other, and the interoperability is poor. It is not possible to interconnect and exchange, nor can it be unified configuration, which will adversely affect the expansion and maintenance of the system. It is difficult or even impossible to ensure that the designed sensor fully satisfies these protocols, which greatly limits the industrial application of networked smart sensors.
The industry urgently needs a sensor interface standard with broad application prospects and wide acceptance to solve the problem of interconnecting sensors and sensors and networks.
1. Intelligent sensor based on IEEE1451 standard
In 1994, IEEE and NIST jointly initiated the development of "Intelligent Sensor Interface Standard IEEE1451". After years of efforts, the IEEE1451.2 and IEEE1451.1 networked intelligent sensor standards were adopted in 1997 and 1999 respectively, and the P1451.3 and P1451.4 working groups were established to further expand the IEEE1451.2 standard. The IEEE1451 standard has received active support from a number of large companies including Boeing and Hewlett-Packard. Using a general-purpose A/D or D/A converter as the I/O interface of the smart sensor interface module STIM, converting the analog quantities of the various sensors used into data with standard specified formats, together with the sensor electronic data sheet TEDS and network The adapter NCAP connection enables the sensor data to be networked as a separate node of the network according to the protocol specified by the network, and has the configuration and interoperability of the network node. TEDS stores all the information needed to describe a STIM: manufacturer, data format, physical unit, serial number, measurement range, and correction factor. These data can be provided to the NCAP or other parts of the system for self-description and correction of the STIM. The application of the IEEE1451 standard greatly simplifies the design of networked smart sensors.
2. IP sensor based on embedded Internet
2.1 Proposal of IP sensor
The IEEE1451 standard has greatly facilitated the development of networked smart sensor technology. However, since the promulgation of the IEEE 1451.2 standard in 1997, the standard has not been supported by the majority of sensor manufacturers. The discussion of this standard is mainly at the level of academic research, and it is difficult to obtain practical applications.
The reasons are mainly reflected in the following aspects.
a. Network protocols are difficult to unify. The network independent information model proposed by the IEEE1451.1 standard theoretically solves the sensor interface problem caused by the incompatibility and non-interoperability of various bus protocols. However, in the case that each bus technology manufacturer still maintains its own interests and is unwilling to promote its use, the standard is difficult to do.
b. Sensor independent interface does not have broad application prospects. The IEEE 1451.2 standard specifies a digital interface standard, TII, based on the serial peripheral interface protocol. This interface will be difficult for high-speed, high-precision A/D and D/A converters and other high-frequency applications.
c. NCAP is too complex to be implemented at low cost. The network independent information model defined by the IEEE1451.1 standard is a relatively complete general model. From the perspective of practical application, the model is too complicated and difficult to implement, and lacks a smart sensor information model with relatively simple functions.
It is worth noting that compared to complex and expensive NCAP, STIM, which can be implemented at low cost, has been favored by many sensor users, and promotes the networked intelligent sensor standard from proprietary bus technology to Ethernet with broader application space. The direction of the network is developing. This development will surely bring a new "de facto" networked smart sensor standard. In addition, the maturity of silicon microelectronics technology has made it possible to realize a complex structure of micro-electro-mechanical systems in a single chip, which not only solves the technical problems of the embedded microcontroller and the Internet connection, but also reduces the connection cost to industrial applications. The extent of acceptance. The development of this technology has led to the emergence of networked smart sensors based on the embedded Internet, called IP sensors. IP sensor refers to a new type of networked intelligent sensor that combines sensors and embedded Inter2net technology in a modular structure based on the standard TCP/IP protocol, and communicates directly with the computer network as an independent network node. Therefore, the on-site measurement and control data is posted on the network and distributed and shared in real time within the reach of the network. After the analog signal output by the sensitive component is processed by A/D conversion and data processing, the network protocol processor implements the encapsulation and network transmission of the TCP/IP data packet. In turn, the network protocol processor can accept data and commands transmitted to other nodes on the network to implement operations on the node.2.2 IP sensor prototype implementation
In order to simplify the design and reduce the cost, the IP sensor has tailored the intelligent sensor information model based on the IEEE1451.1 standard, retains the STIM structure and function of the IEEE1451.2 standard, and extends TEDS to the TCP/IP network protocol. For the carrier, the sensor data is transmitted via Ethernet. The IP sensor is mainly composed of two parts in the overall structure: smart sensor interface module STIM and network protocol processor module NPPM. NPPM is mainly used for sending and receiving TCP/IP packets. On the one hand, the data and commands of other network nodes are received, and the message is parsed and then sent to the STIM for execution; on the other hand, the data of the STIM is received, and the packet is encapsulated and transmitted to the designated network node. [nextpage]
Here, the network node can be either a PC or other IP sensors. Obviously, the IP sensor is essentially an embedded device with Ethernet communication function that integrates STIM and NPPM. STIM is used for the sensor interface part and NPPM is used for the network interface part. In order to coordinate the data communication between STIM and NPPM, IP sensor eliminates the difficult to implement TII interface based on synchronous serial data transfer protocol. Based on ISA standard, a dual port data buffer DPBI is designed to ensure reliable between the two. Data exchange and STIM extensions. The IP sensor developed based on the universal 8-bit microprocessor is based on UBICOM's SX52BD to implement a simplified intelligent sensor information model NPPM. Based on the ADUC812 of Analog Devices, the IEEE1451.2 standard-compliant STIM structure is completed. The ADUC812's built-in flash data memory is used in TEDS implementations to support self-identification and self-description of IP sensors in distributed network environments. IP sensors have the following advantages.
a. Using the most popular network communication protocol TCP/IP as the carrier, use the cheap Internet to transmit sensor data. This means that IP sensors have a broader application space.
b. The application of TCP/IP protocol enables technicians to manage and configure IP sensors online through a browser. This means that distributed networked measurement and control based on Ethernet is possible.
c. IP sensors have a "plug and play" that allows them to be dynamically plugged into an existing system without changing any network structure. This means that the measurement and control system can be dynamically built and reorganized as needed.
d. The openness of the IEEE 1451.2 standard makes IP sensors based on this standard very flexible. This means that IP sensor-based systems have good scalability and maintainability. A typical IP sensor-based distributed measurement control system connects multiple IP sensors, control nodes, and central control units together in a common network. The IP sensor is used to implement parameter measurement and transmit data to other nodes in the network. The control node acquires the required data from the network according to needs, and formulates corresponding control methods and executes corresponding control outputs according to the data. In the whole system, each sensor node and control node are independent and autonomous. The number of control nodes and IP sensors depends on the application requirements and can be dynamically increased and decreased as needed. The choice of network can be either the internal Ethernet of the enterprise or the Internet directly.
3. IP sensor network delay analysis
IP sensor uses TCP/IP protocol as the carrier to transmit data via Ethernet. Its data transmission performance is inevitably limited by network delay, and it will directly affect whether IP sensor can obtain a wide range of practical applications. In a networked measurement and control system, IP sensors and control nodes are connected together via Ethernet. Different routings allow sensor packets to be transmitted along different lines, and the inherent transmission uncertainty of CSM/ACD results in the instability of IP sensor data transmission and the randomness of transmission delay. However, with the use of switched set lines and the increase in Ethernet data transmission rates, this problem has been significantly improved. By limiting the network load, the probability of data collision can be greatly reduced, especially in low-load LAN applications, IP sensors also have a wide application space. In general, in the distributed sensor network based on Ethernet, the total network delay of the IP sensor is TTOT, and there are: TTOT=Tc+Tv+Tp where Tc—the communication delay Tv—the disturbance Delay Tp————Execution delay
Obviously, only Tc and Tv are affected by network communication, which is the main research content of IP sensor network delay analysis. It is worth mentioning that the network delay is strongly dependent on the network load. It is very difficult to construct a precise mathematical model of network delay. There is no law to follow at the micro level, and only the statistical characteristics can be studied from a macro perspective.
4. Test and result analysis
In the Internet application, the control message protocol ICMP is mainly used to test the network reachability of the destination host. Any host that receives the ICMP echo request will form a loopback response and return it to the original sender. The time RTT is actually the sum of the time after the data packet is sent from the source to the destination and back to the source, which can roughly reflect the change of the sum of Tc and Tv.
The 21st century will be the era of embedded Internet. According to relevant experts, embedded devices in next-generation network devices will increase greatly, and 70% will be embedded devices. If the embedded sensor device can connect to the Internet, it can be easily and inexpensively transmitted to any place in the world.
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