Low Power Bluetooth technology applications in modern smart meters

The existing manual meter reading method has heavy workload, poor accuracy, and can only reflect the power consumption once a month or so. Its information feedback speed is far behind the instantaneous operation of power dispatching. Therefore, it is unable to realize the different requirements of high and low peak electricity prices, so the manual meter reading mode needs to be reformed urgently. At present, some automatic meter reading schemes are to transmit the power consumption information of digital electric meter to the receiving end (such as property company or electric power management department) through telephone line or LAN by means of infrared transmission.

There are three problems with such a scheme

(1) Too many infrared control signals in the home (such as: there are many kinds of home appliances remote controls) will interfere with the accuracy of the data;

(2) It is impossible to realize real-time transmission by telephone line, and the cost of modem pool required by receiver is too high;

(3) The use of local area network (LAN) transmission is limited to the re installation of household lines and long-distance transmission of digital signals.

The key to solve the above problems is to find a wireless or wired transmission mode, and timely and accurately transmit the power consumption information to the receiving end. Through a variety of ways of comparison, the combination of Bluetooth technology and CATV equipment is a good way to avoid the above problems and achieve real-time control. That is to say, using Bluetooth technology’s characteristics of wireless transmission, short transmission distance (within 10m) and strong anti-interference, the DC signal of billing meter is first transmitted to the upstream channel of CATV line through the loaded Bluetooth chip CATV network, directly transmitted to the property company or the dispatching department of electric power management, and then demodulated the digital signal, can be input into the computer to complete the automatic inquiry of electricity charges of any user on the network. Of course, it can also be connected with the existing electricity charge network system, and connected with the dispatching and command center of the power grid to form a complete power consumption management network.



2. Anti jamming features of Bluetooth

Bluetooth chip is very small and can be directly embedded in digital meters. The ISM (industrial, scientific and medical) frequency band of 2400-2483.5mhz is used for its work, because there is no fixed signal interference from other systems in this frequency band. At the same time, the frequency band is open to the public and the frequency band is effective worldwide. But at the same time, ISM band is an open frequency band, and any frequency band will encounter unpredictable random interference sources (such as some household appliances, cordless phones and car door openers). Therefore, the interference sources of external and other Bluetooth users are treated with different technologies. For example, the Bluetooth module ms50sfa in the cloud is widely used in electric meters, which can realize various functions.

For example, frequency hopping technology is used to resist signal fading; fast frequency hopping and short packet technology are used to reduce the same frequency interference and ensure the reliability of transmission; forward error correction (FEC) coding technology is used to reduce the random noise impact in long-distance transmission; FM modulation is used to reduce the complexity of equipment. At present, the transmission rate of Bluetooth technology is designed to be 1MHz, and full duplex communication is conducted in time division mode. Its baseband protocol is a combination of circuit switching and packet switching, which fully meets the real-time requirements of meter charging. A frequency hopping frequency sends a synchronous packet. Each packet occupies one time slot and can be extended to five time slots. Bluetooth technology also supports an asynchronous data channel, or three concurrent synchronous voice channels, or a channel that simultaneously transmits asynchronous data and synchronous voice. Asynchronous channel supports asymmetric connection with maximum rate of 72 Kbps, reverse response rate of 57.6 Kbps, or symmetric connection of 432.6 Kbps, which can be flexibly selected according to the situation of interference source.

Bluetooth technology realizes the device’s connectionless work, provides the function of accessing data network, and has peripheral device interface. The Bluetooth system adopts packet based transmission: the information flow is divided into pieces (groups), and only one data packet is sent in each time slot. All packets have the same format, starting with an access code, followed by the header, and finally the payload.

Access codes are pseudo-random, and direct sequence coding can be used in some access operations. In this channel, all packet exchanges are identified by the master unit flag. Only when the access codes match, can they be received. Thus, a data packet is prevented from being received by another unit which is just loaded into the same frequency hopping carrier. In this way, the signals of different users among buildings can be easily distinguished.

3 transmission between Bluetooth and CATV

CATV as a broadband network in urban and rural areas, its uplink channel has rich band resources of 5-65m. We use 34.45mhz as carrier frequency. Using the Bluetooth interface installed at the user’s incoming line, it can be directly sent to the receiver of the property management company or the dispatching department of the electric power management by using the Bluetooth interface defined by the Bluetooth core protocol, and then modulated into digital signals to the corresponding computer.

CATV can transmit real-time data to remote receiver at any time on the network, which is the advantage of PSTN and LAN. For the problem of how to distinguish the data signals from different users at the receiving end, we make use of the unique characteristics of each main unit mark. Although the carrier frequency of each user data transmission on the network is the same, the main unit mark is used as the identification code in the transmission of CATV.

The demodulation at the receiving end can be completed by the existing cable modem, but the cost is high. This can be realized by modifying the special addressable modem, that is, changing the corresponding crystal

Post time: Jul-27-2020
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