Because the infrastructure for data communication in Russia is not highly developed, there is increased interest in making use of the widely available low voltage electricity distribution networks (220V/50Hz) for data transfer purposes. The networks in Russia are different when compared to those in Europe and North America – they have much higher extension and topology bifurcation, higher noise levels and a higher high-frequency attenuation rate. These differences present huge challenges to AMR system developers.
At present, AMR specialists generally recognise that only wideband signals and modulation techniques can provide acceptable noise resistance and transfer data over long distances without the need for additional retransmission stages. Until now, however, there have been obstacles to the use of this class of signal in mass AMR applications, including relatively high costs and high power consumption for both general purpose and special purpose real-time digital signal processors capable of processing signals in bandwidth of tens of kHz.
ASYMMETRICAL SIGNAL SET – KEY TO LOW COST SOLUTION
Some 12 months ago INCOTEX began developing a new approach to the problem of wideband signal application in AMR. Our data collection system, Mercury PLC, consists of a single concentrator and up to 1000 electric power meters with built-in modems. This is similar to many other systems, but data exchange between meters and concentrator is realised by means of wideband, but essentially different signals. The concentrator signal uses a modified PSK modulation type with spectrum expansion by frequency hopping, and the built-in modem signals use OFDM modulation type.
This solution makes it possible to implement the digital transceiver of the built-in modem using an universal micro-power microcontroller (Texas Instruments MSP430F1232) which does not synthesise complex ODFM signals on-the-fly while in transmit mode, as this requires too much processing power. Instead it ‘plays’ one of two pre-loaded signal tables from its FLASH memory. In receive mode the microcontroller implements a simple narrowband digital PSK receiver algorithm with a software-based receiving frequency switch.
The Texas Instruments TMS320VC5409 DSP with 64 MHz clock frequency is used as the main controller at the concentrator. The frequency band used by the data collection system in both directions is 20…95 kHz (CENELEC-A) signals amplitude around 1V.
SUPERSLOW DATA TRANSFER – KEY TO POWER SAVINGS
For most practical purposes an electric power meter can be considered as an extremely low-speed data source, generating only a few bytes of data per day or per month. The Mercury PLC system uses this fact to reduce the speed of data transfer on the built-in modem to as low as fractions of a bit per second. The built-in modem transceiver transmits short (3.2 ms) noise-like ‘flashes’ with long intervals (about 5 seconds) inbetween. This enables a powerful output driver (0.25A in pulse) to be used, which is driven from capacitors that recharge slowly during the time between the tranmission pulses.
This means that the consumption current of the built-in modem is defined almost completely by the digital receiver, which amounts to around 8mA at a supply voltage of 3.6V. This in turn enables the modem to operate from an inexpensive network power supply source using a ballast capacitor. The built-in modem is implemented as a daughter board enabling it to be installed over the meter PCB, and its dimensions – including the network power supply unit – are 70x50x16 mm. The meter can operate both as a stand-alone unit, or with modem installed.
Unlike other data collection systems built using many identical modems, the Mercury PLC system represents an asymmetrical solution. The basic role of the system central unit (concentrator) is to provide the timing synchronisation to the slave unit transceivers, needed to distribute their signals in time. In order to simplify bit synchronisation procedures, all system units use zero level crossing points of the main network voltage harmonic (50Hz) as primary time marks.
In addition, all data in the system is transferred periodically by fixed length packets of exactly 64 bits. This allows the concentrator to start using the signal accumulation feature for an increase in the signal-to-noise working ratio in very noisy receiving conditions. System sensitivity in signal-to-noise ratio is around –20dB, and in attenuation around –60dB, meaning that modem signals can easily operate in 20 storey multi-dwelling houses.
At present a number of Mercury PLC data collection systems are successfully installed in Moscow and the Moscow region. Later this year INCOTEX will begin mass production of inexpensive single- and three-phase multi-tariff meters (Mercury-200 and Mercury-230) based on the Texas Instruments MSP430FE425 micro-controllers with built-in modems and Mercury-225 concentrators.