Electronic Design

European Standard Seeks To Simplify Home Automation While Trimming Costs

When it comes to home appliances, independent home-automation network nodes—including their own power supply and appliance interfaces—shouldn't cost more than $10. In the appliance sector, low cost is at least as important as the appliance's technical features. Of course, home automation is totally different from building automation.

Unfortunately, the most elegant method for keeping node costs down is unreasonable: it's too expensive to install new cables, even in houses that are still being built. Another potential remedy, infrared, only works within single rooms. Therefore, the only acceptable solutions include RF transmission and data transmission via powerline modems (PLMs).

According to the Cenelec standards, PLMs only can operate between 3 and 148.5 kHz. The 3- to 95-kHz band is already reserved for the energy supplier. And, anybody can broadcast between 95 and 125 kHz without sticking to a specific protocol. Babyphones are a typical example of this operation. The frequency range above 125 kHz is accessible only according to a specific protocol.

To address the needs of consumers, service providers, and appliance manufacturers, a group of European companies has founded the European Home Systems Association (EHSA). Headquartered in Brussels, Belgium, EHSA has established the European Home System (EHS) standard. EHS uses the 125- to 140-kHz band, with a carrier frequency of 132.5 kHz.

According to the EHS access protocol, a device may only transmit data for a maximum of one second before it pauses broadcasting for at least 125 ms. Whenever a device wants to transmit, it first listens to the line to avoid collisions. In most cases, the recipient acknowledges correctly receiving the data package. Correct data transmission is then ensured, despite the absence of a collision-detection mechanism. EHS's net data rate is 2400 bits/s.

Spikes and spurious signals aren't the main problems of powerline transmission. The key hurdle is network impedance. According to the access protocol, the transmit power with EHS is set to 116 dBµV (in the future, this will increase to 122 dBµV) at 50 Ω. But in almost every case, these 50-Ω impedances are never reached.

Switch-mode power supplies and many other devices are equipped with filter capacitors to suppress harmonics. These capacitors also shortcut the powerline carrier frequency, producing tremendous impedance problems. In most cases, though, it's enough to add a plug/socket combination with a choke in series to fix things.

Today, EHS protocol software version 1.3 has been established as a standard. The first commercial hardware solution for PLMs built according to EHS 1.3 is now available from STMicroelectronics Inc. in sample quantities. The ST7538 is upwards-compatible with its predecessor, the ST7537, and it offers additional features. For example, the new IC isn't just integrated into a smaller package. The maximum supply current was reduced from 35 to 5 mA, while the carrier peak output voltage increased from a maximum of 3.7 V to a maximum of 14 V.

To facilitate design-ins, STMicroelectronics offers a demonstration and evaluation kit that was jointly developed with French software manufacturer Trialog. The ST7537 HS1 enables data transmission via the mains as well as through mains-independent twisted-pair cables if a separate power supply is provided. This permits the setup of a fully isolated demonstration solution.

Via an API, it's possible to write C code for simple applications. This API contains functions for the handling registration and the enrollment. Both of these are fundamental mechanisms for the plug-and-play feature. Yet there also are functions for transmitting and receiving EHS commands via the EHS powerline network.

EHS and its successor, currently named Convergence, aren't totally focused on powerline transmission. They also allow RF transmission. Designers looking into RF data transmission typically consider Bluetooth (see "With Devices Ready To Go, Bluetooth Is Poised To Make Its Move," Electronic Design, July 24, p. 85). But even though Bluetooth may be used in a universal way, it doesn't seem to be the first choice. Only low data rates are needed for appliance communication, and Bluetooth's networking capabilities are quite limited.

"There is no RF technique covering all requirements," reports Fritz Jauss, R&D manager of home and building technologies at Honeywell, Germany. "Therefore, we have to carefully select the technology. The problem of Convergence is not the technology, the problem is costs." The appliance manufacturers are interested in RF modules that cost less than $5.

Look at the data traffic within a home-automation system for appliances. Short messages are sent out in 98% of all cases. Only 2% of the messages require faster data rates for uploading/downloading. Due to the fact that high data rates are always a tradeoff with sensitivity, range, and costs, it's a good decision to choose a low data-transmission rate for home automation to increase the system's sensitivity and range while keeping costs relatively low.

EHS uses the 868- to 870-MHz frequency band, with preferences for subbands at 868.7 and 869.2 MHz. Within one hour, the duty cycle may reach a maximum of 0.1%. This means that no device can transmit for more than 3.6 s per hour. Furthermore, devices have to stop transmitting at least after 0.725 s and pause sending data for at least 0.725 s. Frequency shift keying (FSK) modulation techniques seem to be the dominating transmission technique for the future in this application. Unfortunately, no dedicated RF semiconductor solutions for EHS are available or announced so far.

The first applications are about to start mass production now. Centrica, the owner of British Gas, will equip its gas heaters at over 7 million customers' locations with an EHS PLM. Another PLM will be installed close to the phone gateway for remote control and diagnosis of these heaters. The advantages are obvious. Only a minimum of installation is required, as the entire communication within the home takes places via the mains lines.

Load-Management System
Another example is the load-management system from BTicino, which uses powerline transmission between 125 and 140 kHz for communication. In Italy, the monthly basic fee for electric power significantly increases with the maximum peak power rate. So, most households voted for a maximum peak power rate of around 3 kW to keep costs low. Before now, blackouts often occurred when, say, the dishwasher and the washing machine were running simultaneously. The load-management system controls different energy-hungry appliances to prevent the load limiter from causing a blackout.

The system consists of a feature controller and up to 16 intelligent sockets. Each one is connected to a load that needs to be controlled. The feature controller can manage the available power coming from the utility and prevent the intervention of the energy meter thermal protection. It also can manage differentiated tariffs to save energy. The sequence for the load switching off is decided by the user, who sets the load priority with a selector positioned on the intelligent socket.

For more information about EHS, point your browser to www.ehsa.com. Also, check out sites like www.st.com, www.trialog.com, and www.bticino.it.

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