Where are you located ?
the main idea of our project is the have a unit that log sensors, transmit data onto our server
Can be done with Arduino,
can upgrade its configuration remotely (including firmware)
upgrade a config, can be done over ethernet; sketch can check new config once a day or so, fetch it and store it in its EEPROM or on SDcard
upgrade firmware over internet, cannot be done => X
Sensors are high quality sensors from the market : hygroclip from rotronic for the temp and RH, kipp&zonen for radiation, young for wind, watermarks from irrometer for soil humidity, decagon sensors,...
Please provide as much details as you can about the sensors, that is the only way to check feasability # IO pins needed
TYPE, NUMBER, WORKING RANGE, ACCURACY, FREQUENCY, LOCAL/ REMOTE, DATASHEET
DS18B20, 2, -10 .. 50 C, 0.5C, once per minute, remote, www.somewher.com/
Transmission, we want to give the choice to our client : gprs (if no LAN), ethernet, wifi otherwise. Is it possible to offer it as an option and pick the appropriate shield ? Can the firmware manage an automated recognition of the shield plugged and us it to transmit data onto our server ?
ethernet and wifi may be interchangeable. Supporting autorecognition of shields takes quite some resources (you need "drivers/libs" for both) which restricts room for sensors.
Remote sensors : we also thing about sensors in the field (like watermark humidity probes) but cable are not welcomed in such environment. Can we use Xbee to communicate between main unit ant such sensors. Do we need an arduino board per sensor or does the Xbee have enough 'intelligence' to deal directly with the main unit ?
Arduino can support XBEE, if XBEE can talk directly to the sensor depends on the sensor
We also care about memory issues and data losses (if the network fails for a while). Any comment on this ? do we need an extra shield ?
Build a 2 stage system,
read sensors and store values to SDcard
once per hour/day upload to server,
if failed try again in an hour
an sdCard can hold 2GB of data (or more) which is enough for quite some samples (depending on #sensores, frequency, dataformat, ...)
We also care about electric consumption (is the arduino efficient on this point ?) has anybody realized projects far from it computer, with battery (and solar panels) ?
The Arduino itself doesn't use very much, but with every sensor it adds up. The frequency of sampling makes sleep modes possible or not. It is hard to say without the compete sensor list
If you check the forum (and the rest of internet) you see that all kind of remote projects have been done. Most use (CAR) batteries, sometimes in combination with solar or wind.
All ideas you have are possible - except the remote firmware upgrade - with standard Arduino, if all options can be combined and still work is a question that needs far more information before it can be answered.