PSN-L Email List Message

Subject: Re: Digging in...
From: Gordon Couger gcouger@..........
Date: Sun, 14 Aug 2005 04:12:30 -0500

ChrisAtUpw@....... wrote:
 > In a message dated 13/08/2005, writes:
 >     To beat long cable runs and the associated problems 
wireless data links
 >     are just becoming a reasonably priced off the rack 
solutions that is
 >     reasonably easy to put in to practice. Using directional 
 >     ZigBee
 >     and BlueTooth can work several hundred yard in the clear.
 >     Maxsteam has good off the rack stuff that I know works.
 > If you want 
plug and play.
 >     Cheers
 >     Gordon
 > Hi Gordon,
 >     Thanks for the feedback.
 >     I am aware of the radio links that have been / are becoming
 > available. If you really need a long link and are happy to 
fund it,
 > fine, but do consider all the options.
 >     UHF radio links do require some skill to implement and 
are not
 > entirely 'fit and forget'. They may impose limits on the 
 > computer, both for RAM provision and on the processing rate,
 > particularly when using encrypted data. You are usually 
running several
 > linked applications for seismic data recording. Check that 
you can have
 > full 'preset' control of the transmitted data rate. This is 
not always
 > available. (e.g. If I run my 56 K modem at more than 38 K, it 
 > out' several times an hour / performs like a geriatric snail 
- due to
 > the length of the line - according to the service provider.) 
You need the
 > link to work 100% in all weather conditions. High speed UHF 
signals may
 > become garbled in woodland locations, especially during rain 
or snow /
 > where you get strong reflected multipath signals.
 >     However, it still leaves you with a remote power supply 
problem and
 > you may need links both ways for seismic work. The link to an 
ADC is
 > usually two way. Low noise amplifiers tend to have 
significant power
 > consumption. Do you need to buy battery operated power 
supplies /
 > batteries / solar panels? You may then fit an ADC and send 
 > signals, or you can generate a frequency modulated signals 
which are
 > demodulated at the receiver, but this may require several 
channels of
 > additional electronics.
 >     Somewhere along the line you have to have a time 
reference on a 24/7
 > basis to 1 second, or preferably to 0.1 second, linked to the 
ADC sample
 > timing. GPS receivers tend to be power hungry, location 
sensitive and
 > still moderately expensive. The internal software clocks which
 > are usually fitted to computers are nowhere near accurate 
enough. On my
 > 'new' computer, I can't rely on it to within 20 secs per day. 
 > the clock every hour is just not adequate. Some of the web 
time services
 > have significant and variable signal delays. (I bought a 
radio corrected
 > LCD quartz clock.) A 1 sec error on a seismic signal 
represents about 10
 > km uncertainty.
 >     Do check on the total $$$ cost / benefit if you are 
considering a
 > radio system. Check if there are similar radio systems 
operating locally
 > over similar distances? Are there any adverse local problems, 
like other
 > radio transmitters / interference sources / obstructing metal 
 > fences? Try to avoid 'buying trouble'.
 >     Regards,
 >     Chris Chapman
Hi Chris,

I don't think we are to the point that it either easy or cheap 
to run out an get a wireless link for home built toys. But it 
time to start thinking about them and including them in thoughts 
and plans.

Doing a wireless link is a large undertaking. I did the first 
work for Datalink Systems wireless stuff and the 
power supplies were designed to for 5 volts at 3 amps to 
accommodate all the various stuff we supported. However 5V 1A 
would  meet most installation requirements.  They were 30% over 
designed hopefully to be longer lived when run in temperatures 
beyond their rating. We couldn't get parts to run at the 
temperature encountered in some installations in the tropics. so 
we were careful to choose chips that had good reputations for 
running hot environments and using the highest temperature 
rated parts we could  find. Using chips like the 68HC11 that 
when run at half speed has a reasonable life at 200 c. We 
strictly warned against installing things in overly hostile 
environments but we did our best to make them live there.

Going wireless some noise problems improve.  Using long lever 
arms made of light beams can mechanically amplify the signal but 
it change the noise problem form electronics to mechanics.

Using microwave particularly the 2.4 GHz band heavy rain and wet 
foliage is always going to be a problem. There is probably not a 
worse frequency for rain interference in the low GHz band than 
they chose for ZigBee. That can be over come with higher gain 
antennas and more power.  I would use Zigbee because it is 
cheap, I know it works and the data format is not set in stone 
and it is easy to implement SLIP or something similar that you 
can use to encapsulate any kind of format you want with minimum 
over head and is easy to modify in the future.

Today I can buy 60 MHz ARM computer board from New Micros off 
the rack for $29 bucks high speed SPI or I2c A/D coverers, 
memory are cheap and 3o yard ZigBee link that I can be 
programmed to use a modification of SLIP to implement a limited to 
store and forward method for capability for digitized data to 
insure data integrity. Made from comments it could be very cost 
effective about $100 for the computer and about the same or a 
bit less for the solar power. Adding GPS for time somewhere in 
the system would cost around $50 bucks. Less for an old Rockwell 
Jupiter but that needs to be off the solar power as it draws 200 
ma with out a amplified antenna but the receiving station could 
use it to send a heart beat time message every second to update 
all the clocks on the warless network. If you make if from off 
the shelf parts it will run about 500 hundred to a thousand dollars.

Going to UHF radios limits your band width to the IF frequency 
of the radio. Of course with ARM computers being a cheap as they 
are you can put more compression and computing power at the 
remote site than I ever dreamed of.

Embedded computing have changed more in the last 2 or 3 years 
than in the past 10 as far and cost and affordability and 
increase in computing power and wireless speed that a serious 
tinker can buy.



Public Seismic Network Mailing List (PSN-L)

[ Top ] [ Back ] [ Home Page ]