Radio telemetry is not suited to all environments, the first primary example being underground. Alternatives need investigation with adverse situations such as these.
Other times the mere locations are not conducive to single frequency, single type installations. A primary example being any where excessive distances are involved or the station situated in an extremely remote location. Three options are open to the engineer:
Each are discussed in full below.
The advantage over normal hard cabling is the fact that the twisted pair may be a spare on a telephone or other comms network thus saving on cabling costs. The signals are usually RS485 and are thus 'balanced' and won't interfere with other sensitive audio channels.
With all signals presented on only one pair of wires (as this pair is cabled to each telemetry module) every signal is available for output at each module where required.
Such systems greatly reduce the cabling issues found with conventional hardwired control systems.
An advantage would be if such systems had radio interfaces allowing all the signal gathering and distribution within a plant to be done with hardwire, and by radio when some distance is involved. An example would be from a mine shaft to a control room a few hundred metres away, the pure nature of a mine not being conducive to cable on the surface and radio underground.
A word of warning (you just knew there had to be a catch!): Many protocols are not conducive to RS485 comms owing to the simple fact that there is no balance between the number of '1' bits and '0' bits. On long transmissions over long distances the line starts to 'bias' in the direction of the predominant bit. MODBUS is not suited to RS485 communications and anyone suffering with an installation remaining unreliable should read the next paragraph attentively.
The answer to badly written protocols is the use of 'base-band' modems. There are two types and both are used but for separate reasons. The first is a 'Full Duplex' type and usually has a separate channel for transmit and receive (and therefore uses two pairs of wires). These are used for single point to point links and cannot be used in multi-drop installations.
'Half Duplex' types come in two flavours, '4-wire' and '2-wire', the first using, as per the full duplex models, two pairs of wires, the latter a single pair making them highly suited to multi-drop installations and therefore MODBUS or similar protocols.
The method of operation of base-band modems is a single frequency that is either halved or patterned during the transition of a bit from 1 -> 0 or 0 -> 1 or simple halved for all '1's and normal for all '0's. Base-band modems run at the frequency = baud e.g. 9600 baud = 9600Hz and the lines must be able to accommodate this.
These systems are usually not conducive to control environments unless simple commands required only few times a day are all that is called for. An example of this is the modules available for turning on your house lights while still enjoying a pint in the pub etc.
These systems do, however, lend themselves to data-logging scenarios where the trend of a monitored value e.g. wind speeds and direction for a wind farm survey. Here the data has no bearing on the 'now' but rather an overall trend or pattern. Collection could be anything from a couple of times a day to a few times a year. As long as the data logger is accessed by (or, itself, accesses) the download computer before it fills its memory then integrity is maintained.
Such systems are widely available with superb examples manufactured by Telog in the States (the 3308, an 8 channel comprehensive logger, pictured alongside).
Potential weaknesses with such loggers, especially in extremely remote locations, is the reliance on the PSTN network to remain operational. Remote sites are prone to telephone line failures as most lines are unprotected against harvesters, cranes, rigging equipment, and crop-spraying aircraft! Even if they manage to avoid these 'life threatening' dangers there is a high likelihood they are noisy and have poor audio quality attributed to the fact they are usually open conductors and are more supported by trees than the poles they were originally mounted on.
In this day and age of wanting everything done faster the old expression 'it was the tortoise who won the race' has more relevance with PSTN based telemetry systems than is realised. Many reliable systems use no more than 2400 baud - those who remember the original 'Internet' as BBSs will also remember when 2400 was regarded lightning! Again, those who remember 2400 will also remember just how reliable the connection was! Almost anything could be thrown at the modem and the data would still get through. Today even the slightest phase problem on an audio circuit will force the modern-day modem to close down and retry to establish the link (re-train).
If the data download or requirement is such that a permanent link need be in place then the use of Private Wire or Dedicated Channel should be investigated. These are equivalent to a 'permanent telephone connection' but bypass the switching equipment thus removing the possibility of the line being 'dropped'. These are by no means a cheap alternative and should be carefully considered.
The very first thing to consider is the setup costs. Cellular modems cost 1 - 1.5 times as much as a standard telephone installation. And it's downhill from here with 'line' rental about twice and calls up to 5 times as much as a PSTN line. If, and only if your data collection is temporary, or there is no chance of a PSTN connection, should such installations be considered. Oh, one more snag, data speeds are 9600 baud max and that's on a good day. As proof, when have you last had a cellphone conversation without a single interruption owing to some difficulty with the network?
UMTS is promising to rectify a lot of these issues but that is all it is at present, a promise. Until the networks roll out across the globe and experience of overloading gained, it has to remain an unknown. What is evident is the satellite based telemetry companies are seeing UMTS as a threat and are trying to sign up as many customers as possible before this cheaper alternative becomes available. They know full well that switching technologies is an expensive business and may deter existing clients to convert to UMTS when available. This seems to indicate that UMTS, according to the satellite based suppliers that is, may very well deliver on its promises. Time will tell.
Cellular links are not all bad news. If you accept the above arguments and realize the pitfalls then there may well be a place where it provides the answer. As long as the criteria of not expecting 'Internet speeds' and the possible dropped call (that's if the network will allow you to make it) then cellular telemetry has some distinct advantages.
Setup is almost instantaneous. Apart from the usual waiting for the network to register the telephone (a brand new installation) then installation is almost 'plug and go'. As long as there is a reasonable signal strength from a 'base station' then the rest should merely be a case of wire up any signals required, configuring the recorder, and finishing off with a cup of tea while admiring your handiwork.
Closing comment: If asked whether personal consideration would be given to a cellular based telemetry system over radio telemetry, the answer would be a simple "No".
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