Design input requested: commercial prune dryer humidity measurement
Ken Gracey
Posts: 7,415
Hey there,
Working with a very capable farmer who we're helping with his project.
He has 10-20 "drying tubes" where we need to measure humidity and temperature (up to 180F). The prunes are shuttled through these large tubes. The tubes are located about 10 feet apart. We'd like to use as much standard Parallax hardware as possible without a PCB design (though at least one is likely, see below). We'd be using the BS2 and SHT1X sensor at each network drop. We're thinking of using RS-485 with twisted pair to provide two communication lines, Vss and Vdd. At the end of the line would be a Propeller for display purposes.
We'd use the BS2 at each dryer simply because it would be the easiest for the farmer to program, with all SHT1x code having been written and debugged quite nicely. Chances are we'd be helping him with the initial setup, but he'd take over after we got him underway. Chip would probably code the Propeller display at the end of the network.
A couple of questions:
(a) Is RS-485 appropriate for this application? Current draw at each location could be 100+ mA though they needn't run concurrently. Voltage drop a problem with 200' of wiring and 40+ connections along the way?
(b) Is there an RS-485 module that could easily be mounted on a BS2 carrier board, containing an RJ-11 (or two), transceiver chip?
(c) All of these connections in an RS-485 network a problem? There could be 40-80 connection points.
We'd like to avoid a wireless approach due to complexity of providing power everywhere, extra hardware requirements, etc.
Does anybody see a better way to do this project?
Thanks,
Ken Gracey
Parallax Inc.
Working with a very capable farmer who we're helping with his project.
He has 10-20 "drying tubes" where we need to measure humidity and temperature (up to 180F). The prunes are shuttled through these large tubes. The tubes are located about 10 feet apart. We'd like to use as much standard Parallax hardware as possible without a PCB design (though at least one is likely, see below). We'd be using the BS2 and SHT1X sensor at each network drop. We're thinking of using RS-485 with twisted pair to provide two communication lines, Vss and Vdd. At the end of the line would be a Propeller for display purposes.
We'd use the BS2 at each dryer simply because it would be the easiest for the farmer to program, with all SHT1x code having been written and debugged quite nicely. Chances are we'd be helping him with the initial setup, but he'd take over after we got him underway. Chip would probably code the Propeller display at the end of the network.
A couple of questions:
(a) Is RS-485 appropriate for this application? Current draw at each location could be 100+ mA though they needn't run concurrently. Voltage drop a problem with 200' of wiring and 40+ connections along the way?
(b) Is there an RS-485 module that could easily be mounted on a BS2 carrier board, containing an RJ-11 (or two), transceiver chip?
(c) All of these connections in an RS-485 network a problem? There could be 40-80 connection points.
We'd like to avoid a wireless approach due to complexity of providing power everywhere, extra hardware requirements, etc.
Does anybody see a better way to do this project?
Thanks,
Ken Gracey
Parallax Inc.
Comments
www.maxim-ic.com/quick_view2.cfm/qv_pk/3735
The board has two multed connectors plus a 6 pin header to pull all the cable connections out in case access is needed. It is set up for USB but you could hack it to do whatever. I have not tested this yet, it will be a few weeks till I can get to it. I can possibly spare two bare boards if you wanted to test it out.
Post Edited (Todd Chapman) : 9/2/2009 10:01:38 PM GMT
a) RS485 is definitely an appropriate solution. I have seen this type of "multi-string-node" style RS485 network many times. Proper power circuitry and quality wiring is key.
b) As Todd mentions, there are several RS485 modules available. I would suggest, however, to look at RS485 Transceiver ICs in a DIP package and use 4 pin terminal blocks for the interconnects. Your farmer probably won't be able to crimp an RJ-11 in the field, but usually always has a pocket knife to strip wire. (just an end-user thought)
c) My company recently built a very large RGB LED video wall that consisted of 108 individual beams that were 3 floors tall. The beams utilized RS485 for the LED data. These were only receivers however and that may be the reason why they ran flawlessly no matter what we did to them. With two way data, there may be other factors.
Is it possible to run individual Cat5 to each tube? I have used my SHT11 modules out to 208 feet with Cat5. You would just need a way to switch between the sensors like I am doing on my home monitoring system. With only 20 sensors, you can group the sensors and still use individual data lines. So you would end up with a display terminal ran by a Prop that has an RJ-11 breakout panel on the back? You can clock the SHT11 as slow as you want to eliminate issues.
I assume you will be using the SF1 filter caps for the SHT11 or at least some sort of debris protection? Will the SHT11 be inside the tube or will a small hole allow the SHT11 to mount to the side of the tube.
Too bad the SHT11 isn't addressable itself.......
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Andrew Williams
WBA Consulting
WBA-TH1M Sensirion SHT11 Module
'485 is definitely a good choice. 75176 chips work really well but some of the newer ones have better overvoltage/static capability.
If you use CAT5 cable, use 2 pairs for power (2 conductors in parallel for + and -). Set up comm as half -duplex, 2 wire. Good for 4000 feet.
There is an upper limit as to nodes, depending on the chip you use. But its easy enough to have the system divided into manageable sections.
Carrier board idea is good. Only a few connections so I'd consider wire-wrap sockets with connections on the bottom. Keeps the boards looking good on the top and you can easily solder the wire-wrapped joints if you're worried about failing connections. Also easy enough to make the connections through the 9-pin connector so each board can be easily changed.
Cheers,
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Tom Sisk
http://www.siskconsult.com
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