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Author Topic: low frequency wireless data transmission  (Read 960 times)

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Offline mstachoTopic starter

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low frequency wireless data transmission
« on: August 16, 2013, 09:17:12 AM »
Hi all, long time, no see.

I'm working on a redundant wireless communications system.  We're using XBee for the most part and this should work fine.  But there is a need for this to be very safe, so in case anything goes wrong we still have some kind of link with the robot.  I'm pretty sure that the big problem will be occlusions like trees, buildings, and even hills, which XBee isn't the greatest for over the ranges we want.

Are there any lower frequency wireless data technologies we can use to get very basic data from the robot?  I'm looking for somewhere on the order of at least 100 bits/second, just to send basic heartbeat info and maybe upload some safe-mode instructions, in case we lose contact.

Any thoughts?

MIke
Current project: tactile sensing systems for multifingered robot hands

Offline waltr

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Re: low frequency wireless data transmission
« Reply #1 on: August 16, 2013, 10:26:40 AM »
There are the 400MHz band ASK (OOK) transmitter/receivers and are good to 1200Baud. These do need a processor to properly encode and decode the data for reliable communications. However, these have less issues with some obstructions.
They are available in 433 and 418MHz which might work for bi-directional communication.

For unlicensed radio data there isn't that much available for use over any great distance.

Offline mstachoTopic starter

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Re: low frequency wireless data transmission
« Reply #2 on: August 16, 2013, 10:48:10 AM »
Hm, well the distances aren't SUPER big (we're talking about maybe 2km as a max it'll ever be).  My main concern is going behind a hill or something and losing all communications.  I wonder if I can even use some kind of analog transmitter like CB to handle it.

MIKE
Current project: tactile sensing systems for multifingered robot hands

Offline waltr

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Re: low frequency wireless data transmission
« Reply #3 on: August 16, 2013, 02:00:22 PM »
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some kind of analog transmitter like CB to handle it.
Not legally!

Offline jwatte

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Re: low frequency wireless data transmission
« Reply #4 on: August 16, 2013, 08:41:44 PM »
Actually, CB packet radio is acceptable under certain situations.

The 400 MHz open band might be better in the US, though. You can get cheap transcievers from China. You're not allowed a ton of power, but with appropriate modulation, it might be able to do OK. 2 km is a long way, though.
Here is a plain RF link for example: http://imall.iteadstudio.com/wireless/im120628014.html Expect significant interference, though.

Or you can just put in a cell phone GPRS module and let the telcos take care of connectivity :-)

Offline dellagd

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Re: low frequency wireless data transmission
« Reply #5 on: August 25, 2013, 11:18:41 AM »
Use ham bands! Depending on your size constraints your range can be as much as you want pretty much. 2km would be a piece of cake for a low end ham radio.

Just go out and get a Technician license, the test is so easy if you have electronics knowledge. 
« Last Edit: August 25, 2013, 11:34:06 AM by dellagd »
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Offline johnwarfin

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Re: low frequency wireless data transmission
« Reply #6 on: August 25, 2013, 06:45:06 PM »
silabs based rfm22b with 100mw @432mhz has no problem with 2-5km. with small antennas too. rfm23bp at 10x more power would be overkill. the important thing is low freq uhf like these will always outperform equivalent higher frequencies like 900mhz, 1.2ghz, or 2.4ghz due to laws of physics. for the same reason 318mhz which is also used for remote control applications has even longer range at same power. being digital also contributes 2x-4x range increase over analog of similar power.

Offline waltr

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Re: low frequency wireless data transmission
« Reply #7 on: August 26, 2013, 01:56:44 PM »
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silabs based rfm22b with 100mw @432mhz has no problem with 2-5km.
and is in the Licensed 70cm Ham Radio band of which you may NOT interfere with.

Offline jwatte

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Re: low frequency wireless data transmission
« Reply #8 on: August 26, 2013, 07:00:14 PM »
It gets a little messier.

First, 433 MHz is actually a specific band allowed for "short range" remote control things in the US (garage door openers, etc.)

Second, even if you're in the HAM/amateur band rules, these bands do allow non-speech communications under various conditions (RTTL/Data, image, etc.)
Thus, you could conceivably find a way to use a stronger transmitter within the 420-450 MHz band (in the US) if you're a licensed operator, as long as your use was non-commercial (for the general amateur classification to apply.)

If you're a technician, operating in the 70 centimeter band, you can theoretically use up to 200 Watts of power! Meanwhile, short-range devices should stay below 100 mW IIRC. In Europe, the specific 433 MHz band is for "walkie talkies" with integrated antennas, and maximum transmit power of 10 mW.

Personally, I would think that if this is an amateur/hobby project, it has just as much right to use the band as two dudes shooting the shit over the microphone :-)
« Last Edit: August 26, 2013, 07:03:11 PM by jwatte »

Offline dellagd

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Re: low frequency wireless data transmission
« Reply #9 on: August 27, 2013, 07:55:23 AM »
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If you're a technician, operating in the 70 centimeter band, you can theoretically use up to 200 Watts of power!

Not quite, at least in the US (Im not sure about Canada). Operation of model craft is limited to 1 Watt of output, but that can still get you to 20 miles or so with a nice antenna.
« Last Edit: August 27, 2013, 07:59:44 AM by dellagd »
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Offline johnwarfin

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Re: low frequency wireless data transmission
« Reply #10 on: August 27, 2013, 10:36:31 AM »
ive had general class license for some time now and also farming myself out as certification engineer i need to stay on top. after burrowing through tons of govt red tape it turns out there is a small slot in the 2.4ghz ism band for medium power control applications. 1 watt is allowed license free only for 50 or more channel hopping though. not a common protocol.

current commercial rx/tx setups and rfm22b/xl7105 are in the order of 100mw where fewer hopping channels are allowed. but no ham license requred in any case.

Offline jwatte

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Re: low frequency wireless data transmission
« Reply #11 on: August 27, 2013, 01:02:39 PM »
Quote
Not quite, at least in the US (Im not sure about Canada). Operation of model craft is limited to 1 Watt of output, but that can still get you to 20 miles or so with a nice antenna.

Right -- unlicensed! The idea was that if you got a Technician HAM radio operator's license, and your application falls within the permitted uses, then you can use Technician class transmission power in the 70 centimeter band, and I haven't found anything saying that the power limitation is less than 200 Watts in that case (except the normal "use no more than necessary to establish and maintain contact.")

Offline dellagd

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Re: low frequency wireless data transmission
« Reply #12 on: August 27, 2013, 07:26:42 PM »
Right -- unlicensed! The idea was that if you got a Technician HAM radio operator's license, and your application falls within the permitted uses, then you can use Technician class transmission power in the 70 centimeter band, and I haven't found anything saying that the power limitation is less than 200 Watts in that case (except the normal "use no more than necessary to establish and maintain contact.")


Ahh... Ok. The FCC states for amateur stations as follows:

Quote
97.215   Telecommand of model craft.

An amateur station transmitting signals to control a model craft may be operated as follows:

(a) The station identification procedure is not required for transmissions directed only to the model craft, provided that a label indicating the station call sign and the station licensee's name and address is affixed to the station transmitter.

(b) The control signals are not considered codes or ciphers intended to obscure the meaning of the communication.

(c) The transmitter power must not exceed 1 W.


This kind of operation gives you exception to the ban on ciphers and codes, so they throw in the 1 W restriction to mediate the risks.

For those interested, the Part 97 FCC Ham Regulations: http://www.ecfr.gov/cgi-bin/text-idx?c=ecfr&SID=336ab7469b61ecbfa15086dbf1bf2c59&rgn=div5&view=text&node=47:5.0.1.1.6&idno=47#47:5.0.1.1.6.1.157.1 (Ugh forgot how to make nice links...) Check out section 97.215

As far as I can see, it stinks, but that is the law for Hams.

PS: I have a General Class License: KB3ZKL is my call.
« Last Edit: August 27, 2013, 07:33:56 PM by dellagd »
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Offline jwatte

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Re: low frequency wireless data transmission
« Reply #13 on: August 27, 2013, 10:16:10 PM »
What I don't understand, though:
- packet radio is also an allowed usage, and is not considered a cipher if it uses a well-known encoding
- you can use packet radio for many things
- a roving robot can receive (and send) packet radio signals

Does this mean that, according to the regulations, if I happen to use packet radio, and happen to have a ground-based rover that partakes of that channel, I can use up to 200 Watts? I think "yes."
Now, what if that rover takes off (say it's a quadcopter or whatever.) Can I still use the same rules and power? Or does the "remote control of aircraft" rule now trump the packet radio use? Even if the link is used for telemetry, not control?

Not that I'd be willing to test this in court or anything :-)

Offline waltr

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Re: low frequency wireless data transmission
« Reply #14 on: August 28, 2013, 08:19:53 AM »
I also have a Ham license, call N3EVV.

If you use a published code, packet, RTTY, AMTOR, etc, you do not have the power restriction noted in 97.215 but you must include your station call to ID the transmission (97.119). When I've worked with experimental digital modes I've have the processor key a CW ID in Morse every 10 minutes to meet 97.119.

Part 97.217 may be applicable for the return transmission from a robot.

If you use these digital modes also be sure to stay within the approved sub-bands where these are allowed. There are also rules for the allowed bandwidth and spectral purity (harmonics) of any transmission.

200 Watts, yes I think that is technically legal but not needed for a short distance (a few km) so could not be strictly legal. I don't think you would have a problem from the FCC unless you were causing interference to either another licensed service or another Ham radio station (97.101). Remember that not all RF transmissions are line of sight, there are many other means of RF propagation even in the hundreds of MHz and GHz frequencies. In the HF bands (1 to 30MHz) I have have good contacts to Europe (I'm on the US east coast) with 1 Watt of output power. I've also worked stations hundreds of kms away with <5 Watts on 1.2 and 2.4GHz.

The FCC's rules are complicated but they need to cover many different technical and international requirements for many different services to share the RF spectrum.

 


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