Society of Robots - Robot Forum
Electronics => Electronics => Topic started by: HyperNerd on January 02, 2010, 05:31:52 AM
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Ok guys, I am in the midst of making my Urban Robot, capable of exploring semi-autonomously and equipped with GPS, environment sensors etc., the basic stuff.
Here is where I hit a stumbling block, radio data comms. I am in the UK, and I believe I have the following options:
- I could easily hook it up to a mobile 3G interenet board with an O2 or T-Mobile SIM (the top 2 3G providers here), but I really don't want to pay the excessive data charge for this service.
- I could use WiFi, but the range is really limited unless I get permission from other people around here to use their WiFi, then put a high powered transmitter / receiver onboard to increase range. I could also use Linux and the Aircrack suite to 'borrow' some protected WiFi :D
- I could use Easy Radio boards, which I have heard are easy to set up and have very good range.
- Bluetooth is out of the question due to the high cost of the TTL boards, and the limited range caused by trees, walls, people, well, just about everything ;)
- Here is my (possible) solution, use walkie talkies. You can get some quite cheaply which claim to have 1 km+ range (0.6 miles-ish) :o, and I am prepared to do whatever is necessary to convert them into radio modules. I have good experience with soldering and circuit assembly, so this could be possible, no?
-HyperNerd
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Great to see you again man!
The walkie talkies are quite possible to use.
You could either:
Replace the microphone with a transistor with the base connected to your mcu or;
If the walkie talkie has a morse code button you could use that.
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Thanks!
The walkie talkies would definitely have a morse code button, as I will be buying them specifically for this project.
So i would just replace the morse code switch with a transistor, then link that to the MCU pin?
What would I do on the receiving end?
-HyperNerd
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Yes, for the receiving end, a transistor would have its base where the speaker would go. And connect +5v to the mcu pin.
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Nice ;D
I'm definitely going to use this now, but would it be possible to send data back, or would the timing be too complex, as only one can be transmitting/receiving at a time?
-HyperNerd
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You could do one of two things.
Program the robot to talk then wait for a response rather than both trying to talk or;
Use two sets of walkie talkies on different channels.
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Okie doke.
I'll probably use two sets of walkie talkies, as this will allow greater data throughput, as they can send and receive at the same time.
Thanks for your help :D
-HyperNerd
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Just out of curiosity, are you just going to use an I/O pin for this?
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Which end? Tx or Rx?
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Hi,
Here is my (possible) solution, use walkie talkies. You can get some quite cheaply which claim to have 1 km+ range (0.6 miles-ish) :o, and I am prepared to do whatever is necessary to convert them into radio modules. I have good experience with soldering and circuit assembly, so this could be possible, no?
Most PMRs (Personal Mobile Radio, operating on around 485 MHz, replacing regular 27 MHz walkie talkies) has got a range of 5 km in open terrain and some goes to 8 km).
However, you'll need a bit more than a transistor at each end and you need to establish a protocol befor you bey them, to make sure that your needed data rate can be realized.
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A few questions:
How much does one of these PMRs cost?
If the range is 5km in open terrain, what would the range be a suburban housing estate, if you have any idea ;)?
What kind of protocol are you talking about?
Also, what kind of circuit would I need at each end apart from a transistor and a microcontroller?
-HyperNerd
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Hi,
I have a feeling that you ain't gonna like the answers ;D
How much does one of these PMRs cost?
It varies a lot, if you buy Motorola or similar, they can be pretty expensive, but they can be bought at around DKR150,- to DKR200,- a pair (here in DK, that's around US$30 to US$40, but mind you, Denmark is a very expensive country, so I'd expect less than half that in the US).
But Google will probably help here.
I don't know where you are, but I assume it's not the US, due to your referencing in km.
If the range is 5km in open terrain, what would the range be a suburban housing estate, if you have any idea ;)?
That depends... Building material, terrain, nearby (salt water sea), etc.
You really have to try it out, as there is too many variables.
The antennas could be modified for longer range, as their physical length is inductively reduced.
What kind of protocol are you talking about?
How you're going to transmit and receive. Will you use a modem, will you use CW (like Morse code, a carrier wave without modulation), or what.
How many bits you want/need to transfer in one transmission.
The timing.
Etc.
Also, what kind of circuit would I need at each end apart from a transistor and a microcontroller?
That depends on your protocol. Define that first, then lets find out what you need to do it (if you present a possible goal ;))
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hey HyperNerd,
check this link for some ideas on what is required:
http://www.fmtv.us/uhf_rc.html (http://www.fmtv.us/uhf_rc.html)
he's using a PSOC but it would be possible to use another microcontroller type to achieve similar results.
alternatively search RCgroups.com for info on audio modems.
a lot of those guys are using the audio channel on wireless video links to return data.
the same principal would work over walkie talkies.
Use two sets of walkie talkies on different channels.
i'm not convinced using a separate pair of radios for 2 way comms would be a good idea.
if the transmitter was too close to the receiver it would cause interference even if it was transmitting on a different frequency.
the alternative is to switch one pair of radios between transmit and receive which is not a very good idea either.
synchronising the change at both ends would be tricky but solvable.
the main problem is the time a radio's transmitter takes to stabilise before it can reliably transmit.
walkie talkies are optimised for voice comms which can handle a few 10ms between the "talk" button being pressed and the channel being used. depending on your project this lag may be unacceptable.
so yea, as Soeren pointed out, no easy answers.
dunk.
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Thanks guys,
@Soeren
30-40$ (£18-25) is quite cheap actually. (I'm in the UK ;D)
As far as the protocol goes, would it be possible to simply connect the output of the PIC's UART directly to a transistor then to the transmit button of the PMR? And the same for the receiving end but the other way round? Basically using standard serial protocol.
@dunk
I had a look on the RCGroups website, but all the stuff I found baffled me :-\
I agree with you on the fact that 2 walkie talkies on close frequencies will interfere, so would it be possible to use an 'over' byte to tell the receiver that it's their turn to send?
A certain amount of lag in the control system would be acceptable as with any teleoperated vehicle, for instance the Rovio webcam thingy has considerable lag in the controls, but it is still fine to drive over the web.
I think that makes a response for everything you guys said ;)
-HyperNerd
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just out of curiosity, is 30-40 is cheap for you, why have you not considered data radios like these:
http://www.sparkfun.com/commerce/product_info.php?products_id=9099 (http://www.sparkfun.com/commerce/product_info.php?products_id=9099)
In open space they have a range of 10km.
I use them on my robot (also a GPS assisted outdoor robot)
http://www.billporter.info/?p=95 (http://www.billporter.info/?p=95)
and have had no problem operating around 3 story buildings and forests.
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Didn't see those...
I always assumed that the XBee modules had crap range, so i shied away from them.
What other stuff do you use on your robot for communication, I see some kind of large antenna at least :)
I might really consider these...
-HyperNerd
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I have 2 antennas on my robot, one is the flat white GPS antenna, one is the tall black Xbee antenna.
Theoretically, 900 Mhz communications works best Line-of-sight but can go farther, and 2.4 Ghz works best around objects and obstacles but has limited range.
I bought a set of both the radios I already linked to you (that say they can go up to 10km)
and these: http://www.sparkfun.com/commerce/product_info.php?products_id=8768 (http://www.sparkfun.com/commerce/product_info.php?products_id=8768)
that say they can go 1.6km max, but would work better around buildings and obstacles (since they are 2.4 Ghz). I have only used the 900Mhz ones, and they seem to work OK at a few hundred meters and around corners of buildings, but that is as far as i tested them. I have not tried the 2.4Ghz ones yet.
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hi HyperNerd,
just bear in mind the 900MHz band is illegal for use in the UK and much of Europe.
whether you care about the legalities or not, many companies will not ship them to the EU. (i don't know if SparkFun will.)
Digi do make a 868MHz module for use in the EU with a range of up to up to 80km but it only runs at 24KBps and is hardware limited to 10% duty cycle to meet legal power restrictions.
what all this means is if you try to send data at the full 24KBps the module will switch it's self off periodically to keep the transmit time below 10%.
i guess another way to say all this is you have to keep *average* transmission rate below 2.4KBps including all the extra data the XBees send as flow control.
Digi's page on the 900MHz module here:
http://www.digi.com/products/wireless/point-multipoint/xbee-pro-868.jsp#overview (http://www.digi.com/products/wireless/point-multipoint/xbee-pro-868.jsp#overview)
and some guys who have issues with the 10% duty cycle here:
http://paparazzi.enac.fr/wiki/Modems#Digi_XBee_Pro_868 (http://paparazzi.enac.fr/wiki/Modems#Digi_XBee_Pro_868)
(that page actually gives a good summary of some of the available radio modems.)
still with me?
personally i think the 900MHz XBee looks like more trouble than it's worth unless you absolutely need the long range and don't mind slow transmission rates.
the XBee-PRO 802.15.4 is worth looking at though. it operates on 2.4GHz.
http://www.digi.com/products/wireless/point-multipoint/xbee-series1-module.jsp#overview (http://www.digi.com/products/wireless/point-multipoint/xbee-series1-module.jsp#overview)
there seems to be a bit of confusion about the exact power output of these modules though...
for example the XBP24-AWI-001 is for the N.American market and has 63mW power output.
the XBP24-AWI-001J is sold in the EU and Japan and is limited to 10mW.
*both* these modules are advertised by Digi as having a 1 mile range...
that's just weird if you ask me....
i am very sceptical that the 10mW version would be readable at 1mile.
i just got curious enough to ask about it on the Digi forum:
http://forums.digi.com/support/forum/viewthread_thread,6878_lastpage,yes#20638 (http://forums.digi.com/support/forum/viewthread_thread,6878_lastpage,yes#20638)
when i return from my trip (around the end of February) i will resume work on my bi-directional radio system
http://sites.google.com/site/mrdunk/ (http://sites.google.com/site/mrdunk/)
but i really can't recommend it until i work some more of the bugs out of it.
anyway, enough of me,
the weather is starting to look dry enough for me to go climbing.
dunk.
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Man, and here I am complaining about the FCC here. l ???
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I hate the UK radio laws now ;D
I think it would be best to stay on the right side of the law, so I will look into the XBee PRO 2.4GHz modules.
Just had a thought... what about a microwave transmitter? Would that do the trick? i believe its what the army/navy/elite team of rc car driving ninjas use for their robots...
-HyperNerd
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Just had a thought... what about a microwave transmitter? Would that do the trick? i believe its what the army/navy/elite team of rc car driving ninjas use for their robots...
-HyperNerd
Technically, the 2.4ghz xbees, are microwave devices, as 2.4ghz is in the microwave band, and is the frequency most actually microwaves operate at.
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update from Digi:
Yes, the range is measured for the US output power. Though the international range values aren't listed, we suspect they would be approximated 75% of the US module's range.
http://forums.digi.com/support/forum/viewthread_thread,6878_lastpage,yes#20692 (http://forums.digi.com/support/forum/viewthread_thread,6878_lastpage,yes#20692)
so yea, looks like even the worldwide version of the 2.4GHz XBee-PRO has the range you are looking for.
dunk.
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Nice.
Definitely gonna order a pair of these now.
While I'm shopping, what else would I need to use these? I'm thinking antennas, breakout boards etc.
-HyperNerd
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While I'm shopping, what else would I need to use these? I'm thinking antennas, breakout boards etc.
that depends a bit on what you already have at home...
there are 4 different antenna types of XBee. make sure you order the right one.
1. the chip antenna is the most compact but it has shorter range. (you'd need to dig around on digi's website to see how much shorter.)
2. the wire antenna seems like a good compromise. better range than the chip antenna, no additional antenna required and still quite a small size. not as versatile as an external antenna though.
3. i'd stay away from the U.fl connector type module if price is a concern. you would have to buy a U.fl cable to connect to it which cost a lot for a simple cable. (i've managed to buy them from digikey.com and off ebay before.)
4. the last option is the RPSMA connector. if you want an external antenna this is the cheaper option. search ebay for "RPSMA connector" for a compatible antenna.
glancing at the datasheet the pin spacing on the XBees appears to be 2mm.
that's not going to fit your standard breadboard so some sort of breakout board is going to be needed unless you can fabricate your own PCBs.
that's all i can think of providing you are going to connect these directly to TTL microcontroller UARTs.
o, one more thing to check, look at the XBee datasheet and make sure the input voltage range on the XBee data pins is compatible with whatever your microcontroller is outputting. (some XBee models can handle 5V inputs and some can't if i remember correctly.)
dunk.
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Thanks dunk.
As this will be the first wireless module I ever buy, I have nothing already, so I will need to buy absolutely everything.
-HyperNerd