Let's Make Robots!

Swarm Robot build

Hey LMRians!

It's been quite a long time that I haven't been here (mostly due to being in the second last year of school and other projects). Recentlly I got interested in robotics again and I came up with the idea of building swarm robots (just a little group of maybe 10 bots, not 40+ that the universities use).

Here I want to describe my thought process a little and then ask some questions.

So my first goal is to make it relatively small, something of the size like the robots seen in this excellent video by the University of Sheffield (see embedded video below).

For the basic case I have thought of the standard 50mm diameter PVC plumbing pipe that I can get cheaply from any hardware store around. The 50mm diameter would allow me to design round PCBs with a 50mm diameter that would just fit into the pipe, plus there are caps for the pipes available so I can fully enclose the electronics inside the pipe.

I plan on using motors like the Pololu 120:1 mini gearmotors in conjuction with their 32mm wheels (the ones that are used on the Pololu 3Pi robot).  Here I don't know how to trade off torque vs. speed, because if I want to let the move things around I will probably need more torque that speed, don't know if speed will be too critical anyway.

The robot should be powered by a Lithium-Polymer rechargeable battery (charger ICs are available) and a LDO regulator (eg. LM1117, 3.3V output voltage).

I will use Microchip PIC 8-bit microcontrollers (more of a random side note here).

As you can see I have got the physical specs pretty much finished.

 

Problem 1: Communication

The problem I have is that I don't know which communication interface I should be using so that the robots can communicate with eachother. 

There are two options:

Option 1: nRF24L01+ 2.4GHz wireless modules

Option 2: Infared comunication via serial data / Manchester encoding like in IR remotes (if Manchester encoding is good enough for 10Mbit Ethernet, it's definitely good enough for me)

Of course Option 2 will be cheaper as it only requires an IR LED and a TSOP31238 (or similar) + some software decoding (nothing that couldn't be done by the PIC18F45K22). My only question is how to handle the interference when multiple robots would be transmitting data at the same time, some sort of token-ring network might be needed.

 

Problem 2: Sensors

The next problem I see is the sensor equipment, especially the collission detection/avoidance sensors. Again there are multiple choices, the Sharp IR sensors, which are relatively cheap (altough I don't know if they would interfere with the commuication over IR or vice versa). Then there are ultra sonic distance sensors, the downside being that these are pretty expensive (20 bucks a pop).

Next up I though of some LDRs for relative light sensing and NTCs for temperature sensing (rough ballpark figures, no exact readings).

Is there any other sensing equipment that should be added for a general purpose bot?

Problem 3: Resources

Because I'm a student and I want to make 10 of these, I will crunch down on cost for an individual robot. Another question I have is what should I use as a front wheel so the bots don't trip over (I don't plan on building balancing bots after all).

So I am asking if you know of any good/reasonably priced sources for:

1. Small size LiPo / Li-Ion rechargeable batteries (here my question would be, what is a reasonable capacity?)

2. Small size motors with 3mm D-Shaft output that would provide a bit of torque (see above).

 

I know that there are a lot of people here capable of answering these questions, so please feel free to do so. If you have any additions, again please feel free to make suggestions!

 

Update April 2, 2013:

I've had a look around on eBay and have found some of the parts I need.

I've decided that I will use the Sharp GP2Y0A21YK as the main distance measurement sensor for obstacle avoidance because the SR04 US sensors can only be powered from 5V.

The motordriver is going to be a 3.3V compatible DRV8833 by Texas Instruments (it's even cheaper than the L293D).

The communication will be done via the nRF24L01+ modules that are cheaply available.

I have found no suitable replacement for the Polou wheels.


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Chris and the other dudes is right, try to find the best and cheapest supplier for the parts. Doing so, you can buy more cool stuff for your budget.

I am also involved in a Swarm Robot Project (now it's kind of pending):http://letsmakerobots.com/node/32511

If you are going to limit your size to 50mm diameter you might check for sensors matching that size. The Sharp IR sensor might be a bit too big. RF communication is a good way but also more expensive than IR. You can limit interferences to a minimum by using different frequencies and timing. Example: if you want to communicate then stop the navigation function for that time. Or define it in the loop like: check for communication, check distance, check something else. This way you will not flood the environment with different IR signals.

 

Run Away of this plastic motors... I have some of this and yellow version too, but they are crap.

So I was using N20 motors with those pololu 42x19mm wheel from EBay surplus for my little tiny robots.

http://www.ebay.com/itm/221027954461?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

I have 4 of this http://www.ebay.com/itm/4sets-smart-Car-Robot-Plastic-Tire-Wheel-DC-Gear-Motor-3v-5v-6v-for-avr-/290779251436?pt=LH_DefaultDomain_0&hash=item43b3cb42ec too but I didint like it.... they are not equals if you powered it with tha same voltage things not happens equal lol.... one rotates more thans others.

Until now its the best deal I find

http://www.ebay.com/itm/2sets-Smart-Car-Robot-Rubber-Tire-Wheel-N20-DC-Geared-Motor-3V-9V-Free-Ship-/321045733914?pt=LH_DefaultDomain_0&hash=item4abfd1221a

So now let talk about bat.

Have you have seen the 18650 standard for LIPO? there are 2400mAH few bigger than AA bat. look for then!

Another thing important is look to your motor current consumption... like I said I was using the first motor that I showed you, They use about 120mAh ( bothe togheter) for run my plataform.

I like to design littel robots so I will be glad to help you!

and look for my start post there are a picture of the small plataform that i Make only  to test the motors

http://letsmakerobots.com/node/37097

anything just reaach me in mail rmhulle@gmail.com

 

 

 

 

 

I have seen very small li-po batteries from micro R/C planes and helicopters, single cell were about 130 to 180mAh.

Not so good, but very very small, 1cm width and about 4cm height. There are also bigger up to 700mAh, just google on micro R/C

flying things.

I have seen a documentary on these a few months back. That's when I got interested in swarm robotics ;)

It's hard to beat the price of the Nordic nRF24L01+ modules. Here are ten for $11.50.

I had a project where I used six of these modules. I just made sure each device had its own ID and then the each device was only allowed to use the RF it an assigned time slice so two devices wouldn't be transmitting at once.

If all of your robots aren't in range of the initial transmission, you can use your swarm to message hop the transmission to make sure it ends up at the right bot.

You'll need a protocol to make sure a message doesn't just keep getting retransmitted. I assigned an ID to each message and each device that transmitted the message added its ID to the message so it would know when the message was a repeat or not. It was a fun project. 

I don't usually just come right out and plug myself like this, but your Ebay-foo does not seem very strong and I think I might be able to save you a lot of time and money.  --As just one example,  if you are going with Pololu motors and wheels, you have already spent 1/2 the price of a whole Tadpole kit. Hell, the motors alone (from Pololu) are 3 times the price of an identical motor on ebay and again, almost a 1/3 the cost of my whole robot (brain included). Not to mention, if you go with a Micro Magician board, you have IR built right in --needing only the addition of a 3-cent LED and some code.

Rocket Brand Studios

 

No, thats fine, I asked for help! :)

And yes you're right I am going to have to hunt some parts down on eBay. I want to make the board myself (I know it'd be easier to buy all the components, especially with cheap Arduino Mini clones around) because that's where I am going to learn from this project (and of course in writing the software/algorithms).

20$ for an ultrasonic sensor? I bought 10 SR04 ultrasonic sensors on Ebay for 15$... http://www.bajdi.com/?attachment_id=857