Wireless PC-controlled robot, a bit confused about a few things...

[MarZer]

Hi guys!

I've always had an interest in building things, my last project was a pneumatic spud launcher. We and a friend had an idea to extend that idea into a computer-controlled turret, and from there the idea turned into building a computer-controlled robot.

The robot itself will be a self-contained VIA EPIA-based computer, encased in an old, gutted Macintosh SE/30 that I found whilst rummaging around in my University's bins . The robot PC will act as a 'server', I'll connect over a wireless network as the 'client' from my PC and control the robot's movements using my Logitech RumblePad 2. I've already written this part of the client/server software, able to connect to the server, capture and send client's controller status to the server and update things on the server appropriately. Other clients can connect and chat with the person in control, and the server can assign control to any other connected client that has a controller. A webcam inside the robot (behind the 'screen') streams the feed back to any connected clients. So far this has all been tested between computers over my home network, using my tablet pc as the server.

That part of the project isn't too much of an issue, once I get the robot hardware it would just be a matter of figuring out the appropriate control method and encapsulating that in the server program. The part I'm having difficulty with is deciding how I'll physically interface the PC with the other components.

I figure that I'll use 4 6v motors + wheels (2 motors per side) to manouver the robot, with 2 signals for the motors (one side per signal, tank-like control) and each motor on an H-bridge (is it possible to use two h-bridges instead of 4, hooking the motors up in parallel? or would this draw too much current for the h-bridge components to deal with?). I've chosen these motors because I have a fair few of them sitting around, and they are all identical.

The power source will probably be one or two 12v UPS/Alarm/Motorised Scooter SLA Batteries, with the PC powered using an M2-ATX mobile PSU or something. A friend has a switching transformer thingy to provide 6v to the other bits and pieces.

I've come across this sort of thing: Velleman USB Interface Kit. I know it's a little pricey, but I have very little electronics experience (hence asking about this ). Would this be suitable for the task? I'm also planning on building 2-axis arms for the robot, but that would come later, so would this be able to accommodate any such expansion?

If you know of any simpler/cheaper ways of doing this then I would appreciate the help!

EDIT: I also plan to build a very small plastic BB launcher in one of the arms, and I was thinking of using a solenoid as the firing mechanism. Would this be strong enough to shoot a very small plastic ball?

[ilovetabasco]

There's sort of a whole long process involved here. Those motors sound pretty small, so before you proceed I would first estimate the total weight of the vehicle and make sure they can drive it (don't forget each motor only must move 1/4 of the total weight, tutorial at http://www.societyofrobots.com/mechanics_dynamics.shtml).

Hopefully they can, in which case you will need the motor controller. If you plan drive this tank style, you can attach both motors in parallel to the H-bridges, but you will need twice the current rating on the bridges. So if the motors have a stall current of 5 AMPS, you need a peak current rating of 10 AMPS.

Now for some more bad news. Motors can really mess with you power (causing brownouts, surges, noise, etc) so most people I have found put their electronics (in your case a full computer) on a second battery. This isn't required but its something to consider.

Digital Output requirements:
4 channels to control two H-Bridges
1 channel per servo (count them up for your arms)
1 channel for fire control
+ any channels to control turrent azimuth, barrel elevation,

The board you found only has 8 outputs and we are approaching that cap very quickly. If you use the same motors for elevation and azimuth, then we're looking at two more H-bridges and 4 more outputs. So.... you might consider jumping to the bigger board (but its 3x the price).

Now I believe you are just about at the same place I was a few weeks ago. I was originally planning on using a PCI card with a whole bunch of digital outputs, but finally decided that I needed to offload all this work to a microcontroller. In fact, those boards you were looking at are simply microcontrollers. You seem to be pretty handy with the code, so I would recommend looking into these things more.

By the way, looking at the size of you spud launcher (really cool) you're probably designing something very similar to my own project
http://www.societyofrobots.com/robotforum/index.php?topic=145.0
http://www.societyofrobots.com/robotforum/index.php?topic=166.0

The main difference is a) mine needs to run offroad (in conditions like you would find at a construction site after a rain) b)I plan on making mine autonomous, which means you have more free cycles on your computer and c) instead of arms I have 2 gimbled cameras. My robot would fit in the heavy weight category of battlebots (~200 lbs) and requires HUGE motors, HUGE H-bridges, HUGE controllers, etc. Hopefully you weigh a lot less. But I'd love to collaborate more if we're working towards the same goal. Let me know if this was helpful.
-ilt

[Admin]

Yea, 6V motors sound a little weak. Ive had two robots fail in the past cause I didnt do the math, just check to make sure.

As for the solenoid trigger idea, sounds like the best option - but again, verify the force first.

As for wireless communication, check this post out:
http://www.societyofrobots.com/robotforum/index.php?topic=104.0

Search the forum for wireless, bluetooth, and easy radio too.

$43 controller is actually a decent price. 
What is your budget?

Also, although H-bridges are the cheapest since you make them yourself, motor drivers are easier and better to use if you have the money.

[MarZer]

But I'd love to collaborate more if we're working towards the same goal. Let me know if this was helpful.
-ilt

Thanks, I'd appreciate having someone else doing a similar thing to bounce ideas off of. I read your tank thread the other day, and I must say its a very ambitious project, I hope it all works out, it would be fantastic to see in action!

As for the second battery idea, I am still considering that, indeed it was part of my initial design, but when I recieved the first battery and saw how damn heavy it was, I figured two would be far too much for this project. Having said that though, based on what Admin has recommended, I may end up going with bigger (12v) motors. That would have the added effect of avoiding inefficient 12v -> 6v voodoo, so I'll certainly look into it.

Thankyou for the advice about the H-Bridges and motor controllers. I was considering the bigger board, but unfortunately my budget is a little restrictive at the moment. I guess because the larger board is so much more, I can simply buy a second smaller one later on and still be financially better off

Oh and I'm glad you like the spud gun, it's an absolute riot, especially considering it's the perfect golf ball launching apparatus. Those things really go!

I'm also planning to make my robot autonomous, once I get the manual control interface working 100%, I'll jump off that cliff. Looking forward to hounding you for ideas then

Yea, 6V motors sound a little weak. Ive had two robots fail in the past cause I didnt do the math, just check to make sure.

As for the solenoid trigger idea, sounds like the best option - but again, verify the force first.

As for wireless communication, check this post out:
http://www.societyofrobots.com/robotforum/index.php?topic=104.0

Search the forum for wireless, bluetooth, and easy radio too.

$43 controller is actually a decent price. 
What is your budget?

Also, although H-bridges are the cheapest since you make them yourself, motor drivers are easier and better to use if you have the money.

At the moment I'm an unemployed uni student, so my budget is pretty small for this one. Well, in fairness, my budget is _essentially_ unlimited, I'm happy to stretch this project out over a very long period of time to save up for things if it means getting a much better robot in the end.

For wireless communication, I was considering just using WiFi over my home network. My router + custom antenna has a very long range (800m+) and the signal strength is generally very good for a lot of that distance (even when roaming using a Tablet or Laptop). I have no experience using WiFi in this field though, so is there anything major I might not know or have not considered?

My plan with the motors is basically to just get it going and see how well it does. They cost me next to nothing and would be handy in something else if they don't cut the mustard here, so its really a win-win.

Fantastic site by the way, when I decided I'd like to build a robot I had a terrible time trying to find a definitive resource until I stumbled upon this place (which is strange, as large and diverse as the internet is, you'd think there would be zillions of robot sites).


Thanks guys, expect to hear from me a lot more in the future when I break stuff!

[Admin]

when I decided I'd like to build a robot I had a terrible time trying to find a definitive resource until I stumbled upon this place (which is strange, as large and diverse as the internet is, you'd think there would be zillions of robot sites).

Yea when I tried to make my first bot, an internet search failed to yield anything useful. 3 years later (summer 2005) I searched again, and still nothing! So I made the site days later . . . Glad you like 

[MarZer]

Well, an update on the motors:

I did some calculations using your RMF equation and learnt something quite pleasing. The current weight of my robot stuff is about 5.1kg. I already have all the heavy components, the only things left to go in are some circuit boards etc, which I dont yet have, and possibly some other stuff later, so I based my calculations on a robot with a weight of 6000g (6kg).

The motors I have (I've since learnt are Mabuchi RS-360RH's) are rated at 47.2 g cm torque and 13700 rpm (~230rps, MUCH more powerful than I thought they were ), giving them a (metric) RMF of 10856. I figure I want the robot to go at least human walking speed (approx 1ms-1) with an accelleration of about 0.5ms-2. To achieve that each motor would need to have a (metric) RMF of at least 5968. Friction notwithstanding, I guess this gives me a large margin of error, as well as leaving plenty of room for additional stuff, perhaps even another battery...

(If those RMF values look very high to you, that's because I did my calculations using grams and centimetres.)

I've also learnt that the motors have a stall current of 7.7A, so if I went the 2x H-Bridge method, they'd need to be able to deal with at most ~16A. You guys know how I'd be able to facilitate that? Could I take an existing H-Bridge design and merely swap components out for higher-current ones? If so, I have no idea what I'm looking for, so a bump in the right direction would be much appreciated

EDIT: Incidentally, ILT (or anybody else for that matter), if you did want to collaborate on this stuff, you can find me on MSN Messenger: marzer_ (at) hotmail (dot) com

[Admin]

7.7A stall current on a 6V motor!?! 
Sounds powerful . . .

this is a motor driver that can handle 2 motors at up to 10A each:
http://www.dimensionengineering.com/Sabertooth2X10.htm
There are others out there too, just browse.

For a H-bridge to support higher current, all you need to do is facilitate a higher heat transfer rate. Due to internal efficiencies from resistance, there is thermal waste generated that can literally melt the components. To support higher currents, you need to do things like add bigger heatsinks, cooling fans, etc. Or you can put two H-bridges in parallel, dividing the current each gets by half.

But I highly recommend getting the driver, as it will save you tons of time and frustration, and you can use it for robots in the future . . .

[MarZer]

Hmmmn, those motor controllers seem to be exactly what I need, the only issue I can see is that they run on 24v DC, the highest voltage I have is 12v DC. Would this be worth a workaround?

[Admin]

Actually, the very first line in the datasheet says:

Input voltage: 6-24V nominal, 30V absolute max.

So 24V is the maximum safe voltage, but it will work just as fine on 6V. It will probably run more efficient at higher voltages, say 12V or above, just by the physics of a mosfet. But 6V will definitely work.

[MarZer]

Actually, the very first line in the datasheet says:

Input voltage: 6-24V nominal, 30V absolute max.

So 24V is the maximum safe voltage, but it will work just as fine on 6V. It will probably run more efficient at higher voltages, say 12V or above, just by the physics of a mosfet. But 6V will definitely work.

Ah, silly me. Teach me for not reading the specs

I reckon I will go with the second battery idea, running the PC and such from the 12v, 7ah battery, and getting a 6v, 12ah battery for the motors, servos etc. I did some rough calculations about the added weight etc, and it would seem that the extra ~2.2kg of another SLA battery will still be well within the capabilities of those motorsl.

I guess that means I need some sort of job during the end of year holidays to pay for all of this now...