Robot on wheels

[VegardAa]

Hello!
I've been playing around with an idea of making my own "robot".
I've got a HP e-pc which uses 19v (3160mA). I would like to put the computer and batteries on a platform with 3 wheels.

Two wheels at the front with one motor each, and a one wheel in the middle at the back (Like those office chair wheels, only smaller).

I would like to control this robot via a webinterface. I want it to turn both ways, and move forwards. I think this is dueable even for a noob like me. I could use opctical mouses to track each of the front wheels, and when I want it to turn it would turn one of the wheels to turn one way.

I'm planning on using linux as the OS. I want to build a "tux" that had webcam in it's eye, wierless connection, speaker, etc.

So, that's my idea atleast...

I would like help from you guys, cause I think you've got a lot more experience then I've got.

Some of my questions:

What kind of batteries should I use?

What kind of voltage should they be, 12v or 24v ? They need to supply the computer (19v) and motors with power.

What kind of motors should I use?

Any help would be great.
Sorry about my English, I'm from Norway.

 

[dknguyen]

You want rechargeable batteries. Lead acid is rugged but heavy. But that doesn't matter much for a robot on flat smooth ground which sounds like what yours will be. NiMH is probably the best for you right now- fast charge, cheap, decent weight, decent power, and decent energy density, rugged, easy to charge. NiCD is similar to NiMH but much slower charging and toxic.

I wouldn't go with LiPo for you even though they are the lightest, have the most power, and highest energy density because they are expesnive. But most importanlty they aren't rugged and are finicky to charge. If you don;t use the proper charger (can be expensive) or misuse it, they can be easily damaged or explode. Yours is a ground robot anyways on flat ground so weight isn't that much of a concern.
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If the on-board computer needs 19V, you probably want 24V batteries. It's MUCH easier to step-down a voltage than to step-up a voltage.

You don't want the battery voltage to be much higher than the required computer voltage either- only high enough so that the minimum battery voltage as the battery discharges is never below the computer voltage (plus some extra headroom/margin voltage so the regulator can function properly, called the dropout voltage of the regulator). Having it too high makes it more difficult and less efficient to regulate to a lower voltage .

And because you have 24V batteries you probably want motors that run from about 24V so you can run the motor straight from the battery without regulating, stepping-up, or stepping down the voltage.

You never want to regulate motor voltages either because motors aren't fussy about the voltage as long as it's not so high it burns out the motor or so low that the motor can't produce useful work. Regulating it would make it difficult and very inefficient because motors draw a lot of current. For that reason, you also don't want to have to step-up/down the motor voltage because it woudl also be very inefficient because of high motor currents.
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We already established ~24V motors. You want gearmotors, cheap ones. Ungeared motors have enough power but its all in speed and no torque. You need a gearbox to turn some of that speed into torque to slowly turn something like wheels that needs lots of torque. The larger the wheel diameter is, the more torque you need and less RPM you need to travel at the same speed (the circumeferance is larger so you travel more in one rotation, but the raduis is larger which is like a larger lever requiriing more torque to produce the same force at the edge of the wheel).

The advantage of larger wheels is they will go over bumps and cracks more easily (but need motors with more gearing because they require more torque and lower RPM to move a certain weight at a certain speed). This isn't that big a concern for smooth ground though so for indoor robots going with smaller wheels will let you use motors with a lower gear reduction ratio which tend to be easier to find which makes it a bit more efficient and lighter too.

Look on the net or locally for cheap DC gearhead motors (motors with a gearbox already attached). You don't want to have to get separate motors and find a matching gearbox. The only real way to do that is to go to a real motor manufacturer and that is very expensive and difficult to source. Depending on the size of the robot, here are soem possible sources/examples:

www.superdroidrobots.com
https://www.robotshop.com/world
www.lynxmotion.com
https://banebots.com/

Don't forget you need a motor controller as well. A computer can make the low power signals to control a motor, but can't output the high current, high voltage, and/or power required to actually run the motor. The motor controller accepts the low power control signals and does the the high current, high voltage, high power output required to run the motor. It's like how you can't lift up a car with your arms, but your arms are strong enough to move levers that control a crane, and the crane uses hydraulic power to lift the car.

I assume you already know a robot with two fixed, front motorized wheels (with undriven, passive support wheels at the rear) where each wheel can go forward/reverse independently of the other wheel is simplest and one of the most manuevarable flat ground robot designs.

Your english is excellent.

 

[cr0sh]

BTW - regarding the wheel at the back (like an office chair) - those are called "caster wheels", and it would be better to use two rather than a single wheel (especially if on a square or rectangular base) as the platform will be more stable and less likely to tip when turning (this tendency will be more likely the higher the center-of-gravity of your robot is, so keep the majority of weight low on the platform). Good luck with it!

 

[dknguyen]

And keep the center of gravity over the driven wheels too, for better traction and more centered and consistent steering. That's true for anything with wheels...and tracks...and legs...even if it's just a table.

 

[VegardAa]

Wow.. Thanks for the very informative answer! It really helps me learn.

I'll go with 4 wheels and 24v then.

But what kind of batteries should I buy? And how many mhA?

I think the robot is going to weigh about 4-5 kg (before batteries). How powerful should the engines be? A link to a product would be great. I'm also not sure how make the engine turn the wheels. On the sites you posted I can see many "kits" but I want to build most of it my self. I mean, buy wheels, gears, motors locally here in Norway. But ofc, if I find cheap motors and batteries in the US, I'll order.

I'm going to sit down this weekend and start making a plan on how it should look. Maybe I'll design the whole thing on the computer first. Got any tips on what program to use?

BTW: I know how to solder and I work with IT everyday.

If you have tips to what the robot should be able to do, please reply to this post. And also, if you have links to similar "DIY projects", please tell me.

Thanks for your help so far! I'll look more in to those links this weekend.

 

[dknguyen]

You want rechargeable batteries. Lead acid is rugged but heavy. But that doesn't matter much for a robot on flat smooth ground which sounds like what yours will be. NiMH is probably the best for you right now- fast charge, cheap, decent weight, decent power, and decent energy density, rugged, easy to charge. NiCD is similar to NiMH but much slower charging and toxic.

I wouldn't go with LiPo for you even though they are the lightest, have the most power, and highest energy density because they are expesnive. But most importanlty they aren't rugged and are finicky to charge. If you don;t use the proper charger (can be expensive) or misuse it, they can be easily damaged or explode. Yours is a ground robot anyways on flat ground so weight isn't that much of a concern.

The first part of the response was me talkign about battery types.

For flat ground you don't have to worry that much about rquired motor power unless your robot has to accelerate and moves unusually fast or has unusually large wheels. You should be able to eyeball it. If it turns out to not be enough torque, you just use smaller wheels (since it's flat ground this won't matter since you have no bumps to go over). It will provide more force at lower speed just like increasing the gear reduction ratio, but without actually changing the gearbox (which likely also means changing the motor).

You can for the most part assume that the force required by the wheels is less than 10% of the weight on flat level ground. The torque required is then the wheel radius multiplied by the force. Then add on some extra force so you can accelerate to your travellign speed within a reasonable amount of time/distance. The RPM the motor turns at while producing this torque is the speed your robot will travel at AFTER it accelerates up to speed. Having a car with a very high top speed but low acceleration is pointless since it will take too long to get up to top speed before you have to turn a corner or hit a red light, so you are never actually using the top speed because you never have enough time to get there. Indoor robots should be slow though...2m top speed probably.

Power is a function of the motor speed and motor torque, but lots of power is pointless if it comes in the form of way too much speed but not enough torque.

It comes from this:
https://www.electro-tech-online.com/threads/motor-sizing-for-moving-robots.23264/#post155131

 

[VegardAa]

Once again, thanks for answering.
I read the post on my HTC Desire, so I guess I missed that part.

I'll read the whole post again tonight.

 

[cr0sh]

One of the first things to work out is your power distribution scheme; it will save you a lot of headaches (and wiring) down the line. Remember to include fuses on all your positive rails. A good way to set things up is to use breaker box bus terminal strips (that might not be the wholly correct term); they're basically strips of metal drilled with holes (they come in different lengths) with screws, used for bringing grounds and neutrals in an AC mains system to common points. If you have a system using a lot of devices or large loads, they can be helpful. Otherwise, look for smaller screw terminal strips. If you are building a metal chassis for your robot, you might consider making the chassis the ground of the system, with a bus bar for the positive, a large fuse from the bar to the battery positive (sized properly for you total load), then smaller fuses for each of your other loads off that bar; those would lead (depending on the load and such) directly to loads, or to regulators (then from there on to loads). Plan this all out first, and you'll save some headache in the future.

As far as what your robot should do? That's a question we probably can't answer - you should have an idea what your robot is for before designing and building it, as your design and the components needed will be affected by what you plan on doing with it. What are your goals for it? What do you see it doing? What do you see learning from it by building and programming it?