Starting on a Robot

[dan500]

Hi Guys,
Been lurking on this forum and reading through threads on and off for at least half a year now and finally decided I will be making a robot. However, there is a twist... I will be doing it in the school workshop as a school project and unfortunately, this means I doubt I will be able to add a weapon until I bring it home due to our health and safety conscious teacher. I will also be adding all sorts of other circuits including a water gun but I do aim to make this all removable and make it easy to be able to add a weapon in the future. I am assuming water is banned?

I will get a CAD representation done as soon as possible, but I already have a lot of questions so here I go:

1) Chassis - I have been looking through numerous threads and have come to the conclusion that HDPE is probably the way to go... Might also add some aluminium I have at school to the front to make it look good Only thing is, should I make a frame at all, or just screw and bolt the HDPE together? Welding probably isn't an option, as none of the teachers as far as I know, actually know how to weld - will ask them in January though. It would be good if it could take quite a bit of weight on top as well (Year 7's in the workshop when I'm not in...), is HDPE still the best thing? I suppose a lot of this still depends on design so I will decide on what I want ASAP.

2) Motors - I think I'll probably go with drill motors. I would like it to be quite powerful though, so will probably go with 4WD. I see some people have used the £10 12V argos ones - are they any good? Or would some higher quality 18v's be better? I am conscious of the fact that I have a smallish budget though.

3) ESCs - In the past, I have always made my own small motor driver however, only up to a current of about 5A. So what brands do people recommend? Or do I become really stupid, and make my own explosive ESC? lol

4) Wheels - The Robochallenge blue wheels look really good. £14 a wheel.... :shock: One thing though - are they suitable for outdoor use, or will they wear?

From what I have seen of the forum so far, everyone on here is very helpful - so hopefully I will have a robot, even if not combat ready, done by the end of next school term. One last thing, I will most likely be trying to source as much as possible second hand - so if anyone has stuff lying around, from old wheels to a nice big sheet of HDPE (lol), I'd be really grateful if you'd be willing to sell to me.

Thanks in advance.

-Dan

[psycho_fling]

Hi there Dan

Welcome to the forum good choice of entering this world

as you have probably seen from the various threads there are many options for chassis. some people (i have done this) dont bother with frames and uses the armour to form the chassis as well as protect the innereds. for your project HDPE is probs the best thing, as it can be screwed or secured with aluminimum angle pieces

drills are again a good choice

definitely go for a ready made esc option you will save yourself a lot of hassle, robot power or dimension engineering controllers are the best choice for a smaller budget

if you go for drills the robochallenge wheels are the best choice easier than anything else

i have some hdpe and some 75mm robochallenge wheels that i can sell on if your interested

once again welcome and enjoy the forum

alex

[RogueTwoRobots]

Welcome to the forum Dan; please leave your time, money and sanity at the door in exchange for a helluva lot of fun

Sounds like you've got a good understanding of what you want/what you need and have taken the time to do some research and read the forum - top marks for a start!
A shame that you can't put a weapon on it in school due to your H&S conscious teacher but he's probably just covering his back. I like the sound of a water gun
Yes water is banned in main events/competitions but can be good for casual fun and soaking of any friends/siblings

To answer some of your questions:

HDPE is cheap and good for making a chassis/frame out of; it's best just to bolt or screw it together rather than making a skeleton frame from something and then affixing the HDPE to that. If you're planning on having any parts that are going to be bearing high loads (weapon mounts etc) it's best to make these out of something more solid. With regard to plastics, that could be Ultra-High Molecular Weight Polyethylene - or UHMWPE for short - or Nylon, or aluminium or steel if you opt for metal. It will all depend on your design but if you are just building a basic pusher to start off with and want it to take quite a bit of weight on top then it may be better to make the robot out of Nylon right from the start; much tougher and stronger than HDPE. But like I said, that all depends on your design and we can advise you more once we've seen what you've got in mind

For what it's worth, and for a first robot, the £10 Argos drill motors will be fine for 4WD using four of them. So that's £40 for your drive motors and if you overvolt them to 18V that will give a bit more power. Couple them with the Robo Challenge blue wheels and you'll have yourself some very respectable pushing power.

ESCs - yeah, buy one
It's much easier, and less stressful, in the grand scheme of things. Options for this include a pair of Electronize controllers (so you'd need two units, one controller per side), a Scorpion XL/XXL from Robot Power or a Sabretooth from Dimension Engineering (with these models, you'd only need one for your robot) These seem to be the main methods of motor control in the featherweight category at the moment.

The Robo Challenge blue wheels are indeed very good wheels. On their own they provide excellent grip but I tend to modify mine by putting a layer of bicycle tyre over my wheels as I feel it improves grip and gives a little extra speed due to the increased diameter.
The £14 price is for a pair of wheels, so £7 each
They will have no problems with wear outside; they will wear over time but that depends how much abuse they're put through and also due to the fact that general wear-and-tear occurs naturally.

I don't have much in the way of second-hand stuff to sell at the moment, but I'll be going through my bits and bobs soon so anything that could be useful I'll post up.

Best of luck

[jonny]

just to let you know you get 2 robo challenge wheels for £14

edit: you beat me to it jamie!

[RogueTwoRobots]

Mwah ha ha ha ha :twisted:

[dan500]

Thanks a lot for your replies - I can already see I'm not gonna have much money left spare at the end of this lol
Alex, I will possibly be forced to take you up on your offer of HDPE and wheels!

Anyway, as to Chassis - I'll wait and see what you guys say when I have a design.
Motors - Am probably going to order some £10 drill motors then, does anyone know the specs of them though? RPM, torque, current etc.?
ESC - Not entirely sure which ones, will probably go for the cheapest if I buy them. Think any of them might like a website redesign in exchange for some ESCs? lol
Wheels - Can't believe I didn't see it was for a pair. Will almost definitely be getting blue wheels - whether new or second-hand is another matter.

Thanks a lot - will get my design done ASAP! Started in Autodesk Inventor but couldn't be bothered to work out how to use it, so gone back to good old sketch up

- Dan

[jonny]

argos drills are 550 rpm (i think)

[Danjr1]

good, cheap ESCs (had no issues with them so far and im running 4 of them) ebay number 160521866706

[dan500]

good, cheap ESCs (had no issues with them so far and im running 4 of them) ebay number 160521866706

Thanks for that - they look like they will do the job.
Favourited for when I decide on which ESC

Edit: One thing with these is they only take up to 12v and I was planning on using 18... Wonder whether they will hold up.

[RogueTwoRobots]

Edit: One thing with these is they only take up to 12v and I was planning on using 18... Wonder whether they will hold up.

In a word, no.
They are good ESCs for starting out with if you're on a really tight budget but the absolute most you can run these on is 14.4V. This means that if you ever want to build a robot that runs on 18V (in your case) or, say 24V, these are useless.

Speed controllers are one of those things where you want to invest some money in because:
a) you can get a good quality reliable unit to control your robot with, and
b) most of the slightly more expensive controllers operate up to around 24V so you will have a great deal of flexibility if you want to use the same speed controller in a different robot (running a different voltage) in the future.

If it's possible, I'd advise you to save up a little more and invest in a decent speed controller from the start. It'll be more expensive in the short-term, but in the long-term you'll be making a saving as you won't need to go and buy a new ESC when you want to change voltage or robot.

Motors - Am probably going to order some £10 drill motors then, does anyone know the specs of them though? RPM, torque, current etc.?

As Jonny said, most drill motors are around the 500-550rpm mark. Other specs tend to be a bit varied when it comes to drill motors; there is no hard and fast data on them as a lot of them are made cheaply in the East and as such, quality, performance and specification can vary greatly.
However, I tend to assume an average nominal current of 8-10A for a basic drill motor. The lowest spec speed controller I've ever used with a drill motor is a 15A Electronize unit and I've never burnt any out. I know some people have used the 5A and 10A Sabretooth models with a pair of drill motors with mixed results. 10A seems to just manage, 5A struggles I believe.

I currently use a Scorpion XL in my robot that can handle 12.5A per side. This has current limiting (so it cuts out if the actual current exceeds the level it can cope with) and I've only ever made it cut out under hard acceleration, which is when the operating current can tend to exceed the nominal value; under normal driving there are no problems, so personally, 12.5A is about the lowest I would go in a speed controller.