What type of Motor would I need and where should I get it?

So, I have just barely got my Arduino, (I got a Mega) and have been looking for some Stepper motors for robotics. I have always been interested in vex robotics but as it is so expensive to get started and continue, I decided to go a less expensive route. Knex. I have a few sets of knex that I would be able to mechanically move, however when I look on eBay (my usual haunt for items like this) I can only find Nema 17 motors, which from what I know are bigger motors for use in 3D printing (or at least that is where I first found them). So my big question is this:

What size of motor would I need to power a Knex robot? I would be going from wheels to arm movements, and would like them to go at around 4 rotations a second, although it does not matter as I can step it up with gears. (If you know a good place to get gears for the motors that would be helpful as well)

And the second thing is where can I get a stepper motor for a reasonable price(with or without the drivers, as long as I can run it with the Arduing Mega)? I have around $10 to get four different motors to drive this thing.

8ByteRemmargorp

I think you will be struggling to get four motors for around $10 all up.

Postage alone will probably be that much.

I've never heard of Knex before, maybe I should get out more. Is this product just for kids?

Ken

Have a look at Stepper Motor Basics. It includes a simple method to determine the required torque.

That Tutorial does not deal with the small 28BYJ-48 unipolar stepper motors - maybe they would be sufficient for Knex.

A Nema 17 motor is quite heavy. I bought these Nema 17 motors partly because the price was good - but they are a lot more expensive than $10 for 4

...R

Hi Robin,

Did you have a look at the video that you linked to?

The presenter advocated that a servo could be driven directly from the Arduino.

I thought that was a no no, your comments?

Ken

Unfortunately the smaller NEMA14 and NEMA11 sizes are more expensive than NEMA17, due to
volume of sales being a lot less, but that's life :frowning:

neksmerj:
Did you have a look at the video that you linked to?

The presenter advocated that a servo could be driven directly from the Arduino.

I don't believe I linked to any video.

I don't advocate powering servos from an Arduino.

...R

neksmerj:
8ByteRemmargorp

I’ve never heard of Knex before, maybe I should get out more. Is this product just for kids?

Ken

It’s no surprise you have not heard of them before, as they are an older toy (even older than Legos) and have lost a lot of popularity ever since the downgraded to a smaller less durable version of the rods they make.

It is mainly used for kids but they have been used for some quite impressive applications such as building a computer that is powered by gravity (let balls fall through them)

The easiest way I can think to describe them is think Legos, but instead of a brick approach it is more with rods that can be used to make structures. If you think of Construction when only the frame was finished that would pretty much describe them.

MarkT:
Unfortunately the smaller NEMA14 and NEMA11 sizes are more expensive than NEMA17, due to
volume of sales being a lot less, but that's life :frowning:

Thanks for letting me know! So from the sounds of it my best bet is to save up another 20 bucks and buy them on auction on eBay. They sell them in lots of 5 or 8. My last question would be would I be able to drive these motors from the arduino? Or would I need to buy the Motor shield? Would I be able to drive the NEMA 17 motors without the drivers that come with them? I'm not afraid to take a programmatically more complex approach.

And then one last item I can think of is where could I get gears that would fit the 5mm NEMA17 rod? It needs to be able to interact with this gear somehow.

http://www.knexusergroup.org.uk/acatalog/K90985.jpg

I just realized my error. Although it will be nice to get a stepper motor in the future for use with armo movement and higher torque applications, what I need now is some servo motors instead. Can anyone suggest one that I could use with the knex gearing?

You are not likely to find an off-the-shelf servo that you can connect a knex gear to directly.

What you will instead need to do is adapt a knex gear to the servo’s shaft, using one of the supplied “servo horns”.

These are plates or small “arms” with a hub that mates with the servo’s splined shaft, and are secured onto the shaft with a small screw. Virtually every new servo will come with a set of these horns. The horns have a variety of holes drilled into them, generally so you can attach pieces of stiff piano wire (bent with hooks on the ends) to transfer the motion. Alternatively, the holes can be attached to a part to move using more small screws. You can also use hot-glues, epoxies, or (in theory) “welding” the part to a compatible piece of plastic using heat.

For a knex gear, the best servo horn to use will be a circular one. You will likely have to modify the gear itself, to - so that there is a flat face to attach the horn to (plus drilling out the holes for the screws). Once you have the gear attached to the servo - then you need to figure out how to attach the servo to the rest of your construction so that the gear will properly engage with other gears…

Servos come in a variety of sizes - the most typical size being called “standard”; these are probably the smallest you’ll be able to use for a knex system. Smaller servos are typically called “mini” or “micro” or “9g” (9 gram); they are fairly small, and likely would be very difficult to adapt to knex usage, but you might find them useful for some lighter weight tasks. Larger servos are also available, but most hobbyists don’t use them (they are mainly for larger scale models, which aren’t as popular mainly due to cost).

Servos also come in a variety of configurations for their internals; cheap servos have plastic gears and plastic bearings (actually, plastic bushings) - while better servos will have metal gears and metal ball-bearings. Great servos have ball bearings on both ends of the output shaft (for better support and less wear under heavy loads). Expect to pay for it, though. However, for many applications these better servos are more than warranted, because by going cheap, you’ll just wear out your servos quickly, and ultimately end up paying the same amount or more in replacement costs as you would have if you had bought the more expensive servo (and generally, when the bearings in these go bad, you can replace them instead of buying a new servo!).

Servos generally only move at most about 270 degrees in rotation; so-called “360 servos” or “continuous-rotation servos”, while made from a servo, no longer have a “servoing” function - instead, they have been modified so that they will continue to run without any end-stops, based on the signal being sent to them. You can buy these pre-made, or you can make them yourself (plenty of info online about how to do this).

A handy reference on servos can be found here:

http://www.bpesolutions.com/bpemanuals/servo.info.pdf

Lastly, be also aware of servo-signal controlled linear actuators (http://www.firgelli.com/) - while expensive, these can be used to move much larger loads (provided you only need limited linear or rotational motion).

A good all-around supplier for servos (and other robotics parts) - while not the cheapest, the quality is there: https://www.servocity.com/

Finally, for some tasks, look into using DC gear motors, instead of servos or steppers (for instance, driving the wheels of a robot). They tend to be cheaper, although they take a small bit more effort to hook up and control (and, if you need feedback - aka, like a servo - you’ll have some work to do, but once you understand how a servo works, it isn’t that great of an issue).

I hope this helps. :slight_smile:

cr0sh:
....
I hope this helps. :slight_smile:

That is more than a help Thank you for all of your feedback that you have given me. For actually purchasing the pieces I found a store while although it is a bit out of my way looks promising enough to look at. The store is called Rencoe and is in Salt Lake City Utah in Amererica. Basically it's a store that takes software components and salvages them. From the reviews I read it was 100 fold greater than Radio shak. Thanks for your suggestion I will still look into servo city, and thank you for all the information about the knex gearing!

Here is an ebay link to five small stepper motors with drivers. I can't speak to how good they are but I found them and bookmarked them recently.

EBAY LINK

~12 bucks for all 5 with free shipping