Is it possible to modify (or maybe just configure) smoothie to "directly" drive steppers using an h-bridge instead of investing in stepper driver boards?
theoretically would be possible. However free/available output ports from MBED I don't think that it's workable. For a H bridge you need 4 pins or 2 + 1 (enable). For a motor you'll need 2 H-bridges.
And at the end what you will obtain ? A Stepper driver able to drive a motor only in full step mode ….
Doing so is possible. You could use smoothie to control directly L298 drivers. Would require some coding though.
It's also a very very bad idea. The stepper drivers are used for a good reason : driver steppers without microstepping or voltage chopping is going to give you absolutely horrible movement ( understand : your machine will wake up the whole block and try to walk off your desk ).
You really really want stepper drivers, there is a reason why everybody uses them. And they are not very expensive …
at a second sought this solution may be improved !!
A SPI DAc and you will get somehow "theoretical" very good and performant drivers !
There is a lot going into what the allegro chip for example do.
I would strongly using something like the A4989, and external drivers.
Driving the steppers directly is no small task, a lot goes into it, just look at the allegro chip's datasheet.
I fully agree with you. And believe me I have played a lot with various stepper drivers.
Indeed inside those chips are complex mechanisms.
On the other side : it's the fact that all allegro chips are for up to 2A per coil. This is more than enough for a 3D printer. For a bigger milling machine unfortunately it's needed more.
On the end as a conclusion I don't see this solution as beeing put in practice.
What I'm trying to do is to build a very inexpensive mill just for the fun of building it. Not certain I would do much with it other than maybe some very light woodworking.
I plan to learn on this project and then decide if I find it useful enough to rebuild a more "proper" version with what I have learned.
I have some 4V 1A per coil unipolar steppers that I got from an old piece of equipment that I am going to use. These were directly driven on the original board simply with transisitors. I know that on the proginal piece of equipment they ran quite smoothly with this sort of drive setup. The only thing is that only one stepper at a time was running because of the rather odd drive circuit setup. There are 7 steppers and the common lines from each motor were routed to two transistors (one for each coil pair) and when the transistor was energized the common lines were connected to ground. Then each motor had 4 other transistors that switched power through the coils.
Not using microstepping is especially problematic on mills, you'll get very high resonnance at some specific speeds and your steppers will block.
You *can* get something that will move, but it won't be very useful.
I strongly recommend spending just a few $ more and get bipolar stepper motors and correct stepper drivers.
I misspoke on the stepper motors. They are 6 lead motors with each winding having a center tap. (not sure if these are classified as unipolar or bipolar)
The ratings are as follows:
Torque @ 800Hz: 100 in-oz
Torque @ 1800Hz: 60 in-oz
Do these look to be satisfactory for a CNC machine?
Mr. Sailing_Nut , don't get me wrong but this path it's too complicated for your level now.
6 lead motors can be driven both bipolar and unipolar.
Indeed most simple method to drive a stepper it's to use unipolar drive with 4 transistors driving to ground.
You can obtain fast and simple a driver capable of half step driving.
100 in-oz would be good for some simple tests.
I'm going to take this slowly and learn along the way. Thanks for all of your help, you have taught me a lot!
FYI, I found a part called the StepStick from Botronicz (would have posted a link but I don't have enough "cred" yet) that looks like a good lower cost option for the Pololu driver. Once I'm ready I'll go with these for my drivers, unless you tell me they are crap! ;-)
StepStick is fine, with one important limitation. I'll get too hot long before you reach 2 Amps, they are designed to go up to approx 1.4A.
If you want to go up to 2A you want to use these : http://www.panucatt.com/product_p/sd82b.htm