Want stepper drivers @ > 2A ?

Electronics related to CNC

Re: Want stepper drivers @ > 2A ?

Postby Enraged » Tue Feb 14, 2012 6:53 pm

Wow.

I also have a mill (a BF20/G0704 model), and most people that convert them to CNC use a 48V system. #5 on this page is a typical system: http://www.kelinginc.net/CNCNEMA23Package3x.html

If you can run higher voltages, your part might be a good alternative to the KL-6050: http://www.kelinginc.net/KLDriver.html
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Re: Want stepper drivers @ > 2A ?

Postby frob » Tue Feb 14, 2012 7:25 pm

Well unfortunately the voltage is limited to what the IC can handle, which is 30V tops.
On the other hand, i've found that by scrounging the usual surplus outlets, i can find good high torque motors that are a good match at 24-30V and 4-7 amps. The trick i think is to start with the driver and find motors to match it, rather than the other way around.

I had in mind to also do a version for higher voltage (50V), but i can only do that with an Allegro IC similar to the a4988 (actually the a4989)
But considering what you can get for $40 from Keling, i cant foresee a lot of demand for it. Still, in an integrated 4+ axis "all-in-one" unit, it might make some sense.

The Keling products look like really excellent value to me. Honestly i couldn't compete with that- its amazing that they can make any money selling those at the price they do. the metal enclosures, pluggable terminal blocks etc, are quite expensive to source, and doing a high voltage board with all the protections and reliability enhancements appropriate to heavy duty CNC use is a lot of work and requires a lot of parts on a much bigger board. If i was doing a big CNC machine, thats exactly what i'd buy too :)

My intent here is to just to advance the state of DIY / hobby grade electronics a little, and help lower the barriers to entry for future "Makers", by providing better "no frills" cheap but well designed modules that help improve the performance, ease of use, and reliability of DIY 3D printers and such while keeping the cost as low as practical.
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View with heatsink

Postby frob » Wed Feb 15, 2012 8:03 pm

Here's a quick rendering with the copper/aluminum heatsink i'd like to use. unfortunately there not cheap!
A much cheaper and probably just as effective heatsink would be an aluminum fin extrusion that's 0.9" X 0.9", so the board would be that size in order to accommodate it.

SOTAstep1-TOPwHeatsink.png
Top view showing heatsink
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Maybe it IS possible....

Postby frob » Thu Feb 16, 2012 7:56 am

I found some much cheaper heatsinks, but a little too big for the first design ( 0.9" X 0.9").
So i wondered, if i made the board as big as the new heatsink,
could i fit the all the parts for a 50V 8A stepper driver in that amount of space?
Then i got a little carried away trying to answer that question. its a very tight squeeze, but...

50V 8A stepper driver in 900x900mil.png
50V 8A pololu "compatible" stepper driver 0.9" X 0.9"

It could work.
The layout is very tricky when things are this bunched up, but got it 70% done, including most of the hard bits.
This will have to be a 4-layer board with 2-3 Oz copper.
The board hangs out 0.3" past the headers over the terminal block sockets on the main board;
There are little problems to solve like how to mount the headers without risking shorting against the heatsink on the other side...
And drilling a hole in the latter to fit the trimpot on the other side. Vcc is still an issue.
Had to leave out the large power supply cap, so that needs to go on the main board, very nearby.

But mostly this leads me to think it would be much easier and better to use a little more space for this one - maybe 1-1/2 square inches or so would be best.
Then it could probably be done in 2 layers. for sure with 2 square inches to work with.
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Re: Want stepper drivers @ > 2A ?

Postby Enraged » Thu Feb 16, 2012 5:03 pm

what about running multiple small standard size, off the shelf heatsinks rather than one large custom on?
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Heatsinks & more - your comments please :)

Postby frob » Thu Feb 16, 2012 6:03 pm

Enraged wrote:what about running multiple small standard size, off the shelf heatsinks rather than one large custom on?


Actually i'm only considering standard sized heatsinks, custom ones are only cost effective if you need thousands of them - not there yet i think :)
The 0.9"x 0,9" is the smallest / cheapest usable heatsink i've found by far, so i just ordered a bunch of them. they're less than 1/10 the cost of the smaller "pin-fin" ones.

The latest layout was more of a design test / reality check-
i don't think I'll try to make it that way because there's too much risk of ground noise and other assembly issues as a result of the crowding.
and I'm not planning on running any batches of other 4-layer boards soon where i could just throw them into the same prototype run.
and i really rather get rid of that pot altogether.

It was useful to help figure out what the optimum size and arrangement might be.
In the end if i did such a module i would really need to add a few more parts on the board for robustness and i'd space things enough to allow a 2-layer only layout.
Size will be about 1" x 1.5" to 2" (if including screw terminals) x 1/2" thick (with heatsink)
Here's what i'm thinking - please comment on this, if enough people like it or show interest will get made, if there's no feedback i'll just drop it for now :)

1- "Pololu" compatibility - too restrictive and inconvenient for this one, so i will likely drop that in favor of a more optimum arrangement - especially considering the currents- (see below)
2- Orientation:
I'm thinking of going with a connector on one edge of the board with pins sticking out in line with the board surface, so the module installs vertically (perpendicular to the motherboard)
This has several benefits:
- much less board area used on the main board, which can be more compact;
- much better natural air convection through the heatsink and around parts on the opposite side;
- Mounting the screw terminal blocks (pluggable) directly on the module:
this i especially like as it makes the whole control system much more compact especially for 4 or more outputs.
In an enclosed box with a fan the box can be 3cm high and the space around the terminal blocks acts as exit for warm air (heatsink rotated 90 degrees)

3- Interface:
I really dislike having configuration dip switches, shorting blocks, jumpers, and especially potentiometers that need to be manually adjusted by the user.
its just too error prone and finicky especially for anyone not "expert" in electronics. One slip and you short pins with your screwdriver and smoke your board.
I would add a small micro or circuit with either SPI or I2C interface to configure it including a DAC for the reference voltage (max current level)
so it could be automatically set up correctly by the host processor.
I like the idea of making it "software compatible" with the first 3-5A 30V driver for serial configuration, if going with a SPI interface.
I2C is simpler electrically though just 2 wires.

One of the shortcomings of this stepper controller IC (still the best available) is the peak current reference has a relatively small range, 0.8 to 2V,
so if 2V = 8 Amps then 0.8V = 3.2A. If you want to be able to select a a lower peak current then you have to change the or mess with the current sense resistor value.
That's why on my test design i had put 4 sense resistors in parallel. So i have 3 choices on how to handle that:
1- follow KISS principle, live with 3.2A to 8A range. If you need less than 3A, you should get the other 100mA to 5A 30V module instead.
2- add shunts / jumpers to select a combination of sense resistor values to expand that range down;
3- add a programmable gain current sense amplifier for the sense resistor.
this has added advantage that lower value resistor can be used thus reducing power dissipation.
the downside is added complexity, a little extra cost (not much) and mostly the risk of greater susceptibility to noise and transients.

Lastly, the other thing I'm tempted to try is adding extra high side high-voltage mosfet drivers to allow much higher motor voltage - up to 180V.
Since these stepper IC's run at relatively low frequencies with very long "dead times" this is unlikely to create problems with shoot through or current sense blanking.
Does that significantly increase the appeal for such a product?

What is your opinion on all this?

Thanks for your feedback! 8-)
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