Kliment Conversations

1

Great to hear from you, thanks for responding. You can see more about our work at this TED talk. Our immediate need is 2.5A and up for serious stepper motors for CNC Torch Table Prototype II (prototype I was shown in the TED talk)

http://opensourceecology.org/wiki/File:Table_Frame.JPG

I would like this to be 1-off producible via CNC circuit mill, such as the open source Shapeoko (I'd like to know if you know of any better off-shelf open source variants of a CNC circuit mill?). That is our preferred route for rapid prototyping, until the point where we stabilize a design. In any case, our goal is to make something designed for an off-shelf CNC circuit mill to enable full control of the technology by the user.

Interestingly, I looked up the driver chip and found this immediately - it appears to be open source -

http://www.thingiverse.com/thing:40540

With fully-assembled board available:

https://shop.germanreprap.com/en/Powerlolu-Stepper-Motor-Driverup-to-10A

Can you help us design a prototype driver equivalent of the above that we can mill with a CNC circuit mill? It appears that the component cost would be about $20 per channel in single units?

2

Yep, that's actually based on a design of mine called STOMP that is GPLv3 licensed, so yes, it is open source. I'm not entirely happy with the lack of link to source on the product page, but at least it's available. The 10A is optimistic, and the current is highly dependent on the current sense resistors, so it's not going to do anything like a 2A-10A range. You have to pick a fairly small range of coil currents and match the current sense resistor value to those. So I'll need to know what motors you plan to use it with. It's a very limited chip in that sense, but the price is right.

Can you help us design a prototype driver equivalent of the above that we can mill with a CNC circuit mill? It appears that the component cost would be about $20 per channel in single units?

Component cost can be less than that, the bulk of cost other than the chip and FETs are connectors. I can definitely try to make a single-layer version of it, but it's likely to end up using many jumpers. What sort of trace/space can you get on a shapeoko? Toner transfer might be a better way for prototyping with chips spaced this densely.

3

The datasheet of the A4989 specifies that the reference voltage (Vref) for the current limiter is 0.8-2.0V.

The current limit is I=Vref/(8*Rsens), where Rsens is the resistance of the current sense resistors. These convert the current flowing through the circuit to a voltage that is fed into the A4989's feedback circuit to drive the power control.

So if we use say 0.05 Ohm current sense resistors, as on that board, we have 0.4V of Vref per A of motor current, so we can use motors between 2A and 5A. This requires that the current sense resistor be able to dissipate 0.05 Ohm * (5A)² = 1.25W. The voltage across the current sense resistor may not exceed 1V, which limits its value as well (the maximum current is 1V/Rsens). So if we want to have, say 20A of motor current, then we would need to use extremely small value current sense resistors, for example 0.01 Ohm. We can then have 20A current limiting by setting Vref to 1.6V (1.6V/(8*0.01)(V/A)=20A). However, the driver would then be unusable for any coil currents below 10A (0.8V/(8*0.01)(V/A) = 10A). The resistors would then need a power rating of more than 4W and that 4W of heat will need to be dissipated somehow. Conversely, if we want the driver to run motors with 2.5A coil current, we would need 0.04 Ohm current sense resistors, which would then be unable to run anything over 6.25A.

When using very small value resistors trace resistance of the copper and the solder joints becomes a major issue, making the current control inaccurate. Irregular trace resistance is made worse by milling as it results in uneven surfaces and variable trace widths.

We had the Xylotex 425 oz in motorst - http://www.xylotex.com/Econo4Ax425.htm

These motors have a 2.5A max coil current. If you use microstepping, you would probably be better served by using a TI DRV8825-based driver for them. That way we can keep material costs at single Qs around the $12 mark. I have a design for those, but it's extremely compact and uses a double-sided board with lots of vias. I can send you a sample of this particular driver if you like so you can try and break it. Source is at https://github.com/kliment/cooldrv. I can look into making a millable design for it, but of course it's also a 0.5mm pitch eTSSOP so it's marginal on a mill.