mawa wrote:@wayoutwest:
have you thought about using 1/2, 1/4, 1/8 or 1/16 step drivers like the trinamic pan.drive for the A axis stepper?
I'm already using the TinyG's 1/8th microstepping. Upgrading to ST's chips is high on my to-do list; they offer 1/32 microstepping but that's not my main reason for liking them.
mawa wrote:
Due to the fact that there is almost no torque load the positioning should be very accurate and with very little slew.
Open-loop microstepping isn't a cure-all for positional accuracy; doubling the number of microsteps will not help accuracy nearly as much as doubling the gearing or halving the motors' step angle.
At rest a microstep is the result of making the motors' two phases fight against each other, with some fixed ratio (1:2, 1:4, 1:8, etc) between the drive strength applied to the two H-bridges. But just setting a particular ratio in drive strength doesn't guarantee you get that ratio of position, and it radically increases the number of factors involved in accuracy.
Think of it this way: with whole-stepping you don't have to exactly match the resistance of the wires leading to the two phases of the motor. If they're mismatched the machine will run a bit choppier but still come to a stop at exactly the same position. If you're relying on microstepped positions, your positioning within a full step now relies on stuff like this! Seriously, now a slightly-weaker solder joint can affect the final resting position of the machine. Or minor ESD stress that slightly weakened one branch of your steppers' H-bridges. Ugh. For 1:2 and 1:4 microstepping this is still an obvious win, since the uncertainty added by all that other gunk is not likely to be more than 12.5%. But as you start pushing towards 1:8, 1:16, etc you have to ask yourself if all of these (likely uncontrolled) electrical properties are more trustworthy than the mechanical properties. Can you make solder joints with 3.125% consistency in resistance? Are you sure the heatsinking of the motors' two coils is identical?
I dunno, part of my aversion here may be as a digital IC design guy. It's been drilled into my head that the relative drive strengths of components are not something to be trusted.
On the other hand ultrafine microstepping would be a very nifty tool for integrating a linear encoder. It can allow really really cheap mechanics to be as precise as the linear encoder if you mainly care about the final resting position (and not the path). But good linear encoders seem to be more expensive than good steppers right now. Trying to figure out some trick for getting a high-resolution linear encoder is also on my to-do list, but further down. Probably somehow leveraging cheap PCB manufacturing. Going closed-loop would make a lot of mechanical stuff matter a lot less.