apt403
Cast Iron
- Joined
- May 24, 2015
- Location
- Yelm, Washington
Been a minute!
I've been handed a linear motion project that's got me down this rabbit hole. My application is far less intensive, but these questions remain.
What are the established methods to achieve high-ish (sub 5mu/2 tenths) positioning accuracy and repeatability?
Figure, industrial servo w/ a ~6000 count encoder has an angular resolution of .06° (1.05x10^-3 rad). Through a 5mm pitch screw, that's .83 micron (30ish millionths) linear positioning resolution. Let's assume it's also got that level of accuracy. So, servos would appear to not be the limiting factor.
But, ballscrews aren't nearly as accurate. A C7 class ballscrew has an accuracy rating of +/- 50 micron (.002") over 300mm (12in). Even fancy C0 grade screws are only good for around +/- 4-5 micron (2 tenths) over 300mm. If your application calls for say, 1000mm travel, that cumulative error starts getting pretty bad. Screw mapping obviously sees quite a bit of use, but screws wear. The error map is only good for so long. Especially when you factor in nut preload, etc.
Okay then, the servo encoder is fast and (hopefully) accurate, but can't compensate for the screw. The screw can be mapped, but the map has a life time. Feeding the servo PID loop the output from a linear encoder (glass/magnetic scale) can compensate for lead errors, and something like a non-contact incremental magnetic encoder is a non-wear item. But, while the resolution and repeatability of linear encoders can be fantastic, +/- 10 micron accuracy seems to be a pretty typical figure.
Best thought I've had so far is to map an absolute linear encoder with some arbitrarily accurate measuring system. No wear, and the absolute encoder always knows where it is, so the map can be applied with confidence. It would appear that's how Renishaw is doing it with their RESOLUTE series of absolute encoders, and they're hitting +/- 1 micron (40 millionths) over a meter (3.3ft).
What other solutions exist?
This is all obviously assuming environmental factors (temp/humidity) are accounted for, motor-to screw couplings are sized correctly to minimize torsional deflection under load (perhaps it's warranted to treat the whole system like a big spring?), some method exists to compensate for thermal growth during operation, a sacrificial goat has been offered to the appropriate deity under a full moon, etc, etc.
Appreciate it,
- Apt
I've been handed a linear motion project that's got me down this rabbit hole. My application is far less intensive, but these questions remain.
What are the established methods to achieve high-ish (sub 5mu/2 tenths) positioning accuracy and repeatability?
Figure, industrial servo w/ a ~6000 count encoder has an angular resolution of .06° (1.05x10^-3 rad). Through a 5mm pitch screw, that's .83 micron (30ish millionths) linear positioning resolution. Let's assume it's also got that level of accuracy. So, servos would appear to not be the limiting factor.
But, ballscrews aren't nearly as accurate. A C7 class ballscrew has an accuracy rating of +/- 50 micron (.002") over 300mm (12in). Even fancy C0 grade screws are only good for around +/- 4-5 micron (2 tenths) over 300mm. If your application calls for say, 1000mm travel, that cumulative error starts getting pretty bad. Screw mapping obviously sees quite a bit of use, but screws wear. The error map is only good for so long. Especially when you factor in nut preload, etc.
Okay then, the servo encoder is fast and (hopefully) accurate, but can't compensate for the screw. The screw can be mapped, but the map has a life time. Feeding the servo PID loop the output from a linear encoder (glass/magnetic scale) can compensate for lead errors, and something like a non-contact incremental magnetic encoder is a non-wear item. But, while the resolution and repeatability of linear encoders can be fantastic, +/- 10 micron accuracy seems to be a pretty typical figure.
Best thought I've had so far is to map an absolute linear encoder with some arbitrarily accurate measuring system. No wear, and the absolute encoder always knows where it is, so the map can be applied with confidence. It would appear that's how Renishaw is doing it with their RESOLUTE series of absolute encoders, and they're hitting +/- 1 micron (40 millionths) over a meter (3.3ft).
What other solutions exist?
This is all obviously assuming environmental factors (temp/humidity) are accounted for, motor-to screw couplings are sized correctly to minimize torsional deflection under load (perhaps it's warranted to treat the whole system like a big spring?), some method exists to compensate for thermal growth during operation, a sacrificial goat has been offered to the appropriate deity under a full moon, etc, etc.
Appreciate it,
- Apt