Hi > On Jul 5, 2015, at 3:17 PM, Jim Lux <[email protected]> wrote: > > On 7/5/15 8:43 AM, Bob Camp wrote: >> Hi >> >>> On Jul 5, 2015, at 8:46 AM, Jim Lux <[email protected]> wrote: >>> >>> On 7/4/15 7:53 PM, Hal Murray wrote: >>>> >>>> [email protected] said: >>>>> Exactly... I've got an array of mirrors on az/el mounts (two servos >>>>> stacked) and the reflection from the mirrors on the wall forms the >>>>> display. >>>> >>>> How many pixels in that display? Or what is the unit of quality >>>> measurement? >>>> >>>> What sort of ADEV are you aiming for? If your goal is solar time rather >>>> than >>>> TAI or UTC, you should be able to get pretty good. >>>> >>> >>> >>> Prototype is 6 pixels to demonstrate concept and work out the bugs. Long >>> term, probably several dozen. >>> >>> Time Accuracy? better than a second >>> >>> Turns out, having done some experimenting, the real issue is angular >>> accuracy. RC servos aren't all that great, and have significant jitter >>> (probably not an issue in their design application which tends to have good >>> mechanical low pass filtering). They're cheap and easy to use (as in, I >>> had a bunch in the garage I could cannibalize out of another project). >>> >>> But if you have 3x3 inch mirrors (call it 7.5 cm), and want to create a >>> picture on the wall that's, say, 10 meters away, you really need angular >>> pointing of 0.007 radians.. that's about 1/2 degree. An RC servo has >>> roughly 270 degree rotation corresponding to 256 steps of PWM (in the >>> Arduino implementation). >> >> Probably a good place to use the “drive a stepper as a selsyn trick. >> Steppers are dirt cheap these days and you can either program the drive >> yourself or get chips that will do it for you. You have essentially zero >> load and zero acceleration. There is no need for anything big. >> > Indeed, microstepping might be the way to go in a production system. > > But steppers don't have convenient mechanical mounting stuff like RC servos > do. I could assemble my prototype with zip ties, double sided foam tape and a > few screws. For a stepper scheme I'd need to design and build (e.g. > fabricate) bracketry. It's also more complex than just plugging a servo into > a pin on the Arduino; that's pretty easy.
*Small*steppers (which is all you need) don’t take much in the way of mounts. More or less that’s why they invented 3D printing. A printed mount is plenty good enough in this case. > > And then you also get into the "do you really want to use an arduino, why not > program a X microcontroller on a custom board you've designed for the > purpose with all the driver components, etc.” Feature creep - that’s my middle name …... > > If I were building up a full scale system, that's probably what I'd do. BUT, > in the mean time, my 6 RC servo az/el thingys are good to fool with and get a > feel for various configurations and what the design issues on a larger system > would be. A *lot* of home built milling machines are lashed together out of steppers with various drivers. It is a bit of a step up from R/C servos, but not *that* bit a step. > > > The virtue of the BBB and Arduino scheme is mostly that it can be cobbled > together without much work. And you can leverage large consumer equipment > volumes for the actuators, servos are <$10 each in any sort of quantity; it > would be hard to find a packaged motor/gear train with a feedback pot for > that much (leaving aside surplus). That’s not all that different than the way home made mills are built. > > I used to have a box of small 200 step/rev motors (floppy drive positioners), > but they had a weird sized shaft, so we're back to the fabrication of mounts: > the servo has a nice splined nylon shaft that mates with cheap other > injection molded stuff. 3D printing ….You *must* have a friend with a printer …. Bob > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
