Bart replied to the topic 'Celestron focus motor' in the forum. 2 months ago

Hi Mohamed,

You don't need all that resolution on a Celestron. it is already 'geared' by the focuser thread inside. 200 steps/rev is plenty. 400 would also be fine.
I use 400 steps/rev on my Maksutov Newton telescope with a Crayford style focuser. This is a much more sensitive focuser, because the translation per rotation of the focus shaft is more. Diameter of shaft: 4 mm, and so: 4 * Pi / 400 = 0.0314 mm/step (full step!) This is a faster system ( f / 4.8 ) which is more critical to focusing than the Celestron ( f / 6.3 ) and even then it -just works- with no problem.

I think I used 16x microstepping with that system. What this is, is that the stepper driver interpolates the electrical current through the phases of the stepper motor, so the magnetic angle can be 'anywhere' within the normal full (native) step angle.

Don't use all that heavy gearboxes and just direct-drive that stuff :-)
The beauty of the Trinamic drivers is that they can be controlled over SPI protocol, so digital communication with the focus controller (arduino like) and can quickly change settings such as stepper current and microstepping. So you could, if you like, change the microstepping value dynamically. This is almost useless for a focuser, because it's a slow system anyway (but fast enough!).
The change in current, however, is exceptionally useful in this regard, because you don't want to heat up everything by holding the stepper at a certain angle with high current. You want the 'hold current' for that, just enough to lock the position.
When you are changing the position, the current is first increased, enabling higher torque, and next a move is initiated. Then it goes back to hold current again. Very strong but minimal use of electricity.

Jo,
I feed the stepper driver directly from the USB (Vcc pin on the Seeeduino Xiao). it is buffered by a 100 uF capacitor. I'm not routing 600 mA -yet- but 300 mA works fine. There is one downside of powering the Trinamic with only 5 V, which is the reduced speed the steppers can get to while keeping high torque.
Luckily, focusers which require a bit more rotational speed (SCT telescopes) need only low torque and vice versa, a focuser which requires more torque (Crayford/ rack & pinion), the speed can be very low.

I intend to design a few PCBs within a month. I may make a few 'kits'.

Cheers,
Bart

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