Soumis par astrokitvy0 le mar, 05/24/2016 - 18:01
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The Sun is tracked on this video with astrokit project, a linux embedded solution for alt-az mounts and working with raspi micro sd card with only a 8Mo linux partition system. Linux kernel is patched with real time patch linux-rt and source-compiled as from scratch thanks to buildroot. Binary performs four PWM-modulation channels per motor with a high level and real time adjustment, to control Dual-H-bridge before motor connexion. In this example, astrokit manages the two stepper motors of a NEQ3-2 mount transformed on alt-az mount : ASC-NEQ3-2 axis is used for azimutal tracking and DEC-NEQ3-2 axis is used for vertical tracking. Vertical delay is due to the imperfections on the declination axis because of gears backlash. Microstepping mode is 1/100, providing less vibration compared to original box control. System inputs are done thanks to infrared remote control, programmed with LIRC Linux package. You can easily install Astrokit as a shield for Raspberry Pi cards. It is preferred to use the Raspberrypi2 card for its cpu quad-core wich is more appropriate to control threads of real time embedded binary. Not a lot of electronics is needed on the shield, so that it is entirely possible to use the raspberrypi card directly thanks to test cards connectors (eg "starter kit" cables). Please visit the dowload page to get the linux embedded system and have the electronic diagram in order to make printed circuit boards. Sorry but french contents on other pages :):)
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You can see below the NEQ3-2 mount transformed into alt-azimuth operating mode
 
 
 

Here the raspi2 with the astrokit shield above.

it is seen that electronics is minimized with only a supply adaptation thanks to voltage regulators dual H-bridge 1A and the infrared LED
 
All other features are manage by the embedded software to sum up :
- microstepping mode with PWM chopper modulation to have the best motor control function with smooth movement (until 1/1000 depending on CPU works)
- calculating the azimuthal speed is refreshed regularly so that motion follows a curve, not a straight
- ability to track objects catalogs but also the planets moon and sun which require more calculations
- the linux kernel is patched soft real time so that it allows the scheduler to average latencies of the system which is sufficient for long pose photography
- it is also possible to use astrokit to drive an equatorial mount very simply

This is the last proto =>

 
 
And the final card with a minimalist electronics =>

The imperfections and delay in the vertical motion of the video are due to gear lash on NEQ3-2 DEC

 

A long pose on EQ-MODE (about one hour) showing that system accuracy is better than EP from VIXEN GP  (wich is about 30" arcsec)
The circle-shaped tracking is normal and due to the rotation of the DEC error vector,
due to the difference between the north pole-object circle radius and mount axis-object circle radius
you may notice than EP forms a periodic curve with 10 minutes period =>