tizianoniero.it Solar system HiRes images

Focuser controller project

Here I describe my self made focuser controller. A generic hybrid unipolar or bipolar stepper motor of 200 steps/revolution is used to move intra and extra-focal my crayford focuser (a Baader Planetarium 2" Steeltrack for SC OTAs). Using a micro-switch inside the focuser it is possible to achieve absolute positioning and by using a simple NTC thermistor also temperature compensation can be performed. The motor is controlled both by a remote PC through USB and by a local keypad in order to mantain the step count and thus the absolute position.

The project uses a Arduino Micro board which controls a POLOLU stepper motor driver based on the Texas Instruments' chip DRV8825.

For the purpose I wrote both the firmware for the Arduino, a main control application (for Windows) and the ASCOM driver for this focuser controller.

Project links:

on the C14 - 1
The focuser and the controller mounted on C14. All the cables from the controller are tied with plastic ties to the controller box, specially the Arduino USB micro plug which is very fragile.

on the C14 - 2
Particular of the belt and the pulley. On the market you can find aluminum pulleys already CNC machined with a 5mm hole which can be inserted into the shaft of the motor. The motor is a NEMA - 14 unipolar stepper retrieved form an old printer.
Glued to the focuser you can see the low-profile micro-switch used to find the zero position for knowing the absolute position.

the keypad
The keypad used to move the focuser in loco without losing the step count and thus the absolute position. The two big buttons move intra and extra focal, the little red button is used to toggle slow / fast speed.

the thermistor
The thermistor wrapped to a faston (with a little of thermal grease) with a heat shrink tubing.

testing the assembly
Here the focuser assembled with the controller during testing.

testing the assembly

particular of the electronic assembly
The controller plastic box with the Arduino and the Pololu current driver.

the NTC thermistor
The NTC thermistor. It is a 10 kΩ ±0.9 %, 10 s, from RS (code RS 151-237).

the micro-switch
The micro-switch. It is an ALPS tactile SMD switch tipically used as reset button on microcontroller boards. It has a very low profile of about 1.5mm.