Vixen Polaris - polar axis drive

A Lidl mount and a TS Eq-2 drive just don't work when you want to do long exposure photo's. The mount in itself is OK, albeit a bit unpredictable due to it's flimsy make. The Chinese Eq-2 drive is complete crap when used for anything else than visual observation. The speed of the DC motor is controlled by a simple battery-supplied open-loop current source, which is not sufficiently stable.
So I decided to whip up a quick (but not so dirty) driver for my old Vixen Polaris mount. The design and results are detailed below.
As a result of the prototype described on this page, I have made a final design which can be used to drive any RA axis with a unipolar stepper motor.

Mechanics

Biggest challenge were the mechanics: electronic design and software are pretty much straightforward, but fitting the lot onto the RA axis was a small challenge. I happened to have a 200 teeth gear with a modulus exactly matching 1/4 inch Whitworth thread. These form the basis for reduction of a reasonable stepper speed to the required 6 rev/hr of the RA axis.



Other parts were acquired from Conrad, specifically the bearings and belt/wheels. The belt can be tensioned by rotating the whole drive assembly. The thread is suspended in a couple of 22mm bearings, which snugly fit in a 25mm U profile. The threaded rod is sanded down to about 4-5mm on one end, and flexibly coupled with the motor axis using a piece of rubber hose. The U-profile can be adjusted so it fits the gear with minimum backlash.

The resulting construction is shown in the image below:

Electronics

The schematic is built up around a PIC 16F628 and a ULN2004 aray of TTL-Darlington drivers. The software timing is controlled by the 20MHz crystal clock, and can accurately time the stepper pulses.
To be able to control the drive software more intelligently at a later stage, an RS232 interface is provided as well. Finally, in the future buttons will be added to be able to speed up and down.



The board layout has been designed in such a way that it can be implemented on a simple breadboard:

Software

The software is extremely simple. It consists mainly of an interrupt service routine, that handles timeout and UART interrupts.
The timer TMR1 has been used in combination with a comparator (CCP1) to make it self-reloading and hence very accurate. It interrupts 10x per msec, which provides enough accuracy for the driver: The stepper should rotate at 1/200 of the HA rate, this is 1 rev per 3 seconds. In half step mode the stepper makes 400 halfsteps per revolution, and hence it must be driven at one half step per 7.5 msec. This adds up to 75 timer interrupts.
The UART driver uses a pair of queues for receiving and transmitting side. This may be a bit luxurious for such a small device, but in the end it pays off in debugging and efficiency in CPU use because waiting loops are prevented.

Result

After some initial problems with slipping gears the drive seems to track very well. Onwards to the long-exposure photo's!
The resulting drive as mounted onto the Vixen Polaris: