Digital, Temperature Compensated Focusser.

Just a heads up on my latest project. I've now finalised the design for a DIY focus controller. Just thought I'd give you a preview of some of it's features and ask for your input on any desirable feature I have missed?

Microstep capable - Using a 1.8degree stepper motor in microstep mode can produce an actual movement of the focusser as small as 1/100mm depending on what focusser you use it with. Step size will be variable by setting internal jumpers.

Temperature compensated. Built in digital thermometer can report back to any software capable of dealing with it.

Serial/USB PC interface: A standard RS232 port for connecting to the PC. Compatible with Serial/USB adaptors. ASCOM drivers.

Intelligent microcontroller based means it will have the ability to upgrade firmware.

All based on easily obtainable components. I've limited the number of components to make it small in size and easy to build.

Uses a drive belt to drive the focus mechanism on the focusser so it will be compatible with a very wide range of focussers. You could of course connect the stepper directly to the focusser too.

I'll be building/testing it over the next couple of days. It will then be a straightforward matter of writing an ASCOM driver so it can be used with all popular software which provides auto-focus. Once it's done and dusted all the schematics and plans will of course be published for the DIY enhusiast.

Suggestions are most welcome!

Dave,

Sounds interesting even without the temperature compensation.

Is your target focus mechanism a crayford or similar design?

Around how much will the components work out and can they all be got from the likes of Radionics?

Why use the RS232 interface? Very few laptops have RS232 interfaces any more, it is all USB but it would probably mean that there would have to be a USB interface in the microcontroller which would add to the cost.

Looking forward to seeing the plans, would love to try it myself

Darragh

Hi Dave,

I agree with Darragh - go for a USB interface if possible.

Any word on the dew controller?

Michael.

Dave, that sounds like the bees-knees! I'll be knocking up a couple of those for myself!

Quick Q: - What condition are you leaving the motor stators in after a focus move - are they energised (to lock the motor/focuser in place) or does the circuit simply cut power to the motor? Just be aware that if you are keeping the motor energised all the time it can cause it to hum slightly (and warm up!) and may be a source of vibration. This is the default condition in some stepper controllers. You'll have to check this out the hard way... at the eyepiece to see if it is going to be a problem.

Phil.

Phil,

I used an SLA7062M IC in the design. The 7062 is black box package which has pretty much everything you need on board including PWM so no more need for complex microstepping in software.

The IC has a sleep mode which is activiated simply by setting one of the pins to TTL high. This de-energises the windings. The IC includes current control so you don't even have to worry about the voltage for the stepper. The IC manages the voltage and keeps the current within tolerance. All you need is a voltage divider and 2 sense resistors to manage the current.

Details for the 7062 (incl datasheet) here;

ie.farnell.com/jsp/Semiconductors/Driver...duct.jsp?sku=1329628

Just think of the possibilites for drive systems, yum yum!!

Is your target focus mechanism a crayford or similar design?

The design of the unit is really indepenent of the focusser. Crayfords and the like will be ideal since they need very little torque to turn. Big sticky cheap and nasty rack and pinions will probably be too hard for the stepper to turn but wouls probably still work with appropriate gearing.

Around how much will the components work out and can they all be got from the likes of Radionics?

The stepper motor is the most expensive bit at about €40 but any unipolar (5 or 6 wire) stepper will do. You can get salvage suitable motors from old dot matrix printers to save a few bob. The next most expensive part is the stepper motor driver IC which is a tenner, then rest is just standard components costing small amounts. All are freely available from RS, Farnell etc.

Why use the RS232 interface? Very few laptops have RS232 interfaces any more, it is all USB but it would probably mean that there would have to be a USB interface in the microcontroller which would add to the cost.

Yep USB capable controllers cost more. Also need specific drivers to be written. Also serial/usb converters are so cheap and available that using RS232 allows the best of both worlds and also simplifies the design and build.