Levesdome, Encoders and Shutter Motors

[pmlogg]

The mechanicals for rotating my Pulsar dome are finally working OK - not really DIY but a 2nd hand system from an old model 2.2m dome adapted to fit an old 2.7m version.  The aim, as mentioned in other posts is to move on to automation and I've begun gathering the parts but haven't got too far yet.

In parallel I'm looking towards motorising the shutter.

My first question is about encoders for the rotation system.  I have used these before as part of Digital Setting Circles but not starting from scratch.  I think that I read about a limitation with Levesdome on how many pulses/tics it can cope with so I don't want to try to use an encoder that it wouldn't work with.  Could anyone advise me as to what the limit might be?

My second question is about the gearmotor for the upper shutter; my dome having the old two-part one.  My thought is to copy the method used by another SGL member.  For the upper shutter he uses parts from Explora-dome rotation system.  He has fitted a gearmotor with 8 inch diameter Acorn type sprocket at the top of the dome which interfaces with a strip of the Explora-dome track mounted centrally along the length of the upper shutter to raise and lower it.  Apparently that is working very well for him.  

I've asked Explora-dome for a quote for the bits but have a horrible feeling I won't like the price of the gearmotors.  So finding a cheaper, but still good quality motor might be necessary.  The Explora-dome gearmotor specifications appears to be: 25rpm, continuous torque 79 lb-in, 443 in-lbs peak torque, IP54.  I think the Explora-dome is of heavier construction than the Pulsar so perhaps my shutter would need less torque to move.  A complication with using a different motor is that the one that mates with the sprocket has a 12mm diameter shaft with a protruding 4mmx4mm key.  The motors I've been finding online either have flats or internal keyways and the shaft adapters all seem to be too big.  Suggestions regarding the gearmotor, including required torque and adaption of the shaft, would be much appreciated.

 

[hughgilhespie]

Hi,

There is a lot of information about maximum encoder pulse rates on the Lesvedome web site, including a useful spreadsheet. Also look on the Lesvedome Yahoo forum, the same question was answered very recently. If you are thinking of using a Bourns encoder, most people have found an 8 ppr encoder works well BUT it will depend on the gearing between the dome and encoder and the dome rotation rate.

For a powerful low speed motor, a lot of people have used vehicle windscreen wiper motors with success. You will face the problem of mating the sprocket with the shaft of the motor - if you can find the legendary 'small, local, friendly engineering workshop' it would be simple operation for anyone with a lathe to make.

HTH

Regards, Hugh

[tony210]

Hi - I used the 8 ppr Bourns encoder with LesveDome and a windscreen wiper motor and my old style 2.2 Dome will track an object I am imaging pretty well which is all I wanted - the problem was not attaching a sprocket but building the dome drive system that took many prototypes as I like to call them {failed ideas others might say ) but you seem to have done this so the rest should be relatively easy - Tony.

[sloz1664]

I user the Bourne EM14 Encoder on my Levesdome system driving my Pulsar Dome.

Details here:- https://eu.mouser.com/ProductDetail/652-EM14A0D-C24-L08N?r=652-EM14A0D-C24-L08N

As Hugh as pointed out. It depends on the motor speed of your dome. I use an old windscreen motor attached to a motor controller.

Steve

[hughgilhespie]

Just a quick heads up. If you do decide to use a Bourns encoder - the one Steve recommended is perfect - I strongly recommend you buy the connector and cable to match at the same time. Mouser shipping is pricey so buy both at the same time. Part number from Mouser is 652-H-290–2

Regards, Hugh

 

[sloz1664]

Totally agree Hugh. I had to purchase a second encoder after I damaged my original. Postal costs were a similar price to the encoder.

Steve

[pmlogg]

Thank you for all those good points.  I hadn't found the Levesdome Yahoo group and having now done so I see that there is a lot of material to look through. I've spotted on of the conversations on encoders but not yet found the spreadsheet. 

It looks as if most/all of the off the shelf optical encoders, seen on the internet complete with housings and mountings at quite low prices, have much higher pulse rates - as do the encoders on digital setting circles, e.g. US Digital.  I have one that probably works from a drive motor that failed but it will be a higher pulse rate than 8 or 16.  The Bourne ones seem very reasonable in price and the example circuits pretty easy to make.

On the shutter, the motors in the setup I was minded to copy are inline gearmotors whereas most (all?) wiper motors look as if they are right angle with relatively high rpm. A different mounting I suppose is just something to work out and I have now seen some postings on the use of Pulse Width Modulation Control to reduce rpm as required.  The electronics in all this is where I am very weak.  The same for programming - despite having done some many, many moons ago at school.

I do have one contact who might be able to help with a bit of machining for the motor coupling - or at least point me in the right direction.  When in the past I contacted local machining companies the cost made it cheaper to buy ready-made instead.

I've read a couple of interesting posts on home made acorn drives and trackway although those are far and away the cheapest bits to buy from Exploradome or Opt.

I'll get on with reading at the Levesdome group and have a think about mountings for wiper motors.