The Sky's the Limit (Switch)

[hughgilhespie]

Hi, I'm starting to get all the bits I will need to motorise my Pulsar dome. I want to use limit switches or sensors to give an 'absolute' dome position. The switches will just be low-current types to send a signal to the Arduino running the rotator. I suppose that technically they won't be limit switches as the dome will be able to rotate past the switch location, so any type of sensor that gives an output when the dome passes it would be OK.

Any ideas for a good switch / sensor for this use?

Regards, Hugh

[martin_h]

Hall effect sensor, with a set of magnets fixed to the moving part of the dome perhaps?

[steppenwolf]

Why not just use a rotary encoder driven by a soft rubber wheel? What I know about Arduino systems could be written on the back of a postage stamp but a rotary encoder and a homing switch would surely be accurate enough?

[hughgilhespie]

Hi,

Martin, Never used Hall sensors but they would be a possibility. I will have a look at what's available.

Steve, I will be using a rotary encoder and that is a very accurate way of measuring dome movement but it's not absolute. In other words, the encoder doesn't know where the dome is when it's first powered up. So my scheme is to have a start routine that will just rotate the dome until it triggers a switch or sensor. At that point the Arduino 'knows' where the dome is pointing and will thereafter use the rotary encoder to track it's position and slave to the telescope azimuth (I hope!). If I can find cheap enough switches / sensors, I will put in more than one, maybe 4, so that worst case, the dome only has to rotate a quarter turn to establis a 'home' position.

Hugh

[Horwig]

I'm running a Lesvedome system, which is controlled by a grey code wheel and a position/home switch (a Maplin's patio door burglar alarm switch)

I have pictures if your'e interested

H

[hughgilhespie]

Hi Horwig,

I would love to see pics of your set-up. I will also be using Levesdome / Vellesmann board to control the dome and shutter but I intend to use a stepper motor/pinion/T10 timing belting for the dome rotation drive with a home brew Arduino controller rather than a relay board. The advantages - at least as I see them - are a 'simple' way of getting accurate dome position and the chance to have a safety check that the dome is actually rotating when the motor is running. I have a Bourns rotary encoder that will feed pulses into the Vellesmann board but also into my controller.

Regards, Hugh

[steppenwolf]

Steve, I will be using a rotary encoder and that is a very accurate way of measuring dome movement but it's not absolute. In other words, the encoder doesn't know where the dome is when it's first powered up.

Quite true. However, I always 'home' my mount at the end of a session (therefore, by definition, it is 'homed' at the start of the next session) to place my solar panel due south for the best chance of a good daily top-up. Would this not be a suitable known start point for your sessions too? Whilst this is certainly not an 'absolute' encoded system, it does seem to work very well.

[Horwig]

OK, can't seem to find any pics at the moment Hugh, so much for a tidy filing system

Will take some fresh ones in the morning for you

Huw

[hughgilhespie]

Hi Huw,

I prefer a piling system!

Takes time but you know its there somewhere - or it was definitely there a few days ago - unless I put it in the other pile........

Hugh

[Gina]

I use boxes but the principle is the same   Can never find anything when I want it

Re. Hall effect switches - think I got some from ebay - pretty cheap.  Easy to use too.  Just give them a 5v supply and they give a digital output that you can connect directly to an Arduino.

[Horwig]

Right here's some new pics:

The encoder is the tiny blue thing in the bottom, totally dwarfed by the mechanics, it's a Bourns 6 position rotary from Farnell.

The home switch as I said above was a Maplins (or could have been B&Q even) patio door  burglar alarm contact set.

The rubber wheels are Banebots wheels, used to be available from Technobots, but they have stopped selling them.

My filing system is better than my desk. My wife keeps asking me if my desk needs to be so messy, can't see the problem personally

Huw   

[Gina]

It's no worse than mine

[hughgilhespie]

Hi Huw,

Desk? What desk? Where?

But - many thanks for the pics, a good encoder system you have. Can I ask about the little bearings you are using for the encoder shaft? They look just right for my encoder mounting.

Regards, Hugh

[Horwig]

Sure thing Hugh, they are Igus pillow blocs:

http://www.technobotsonline.com/bearings/igus-plastic-bearings/igus-pillow-block-bearings.html

Huw

[Stub Mandrel]

The simplest limit switches to use are microswitches with roller levers: robust, reliable, very tolerant of misalignment, easily wired as NC or NO and high current capacity - enough to directly switch many motors.

Hall sensors are great, if you can be confident that the sensor-magnet gap will be stable.

I'd show a photo of my desk, but I can't find it (desktop, not photo)

[Horwig]

Since there are threads on all the other equipment boards for SHOW US YOUR... should we atmers start a show us your workshop/desk thread?

Huw

[Gina]

Glimpses of mine can often be seen in my photos though I try to clear a space when I photograph smaller items

[hughgilhespie]

Hi, I agree about the roller lever microswitches. The point about misalignment is a very good one as the fibreglass flange on my pulsar dome is a bit wavy. Nothing too bad but not millimetre accuracy either. There are some nice Honeywell vane type Hall effect sensors that aren't too expensive - about £6 from RS. These have two arms with a built in magnet in one arm and the Hall chip in the other. Switching occurs when a ferrous metal vane passes between the two arms. They are quite accurate and could give a position to within a couple of millimetres but they do need the vane to be well aligned betwen the arms and that might be difficult. The joy of the Hall devices is as Gina said, a nice clean logic output with no switch bounce to worry about. 

I'm thinking of getting both types. Just one Hall effect thingy and four roller lever microswitches. They are a lot cheaper,about £2 each I think. If possible I could use the Hall effect detector to actually 'set' the home or park position for the dome. The lever switches would be more use for the shutter travel - which I think will be a much more difficult thing to motorise.

Then of course I should also be looking at optical detectors...... I wish you could still get the good old PRINTED ON PAPER RadioSpares catalogues. 5 or 6 volumes of delightful bedtime reading. Browsing a web site is NOT the same!

Regards, Hugh

[Stub Mandrel]

Hi, I agree about the roller lever microswitches. The point about misalignment is a very good one as the fibreglass flange on my pulsar dome is a bit wavy. Nothing too bad but not millimetre accuracy either. There are some nice Honeywell vane type Hall effect sensors that aren't too expensive - about £6 from RS. These have two arms with a built in magnet in one arm and the Hall chip in the other. Switching occurs when a ferrous metal vane passes between the two arms. They are quite accurate and could give a position to within a couple of millimetres but they do need the vane to be well aligned betwen the arms and that might be difficult. The joy of the Hall devices is as Gina said, a nice clean logic output with no switch bounce to worry about. 

 I wish you could still get the good old PRINTED ON PAPER RadioSpares catalogues. 5 or 6 volumes of delightful bedtime reading. Browsing a web site is NOT the same!

Thinking about this a bit more an ideal solution could be strips of black insulation tape or even paint at regular intervals and an IR detector. Cheaper than magnets all the way round! That way you could turn the whole dome into its own encoder. two detectors and you could use quadrature detection, although you will know which way the motor is going (hopefully!) Perhaps on detector to spot the angle pulses and another 'home switch'.

Debouncing is so easy in Arduino/AVR software I never worry about switching quality.

When I was at school my dad used to let me have his old RadioSpares catalogues - in those days they were about the size of a long magazine!

[Horwig]

I wish you could still get the good old PRINTED ON PAPER RadioSpares catalogues. 5 or 6 volumes of delightful bedtime reading. Browsing a web site is NOT the same!

Regards, Hugh

A good friend of mine used to say "pornography is whatever gets you to go phwoooar", for him it was boats, some of us have simpler tastes, yes I too miss the old RS catalogue

H