DIY - Dome Automation

[mattifor]

Hi Rich,

I have bookmarked you web page and will view with interest.

best regards

mattifor

[Reggie]

Hi George, I believe it would be possible Not entirely sure how but something like a transistor to switch in the 240v for the motor as the arduino really only runs between 3.3v-12v.

[George]

Hi George, I believe it would be possible Not entirely sure how but something like a transistor to switch in the 240v for the motor as the arduino really only runs between 3.3v-12v.

Seems what I'm wanting has already been done

Login

I picked myself a Veleman USB K8055 interface board off Ebay for £10.50 that will work out of the box with the Motorlift garage door opener I'm using.

The software to interface the 2 can be found here

Thats me sorted.....I think

[Psychobilly]

Nice one George

Peter....

[Reggie]

That's a very nice price George....

[ribuck]

I also found the link for LesveDome earlier today when searching for ascom user groups on yahoo.

I'm still going ahead making my own purely out of interest for myself, and i think i'll really enjoy tinkering with the electronics and the programming. Least this way i can design my own to be very modular so i can add the things i want as i go along like cloud/rain sensors etc.

I'll be starting the ascom coding this weekend.

Rich.

[ribuck]

Ok the arduino Interface finally turned up today, with a host of other goodies. I've got in from work late,so couldn't really test reading values from the mount. However, i've done an anitial test reading values from from a 10k potentiometer to the pc, which proved fairly straight forward.

I'm going to make up a mock mount with 3 motors for initial testing. 2 motors to simulate RA & Dec movement on a real mount and a final motor to similulate a dome moving.

Had some thought about about how to always auptmatically calibrate the position of the dome on power up, and i think a small magnet and a reed switch will be a good easy way to achieve this.

watch this space over the next few days, and i'll post some pics.

Rich.

[ribuck]

Forgot to mention that i've also been looking at the math behind converting the scopes position from RA / Dec to Alt /Az.

I found this informaiton on the web, which should be fairly easy to transpose to the programming of the interface.

1) First the Local Mean Sideral time need to be calculated.

The mean sidereal time (MST) is calculated from a polynomial function of UT since epoch J2000. This formula gives MST, the sidereal time at the Greenwich meridian (at longitude 0°) in degrees. To get local mean sidereal time (LMST), add longitude if East or subtract longitude if West.

MST = f(UT)

LMST = MST + LON

2) Now to calculate the Hour Angle

The hour angle (HA) is the angle between an observer's meridian projected onto the celestial sphere and the right ascension of a celestial body. It is used in coordinate conversion.

HA = LMST - RA

3) HA / Dec conversion

Conversion of HA and DEC into ALT and AZ

Using the RA, DEC and HA for the object, and the latitude (LAT) of the observing site, the following formulas give the ALT and AZ of the object at the time and longitude that was used to calculate HA.

sin(ALT) = sin(DEC)·sin(LAT) + cos(DEC)·cos(LAT)·cos(HA)

sin(DEC) - sin(ALT)·sin(LAT)

cos(A) = ----------------------------

cos(ALT)·cos(LAT)

If sin(HA) is negative, then AZ = A, otherwise AZ = 360 - A

This gives the computed horizon coordinates without correction for atmospheric refraction.

[ribuck]

My last post of tonight,

Looking at the usb interface and it's programming language, i think it may be possible to design the interface to work either through a pc or completely independant of the pc.

My thinking is that is i create a pass through interface i should initially be able to get a readout from the pins from the hand controller for the RA / Dec and then feed them directly to the microprocessor to calculate the correct movement based on the maths and then move the stepper motors accordingly, whilst still retaining full functionaility for pc control from an application if you want it.

Rich.

[Reggie]

Hi Rich, it's very possible to make it completely standalone or intelligent enough to be able to know when it's connected to a serial port and send and receive data to and from a PC app while still being controllable from the unit at the same time

Shouldn't the dome be at 'park' when you shutdown for the session as it's keeping up with the scope? No need to calibrate it if that's the case as all it has to do is turn on and track your ra/dec conversion from the mount?

In fact do you even need to track, once you've moved the scope to the target and the dome has caught up you can run it sidereal until the scope slews again?

[ribuck]

Hi Reggie,

I just wanted to have the calibration feature as a "Nice to Have" option, where it just double checks to make sure that the scope and dome haven't got out of Sync for whatever reason.

The bit i'm stuck on at the moment is finding a suitable motor to move the dome. Ideally i had hoped to use a large stepper motor but finding one with enough torque to push the dome might be a problem.

I may possibly just have to use a more powerful DC or AC motor and mybe just use a stepper motor as the position encoder.

Steve was going away and work out the Torque calculations, so just waiting to hear back from him to decide if using a stpper motor is possible.

If not

[Reggie]

that's a pity, but reasonable position data should be achielvable. I don't have a dome, so the most I'm working on at the moment is a full rotation servo for my focuser, incredibly simple to drive. Looking at photo cells for detecting cloud cover at the moment.

[Chris Rowland]

A few quick comments:

My Astro society dome is motorised using a 12V windscreen wiper motor. The drive is using a cam pulley with cam belts cut up and nailed to the dome ring. The motor is spring loaded against the dome ring. The max current draw is about 5A.

I'm in the process of automating it. I'm using an Arduino (a Freeduino to be precise) for control and position sensing. Position sensing is probably the most difficult bit.

Chris

[ribuck]

Hi Chris,

Can i just ask what motor driver board you are using to drive your motor or did you just make your own ?

Regards,

Richard.

[callump]

I have heard it done, but not seen any description. But you don't really need precise dome control. So why not have a sender on the end of the scope (ir maybe), and a receiver on the dome, and the dome just follows the sender around ?

Just a thought...

/callump

[ribuck]

Hi Callump, be sender and reciever do you mean RF ? I dont think RF is Uni-Directional, I'm pretty sure it just floods out in all directions which would make it impossible to determine the position of the scope. This is why it needs to be calculated from the RA/Dec and then transposed to Azimuth to be fed to the motor.

[callump]

Well not sure - i thought IR (infra-red).

There may be other proximity detectors that could be used.

I would have thought you could have a narrow beam rf signal, but it would need to be high frequency.

Might be able to do something with magnet and hall effect sensors.

As i said, it does not need to be really accurate, just sufficient to line up the slit with the scope.

/callump

[ribuck]

Hi Callump,

Not sure Infra-red would be a good idea anywhere in a dome, especially if you are an imager. I think a magnect reed switch is the option i will go for.

At the moment, i'm more concerned about sorting a motor and it's driver.

Regards,

Rich.

[Chris Rowland]

I did my own motor controller, a couple of power MOSFETs, one driving a two pole change over relay wired to set the direction and another one to switch the power to the motor. They can be driven by the TTL outputs from the micro controller.

For a dome reference position use a reed relay switch and magnet. They are easily available as security switches.

I also used an IR 2 button key fob controller and receiver (a couple of Velleman kits from Maplin) for manual dome control. If I was doing it again I'd use RF control. I think they do this as well.

Position detection seems to be one of the more difficult things. I'm currently using a home made rotation detector made from an old potentiometer mechanics and a couple of opto switches to give two signals in quadrature. You can get rotary position detectors as well. The trick it to arrange them so they rotate as the dome rotates.

Personally I'd use a lower powered motor and make sure the dome rotates easily. That way when something jams the motor stalls and no damage is done. If you have a powerful motor bad things could happen if something gets in the way.

The whole automation issue is very complex. Everything is a lot simpler if you are there or can wander down the garden to sort it out. Once it's truly remote things get a lot more difficult.

Chris

[ribuck]

Hi chris.

I agree the reed switch and magnet is the best option in my mind to set the dome home position. I've also thought about the dome position control, I think for me, i'm just going to use a stepper motor and a wheel as a position encoder to feed back to the processor.

I also thought about the motor jamming or getting stuck. My plan is that i read the RA & Dec of the mount and compare this with the encoder feedback. e.g. if the RA/Dec values are moving, but if the encoder (Stepper motor + Wheel) are static then that means there is a jam, and i will use it like a kill switch.

Rich.

[Psychobilly]

If its torque your after then use gearboxes...

[ribuck]

Hi Billy,

I'm looking at a few motors in the range of 150w 1/10hp with 80-100nm of torque with a gear ratio of around 15/55 giving a nice slow steady 6-7rpm.

Next up i need to find a suitable Ac Inverter and a dc controller.

Other updates

===========

I've currently got the Arduino board wired up to an LCd screen, reading sensor inputs and displaying them on the screen. Tonight/tomorrow i'm planning testing reading data directly from the mount to see what format the data is in.

[mattifor]

this was posted to-day on a dome automation thread on a Yahoo group.

Paravalux

Model PM1LWS Parvalux Electric Motors

24vdc

7. amps

120watts

rpm 4000.

gearbox:-

ratio 9/56

output rpm 7.6

Nm 85.

they are used to raise wheelchair access lifts on disability vehicles.

it was sugested that they were stuitable for a 8ft dome of fiberglass construction weighing 95lb

[ribuck]

Hi All,

For those of you interested i've just posted an update on the project on my site, just to give a quick update on where i am on the porject. I'm posting details on there just to make it eaier to view all the information in one place, kind of like a blog of sorts.

Astrosky Dome Runner Poject

If anyone has any qustions, just post themon here so we can discuss things for the benefit of everyone else.

Rich.

[Chris Rowland]

Synchronising a dome shutter to the scope position is pretty complex if you take everything into account - offset of the scope position relate to the dome, offset of the OTA relative to the scope Ra Axis and the pointing mode of the scope (aka the side of the pier).

This link DomeSync seems to be the least complex explanation I've seen.

You may be able to ignore most of this if you assume the scope is at the centre of the dome and the shutter opening is wide enough that you don't need to move the dome dependng on the pointing mode of the scope.

There's some example code there and the ASCOM POTH scope/dome driver will have this as well.

Chris