Power to Rotating Dome - How?

[hughgilhespie]

Hi, 
I'm looking for suggestions about a good way to get power to the rotating dome of my Pulsar observatory. The power I need is from the output of a 'smart' battery charger that is used to charge a 25 Volt LiPO4 battery. The battery then supplies power to the motor that opens and closes the shutter and to the associated control electronics. The maximum output from the charger is about 4 amps.
The set-up I use includes a a remote power switching box and the charger is only switched on when the imaging session has ended, everything is shut down and the dome is in the home position. When the charger is on, the 25 volt DC output activates a relay that disconnects the shutter motor and controller from the battery and connects the charger and battery. A 'super diode' is used to prevent the battery from keeping the the relay coil powered after the charger is turned off again. The battery has plenty of capacity to operate the shutter for a full night - probably several nights before it needs charging again. 

The Mk. 1 schematic is attached.

Because the charging only takes place at a specific location, I only need / want to connect the battery charger when the dome is in the home position plus or minus say 30 mm. and there is no need for a continuous connection as the dome rotates.
 
I have bought a couple of types of spring loaded carbon motor brushes as I thought this would be a good way of making the connections. However, when I measured the resistance pushing a brush against a brass strip it was very high - typically over 30 ohms, even when pushing hard. I got much lower resistances when I tried a piece of 2 mm silver solder rod (34% silver) against the brass. Looking on Google, it is easy to buy sterling silver as a thin bar, say 6 mm by 1 mm and I believe sterling silver can be thermally hardened to spring-hard. But before going down that route I would welcome any suggestions of a way to make a connector (2-pole) that gives a low resistance contact reliably over a short distance.

Thanks, Hugh

DomeBox1.pdf

[Geoff Lister]

Hugh,

Clearly, many people have had a look at your problem, but no replies. I think this is because it is a real challenge to get working. I come from an avionics background, and passing power across a rotating joint is a major source of equipment unreliability.

My only suggestion is to have a fixed socket, and a floating plug on a short cable, with some form of mechanical fool-proofing; so that you cannot move the dome from its "home" position, with the plug connected. Your intelligent charger needs a reliable, low resistance, connection to the battery, otherwise it becomes a "dumb" charger, and my even damage the battery.

I am sorry that I don't have a magic solution.

Geoff

[Horwig]

This is where I'm at with my dome at the moment, and I'm with Geoff on this one, a slip-ring system would be very clever but probably hit and miss in a cold/damp environment.

Far safer to make a 'charge at home' system, with interlocks so that when the charge cable is connected the dome cannot rotate.

 

Just my 2d (showing my age)

 

Huw

[hughgilhespie]

Hi Geoff and Horwig,

Thanks very much for replying, I had pretty much forgotten about my post. In the absence of any suggestions  I have started making my own slip ring system. This seemed like a simple idea when I started but it has proved to be anything but simple. I will post some more details and pictures of where I have got to tomorrow.

 I already have the floating plug on a short cable. I have modified my dome controller so that rotation is inhibited unless the floating plug is disconnected from the shutter controller and plugged into a separate socket on the static part of the dome. This works very well - although it's not so good when you try and rotate the dome by hand - guess how I know that! However, my goal is as close to full automation as possible so I do want to use a slip ring type connector. In the finished version, I will re-purpose the existing switch box that I use to connect the LIFePO4 battery to either the charger or the shutter controller and I will replace the toggle switch with a relay so that the battery is connected to the charger only when the charger is on. I will use a power FET as a  'super diode' so that the battery cannot power the relay coil - and the relay will switch according to whether the charger is on or off. The charger itself is connected to another remote switch box and is turned off when the observatory power goes live.

I do take your point about the possibility of damage from the slip rings causing too high a voltage drop. I will check this carefully when I first install the system. As built, my shutter controller monitors the shutter battery voltage fairly accurately (Teensy 3.2 with a theoretical 16 bit ADC - probably working at about 12 bit resolution) If it looks like there might be a problem I do have the possibility of leaving the shutter controller permanently connected and measuring the charging voltage at the battery terminals. I could then arrange yet another relay / solid state switch to disconnect the charger if the voltage goes too high.

Regards, Hugh

[Geoff Lister]

Just a thought:- would it be possible to use a photovoltaic charger, attached to the dome, and avoid slip-rings altogether?

Geoff

[hughgilhespie]

The system that Pulsar supply uses a PV panel to keep the shutter motor battery charged. I have heard some reports from users of this system that it is ineffective in winter - and here in North Devon - natural habitat of the raincloud - it wouldn't surprise me if it didn't work in summer either! 

Regards, Hugh

[Geoff Lister]

Wind generator? The Atlantic gales that we get in North Somerset still have a lot of energy left, after you guys in North Devon have taken the edge off them.

Regards, Geoff

[hughgilhespie]

Would love to! A really interesting engineering challenge, 3 metre high mast on top of the dome - woohee!!

Hugh 

[hughgilhespie]

As promised, here are some pics and notes about my still to be tested solution to transferring power to my rotating dome.

The first 3 pictures are of the static block that will be attached to the observatory wall. I used some mahogany type wood I had available. The block size is 20 mm thick, 44 mm wide and 90 mm long. The moving block has the same dimensions. The block was milled to have two 6 mm wide slots, 11 mm deep. The four aluminium pieces are guides that engage with matching slots on the moving block that is attached to the dome. The contact rings are sterling silver.

The mounting plate is a piece of 6 mm aluminium plate and the block is attached using 4 compression springs slipped over M4 studs. The spring rates were selected so that there will be a total of about 2 kg force when the two blocks are lined up. When I tried the blocks with a multimeter, this amount of pressure was enough to show 0 ohms. Not a very accurate measurement but better than nothing.

The next two pictures are the moving block. This has two 3 mm high by 6 mm wide pieces of silver bar fitted into 6 mm wide grooves. The metal bar was supplied as fully annealed and it was easy to bend it to follow the tapered grooves. Each piece of bar is secured by two countersunk M3 machine screws. These will also be used to make the electrical connections.

Then a couple of pictures showing how the system is meant to work. As it is spring mounted, the Static Block can move around in all directions by a few millimetres. The entrances to the contact channels on the Moving Block are tapered and the theory is that as the contact channels engage with the aluminium guides on the Static Block, they will automatically position the Static Block so that the silver contact ring is wiping against the silver bar in the Moving Block. The springs force the two together. Bear in mind that the electrical circuit will only have to work when the whole system is static after the dome has been sent to its Park position and the dome drive stopped. This should be an easier proposition than attempting to keep a good electrical contact when one of the blocks is moving. The dome goes to its Park position with a repeatability of about 1 mm so the blocks are plenty big enough to ensure they will make electrical contact when the dome is parked.

The final picture shows some of the bits I needed to make the silver contact rings. I do NOT recommend this method to anyone else, it became ludicrously complicated and I still don't know if it will work. The bits in the picture are a jig I had to make to fabricate the silver contact rings. It allowed me to drill 3 mm holes whilst securely holding the thin metal strips and it also allowed trimming the strips to identical lengths.

The dark grey flattened circles are mandrels turned from silver steel rod I had. The rod was turned to 22 mm diameter then a 10 mm wide flat was milled and then parted on the lathe to give two 6 mm sections. Then each section had an  M3 hole tapped into the centre of the flat. Next the pre-drilled silver strips were fitted around the mandrels and screwed into place.

The final operation to make the silver rings was to heat treat the silver to increase its hardness to 'spring' hard. I have had a very secondhand muffle furnace for about 8 years (cost more in fuel to collect than the purchase price)  and for the first time I was able to put it to use. But only after buying some special flux to prevent oxygen damage to the silver when it was heated. So, heat to 730 C for 45 minutes and then quench in water followed by 30 minutes at 330 C and air cooling. Then lots of cleaning and gentle polishing to remove the flux and get back to a smooth surface.

I do have my doubts about how effective the heat treatment was. The rings are fairly springy but they have already deformed more than I expected. So, after all that I have now ordered some spring brass to try if the silver doesn't work. The rings shown in the final picture are first go rejects!

So, fingers crossed and now onto mounting the bits in the observatory.

Regards, Hugh

[Geoff Lister]

Impressive!

If you need to make the silver rings more springy, would a circle (or more) of old clockwork spring inside the silver, help?

[hughgilhespie]

Thanks Geoff!

Good idea about making the silver rings more springy. I was thinking about making new brass multi-coil springs and soldering on a silver stud for contact but the soldering would almost certainly affect the temper of the brass. Putting the new spring inside the old one avoids the need to solder so should work better.

In the next couple of days I will finalise the design for and hopefully fabricate the support strut for the fixed block. This will let me do some meaningful tests on how it all works mechanically and electrically. 

Regards, Hugh

[Gina]

Good luck   The design looks good but as you say "the proof of the pudding is in the eating".   I have tried rotating contacts for a different project without much success - it's a tricky one!

[hughgilhespie]

1 minute ago, Gina said:

Good luck   The design looks good but as you say "the proof of the pudding is in the eating".   I have tried rotating contacts for a different project without much success - it's a tricky one!

Hi Gina,

Thanks! All the time I have been fiddling about with this project I have been very aware how this would be so much easier if I had a 3D printer. The use of bits of wood is far from ideal.

Regards, Hugh

[Gina]

Have to say I've found a 3D printer invaluable for printing astro parts and that includes a micro dome observatory which I hope to have completed for next main DSO imaging season.