Wireless data transmission

[Grumps]

Hi
I'm trying to find a way of transferring power and data between two rotating
devices (1500 rpm). The two devices rotate along a common spindle and are
separated by 5mm.
Connecting power between these two is easy by the use of some form of
commutator arrangement (you know, a couple of brushes and concentric copper
rings), but I don't know if this is a good approach to send the data as
well. I can envisage noise appearing on the data due to dirty brushes or
copper rings, or should I just try this and add more brushes for the data
ring?
The data rate is less than 1Mbps.
Ideally an inexpensive solution.
Thanks for any ideas.

 

[Tim W]

Hi
I'm trying to find a way of transferring power and data between two
rotating devices (1500 rpm). The two devices rotate along a common spindle
and are separated by 5mm.
Connecting power between these two is easy by the use of some form of
commutator arrangement (you know, a couple of brushes and concentric
copper rings), but I don't know if this is a good approach to send the
data as well. I can envisage noise appearing on the data due to dirty
brushes or copper rings, or should I just try this and add more brushes
for the data ring?
The data rate is less than 1Mbps.
Ideally an inexpensive solution.
Thanks for any ideas.

Air bourne optical link for the data? I thinking 3 or 4 TX leds with common
drive spaced around the axis and one photoreceiver on the other end. Repeat
in reverse if duplex is required.

Or use the brushes and perhaps use a balanced current driven signal with
plenty of error detection. Carbon brushes might be better than small contact
area wipers.

 

[Sylvia Else]

Hi
I'm trying to find a way of transferring power and data between two rotating
devices (1500 rpm). The two devices rotate along a common spindle and are
separated by 5mm.

I'd be thinking of transferring the power by means of a transformer
consisting of two coils sharing a common axis, about which one is
rotating and the other isn't. I can't see why data wouldn't pass the
same way. Perhaps not so effecient in transferring power, but no wear.

Sylvia.

 

[Grumps]

Thanks for the idea. I was thinking of some form of optical interface too.

Interesting. There might be issues with channeling the light so that
the receiver can always pick it up; 1Mbps @ 1500rpm works out at about
42000 bits each revolution, so any 'dead zones' would cock things
right up.

Maybe the spindle that everything's mounted on could be hollow? That
way you could have one TX LED and one detector mounted inside, with
one
part rotating relative to the other.

Ooh, I like that idea! Thanks.

 

[Grumps]

I'd be thinking of transferring the power by means of a transformer
consisting of two coils sharing a common axis, about which one is
rotating and the other isn't. I can't see why data wouldn't pass the
same way. Perhaps not so effecient in transferring power, but no wear.

Thanks. I know of one patent which covers this approach, so it's not going
to be good for the finished product.

 

[Jules]

Air bourne optical link for the data? I thinking 3 or 4 TX leds with common
drive spaced around the axis and one photoreceiver on the other end. Repeat
in reverse if duplex is required.

Interesting. There might be issues with channeling the light so that the
receiver can always pick it up; 1Mbps @ 1500rpm works out at about
42000 bits each revolution, so any 'dead zones' would cock things right up.

Maybe the spindle that everything's mounted on could be hollow? That way
you could have one TX LED and one detector mounted inside, with one
part rotating relative to the other.

cheers

Jules

 

[The Natural Philosopher]

Hi
I'm trying to find a way of transferring power and data between two rotating
devices (1500 rpm). The two devices rotate along a common spindle and are
separated by 5mm.
Connecting power between these two is easy by the use of some form of
commutator arrangement (you know, a couple of brushes and concentric copper
rings), but I don't know if this is a good approach to send the data as
well. I can envisage noise appearing on the data due to dirty brushes or
copper rings, or should I just try this and add more brushes for the data
ring?
The data rate is less than 1Mbps.
Ideally an inexpensive solution.

1 Mbps is pretty high..you need a carrier up at least 27Mhz, which would
be my first thought, since its essentially cowboy territory. Anything
goes below a half watt or so.

...but then you ned all that mod/demod kit.. Yuk.

probably time to grab a couple of WiFi chips and pics and get
coding/soldering.

 

[The Natural Philosopher]

I'd be thinking of transferring the power by means of a transformer
consisting of two coils sharing a common axis, about which one is
rotating and the other isn't. I can't see why data wouldn't pass the
same way. Perhaps not so effecient in transferring power, but no wear.

Sylvia.

well that's just using the M part of teh EM wave instead of the E part.
Frequencies and modulation schema are the same for either.

 

[Grumps]

How much less than 1Mbps?

691,200Hz at present. I may want to increase this (x2 or x4) later.

Digital model railways send power and data (much lower rate however)
through rolling contacts between wheels and rails and wipers on the
back of the wheels quite successfully. A simple XOR error check is
used.

Any idea of the data rates that these use.

 

[Grumps]

2 halves of a potcore, each with a coil in it (rotating transformer,
by 5mm is a rather large gap, make it smaller.

I can try.

Or optical.

This is my current favourite solution.

 

[The Natural Philosopher]

691,200Hz at present. I may want to increase this (x2 or x4) later.


Any idea of the data rates that these use.

about 50 baud ;-)

 

[amdx]

Hi
I'm trying to find a way of transferring power and data between two
rotating devices (1500 rpm). The two devices rotate along a common spindle
and are separated by 5mm.
Connecting power between these two is easy by the use of some form of
commutator arrangement (you know, a couple of brushes and concentric
copper rings), but I don't know if this is a good approach to send the
data as well. I can envisage noise appearing on the data due to dirty
brushes or copper rings, or should I just try this and add more brushes
for the data ring?
The data rate is less than 1Mbps.
Ideally an inexpensive solution.
Thanks for any ideas.

If this is a one of, you can pickup a junked VCR for next nothing and
use the rotory transformer from the video head assembly. It has good
bearings
and is very stable. I'm not sure about the frequency response or the power
level it is capable. Opps, your 5mm spec. means you can't use the whole
assembly. Oh well the ferrite pieces are there.
Anyone have thoughts about it?
Mike

 

[Gordon Henderson]

Hi
I'm trying to find a way of transferring power and data between two rotating
devices (1500 rpm). The two devices rotate along a common spindle and are
separated by 5mm.
Connecting power between these two is easy by the use of some form of
commutator arrangement (you know, a couple of brushes and concentric copper
rings), but I don't know if this is a good approach to send the data as
well. I can envisage noise appearing on the data due to dirty brushes or
copper rings, or should I just try this and add more brushes for the data
ring?
The data rate is less than 1Mbps.
Ideally an inexpensive solution.
Thanks for any ideas.

How much space on each 'wheel' do you have and what sort of weight can
it carry before causing some sort of imbalance... ?

I'm thinking of xbee modules... but you might need a smart sort of
microcontroller on each do do the data caputre/transmit - but maybe you
have that already?

Then again, I've just looked and they're 250Kb/sec max. however there
are then things like this:

http://www.skpang.co.uk/catalog/product_info.php?cPath=110_124&products_id=396

up to 2Mb/sec, but you will need some sort of smart device to push data
into it and get it out again...

Gordon

 

[Grumps]

How much space on each 'wheel' do you have and what sort of weight can
it carry before causing some sort of imbalance... ?

I'm thinking of xbee modules... but you might need a smart sort of
microcontroller on each do do the data caputre/transmit - but maybe
you
have that already?

Then again, I've just looked and they're 250Kb/sec max. however there
are then things like this:


http://www.skpang.co.uk/catalog/product_info.php?cPath=110_124&products_id=396

up to 2Mb/sec, but you will need some sort of smart device to push
data
into it and get it out again...

Thanks, that looks a good little module. My system could easily support that
weight.
But I'm tending to the optical approach.

 

[Paul Keinanen]

I'm trying to find a way of transferring power and data between two rotating
devices (1500 rpm). The two devices rotate along a common spindle and are
separated by 5mm.
Connecting power between these two is easy by the use of some form of
commutator arrangement (you know, a couple of brushes and concentric copper
rings), but I don't know if this is a good approach to send the data as
well. I can envisage noise appearing on the data due to dirty brushes or
copper rings, or should I just try this and add more brushes for the data
ring?

I once worked with a slowly rotating system using 20 mA current loop
at 9600 bit/s through two sets of rings and the third pair of rings
carrying 220 V at a few amperes.

Originally it used a half duplex (command/Ack) protocol, but since
both the command or the acknowledge message could be lost, due to
individual bit errors at that data rate, it created quickly quite a
nasty backlog of messages due to retransmissions. I wrote a broadcast
(no ACK) protocol with heavy error correction coding (ECC) and after
that the system worked OK.

The data rate is less than 1Mbps.

At that data rate several consecutive bits will be lost due to dirt
etc. so you also would have to use bit interleaving to convert burst
errors into random errors, that can be readily corrected by the ECC.

Exactly for the same reason ECC and interleaving is used for instance
in CDs or digital radio links.

Adding 30-50 % of ECC bits with interleaving should help lot with the
reliability and since the data rate is only about 1 Mbit/s and hence
the wavelength is in order of 200-300 m, ring assemblies less than a
few meters in diameter should not be a problem.

 

[Harry Bloomfield]

Grumps brought next idea :

Hi
I'm trying to find a way of transferring power and data between two rotating
devices (1500 rpm). The two devices rotate along a common spindle and are
separated by 5mm.
Connecting power between these two is easy by the use of some form of
commutator arrangement (you know, a couple of brushes and concentric copper
rings), but I don't know if this is a good approach to send the data as well.
I can envisage noise appearing on the data due to dirty brushes or copper
rings, or should I just try this and add more brushes for the data ring?
The data rate is less than 1Mbps.
Ideally an inexpensive solution.

An infra red link could do that.

 

[amdx]

If this is a one of, you can pickup a junked VCR for next nothing and
use the rotory transformer from the video head assembly. It has good
bearings
and is very stable. I'm not sure about the frequency response or the power
level it is capable. Opps, your 5mm spec. means you can't use the whole
assembly. Oh well the ferrite pieces are there.
Anyone have thoughts about it?
Mike

Nice picture.
http://commons.wikimedia.org/wiki/File:Rotary_transformer_2.jpg

http://en.wikipedia.org/wiki/Rotary_transformer

 

[Grumps]

That's not a commutator unless it has sectors that make and break
connection.
It's 'slip rings' if it's just a connection to the rotating part.

Ah, slip rings. That term escaped me, thanks.

If you use two
brushes onto a slip ring, probably the data and power transfer will be
continuous (and a little ECC treatment can keep the data clean).
Redundancy of brushes is... cheap. Three slip rings, power/ground/data,
would
that be enough?

Yes, 3 rings would be enough.

 

[Andrew Gabriel]

Thanks. I know of one patent which covers this approach, so it's not going
to be good for the finished product.

Every spinning VCR head assembly works this way, at 20-40MHz IIRC.

 

[Theo Markettos]

An infra red link could do that.

4Mbps Fast IrDA should do the data rate, and the chips and transceivers are
off-the-shelf.

Theo