Data over battery line

[Marco Trapanese]

Hello,

I need to transmit few bits on a cable connected to the battery of a
motorbike. The communication must be bidirectional. Very very low bit
rate. The cable lenght is less than 2 m.

I'm thinking about a quite high-frequency PWM (about 100 kHz) with a
simple on-off modulation. Passive detector on receiver side with a
comparator.

Cost is the primary target thus I'd avoid to use a pll.

I'm wondering if the battery will "eat" the carrier, like a big
capacitor. Because on the cable flows up to 30A it may cost a lot using
inductors on battery side.

Any hint?

Marco

 

[Marco Trapanese]

Il 07/02/2012 11:42, Jan Panteltje ha scritto:

At 100kHz you would nee an inductor to prevent the RF from shorting.
Its resistance would be prohibitive at 30A.

I agree.

Maybe better use some of the shelf 430 MHz modules and go wireless?
Or at least use a much higher frequency, 100 MHz or up.
Sometimes an extra wire (copper) is the simplest solution,
bet there is space where there is a 30 A conductor.

I know this, but the customer requires neither extra conductor nor
wireless... So I'm looking for a solution, if any.

Marco

 

[Jeffery Tomas]

"Marco Trapanese" wrote in message news:[email protected]...

Hello,

I need to transmit few bits on a cable connected to the battery of a
motorbike. The communication must be bidirectional. Very very low bit rate.
The cable lenght is less than 2 m.

I'm thinking about a quite high-frequency PWM (about 100 kHz) with a simple
on-off modulation. Passive detector on receiver side with a comparator.

Cost is the primary target thus I'd avoid to use a pll.

I'm wondering if the battery will "eat" the carrier, like a big capacitor.
Because on the cable flows up to 30A it may cost a lot using inductors on
battery side.

Any hint?

Marco

Of course the battery will act as a capacitor.... that’s what they
effectively are.

The issue is not the battery as that can be disconnected, it is the devices
being power by the battery. They must have enough capacitance to deal with
the added noise on the line.

You just need a way to disconnect both ends(because of bi-dir) and put your
comm stubs inbetween.


device----+-------wire----------+----battery

where the + are the comm stubs.

Essentially you disconnect the wire when transmitting a low which causes the
voltage to drop.

Dealing with the switching, the high currents, etc really isn't worth it
though. It doesn't cost much do add a 30AWG comm wire. It will be much
cheaper, easier, and safer.

(The above scheme is basically a TDM but 1 signal is actually power)


You can get around some of the issues with switch by not completely
disconnecting the wire(so your low is not 0V but maybe Vcc/2). Since you are
not transmitting much data and you can do it at relatively high rates you do
not need much capacitance added after the stubs. Just enough to keep the
voltage from drooping too much during transmission.



--

 

[Marco Trapanese]

Il 07/02/2012 11:31, [email protected] ha scritto:

Do a google search for 1-wire. Maxim and others make inexpensive
developers kits and interface products.

I know 1-wire protocol. I'm concerned about the battery connected to the
bus. Do you think it isn't affected?

Marco

 

[hamilton]

The cable may be less than 2m but the line of sight distance should be
more like 1m. Using some 555s, an opamp active bandpass at the carrier
frequency (~100KHz band), and some ferrites, you can make an NFMI
transceiver. It's done all the time for shortrange communication and
low throughput telemetry systems. NFMI stands for Near Field Magnetic
Induction, which just means magnetic field generation and pickup,
usually using ferrite rods. These systems are relatively low power
too, it doesn't take much to communicate over 1m.

It would seem to me a 2m piece of wire and one-wire comm would be far
easier.

 

[Grant]

At 100kHz you would nee an inductor to prevent the RF from shorting.

Not really. I made a circuit 35 years ago that sent a signal up
300A+ charging leads to the charger, was a high temperature interlock.

Its resistance would be prohibitive at 30A.

Not at all. Batteries have a complex impedance you can play against
at fast rise times.

Maybe better use some of the shelf 430 MHz modules and go wireless?
Or at least use a much higher frequency, 100 MHz or up.

Maybe? Technique I used basically shorted the battery for a very
short time, resulted in pins on the DC line that could be picked
up at the charger, a couple metres away down the charging leads.

IN a dirty noise environment as the DC converter was mains
commutated SCR controlled bridge. A huge inductor in series with
the charging lead probably helped, but tests showed the battery
was not a short circuit for high frequency.

I remember the recovered pulse had to be lengthened to trigger a
555's reset pin, it was less than 500ns or so, long time ago.

Sometimes an extra wire (copper) is the simplest solution,
bet there is space where there is a 30 A conductor.

No space for a special connector assembly with an extra wire?

Grant.

 

[mike]

use a device similar to a clamp on current probe to inject THE 100 kHz
signal onto the batter circuit and receiver it at the other end with
similar current probe. THis way you don't care how low the batty Z
is.

Mark

That sounds really good on the surface, but you're building a transformer
that will have a high turns ratio. Works fine for the data, until
you hit the starter and zap all the electronics on the other side of the
transformer...or turn on the turn signals or...or...

SNR is gonna be horrible, even if you clamp all the voltages so the chips
don't explode.

Quoting from your original post
Cost is the primary target
end quote.
Nuff said.

We don't know all the constraints, but my advice is to tell your customer
to add the extra wire. It's gonna be easier==cheaper than all the stuff
you're gonna have to add to make it work over the existing battery wire.


Using a wireless link to replace a fixed 2m wire on a motorcycle frame,
is insane.

Contrary to popular opinion, it's the system engineer's job to tell the
customer when he wants the wrong thing.

 

[Grant]

Seems like amateur crap to me.
Shorting a battery.

Company got a patent on the technique, so it must've been bad

Grant.

 

[Marco Trapanese]

Il 09/02/2012 12:59, mike ha scritto:

We don't know all the constraints,

I don't, either

but my advice is to tell your customer
to add the extra wire. It's gonna be easier==cheaper than all the stuff
you're gonna have to add to make it work over the existing battery wire.

I see.

Using a wireless link to replace a fixed 2m wire on a motorcycle frame,
is insane.

Agree.


Contrary to popular opinion, it's the system engineer's job to tell the
customer when he wants the wrong thing.

Yep. I always tell the customers when they're wrong. Sometimes they
don't care, though. And I don't accept the job.

Marco

 

[Jasen Betts]

There have been patents issued on perpetual motion machines.

You would not want to 'short' the sort of batteries I have been around,
size of a room, the switch and wiring would evaporate immediately.
1 uS would be far too long,

assuming 6.3kg of copper wiring
it's about 30Mj to vaporise it, giving your batteries a peak power
ofOVER 30TW, impressive.

How did they get the inductance so low on a battery that size?