02 June 2009

DC (and AC) power control

Our new sailing boat is proving to be a new playground for exercising some skills that I haven't used for 25 odd years. In particular, designing the DC and navigation systems that are going on board. I have slowly been coming up with what I think is a good mix of reliability, ease of use and high tech. This last bit just for the fun of it. But, let me start at the beginning...

We really like sailing, and sailing should be simple and fun. To make that come true you need to be comfortable as well. Over here in Northern Europe this means that during the colder months of the year you want heating on board. That and shelter from the rain. After some to-and-fro we decided to heat the boat using an ultra-reliable diesel-fired Kabola stove that is going to distribute the heat using hot water. As space is at a premium and we didn't like to have normal radiators in sight, we decided to go for floor heating. There is a Dutch company called Yacht Floor Heating that has come up with a solution that 'works' for yachts.

Floor heating does have the disadvantage that it takes a long time to heat up the interior initially, so we really needed a method of being able to switch the heating on remotely before we get to the boat in winter. Once you can remotely switch on the heat the question soon came up why the system should be limited to just that. Why not be able to do more, like monitor bilge water, battery charges and switch on fridges, deck lights? Why can't I check what the wind conditions are remotely? Maybe even put a camera on board?

Looking at the various options it soon was apparent that there are many systems on the market, but not one that integrated everything that I wanted from such a system.

What I wanted is something that:
  1. Does not interfere with manual operation. If the fancy electronics fail, the system should still be operable by hand. This rules out all new technologies that do distributed DC switching, as they rely on the electronic DC bus to be able to switch stuff on and off.
  2. Should offer a simple switches-and-lights interface that is easy to understand for people that are not comfortable with computers.
  3. Allows integration with modern technologies such as remote controls (locally and remotely) and Google Maps mash-ups.
  4. Can be replaced, extended and modified over time.
The DC wiring and all consumers are going to be installed by the builder. They pointed out that the relatively low complexity of the boat in terms of electrical consumers meant that a distributed DC layout was not going to provide many advantages, if at all. A single cabinet that is centrally located will suffice. This will also simplify fault detection and maintenance: all circuit breakers are located in the same spot.

The system that I have come up with is as follows:
  • The distribution cabinet will contain circuit breakers and old fashioned mechanical latching relays. As they are latching they don't use power in use. It also gives the opportunity to have multiple switches that toggle the relay state.
  • The switches can be located anywhere in the boat. They are momentary action switches coupled with a feedback LED. We've chosen to centralize them in a single board near the entrance, hidden behind a frosted glass panel.
  • A small Linux driven PLC contains digital I/Os as well as analog inputs. It can toggle the relays and measure their state. The analog inputs are used to measure all tank levels and possibly bilge levels. The WAGO PLC is modular and extendable but proven technology.
  • A 2nd computer will provide on-board WiFi and a cellular link to the Internet. This can be extended to a satellite link later on. This computer will also have a CAN bus interface so that it can talk with the NMEA 2000 bus that all the navigation systems are connected to. Finally it will also talk directly to the Mastervolt DC and AC power systems.
  • Local control is via an interactive AJAX driven website that is accessible, for instance, via an iPod touch or iPhone over WiFi, as well as any other computer that is onboard -- including the navigation computer.
  • Remote control is via any computer or phone connected to the internet. The actual connection will be via a proxy located in a data-center, as the cellular end of the boat does not have a fixed IP address. Access to the data will be using a form of authorization, of course.
I've started some prototypes for all this, and it seems that this will come together nicely. I'm working on a NMEA 2000 packet sniffer as well as a web application for the iPhone/iPod touch. You can see the "Fluid and battery level" screen of the app in action at the top of this entry.

6 comments:

  1. Not quite following ... are the circuite breakers on a typical marine DC Panel? Are they on all the time ?

    If the computers go kuput, how are you turning on and off a DC circuit?
    (i) Relay is in the on-state, circuit breaker turns it off, right ?
    (ii) Relay is in the off-state, how do you turn on the circuit ? Another set of switches? Toggle breaker off then on ?

    Overall Very Intriguing,
    b393capt from Panbo

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  2. Hi Dan,

    The relays are bi-stable: a pulse (either from the manual switch or from the PLC) will toggle its state from on to off or vice versa.

    This does mean that you can't have switches that physically show whether the relay is switched on or off, and that's why there will be a LED next to each switch. That LED will show actual circuit power, e.g. when the circuit breaker has popped it will remain off.

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  3. Awesome !

    What application are you using on your iPhone to display the battery/fuel/water levels?

    Does it also do environmental data?

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  4. Hi Kees.

    I'm looking at a similar project. Can you please let me know how best to get in contact with you. I'd very much appreciate your advise.

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  5. Benjamin,

    You can mail me at kees@keversoft.com.

    Regards,
    Kees

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  6. Hi Kees. I am also very interested in your N2k to Iphone interface. Can you tell me more please?

    Regards
    Tim

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