The 24 vdc power cables are essentially run up both sides of the boat. Tapping into these cables at various points are distribution nodes. These nodes have two functions.
1. distribute the 24 vdc power to the local devices that need it, e.g. pumps, fans, lights etc.
2. collect signals and settings from local devices for transmission back to the computer system.
3.
Each node is connected to the computer systems through USB. This enables the computer system to turn on or off any of the devices connected to the node. The USB connection also allows information to be sent back to the computer system e.g. current amps and switch settings. Power 24 vdc and USB cables are the only incoming cables to these nodes.
Each distribution node consists of a
1. aluminum skinned box
2. 24 vdc power inlet unit
3. a variable number of power control units
4. a variable number of digital I/O units.
5. one or more USB interfaces and hubs.
There is a “standard” distribution box and a “large” distribution box. The standard box has
1. A High-Power card with 4x 15 amp fused circuits under digital control
2. A Low-Power card 8x 3 amp fused circuits under digital control
3. 24x optically isolated I/O channels that can be either inputs or outputs.
4. A USB connector for control and communication
5. Current feedback from each Power card. There is one current sensor for each card. The total current is reported, not the current for each channel on the card.
6. Space for 4 2-pole circuit breakers.
7. The standard box is 610 (length ) x 260 ( width) x 125 (height ) mm. This includes the cover but not the space required for the external cable connectors.
The “large” distribution box has an additional High-Power board. Its dimensions are 870 (length) x 260 ( width) x 125 ( height) mm.
Aluminum skinned box.
There is a file dealing with the manufacture of these boxes. They are designed to
1. protect the internal components from the environment, i.e. water splashes, mist. The boxes are not immersion-proof.
The boxes have simple rectangular sides. The incoming 24 vdc power comes in through waterlight cable feedthroughs. The USB control cable has a watertight fitting. The outgoing cables to the various devices go through slots in the sidewalls that will be protected by compression foam. The lid of the box is completely removable and there is a foam seal between it and the body of the box. We also have a “breather” plug to try out on the boxes. This is supposed to allow the pressure inside and outside the box to equilibrate without allowing particulates ( e.g. salt ) to enter the box.
2. protect the internal components from electrical upsets. The boxes are made with a double aluminum skin. Each skin is electrically insulated from the other.
Large differential voltage surges on the 24 vdc lines will be devastating to the electronics. Protection of the electronics takes place at several places in the circuitry ( and we never believe that it can be 100% successful ). At the box level the incoming 24 vdc lines are connected to the outer aluminum skin though MOV and gas discharge units. The MOV units are EPCOS S20K25s and the tubes are EPCOS . They should short any spike over 77 volts to ground and recover when the voltage drops below 77 volts. The two devices are used in parallel because the MOVs are faster than the tubes but carry less charge.
These devices operate on a voltage difference between the cable and ground, not the inter-cable voltage. Normally the hot cable will be 24 volts above ground and the return line should be very close to ground. During an impulse event there is only a local ground. If the 24 vdc cables and the local aluminum hull all go up, with exactly the same voltage profiles during an event ( very unlikely ) then these devices will not function.
The outside aluminum skin of the box is connected to the boat hull by as short a cable as convenient.
3. The boxes are make of thin aluminum skin laminate. They are not designed to take any physical abuse. The aluminum is easily marked.
4. The boxes are expected to be able to passively dissipate the heat generated within them. That heat depends upon the “no-load” dissipation inside the box and the effect of load on the MOSFET switches. The “no-load” load for the various components is shown below
a. 4 chan High Current board …. 0.3 watts
b. 8 chan Low current board……0.3 watts
c. MCC USB IO board…………~0.5 watts
d. NI USB IO board …………~0.5 watts
e. USB hub…. Externally powered….3 watts, unpowered ~0.5 watts.
We will use the Omega temperature data acquisition units to test how hot the boxes get in practice.
24 vdc power inlet unit.
The 24 vdc power lines enter the box through two watertight connections. The cables immediately connect to two copper busbars that feed a row of Carling Technologies CA2 two pole circuit breakers. The value of the requirement to have a two pole circuit breaker is unclear since the return side of the 24 vdc should always be close to ground. However with the 2-pole option it is always very clear that the downstream circuit is completely disconnected from the power source. There is space for 4 circuit breakers in the standard panel. One of the breakers ( on the extreme right hand side ) is always for a 5 amp breaker. This supplies local power for the distribution box, for example, a USB hub, a 5 vdc power supply etc. The other circuit breakers are sized according to the card that they control, usually they are either 40 amps of 20 amps.
One of the shortcomings of the Carling circuit breakers is that it is not apparent, from the front, what the amp rating is. We inscribe this on a plastic panel Velco’ed to the aluminum plate to which the breakers are mounted.
The connectors for the cabling exiting the breakers are homemade. Because of the geometry of the box components we take normal cable connectors and undo one of the 90 deg bends. This cracks the surface of the copper. We solder over this crack. I hope it holds up.
USB Interface Devices
There are two types of USB interface used in these distribution boxes. The “standard” distribution box has 1 High Power Board and 1 Low Power Board. To control them there is a MCC 1208 device. This is a USB 1.1 compliant. It is mounted on its own circuitboard with Phoenix connectors to the boards. The MCC 1208 also has the analog inputs that are used to read the current flow though the High and Low Power boards. In addition there is a NI6501 board ( USB 2.0 compliant ) with 24 IO channels that can be individually configured as either inputs or outputs. This also is mounted on its own card. Connectors on this card normally connect to 24 optically isolated 2 wire Phoenix terminal headers. Plugs to these headers can be used for controlling external digital levels of reading external digital signals. They cannot be used for powering anything more than a milliamp LED.
There is an unpowered 4- port USB 1.1 Hub from RadioShack. This connects the USB devices in the distribution box to the single USB watertight connector on the outside of the box.
We do not anticipate any requirement for high speed data transport into and out of these boxes. Therefore we think that USB 1.1 will be adequate. There is space for a USB 2.0 hub inside the box but, at the moment, these appear to require considerable external power and so they are not standard.
Power Control units.
The individual power control circuit boards are described elsewhere.
