So in a couple of previous forum posts, I've hinted that at OpenROV HQ we were playing around with a circuit board design to clean up the wiring in the ROV and provide a power switch. We just went to fab with a new version of this board that is (hopefully) suitable for wide distribution, so it's time to spill the details!
Things all started a couple of months ago here, where Brian, Eric, and Thomas were looking at ways of measuring the current draw of the entire ROV (as opposed to just the Cape), as well as cleaning up some of the wiring around the ESCs. Thomas went ahead with that design, and at the same time Eric and I started discussing ways to expand upon the concept. We decided fit the board into the 3"x3" bay beside the ESCs, which would allow us to integrate the Molex connector for the vehicle wiring harness, and add some other functions as well- the first priority being a power switch for the vehicle.
I laid out a first prototype of the board, which came back from the fab a couple of months ago. Here's a picture of it in an E-chassis at OpenROV HQ:
The initial design had a bunch of different options on it for power switching. After playing with the prototypes for a while, the design we settled on was to turn the vehicle on by placing a small DC voltage on the tether. By choosing this, I was able to simplify the board and add some additional features.
So I've attached a schematic of the revised board, as well as a layout. As soon as I figure out how, I'll get the Gerbers and the Eagle CAD files posted up on GitHub.
The board has the following functions:
1.) It cleans up the wiring on the back of the E-chassis.
2.) It provides a power switch for the ROV. The principal switch method will be to place ~5V DC on the tether to turn the vehicle on. A set of solder pads (J4) is provided if the user wants to use a bidirectional reed switch (like here), or some other type of switch.
3.) An Allegro ACS711 current sensor is used to measure the power draw of the ROV.
4.) The board has an 8-channel 12-bit Analog-to-Digital converter on it. One channel is used to measure the battery current and one to measure the battery voltage, leaving six spare channels for the user.
5.) The board extends the I2C bus from the cape, sending it out the 18-pin Molex connector to the outside world, as well as providing solder pads on the board itself. The A/D converter runs on the I2C bus as well.
6.) The board uses an ISO1541 chip to isolate the I2C bus from the cape. Power for the sensors comes from a local voltage regulator as opposed to from the cape. So if you do something to muck up your sensors, you don't fry your Cape at the same time.
7.) Some circuitry is provided to allow us to start playing with power-over-tether. It's pretty experimental right now, but you've got to start somewhere.
In about two weeks we'll have a couple of assembled boards to start playing with. Take a look at this, and let us know if you have any ideas to improve on it, or would like to get your hands on one. If enough people think this is a good idea, maybe I can convince David to put the bare boards in the OpenROV store.
-Walt1382-PowerController2.1FinalSchematic.pdf (32.6 KB) 1383-PowerController2.1FinalLayout.pdf (29.3 KB)