So I've been doing a lot of work on redesigning the OpenROV power system to use lithium batteries and charge over the tether, and one of the things I keep coming back to is that we need to fundamentally rearchitect the power system to properly balance the available battery power with the required motor power. There are many comments on the forum about how overpowered the vehicle is, that we need to add throttling software, that sudden throttle movements kill the BeagleBone because of excessive current draw, etc. What this is telling me is that we are running the motors at too high a voltage; the battery pack voltage is not properly matched to the required electrical characteristics of the motor.
I'm hoping that in the near future we can do some tank testing of the motors and props, to get a better handle on what is really needed to drive the vehicle. But in the shorter term, I began thinking about the consequences of changing the alkaline battery pack from a 12 Volt design to a 6 Volt design.
The Turnigy 18A contollers are rated for use down to 4 NiMH cells, which is 4.8 Volts. Cutting the voltage to the ESCs in half would cut the peak power by a factor of 4, and would likely solve all of our issues with throttling too fast and twitchy response of the motors. A 6V pack would hopefully be a simple configuration change for people who already have kits, so perhaps they would not need to buy a new cape design. In addition, if we came up with a good battery/motor configuration now, it would allow us to do a lot more driving and testing in the near-term, and take some of the pressure off the Lithium battery redesign, so that we have more time to do a good job there.
Here's the current configuration of the vehicle:
The 8 C-Cells that are used by the ROV are connected in series, providing a 12V output. This voltage is fed directly to the ESCs, and also goes to a buck converter on the BeagleBone Cape that drops the voltage to 5V for all of the electronics. When the input to the buck converter goes below ~7V, it shuts down and the entire ROV crashes. This apparently happens at relatively low throttle settings when using alkaline cells.
One possible avenue of improvement is to lower the input voltage requirement at the input of the buck converter. We are unlikely to get this below about 6 volts for a 5V output, so this architecture has a fundamental limitation that anytime the ESCs pull the battery down to about half the initial voltage, the ROV is going to die.
Here is a sketch of what I think would be a better layout:
This time the two battery tubes are connected in parallel, for a 6V output. The Turnigy 18A controllers are rated for use down to 4 NiMH cells, which is only a 4.8V pack, so they should do just fine. The problem here is that we need a different type of converter to get 5V power to the BB and Cape, a Buck-Boost converter that can handle input voltages that are above or below the output voltage. You can find converters that will run down to about 2.5V, so this configuration will keep the BB running when the batteries are pulled down to just 40% of their original voltage.
It may be that 6V is not quite enough voltage to drive the vehicle well. In that case, we can add a fifth cell to each battery pack:
There are buoyancy and balance issues that need to be worked with this, but I believe some people have already done this to support using NiMH cells (10 cells in series = 12V). Note that by starting with a slightly higher initial voltage, the input requirements on the buck-boost converter can be relaxed slightly. A converter that will work down to ~3.5 volts or so is somewhat less exotic than one that works down to ~2.5 V.
So if we were going to build new capes the above configuration would be easy to support, but what about in the near-term, with existing capes? Here is a configuration that can be rapidly tested in the lab:
In essence all we're doing is rewiring the two battery packs in parallel, and then we're adding a small boost converter to the battery voltage before it goes to the cape. Pololu actually makes such a product here and here, though they are not powerful enough to drive the cape. I've got some on order that are supposed to arrive tomorrow, and I'm going to hook two of them in parallel with some blocking diodes and balancing resistors, and see if I can power the cape that way. My goal is to be able to support testing of this configuration during Saturday's build day, if there's a vehicle available and somebody wants to give me a hand. There are a number of interesting questions that can be answered immediately, such as:
- Is running the motors off of 6V sufficiently powerful to drive the vehicle?
- Does this configuration eliminate the problem of crashing the BB with the motors running hard?
- Do the ESCs run correctly if the motor voltage drops below the servo voltage (+5V)? I believe the answer is yes, but it needs to be tested.
If this configuration seems to work well as a test, my end goal is to do something like this:
So it turns out we already have a boost converter on the BB Cape-- it's the +12V supply for the PWM switches! Through making some minor modifications to the existing cape, I want to drive battery power first to the 12V boost converter, and from there into the buck controller that powers the BB and the Cape. There are a number of components that need to be changed around the +12V regulator, and it's not clear how low an input voltage it will support- it may go as low as 3V, but I'm not counting on it. If we can get it to work down to 3V, then this is probably low enough for a decent ROV configuration. If not, here is a configuration that will definitely work:
Here we're using the 5-cell battery packs, with the appropriate buoyancy and balance adjustments made. The +12V boost converter now only needs to work down to ~3.5V or so to provide plenty of margin for voltage drop while running the motors, and I'm fairly confident that can be done. The 5-cell packs will give us 20% more energy than the original design, lengthening the run time.
Anybody want to play with this on Saturday?
P.S. For those EE's on the forum who like to tinker with this kind of stuff, here's my first cut at the changes that need to be made to the cape to get to the configuration above. Doing this is highly experimental right now, and don't be surprised if you smoke a cape in the process. You should definitely double check all this against the schematic.
- Remove R12 to isolate the 5V buck supply from Vbat.
- Connect 12V_Reg to the input of the 5V buck converter by the best available solder pads.
- Revise the value of R82 to change the +12V UVLO threshold.
- (Maybe) Revise R88 to change current limiting on the +12V supply.