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Better to run motors long or short?

Hi there,

I am finalizing the design for my autonomous ocean drone for the Microtransat Challenge, a contest to see if your drone can cross the Atlantic ocean.

My design calls for 2 - T200 thrusters … not for steering, but one as the back up to the other. In your opinion, do you think I would be better switching between the two regularly, or only switching if the first one fails (great product, but we are talking about running non-stop for 3 months). If it is better to switch, at what interval would do you think (i.e. hourly, 24 hrs, etc.)?

Thanks in advance!

Looking at the technical details section they run most efficiently at lower speeds and lower voltages. Lower voltage also results in a lower RPM, which would reduce any mechanical wear.

Is it possible to run both at the same time, but at reduced voltage/speed to match the power you were planning to use for a single thruster? That way you can operate at optimal efficiency, and then if you happen to have a failure you can switch to increased power on the remaining one.

Switching would be primarily beneficial to allow the electronics to cool down, which should be fine if the water is cool, but if you’re concerned you could have something like 15-min breaks for one thruster at a time every few hours or thereabouts.

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First, thanks for the response.

Second, with a 175W panel, and usually only about 6 hrs of good day light on the water, I really only have the juice to run one motor at a time. Ideally, a motor is running the entire time to reduce the amount of dead floating which will take me off course most likely. My concern is that anything can happen to the power train (motor failure, broken prop, entanglement, etc). I would hate to not finish due to that. That being said, there are 100 other things that could go wrong … just need to pick which to focus on. From the other projects I have seen attempt this, motor/prop issues and rudder are the two biggest points of failure, which is probably obvious as they are the only two exposed moving parts.

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Not sure how this makes sense - if you run them at a lower speed you should be able to get the same power usage but with greater efficiency.

Might be worth implementing a low power mode that activates once you’re predicted to run out of battery before sunrise - you can try to have very short bursts of power just to correct heading and any significant drift noticed from the accelerometer. Also presumably you’re using custom control stuff, but if you haven’t already it would likely be helpful to turn off or find low power options for any services that aren’t helpful (e.g. avoid checking for sensors that you’re not using, and use mavsdk instead of mavproxy if you even need to send mavlink messages)

We use stainless mesh in front of the props to avoid any seaweed or fishing line or whatnot getting caught in the props. Not sure how much that reduces the efficiency due to added resistance, but might be worth considering.

  1. I should have been more specific on the juice comment. During sunlight, I am hoping to get 6 hrs on 175 watts, or 1,050 Wh (assume 100% efficiency which is unlikely). This will need to recharge the battery pack of roughly 50Ah, while also running the motors. My assumption until I can test is that I need to run them at least 35 watts to move the vessel against current, etc. If I ran both motors, it would pull 70 watts which I don’t have the “juice” for or power in storage without stopping overnight (70 W x 18 hrs = 1260 Wh). I am also assuming it is better to favor continuous movement vs. sprint/stop.

  2. I love the idea of having a low power mode for keeping course. I will have a general low power mode to keep power to critical systems, but I can incorporate this as well.

  3. I do plan to putt some SS wire spokes in front of the props to reduce chances of debris entering. it is just 6 spokes for bigger pieces so hopefully doesn’t create too much drag.

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Continuous rotation causes wear on the plastic bearings, which means greater and greater power consumption.
Discontinuous operation is recommended. Intermittent operation is required to ensure that biofouling is not prepared to cause blockage.

In addition, separate thrusters can cause reverse torque, which can change if the direction of motion is not corrected. Stator blades can be added to the tail to make it a pumping propeller with no reverse torque.

If you’re ok with one running at at least 35W, could you not run two at 17.5W each to get the same total power usage and slightly higher thrust output? For 10V power on the Thrust-Power chart (easiest to see values for) 35W -> 1.21kgf whereas 16W -> 0.76kgf, so 2x16W = 32W and 2x0.76kgf = 1.52kgf which seems to be a 25% thrust improvement while using less power. Likely that would get reduced somewhat by fighting for the same water and by resistance from slightly more wiring, but as far as I can see it seems to be beneficial. You’d need to factor in the additional weight from an extra thruster and the relevant attachment hardware to see if it’s worth it for you, but my suggestion stands.

Fair enough, cool :slight_smile:

Sounds decent, hopefully there are no tiny fish that come too close, and no seaweed or fishing line floating around where your craft is.