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Faulty current value reported by QGC


#1

I have been testing my ROV extensively and I have noticed that in my ROV, the current value is always capped at 56 amps in QGC. I tested with the wattmeter and saw the value of the current is about 86 Amps. Why is there this anomaly.?
2) I am not getting proper thrust value as promised by T200 ( I have 4 vectored ones). Will the current limiting be a reason for this?


(Adam) #2

Hi @rnr,

The is normal with the original Pixhawk power module. That one could only measure up to 56-60A current, even though it couple carry over 90A. There are more details in the documentation here. Our own Power Sense Module that has been shipping with the BlueROV2 for a little while now will measure up to 100A.

How have you determined you are not seeing the expected thrust? Can you share more details on your power supply, control system, and thrust measuring setup?

-Adam


#3

@adam I never knew the 60 amp limit on the sensor.
But the problem is I am not getting the thrust as guaranteed by T200.
I determined the thrust using a spring balance attached to the end of ROV using a rope to imitate a bollard pull condition.
I can at best get around 9kgf with full forward at 100% gain.
Power supply: I am using bluerobotics Li-ion battery (14.8V,18 Ah).
All the rest are same as Bluerov 2.

Upon inspecting the T200 characteristic chart I saw that it produce max thrust at 16V. But the battery BlueROV 2 uses is 14.8V and yet it gets around 14kgf thrust.?

What maybe the root cause of the diminished thrust then?


(Adam) #4

Hi @rnr,

Ah, I see where you are getting the 14kgf thrust number from on the BlueROV2 documentation now, thanks for pointing that out! It looks like we missed updating that number, its left over from the initial development of the BlueROV2 almost 3 years ago when we were estimating thrust by simply adding the thrust of four T200s at 16 V with the cosine losses. The design of the BlueROV2 was still in heavy flux at that point, the nuymbers have been updated.

Yes, the nominal voltage of the battery is 14.8 V and is not typically at 16 V except for a short time when using a fresh battery. In addition, in our initial estimates we forgot to account for voltage drop in the battery and power distribution system under load and how that would affect thrust. At full throttle 100% gain, the BlueROV2 draws quite a lot of current, and the four horizontal thrusters all out can draw over 70A. When taking into account the XT60/90 and bullet connectors, wire, solder joints, PCB copper (power module), terminal block and crimp connections between the battery and ESC there is noticeable voltage drop under load. All out, a battery that has been run for a bit will drop to 12-13 V at the power module, and a bit less at the ESC. A good estimate is 12 V at the ESCs under full load for the majority of the battery run time.

Assuming all four thruster are in the 45° mounting positions and each thruster is running at ~12 V under load, total thrust will be (cos(45°)x2x3.5kgf) for the rear thrusters, and (cos(45°)x2x3.0kgf) for the fronts. This comes to about ~9.2kgs total, which is very close to the ~9kgf number you measured. Actual voltage will vary over time, and the thruster angles are a little off a perfect 45° for stability reasons, but this estimate should be close to the actual most of the time with those mounting holes. We’ll run some new tests in the future to confirm these numbers.

-Adam