False readings from IMU depth

electronics
software

#1

While I have been bench testing my unit 2.8 kit and am seeing a great deal of fluctuation on the depth sensor. It will sit between .33-.66 on the desktop then bounce to 233 or 467 and then back again.
What would cause the false depth readings? Is there a way to calibrate the depth from the cockpit?

Once I get it in the water, I may need to calibrate the pitch and roll. Can that be done from the cockpit also?


#2

@Jim_Scholz,
The I2C bus that the IMU Module is connected to cannot tolerate much in the way of noise on its bus, especially with long wire runs, which is probably causing your pressure fluctuations.

The I2C bus was never intended to be used with long cable/wire runs. It was intended to be used for short runs between PWBs. I believe that the I2C spec calls for one twist every inch or so in the cable to reduce noise susceptibility. I was using the I2C bus for thruster control and had a thruster that was 18 inches from the controller. I had to reduce the I2C bus clock frequency from 100kHz to 75kHz to maintain consistent communication with the thruster.

As an example, I have a test bench setup of just the wire harness, motors, and a IMU Module. When I kept the I2C bus wires at the original wire harness length, I saw the fluctuations in the pressure reading that you are experiencing. So I cut the test bench I2C bus wire harness wires to the shorter length that I cut them to on my ROV chassis. After shortening the I2C bus wires, the readings became stable.
Regards,
TCIII AVD


#3

Thanks for the response. I think my problem may have been with movement of the wires. I resecured the wires to the outer housing and it seems to have improved the situation.

Any advice on re-calibration? My pitch and roll are off a bit.


#4

Hi Jim,
Since I was building a hybrid ROV I built a platform for the IMU Module that I could adjust in the roll axis unlike the OROV mounting scheme. Also, my IMU Module platform was parallel to the longitudinal axis of the ROV frame so it was level in the pitch axis.
During the potting of the IMU Module in its case I found that the solder joints on the bottom of the Module were about 0.030 in high. So I cut a very small piece of 0.030 in thick abs plastic and CA’ed it on to the bottom of the opposite end of the Module pwb from the solder joints. This guaranteed that the pwb would be level in the acrylic case during the potting process.
My understanding is that OROV plans to add an offset calibration function to the next Cockpit firmware release to correct for minor offsets in the IMU Module pitch and roll axes like you are experiencing.
Regards,
TCIII AVD