I have been using the AttoPilot 90A sensor on my 3DR quad copter, from the start. It works, even though AC firmware requires some software lines to define calibration ratios (I looking forward to that to be set in the Mission Planner configuration).
Anyway, a few days ago I read a post from Arnt-Inge about AttoPilot sensor noise was interfering with sonar signal. That got me thinking about how the Atto-sensor is installed, and I started to worry about if the black ground wire in the Atto-sensor signal cable might act as a parasitic shunt to the main battery ground lead. Simply put, the motor current and other power consumers on the copter should have their current returning via the battery pack cable, not via the tiny Atto-sensor ground wire (and the APM board cirquit).
This evening I set up a test to find out whether the Atto-sensor three wire signal cable, black ground wire, acts as a parasitic shunt or not. I simply cut the black wire and connected a low impedance digital current meter.
Test turned out positive. At rest there is a 30mA current from the APM board to the sensor ground.
At full motor power it mesures 450mA. Half of an ampere. Hmm.
For me this is slightly disturbing. Having parasitic current like this in low level signal cables is simply dirty, even though I am not sure what significant implications it has.
I would like to know what you hardware guys have to say about this, please??
(this issue was also posted in the hardware issue forum, but that is a bit too narrow, since it is meant to be a tech support forum of the DIY store, sorry)
Replies
I agree with the both of them Tomas, However, I still think you need a capacitor inline with the output or ground of the sensor... Also now that I know there is an op amp involve, one should consider if it has enough reference voltage to push this op amp, also it depends which op amp they are using (inverted, non-inverted, current to voltage, voltage to current, Integrator ETC...) I could be wrong, because I haven't been in Electronics class for a long time however, I still feel that you need a capacitor inline with the ground of your sensor... This will smooth the signal and help get ride of noise... Also you have to figure out if the noise is coming from another component or not...
Also consider that it actually could be a APM design issue...
Hmmm.
Now I've looked at the Atto sensor datasheet - which I should have done before - I see that it does implement a 4 wire interface, and brings out the I and V signals as single ended (referenced to ground). The shunt is floating with 60V of common-mode range.
The Atto current sensor board needs the shunt to be on the high-side (at least 2.7V), as it uses this to power the INA169.
You do need to connect ground otherwise the board has no ground reference and will not work.
But you should not see 450mA of ground current. There is something wrong.
Not owning any of this hardware I should perhaps pull my head in now. Sorry for intruding :-)
Cheers,
Andrew
Hi Tomas,
I don't own an ArduCopter - or indeed any DIYDrones product - but I do feel qualified to comment because I'm an Elec Eng.
Your suggestions as to what is going wrong and how to fix it are exactly correct. In an ideal world, for sensing large currents with a shunt resistor you need a 4-wire interface: 2 to carry the actual current and 2 to sense the drop across the shunt. Obviously the sense wires feed a differential amplifier.
The situation you are experiencing is an entirely predictable result of connecting 2 grounds in parallel to sink a large amount of current. They current will divide itself up according to the impedances. You are getting 29A/0.45A, so this is roughly the impedance ratio of your 2 ground paths.
I think you should consider the possibility of large current flows through your servo cable. As the system stands this is potentially a safety concern, as obviously servo cables are not designed to handle motor current. I'd be tempted to tolerate the sensing error and disconnect the servo cable ground.
Cheers,
Andrew.
Tomas, looking at the circuit diagram, it seems the battery negative, or to be precise, the ground wire, merely passes through the board for convenience.
If you have that huge current flowing in the signal ground, I respectfully suggest that you have a wiring problem.
Measure the VOLTAGE over the battery ground wire, from battery itself, to each connection point, right up to the PDB.
Use a needle to stick through insulation.
Your description implies you have a large voltage drop along the ground wire.
And that is really going to mess up the readings from your sensor.
I put a ferrite on the atto cable, the noise was more or less gone that way. This can be read in all baro if I remember right. I will not do more before I get apm2, hope the program is quite stable when it arrives.
I wonder if this has anything to do with the random lack of gyro stability when I plug in the battery when the AttoPilot sensor is already attached to the APM board? I find if I have the AttoPilot sensor disconnected when I connect the battery the APM board initialises correctly and is stable, otherwise I randomly get barely any attitude stabilisation. After that I connect the AttoPilot sensor and all is OK, including alt-hold (2.1alpha). I did end up shielding all three wires to the sonar with the shield only connected at the APM ground end. Bit hard to see how a current could be present between the ground pin on the APM and the ground pin on the AttoPilot board when both are connected to the ground of the battery.