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Myrio accelerometer

Hi
I am using MyRio to build small autonomous robots. I am using a battery as a power supply and I already have 3 working robots. The robots have 4 small DC motors that get their power directly from the battery while to command comes from MyRio DIO.I need acceleration measurement to increase the accuracy of the robot and I notice something very strange,whenever I am using the accelerometer without the motors I am getting reasonable values. But when I am reading the accelerometer while working with the motors I am getting some very large reading (10g) even due the robot is not moving at all. is it possible that due to the peak power consumption the accelerometer is not working properly? I even try to give MyRio power from a different source (but still connected to the motors by the DIO) and I got the same result.
Any ideas of what effecting the onboard accelerometer?
Thanks
Eviatar

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Eviatar98,

 

What are the reasonable values returned without the motors?

 

What values are you expecting to be returned from the accelerometer? The specifications for the myRIO 1900 indicate that the accelerometer range is +/-8g.

 

When you say that you’re using the accelerometer with the motors I’m making the assumption that the myRIO is in motion. Is this correct?

 

Are you using the accelerometer express VI or lower-level FPGA to calculate the acceleration?

 

Additionally, what direction are you measuring acceleration in with the tri-axis accelerometer?

Will M.
Applications Engineer
National Instruments
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Hi
Without the motor I am getting near zero values on X anY axis and ~1g on Z axis.
Our robots float on an air and the acceleration is created only by the thrusters which provide 180mN. Let say the mass is 1Kg and the thrusters have 45 deg from the body axis (see attached figure) if two of them are working together we need to expect acceleration of ~0.25m/s^2.
Now even if the robot is standing on the table and does not move at all, when the motors are working we get 2-3g on the Z axis which is not in the thrust vector direction. I am using accelerometer express VI, I am also using Pmod GYRO which is working perfectly.

 

Any ideas?

 

 

Regards,
Eviatar Edlerman
Aerospace engineer
Lab and Systems Engineer at Distributed Space Systems Lab
Technion - Israel Institute of Technology
http://dssl.technion.ac.il/
972778875603

 

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eviatar, 

 

If you rotate the myRIO on the directional axes (without the motors attached) what readings are you getting on the X &Y axes? The 1g reading on the Z axis is expected. I would expect you to read ~1g on the X and Y axes when the accelerometer is aligned in the resepcetive directions. 

 

If you use the myRIO getting started Wizard, you should be able to view the accelation values through the GUI (if you're not doing this already). 

Will M.
Applications Engineer
National Instruments
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Will,

 

If I point X or Y axis to the gravity direction I get ~1g but as I mention before everything work fine when the motors are off.

 

Eviatar

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Eviatar98,

 

If you’re getting the expected results with the motors turned off, that indicates to me that the accelerometer is working correctly. You may be introducing noise into the system from the motors or some kind of other interference that is throwing off the accelerometer readings. It could be a different in the way the two sensors measure acceleration.

 

Do you have a link to the product page and spec sheet for the Pmod GYRO you’re also using?

 

Additionally, how is the myRIO connected to these DC motors?

Will M.
Applications Engineer
National Instruments
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Hi

 

link to the gyro spec, The gyro is connected through I2C.

 

The motors are connected directly to the battery and their commands come from digital output on the MyRio.

 
Thank you
 
Eviatar
 
 
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As I mentioned, the “bad” readings are likely coming from noise introduced by the motors at the base of the board mount. I’m following up internally for more specifics about the myRIO’s on-board accelerometer to see how that sensor compares to the one linked. I will post an update here once I have more information.

 

It sounds like the accelerometer reading is not preventing you from moving forward with the project because you are able to obtain accurate readings from the gyro. Is this correct?

Will M.
Applications Engineer
National Instruments
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Indeed, the project continues and robots working thanks to the sensors that do work properly. In addition, I get a fairly accurate position measurement from an external system and therefore by using Kalman filter, I can estimate the accelerations. Getting the acceleration directly will obviously increase the accuracy of the system.
 
I think this issue is worth a more thorough check so future customers will be able to enjoy these sensors.
 
Thank you
 
Eviatar
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A few more follow up questions for you: 

 

1). Can you provide a schematic for how everything is wired (which pins are writing/reading data)? 

 

2). If you were to generate another DIO signal, is there noise on that channel as well? 

 

3). Have you including anything to isolate/dampen the effects of the noise that could be generated from the motors? It's possible that the noise is either mechanical or electrial. 

Will M.
Applications Engineer
National Instruments
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