Re: Noisy Accelerometer
 

Good Call, Al. It is good practice to check for chassis faults when this sort of thing happens.

LabVIEW has filters in its Signal Processing pallette, I'll try to write a blurb about them when I get time.

Re: Noisy Accelerometer
 

If you read the accelerometer datasheet, the output signal can have a noise according with the internal frequency....or the noise from power supply

it's on page 7 on datasheet.

Re: Noisy Accelerometer
 

Quick update on our situation.

Today we accomplished many tests. Got assistance from another team who aided us in debugging our situation. There is not a current going through the robot. The "noisy-ness" still occurs whether we push the robot (without power) or drive the robot (with power). However, the noise does not occur when we "swing" the robot with our humans. We also let the robot sit in free-space and in a anti-static bags and it did not fix it. We tried a foam attachment to cancel out vibrations, not much help there. So we have found nothing to filter out this "noise", and we believe to have "broken" our accelerometer, as it returns nothing now but -5 G's (which apparently FIRST won't be selling).

So.... we're out of ideas...

-Tanner

Re: Noisy Accelerometer
 

If you are getting noise when the wheels are turning (by pushing or battery) but not getting it when the wheels are not turning, then it is most likely noise from your motor power lines. When you push the bot, the motors generate electricity because a DC motor is both a motor and a generator (depending on how you use it).

To reduce this problem, it would be best to locate the sensor near the controller to minimize the wire length. Also, avoid putting any major power supply line near the sensor. Consider shielding the sensor and the signal lines as well (an antistatic bag with an open end is not a quality shield).

If you need a replacement, there are many retailers who sell accelerometers. Sparkfun.com is a good supplier at a reasonable cost. However, this would count against your non-KoP spending limit.

-Jeff Erickson, FRC 41

Re: Noisy Accelerometer
 

It doesn't sound as if you've run the "up on blocks" test that I recommended - lift the robot off the ground and electrically spin the wheels. In this configuration, you should read zero acceleration (with the exception of gravity and any vibrations that couple the motors to the accelerometer).

Reading -5G is obviously an indication that something is broken. You'll need to either fix that problem (broken PWM cable?) or replace it before doing anything else.

Russ

Re: Noisy Accelerometer
 

-5G is probably 0V. This means you are either not powering the module or are not receiving the signal back.

Is the LED on the analog breakout lit?

Re: Noisy Accelerometer
 

What types of shielding would be used to shield the sensor? I know what we can use for the wires, but don't know what we'd use for the sensor.

Yeah, we ran that test yesterday, one of the first things we did. In that configuration we still had the noise (which probably just reinforces the fact of noise going through the motor wires).

Yes, the LED in the analog breakout is lit.

Think we'll order another sensor or two, if this is the way we'd like to go.

Thanks
-Tanner

Re: Noisy Accelerometer
 

I am using a smoothing filter and it's working well, just don't try integrating it to get the velocity. The noise, insignificant on acceleration readings, always ruins velocity for me.

Btw, has anyone obtained a good velocity reading using the accelerometer? through which method?

Re: Noisy Accelerometer
 

Tanner,
Can you upload some test data that shows the noise (with sample rate/timing info)?

I'd like to see how much noise you're encountering. Please provide both "static" noise (no motors running) and "dynamic" noise (with motors running).

Also, can you put a jumper on an open analog input and measure your 5V power supply at the same time? I'd like to see that noise AND your battery voltage as well. More data is good. Maybe there is a correlation between your observed noise and noise on either the battery or 5V supply.

Russ

Re: Noisy Accelerometer
 

any accelerometer is going to give you a good deal of noise. Try using one that supports I2C. There are ports for I2C right on the digital sidecar, and I have heard that the these accelerometers eliminate most of the noise by themselves.

A affordable method I've thought about is using the Wiimote's Nunchuck. It uses the I2C protocol to interface with the wiimote, it only costs $25, and the accelerometer inside it goes for $40 last time I checked.

There are a lot of resources on the internet geared specifically towards interfacing with the nunchuck.

Re: Noisy Accelerometer
 

Daniel,

We've gotten velocity readings from our accelerometer.

We modified the Gyro class, and made the m_analog member public. That way we can use the accumulator support in the Gyro class for the Accelerometer instead.

We manually specified accumulator centers and deadbands.

We also tweaked the averaging and oversampling bits on the analog module.

We are still having some problems with precision, but the numbers as is are useable for some basic autonomous movements. Maybe even good enough to accumulate again to get displacement data... we'll see!

Re: Noisy Accelerometer
 

Hi everyone,

I'm looking to do the same thing as you SlimBoJones, but in Labview. I have attached a VI with something that started as the 'Get Gyro Angle' vi, but made the changes I could see to the inputs to look at the accelerometer instead of the Gyro. I haven't tried it, as there is an error.

Two questions come out of this:

1. How can I modify the VI that changes the accelerometer DevRef global to the AI Channel cluster so that it works? At the moment, the error dialog says it's being referenced from outside the library, so it won't run. Is there another way to do this?

2. I don't understand the process of what the VI is doing with the accumulator to come up with the integrated value...is it enough to just feed it an accelerometer rate instead of angular rate, as well as the scaling and center voltage, and hope for the best?

Thanks for any and all responses...

Evan

Re: Noisy Accelerometer
 

We're seeing the same kind of jitter in the accelerometer from this years KoP. In all directions (x, y and z) we are getting odd values for an at rest bot. The nominal values are suppose to be 1.5V at rest. X is 1.84, Y is 1.76, Z is 1.74.

I tried resting the accelerometer on the ground, away from the bot and no change. I swapped the accelerometer with one from 2005, and while there's a little jitter it's in the +/- .002 range.

Now the new on is more sensitive (300 mV/G) and it's possible that we need to replace the bandwidth filter on the board, and isolate the board better but I'm leaning more toward that the board is broke.

Re: Noisy Accelerometer
 

Your post is very informative, and a refreshing break from "It don't work". Thank You.

At rest, you shouldn't be able to see those values if everything is working - it corresponds to about 2.5g acceleration towards a corner. Something is wonky.

Do you have an oscilloscope? It could make debugging easier.

Can you plot the values over time? You may be able to see noise.

I would hold the accelerometer in each of the 6 basic positions (Z up, Z down, Y up, Y down, X up, X down) and record the values given. This may give you better insight into what is wrong.

Re: Noisy Accelerometer
 

While our team is planning on using WindRiver, I hooked up the LabView Accelerometer VI to get the pretty pictures of what was going on with the sensor. It is a very jagged plot with a +/- 0.15 V, making it virtually useless. This test is being done on a bench test setup, the cRIO is on a piece of plywood, and the sensor is using a PWM cable that is long enough so that the sensor is resting on the floor. (and it a concrete floor in a manufacturing facility) We do have an oscilloscope and could test things.

I'm a mentor, and as such I'm at work at the moment so getting data will have to wait till this evening. For us, I suspect that we're going to use the older sensor because a) it works, b) only need x and y axis, and c) I doubt that were going to see 3G accelerations to warrant messing further with the newer device.

I just wanted to add some data to the discussion. I like the theory that it's just some noise artifact that could be corrected by either software or a hardware fix. But it seems to me that the board is bad.