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Unread 14-01-2008, 01:18
comphappy comphappy is offline
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Re: Filtering out Vibration while using a KOP Accelerometer

Your calculations look very wrong, i would check them first.

<R68> is what i was referring to, the beginning makes it sound like they are not allowed, because they are not COTS. But if all of the components are COTS and you put them on the PCB is that OK?
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Unread 14-01-2008, 01:36
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Re: Filtering out Vibration while using a KOP Accelerometer

Quote:
Originally Posted by comphappy View Post
Your calculations look very wrong, i would check them first.

<R68> is what i was referring to, the beginning makes it sound like they are not allowed, because they are not COTS. But if all of the components are COTS and you put them on the PCB is that OK?
Let's look at R68:

Quote:
Originally Posted by <R68>
Additional electronic components for use on the ROBOT must be either COTS items or assembled from COTS items. Additional electronic components include any object that intentionally conducts electricity, other than Innovation First Inc. relays and speed controllers, wires, connectors, solder, and fabricated printed circuit boards.
So as long as your custom electronics is either COTS (such as an encoder) or assembled from COTS items, it's allowed. A separate PIC, resistors, capacitors, PCBs, etc, are all examples of COTS items which can be assembled to your own circuit.
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Unread 14-01-2008, 01:41
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Re: Filtering out Vibration while using a KOP Accelerometer

As for data. I guess one question I have is: what motion (if any) was occuring while this data was recorded.

The next question is how to interpret your accelerometer values, which are in the range of 475 to 569 or so. How does this relate to millivolts coming out of the accelerometer? Sorry, I havn't played with the controller in a while... and when I did, back in 2002... well... everything was different

Anyways, according to the Sensor Manual, it would appear that horizontal orientation under no acceleration should be outputting 2.5 volts.

According to the Data Sheet, it would appear that the 0g corresponds to 1.65V given a 3.3V input. I think first is driving it with a 5V input. So a 0g output of half of Vs seems to be self consistent here.

So the first thing we want to verify: is the meaning of your numbers there somehow consistent with the ~2.5V +/- 1.7V that we expect to see from the output of the device?
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Unread 14-01-2008, 01:45
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Re: Filtering out Vibration while using a KOP Accelerometer

Quote:
Originally Posted by ay2b View Post
Let's look at R68:



So as long as your custom electronics is either COTS (such as an encoder) or assembled from COTS items, it's allowed. A separate PIC, resistors, capacitors, PCBs, etc, are all examples of COTS items which can be assembled to your own circuit.
my concern stems from the fact that I cannot come up with something that would be in violation of this rule if it is not a drive circuit which are explicitly prohibited. So if I fully design my own integrator and PCB, then I am ok?
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Unread 14-01-2008, 08:05
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Re: Filtering out Vibration while using a KOP Accelerometer

Quote:
Originally Posted by jleibs View Post
The next question is how to interpret your accelerometer values, which are in the range of 475 to 569 or so. How does this relate to millivolts coming out of the accelerometer?...So the first thing we want to verify: is the meaning of your numbers there somehow consistent with the ~2.5V +/- 1.7V that we expect to see from the output of the device?
The RC's analog inputs have ten bits of precision. That gives results from 0-1023 for a voltage from 0-5 volts. A value of 475 translates to about 2.32 volts, and 569 represents about 2.78 volts.
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Unread 14-01-2008, 08:12
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Re: Filtering out Vibration while using a KOP Accelerometer

Try a Kalman filter. There is source code to some optimized versions floating around on the internet. (The full filter is a bit labor intensive.) This filter was designed to read noisy signals and make estimates of true value based on it.

http://en.wikipedia.org/wiki/Kalman_filter
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Unread 14-01-2008, 08:22
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Re: Filtering out Vibration while using a KOP Accelerometer

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Originally Posted by joshyboy9987 View Post
won't it take out things like the robot crashing into the wall and stuff like that. That would change the acceleration really fast. I mean, I can't assume I'm not going to hit anything in autonomous (hybrid) mode.
Hey, that's a safety feature we're having in hybrid mode! We're using one of the programmable comparators to detect (and generate and interrupt) if we get a G-spike indicating a crash (a lot like a car with airbags) and in the event of a crash will kill all the motors to protect the robot. Now if we can just get it to call OnStar...

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Unread 14-01-2008, 08:24
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Re: Filtering out Vibration while using a KOP Accelerometer

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Originally Posted by joshyboy9987 View Post
Hmmm... The design team is gonna have a fit if I tell them to put an extra wheel on the robot, maybe I'll just make 'em deal with it.
uhhhhh..... just put an encoder on one of the wheels you already have? I realize this can slip, but its probably a better probability of it working than an accelerometer with error squaring... eh... i mean double integration.

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Unread 14-01-2008, 11:34
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Re: Filtering out Vibration while using a KOP Accelerometer

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Originally Posted by Tom Bottiglieri View Post
Try a Kalman filter. There is source code to some optimized versions floating around on the internet. (The full filter is a bit labor intensive.) This filter was designed to read noisy signals and make estimates of true value based on it.

http://en.wikipedia.org/wiki/Kalman_filter
First of all, Kalman Filters are great

Second of all, the rest of this post will likely sound like non-sense unless you've read a couple articles in Kalman Filters. If you think you're interested in Kalman Filtering, this is a good place to start.

Now onto what I was going to say: I've never had a Kalman treat the accelerometers as direct measurements in a nav system before. There are a couple reasons for this. Keep in mind, I use a full 6DOF system where all of this is more complicated. For one, it requires running the Kalman faster than we want to (at the full 400Hz IMU rate). Additionally, doing so requires the linearization of a fairly gnarly prediction function (since the prediction function is doing an implicit double-integration).

Finally, and I think this is the most important one: if your state vector includes acceleration and you are Kalman filtering your acceleration. You need a MODEL of how you expect your acceleration to change from one time-sample to the next. It's comparing this prediction to what you actually measure that allows a Kalman to function well. You have no means of acquiring such a prediction, especially when you want to measure things like crashes and bumps (outside the scope of your controlled values). So basically, to tune your Kalman to reflect the unpredictable nature of your accelerations, the process noise will have to be high enough that very little smoothing will actually happen.

What we tend to do is leave acceleration out of our state vector. We track position, orientation, and velocity inside the Kalman. We then integrate the accels/gyros using the inertial nav equations essentially as part of the prediction phase, and use this to update our predicted position/orientation/velocity values. Then, we use the Kalman measurement update to incorporate additional sensors such as GPS/IMU and correct these terms.

Calibrating and working out bugs in this kind of system is the kind of nightmare that can take an experienced engineer months (or years) to get right. At work, the Kalman Filter we use for our nav system has a state vector with 150 elements in it. Needless to say, it is still being debugged.

But you guys are working in 3DOF and I expect the problem is likely more tractable in that case. Maybe I'll review the equations when I get a chance this afternoon.

Where a Kalman would really shine is fusing measurements from your accelerometers with measurements from your shaft encoder or other sensors.

Anyone, please feel free to contact me if you would like to discuss Kalman filtering in more details.
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Unread 14-01-2008, 23:20
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Re: Filtering out Vibration while using a KOP Accelerometer

Some people might find this article of interest.
http://www.freescale.com/files/senso...ote/AN3397.pdf
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Unread 14-01-2008, 23:36
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Re: Filtering out Vibration while using a KOP Accelerometer

Quote:
Originally Posted by comphappy View Post
Some people might find this article of interest.
http://www.freescale.com/files/senso...ote/AN3397.pdf
Definitely a great starting point for anyone looking at doing this. Thanks for the great find, comphappy!
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Unread 17-01-2008, 23:15
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Re: Filtering out Vibration while using a KOP Accelerometer

Yeah, my calculations were off a bit... I forgot to keep track of the stupid velocity constant when I did the first integration... Now the data is better, but still bad... I think we might go with the encoders... We even shock mounted the accelerometer, and that didn't help all that much. (Magnetic shock)
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Unread 17-01-2008, 23:17
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Re: Filtering out Vibration while using a KOP Accelerometer

you may consider low pass filtering out the freq range from the motors and/or compressor.
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Unread 18-01-2008, 07:42
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Re: Filtering out Vibration while using a KOP Accelerometer

I looked at the data in the spreadsheet. I plotted the dx values and the dy values (I assume these were the result of double integration and represented position). Each seems to have a recognizable trend. If this roughly corresponds to the movements you were applying to the robot (you didn't say), I'd be encouraged that the data could be useful. Sure, it's a little fuzzy. The trick is to filter out the fuzzyness without delaying the data so much that it's no longer useful.

If you need instantaneous location to within an inch, it's probably not going to be useable. But if you just need to know if you are probably past the wall and should turn left, it might work.
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Unread 21-01-2008, 02:55
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Re: Filtering out Vibration while using a KOP Accelerometer

Hi, I'm currently trying to decide between using wheel odometry and a double integration of accelerometer data. So this is a very interesting conversation!

One thing that I've failed to see is any mention of the gyroscope data used in conjunction with the accelerometer data. I see this as important because the robot's frame of reference translates and rotates with respect to the absolute coordinate frame of the field. Pretty much what I am saying is that the x and y coordinates of the accelerometer won't always remain parallel to the x y axes of the field therefore the orientation of the robot must also be monitored. Has anyone considered this as a probably source of error? I might've missed something, but there is my two cents!

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