01-22-2019 04:31 PM
Hi all,
Consider a DC accelerometer that reads 1g when sitting stationary on top of a table. We have a couple of questions about what it would read in following scenarios (we are trying to replicate a signal in a CAE model that would look like DC accel's signal in testing and would like to make sure that our formulae etc are correct).
- What would a DC accel read in Z (as function of time) if it is allowed to free fall without rotation.
- What would it it read in Z (as function of time) if it is attached to an end of a perfecty simple pendulum (all mass at a point) intially held horizontal with the DC accel attached at the center of mass with Z pointing upward (Z of DC accel is perpendicular to length of pendulum). (see pic below for the second case)
Thank you all!
01-22-2019 06:00 PM
Hello MakD,
If the DC Accelerometer is at rest it will see 1G. Since you have it on a pendulum I would expect it to be somewhere between 1 and 0 and -1 (when upside down at 180 degrees) at 0 Hz. It would depend on how much the pendulum string constrains the free fall and what angle the accelerometer is at because as it inclines it will not see 1 G but a vector.
Per your formulas the acceleration is probably some form of Acceleration * cosine(theta) where initially it has cosine(0), at the bottom a cosine(-90) and at end of the swing cosine(-180). Not sure how much the string constrains it through.
I found some information via google search:
https://en.wikipedia.org/wiki/Accelerometer
https://www.digikey.com/en/articles/techzone/2011/may/using-an-accelerometer-for-inclination-sensing
Maybe others can add responses as well
01-23-2019 08:40 AM - edited 01-23-2019 08:41 AM
Hi Kevin,
Thank you for your response.
Our DC accel formula does what you said: offset the acceleration by g*cosine(angle) but we still wanted to make sure that it is correct and is giving outputs as we expect. In certain cases, expected output is easy to visualize. When we have it stationary, ot DC signal does meaure 1g. When under free fall, it measures 0g. But for more rigor we wanted to test it on situations that are little more complicated (but at the same time, that makes it hard to guess what the right answer should be). For the pendulum case as shown above, when the pendulum is let go and swings back and forth freely, our DC accel measures 0 all the time. We saw that the g part of the formula and its actual acceleration in its local Z are equal and opposite and hence producing a reading of 0. But we want to make sure that this is indeed correct and such a thing would happen (note that the pendulum has concentrated mass at the end; when the cg is not at the end, then DC accel formula doesnt show 0).
Thank you for the links. The second link is useful because ultimately, we wanto to use this for some distance/angle calculation (and I will post soem questions about that as well).
Thank you!
much appreciated.
regards,