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### Topic: Determining accleration due to gravity (Read 17145 times)previous topic - next topic

#### michinyon

#30
##### Oct 31, 2012, 08:03 am
"I do understand the units' equivalency, and indeed have understood this stuff since  about 1971.... I doubt if I'd have graduated as a civil engineer without having understood the relationship between force, mass and acceleration.."

You might have,  if you were an American.   The distinction between force, mass and weight becomes rather obfuscated in their system.

#### michinyon

#31
##### Oct 31, 2012, 08:08 am
OK, so suppose I am not interested in measuring small fluctuations in the force of gravity,  because I am not looking for buried iron ore deposits or big meteors.

Suppose I want to measure the direction of the graviational force,   and I want to do this from a moving platform like a car or an aircraft.

If I get a 3D MEMS accelerometer,  that will do a good job if the vehicle is stationary or moving at a constant velocity,   but otherwise,  it won't.

Is there another way to measure the direction of the gravitational force which will work in an erratically moving vehicle ?

#### liudr

#32
##### Oct 31, 2012, 01:43 pm

OK, so suppose I am not interested in measuring small fluctuations in the force of gravity,  because I am not looking for buried iron ore deposits or big meteors.

Suppose I want to measure the direction of the graviational force,   and I want to do this from a moving platform like a car or an aircraft.

If I get a 3D MEMS accelerometer,  that will do a good job if the vehicle is stationary or moving at a constant velocity,   but otherwise,  it won't.

Is there another way to measure the direction of the gravitational force which will work in an erratically moving vehicle ?

Thanks to Einstein's general relativity, you can't tell between gravity and acceleration. You can't find gravity with accelerometer if you yourself is accelerating. If you have a gyroscope, you can spin it in the direction of gravity and see how your down direction compares with it, I suppose.
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#### wwbrown

#33
##### Oct 31, 2012, 04:30 pm
How about a magnetometer if you only want to measure the direction of the acceleration due to gravity.  I am pretty sure that a magnetometer will not be affected by linear or rotational acceleration.

#### liudr

#34
##### Oct 31, 2012, 04:40 pm

How about a magnetometer if you only want to measure the direction of the acceleration due to gravity.  I am pretty sure that a magnetometer will not be affected by linear or rotational acceleration.

Is that a magnetic sensor? It can only measure your angle to local magnetic field, not your angle to local gravity. Facing north while tilting up 30 degrees and facing south while tilting down 30 degrees will read the same on a magnetic sensor.
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#### liudr

#35
##### Oct 31, 2012, 04:46 pm

This reminds me of how we measured it back in high school...

It involved a battery operated bell and a huge long strip (as long as the drop from the window to the ground) of carbon paper with a weight on the end. Then dropped the weight while the bell was ringing, with the strip between the bell and the ringer lever thingy. The impact of the ringer made marks on the carbon paper, and of course the marks got further apart as the weight accelerated.

Measured the time it took to drop from the science lab window to the ground. Counted the total number of marks. That gave the number of marks per second or seconds per mark.

Measured the (varying) distance between marks. That, with the now known time between marks, gave the (varying) velocity at any instant.

And hence the acceleration....

But using a pendulum is by far the easiest way to do it... (As long as the bob is very heavy compared to the string, the whole mass may be deemed to be at the bob's CoG.) Beauty of the pendulum method is that since the period T is constant regardless of how wide the swing is, it can as suggested above be measured over 100s of swings and that reduces the impact of the reaction time when the stop watch is started and stopped.

Oh the old memories. They were probably called impact timer?! Later they were replaced with spark timers that generated sparks with car ignition coils at 60Hz so wax tapes will have regular burn marks on them. A bit less friction. Then photo gates became more popular so did sonic rangers and then video cams. You can measure gravity with any method that involves a formula where g is a part of. Pendulum is definitely easy and accurate (you mentioned about response time!). Not doing it easy way can have instructional values such as seeing the dots separating further and further away or video frames doing the same thing. There is another way, just to toss in the discussion, is to use sound. You record sound clip of scissors cutting string and ball hitting floor. I've been doing this for a while. Results are pretty good with 0.x m/s/s as error bar. Some kids these days can't even cut with scissors. That's a whole other discussion topic.
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#### wwbrown

#36
##### Oct 31, 2012, 05:54 pm
Yes I made a huge error I thought for some strange reason the poster wanted to determine the direction of the magnetic field and not the acceleration due to gravity.

I believe with a full-up IMU you could get the direction of the acc. due to g.

#### michinyon

#37
##### Nov 01, 2012, 07:15 am
Well the idea is, if you know the accelerations and gyro readings in 3D,  then you can calculate the change in orientation.  But you get variations and drift in the gyro readings which make this not actually work in practise.

You need to keep correcting the gyro-derived orientation using some other means,   and two of the available means are the direction of "down"  and the direction of the earth's magnetic field.

The problem is,  a lot of the feasible schemes for doing this ( for example,  read Magdwick's papers ) kind of assume that the acceleration of the body is small and that therefore the accelerometer reading indicates the direction of gravity ( either "up" or "down" ).   I've developed another scheme which also works and has the advantage ( for me ) that I can understand it.

It would be useful to have some means of identifying the direction of gravity independently from acceleration but I guess if Einstein says you can't,  we it is probably right.    Now a very small higgs boson detector might be able to detect the direction where they are coming from, independent of the actual motion of the device.

#### liudr

#38
##### Nov 01, 2012, 05:36 pm

Well the idea is, if you know the accelerations and gyro readings in 3D,  then you can calculate the change in orientation.  But you get variations and drift in the gyro readings which make this not actually work in practise.

You need to keep correcting the gyro-derived orientation using some other means,   and two of the available means are the direction of "down"  and the direction of the earth's magnetic field.

The problem is,  a lot of the feasible schemes for doing this ( for example,  read Magdwick's papers ) kind of assume that the acceleration of the body is small and that therefore the accelerometer reading indicates the direction of gravity ( either "up" or "down" ).   I've developed another scheme which also works and has the advantage ( for me ) that I can understand it.

It would be useful to have some means of identifying the direction of gravity independently from acceleration but I guess if Einstein says you can't,  we it is probably right.    Now a very small higgs boson detector might be able to detect the direction where they are coming from, independent of the actual motion of the device.

You can still device an external measurement, say a person standing on the ground telling the device its orientation wrt gravity. You just can't have the device tell itself so. If you have a flat floor with regular dots, you can use that and a bottom facing camera to tell orientation.
Serial LCD keypad panel,phi_prompt user interface library,SDI-12 USB Adapter

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