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Science Forum Index » Mechanics Forum » Load cells and accelerometers...
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 Posted: Thu Jun 12, 2008 5:28 am |
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Guest
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Hi,
I am not a mechanical engineer, and I got a bit confused about the
terms "load cells" and "accelerometers":
If I understand it correctly, a load cell is a force sensor that
requires physical contact. The force is being measured by converting
strain (in the sensor) into an electrical signal. The strain must be
generated by an object that is in contact with the sensor. Is this
correct ?
An accelerometer measures acceleration in units of m s^-2. Can it be
also used to measure a force?
Thanks in advance for all clarifying answers!
Willy |
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| Posted: Thu Jun 12, 2008 10:50 am |
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Guest
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On Thu, 12 Jun 2008 08:28:18 -0700 (PDT), in sci.engr.mech
wilhelm.rainer at (no spam) gmx.net wrote:
Quote: I am not a mechanical engineer, and I got a bit confused about the
terms "load cells" and "accelerometers":
If I understand it correctly, a load cell is a force sensor that
requires physical contact. The force is being measured by converting
strain (in the sensor) into an electrical signal. The strain must be
generated by an object that is in contact with the sensor. Is this
correct ?
In general, yes. There are of course more elaborate magnetic
suspension/force balances that have been investigated for wind tunnel
testing.
Quote: An accelerometer measures acceleration in units of m s^-2. Can it be
also used to measure a force?
Force can be implied by applying Newton's Second Law, F = m*a. |
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| Brian Whatcott... |
| Posted: Thu Jun 12, 2008 8:16 pm |
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Guest
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On Thu, 12 Jun 2008 08:28:18 -0700 (PDT), wilhelm.rainer at (no spam) gmx.net
wrote:
Quote: ... a load cell is a force sensor that
requires physical contact. The force is being measured by converting
strain (in the sensor) into an electrical signal. The strain must be
generated by an object that is in contact with the sensor. Is this
correct ?
Yes
Quote: An accelerometer measures acceleration in units of m s^-2. Can it be
also used to measure a force?
Thanks in advance for all clarifying answers!
Willy
Force can arise in several ways: a nut tightened on a bolt, for
instance. Any way it is provoked, a load cell can be arranged to
measure it.
The kinds of acceleration we are familiar with, every day are
1) putting the pedal to the metal - or stamping on the brake pedal,
2) Jumping off a building. If you stand on the penthouse parapet,
you feel your weight, your feet get tires...etc., etc.
This is a force felt from resisting the acceleration.
If you slip over the edge, your feet no longer hurt, and you don't
feel your body weight.
If we arrange a simple accelerometer consisting of a mass on a spring,
when we stand on the edge, the weight of the mass is pulling down, and
registering 1g or 9.8 m/s2 or 32 feet per second squared.
When we step off, the weight springs back, and the accelerometer
indicates no acceleration!
No wonder you can get confused!
Anyway. there is a relation between force and acceleration
which we label inertia. In the simplest form. the equation reads
force = mass times acceleration
If you use units like kg for mass, Newtons for force, and meters per
sec per sec for acceleration, there are no scaling factors built in so
you can say
a force of ONE Newton goes along with a mass of ONE kilogram
speeding up by ONE meter per second per second.
The weight of an apple in your hand is about 1 Newton, about a tenth
kilogram or a quarter of a pound.
Clear as mud now?
Brian W |
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| Spaceman... |
| Posted: Mon Jun 16, 2008 5:18 pm |
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Guest
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Brian Whatcott wrote:
Quote: If we arrange a simple accelerometer consisting of a mass on a spring,
when we stand on the edge, the weight of the mass is pulling down, and
registering 1g or 9.8 m/s2 or 32 feet per second squared.
When we step off, the weight springs back, and the accelerometer
indicates no acceleration!
Why would it read 0?
That is silly.
Don't they read the Earths acceleration rate?
After all it is not in a perfect orbital path with 0 acceleration.
you better re-calibrate that thing.
It will make you crash into a planet you are freefalling towards,
and accelerating into.
--
James M Driscoll Jr
Spaceman |
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| Brian Whatcott... |
| Posted: Mon Jun 16, 2008 5:32 pm |
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Guest
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On Mon, 16 Jun 2008 18:18:59 -0400, "Spaceman"
<spaceman at (no spam) yourclockmalfunctioned.duh> wrote:
Quote: Brian Whatcott wrote:
If we arrange a simple accelerometer consisting of a mass on a spring,
when we stand on the edge, the weight of the mass is pulling down, and
registering 1g or 9.8 m/s2 or 32 feet per second squared.
When we step off, the weight springs back, and the accelerometer
indicates no acceleration!
Why would it read 0?
That is silly.
Don't they read the Earths acceleration rate?
After all it is not in a perfect orbital path with 0 acceleration.
you better re-calibrate that thing.
It will make you crash into a planet you are freefalling towards,
and accelerating into.
take a look at an aerobatic airplane if you can. They have g
meters to measure acceleration. On the ground, they read 1 g.
If you hold a meter like that upside down, it reads -1 g.
However, if you jump off a parepet, it reads 0 g.
Brian W |
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| Spaceman... |
| Posted: Mon Jun 16, 2008 5:52 pm |
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Guest
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Brian Whatcott wrote:
Quote: take a look at an aerobatic airplane if you can. They have g
meters to measure acceleration. On the ground, they read 1 g.
But when on the ground, they are not being accelerated at 1 G.
They are "at rest" with Earth
they are being accelerated at only the acceleration changes
that occur in Earths orbit and maybe the solar systems orbit
arond the Galaxy and maybe even a tiny bit for the Galaxys trip around
the Universe. anyways... .
such is surely not 1 G at all or we would be able to float when
on the far side of the furthest point in orbit.
If anything, on the ground should be the 0 acceleration rate.
and 1 g-force constant. (not accelerated)
2 different things.
:)
Quote: If you hold a meter like that upside down, it reads -1 g.
However, if you jump off a parepet, it reads 0 g.
That is bad,
as I said. you better re-set it for "real" acceleration changes
or one day you will crash into the planet you are trying to land on
since the accelerometer will not measure the freefall acceleration rate.
:)
--
James M Driscoll Jr
Spaceman |
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