detecting weightlessness with Hitachi H48C 3-Axis Accelerometer
bmp
Posts: 1
i am using the Hitachi H48C 3-Axis Accelerometer to detect brief moments of weightlessness in every-day life. i attach the sensor to my chest and monitor for weightlessness throughout my day. i have connected it to a micro-controller and clock chip so that both the time and duration of the weightlessness is logged.
the specs of the accelerometer say that the sample rate is 200Hz, so you wouldn't expect to see a duration of weightlessness less than 0.005 seconds, but i am getting many readings between 1-5ms. can someone help explain this? i don't see how this is possible given the accelerometer sample rate. what does this mean?
any thoughts would be greatly appreciated.
thanks
the specs of the accelerometer say that the sample rate is 200Hz, so you wouldn't expect to see a duration of weightlessness less than 0.005 seconds, but i am getting many readings between 1-5ms. can someone help explain this? i don't see how this is possible given the accelerometer sample rate. what does this mean?
any thoughts would be greatly appreciated.
thanks
Comments
My first thought is that it's a timing issue between the Stamp(?) and the accelerometer (sample taken during, rather than at transition to, freefall) . You don't mention MCU flavor; this would help, as would code. Have you tried varying the sample rate to see if your readings normalize? What about 'scoping the chip output?
One other thought... If I was walking around with a weightlessness monitor strapped to my chest, I'd be careful in suggesting that "any thoughts" would be appreciated [noparse]:)[/noparse].
Mark
To test your device, you might try dropping it (i.e. freefall) from a known height, into a pillow.
You should get the following results:
1" fall = .072 sec of weightlessness
6" fall = .177 sec
12" = .250 sec
36" = .433 sec
16 ft = 1.0 sec
Time to Fall(in sec) = .25 x (sq root(Distance(in ft))
I would be interested in hearing about your results.
DaveG
Post Edited (DaveG) : 10/30/2006 1:36:52 PM GMT
I thought it was 32 ft per second, per second?
TT
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
(Frequently heard from other's)
Tommy, I know it wasn't designed to·x, but can you make it·do x·anyway?
·
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Paul Baker
Propeller Applications Engineer
Parallax, Inc.
Here's a simple way to understand why a falling object doesn't go 32 feet in the FIRST second.
The instantaneous velocity and distance can be found by the following equations:
v = g * t, where v = velocity, g = 32 ft/sec^2, and t = time
d = 1/2 * g * t^2, where d = distance, g = 32 ft/sec^2, and t = time
Therefore:
At Time = 0.00 sec, Velocity = 0 ft/sec, Distance = 0.0 ft
At Time = 0.25 sec, Velocity = 8 ft/sec, Distance = 1.0 ft
At Time = 0.50 sec, Velocity = 16 ft/sec, Distance = 4.0 ft
At Time = 0.75 sec, Velocity = 24 ft/sec, Distance = 9.0 ft
At Time = 1.00 sec, Velocity = 32 ft/sec, Distance = 16.0 ft
At Time = 2.00 sec, Velocity = 64 ft/sec, Distance = 64.0 ft
At Time = 3.00 sec, Velocity = 96 ft/sec, Distance = 144.0 ft
At Time = 4.00 sec, Velocity = 128 ft/sec, Distance = 256.0 ft
You can see that the object is going 32 ft/sec only after a FULL second has gone by.
The average velocity during the FIRST second is 16 ft/sec.
In seconds 2, 3 and 4 the velocity also increases at a rate of 32 ft/sec/sec.
DaveG
Post Edited (DaveG) : 10/30/2006 1:42:51 PM GMT
You should get between 45 secs to one minute of freefall...
As long as it's not big and bulky and you put it in a case you should probably find someone to carry it for you...
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Living on the planet Earth might be expensive but it includes a free trip around the sun every year...
Experience level:
[noparse][[/noparse] ] Let's connect the motor to pin 1, it's a 6V motor so it should be fine.
[noparse][[/noparse] ] OK, I got my resistors hooked up with the LEDs.
[noparse][[/noparse]X] I got the Motor hooked up with the H-bridge and the 555 is supplying the PWM.
[noparse][[/noparse] ] Now, if I can only program the BOE-BOT to interface with he Flux Capacitor.
[noparse][[/noparse] ] I dream in SX28 assembler...
/Bamse
·s=vit+1/2(at2)
s = distance (scalar)
vi = initial velocity (scalar)
t = time
a = acceleration
(keep units consistent...i.e. distance in meters and acceleration in m/s2 or ft and ft/s2)
so for a=32ft/s2 and initial velocity of 0 (dropped), after 1 second
·s=(0)(1) +1/2((32)(1)2)
·s=1/2(32)
·s=16
HTH
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Truly Understand the Fundamentals and the Path will be so much easier...
For those not familiar with "terminal velocity" this is the point when in "free fall" that the resistance of the air and/or combination of resistance and lift become high enough that you can't fall any faster, no matter how hard Gravity is pulling you down. As an anectotal example, as proven on "Myth Busters", this is why a penny dropped from the Empire State building will not kill someone on the sidewalk. It tumbles, and reaches a "terminal velocity" that is non-lethal.
Again, for finding moments of "Zero G" in everyday life, the friction, lift and terminal velocity are negligable, and can be safely ignored.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
John R.
8 + 8 = 10
Without really giving it thought, I "assumed" that at the completion of the first second 32 feet would be covered, + 64 for the 2nd second·totaling 96, and so on until terminal velocity was attained.
Learn something new every day.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
(Frequently heard from other's)
Tommy, I know it wasn't designed to·x, but can you make it·do x·anyway?
·