Re: Accelerometer wiki page erroneous axes ?
So, the fact that when idle the accelerometers report a positive Z value implies that the Z axe is actually upward. Agree ? Yes I think that's right. If you hold the Freerunner that any arrow of an axis points to the earth middlepoint then the sown value must be negative. Michael ___ Openmoko community mailing list community@lists.openmoko.org http://lists.openmoko.org/mailman/listinfo/community
Re: Accelerometer wiki page erroneous axes ?
Michael Tansella michael-tanse...@gmx.de writes: So, the fact that when idle the accelerometers report a positive Z value implies that the Z axe is actually upward. Agree ? Yes I think that's right. If you hold the Freerunner that any arrow of an axis points to the earth middlepoint then the sown value must be negative. I beg to disagree. If you are talking about acceleration of the sensor itself then you'll see that: 1. it's not actually accelerating if you attach a reference frame to the Earth (the freerunner is laying still). 2. The gravitational force applied to the sensor is directed to the center of the Earth, so the reported acceleration should be positive too. -- Be free, use free (http://www.gnu.org/philosophy/free-sw.html) software! mailto:fercer...@gmail.com ___ Openmoko community mailing list community@lists.openmoko.org http://lists.openmoko.org/mailman/listinfo/community
Accelerometer wiki page erroneous axes ?
Please have a look at the accelerometer data retrieval wiki page at http://wiki.openmoko.org/wiki/Accelerometer_data_retrieval . It is said that the Z axis is pointing from the display downwards to to the back of the openmoko. In my opinion this is false and all other axes my be inverted as well. The confusion seams to originates from the Z value you read from the accelerometer which is positive when the phone lies horizontally. But the fact that the gravitation vector G points downward means that the acceleration measured by the sensor is actually upward : gravitationnal force is equivalent to an acceleration in the other direction (remember the elevator analogy : beeing pulled down toward the ground by gravity is equivalent to standing in an elevator that continuously accelerate _upward_ in the absence of gravity). So, the fact that when idle the accelerometers report a positive Z value implies that the Z axe is actually upward. Agree ? ___ Openmoko community mailing list community@lists.openmoko.org http://lists.openmoko.org/mailman/listinfo/community
Re: Accelerometer wiki page erroneous axes ?
Paul Fertser fercer...@gmail.com writes: ? Michael Tansella michael-tanse...@gmx.de writes: So, the fact that when idle the accelerometers report a positive Z value implies that the Z axe is actually upward. Agree ? Yes I think that's right. If you hold the Freerunner that any arrow of an axis points to the earth middlepoint then the sown value must be negative. I beg to disagree. If you are talking about acceleration of the sensor itself then you'll see that: 1. it's not actually accelerating if you attach a reference frame to the Earth (the freerunner is laying still). 2. The gravitational force applied to the sensor is directed to the center of the Earth, so the reported acceleration should be positive too. Thinking about it a bit more, i now understand that if you insist on talking about accelerations, the Z axis should point upwards. -- Be free, use free (http://www.gnu.org/philosophy/free-sw.html) software! mailto:fercer...@gmail.com ___ Openmoko community mailing list community@lists.openmoko.org http://lists.openmoko.org/mailman/listinfo/community
Re: Accelerometer wiki page erroneous axes ?
-[ Wed, Apr 08, 2009 at 04:32:41PM +0400, Paul Fertser ] I beg to disagree. If you are talking about acceleration of the sensor itself then you'll see that: 1. it's not actually accelerating if you attach a reference frame to the Earth (the freerunner is laying still). 2. The gravitational force applied to the sensor is directed to the center of the Earth, OK untill here. so the reported acceleration should be positive too. The reported acceleration is positive, this is a fact (cat /dev/input/event3). The question is : is a positive acceleration accelerating downward or upward ? The wiki tells that a positive acceleration is downward (Z axe pointing down). This is not true, in my opinion. When standing still, the accelerometer must report an acceleration directed in the _oposite_ direction the the gravitational force. As it is reporting a positive acceleration, then the Z axe goes up. And I believe the other axes are inverted as well (which, as suggested by Michael, is easily provable by turning the FR around so that each axes points down). ___ Openmoko community mailing list community@lists.openmoko.org http://lists.openmoko.org/mailman/listinfo/community
Re: Accelerometer wiki page erroneous axes ?
I beg to disagree. If you are talking about acceleration of the sensor itself then you'll see that: 1. it's not actually accelerating if you attach a reference frame to the Earth (the freerunner is laying still). 2. The gravitational force applied to the sensor is directed to the center of the Earth, OK untill here. OK so the reported acceleration should be positive too. The reported acceleration is positive, this is a fact (cat /dev/input/event3). The question is : is a positive acceleration accelerating downward or upward ? The wiki tells that a positive acceleration is downward (Z axe pointing down). This is not true, in my opinion. When standing still, the accelerometer must report an acceleration directed in the _oposite_ direction the the gravitational force. As it is reporting a positive acceleration, then the Z axe goes up. And I believe the other axes are inverted as well (which, as suggested by Michael, is easily provable by turning the FR around so that each axes points down). What is drawn in the wiki is a reference system; in that reference system the gravity acceleration take the right sign, being in the same direction as the z axis. If you accelerate upwards your phone (with the screen pointing up) with an acceleration a you see a greater acceleration a_tot (along the z direction) that is the sum of gravity acceleration g and your imposed acceleration a (a_tot=a+g). That means that the force acting on the phone is equal to the mass of the mobile m time the total acceleration and you can verify it with a dynamometer. Giacomo -- /_\ The ASCII Per comunicare in modo riservato: \_/ Ribbon Campaign gpg --keyserver pool.sks-keyservers.net \ X Against HTML--recv-keys 20611EAD /_\ Email! -- Please avoid sending me Word or PowerPoint attachments. See http://www.gnu.org/philosophy/no-word-attachments.html ___ Openmoko community mailing list community@lists.openmoko.org http://lists.openmoko.org/mailman/listinfo/community
Re: Accelerometer wiki page erroneous axes ?
-[ Wed, Apr 08, 2009 at 04:26:02PM +0200, giacomo giotti mariani ] If you accelerate upwards your phone (with the screen pointing up) with an acceleration a you see a greater acceleration a_tot (along the z direction) that is the sum of gravity acceleration g and your imposed acceleration a (a_tot=a+g). Well, some confusion here. OK, let's imagine a _perfect_ accelerometer. This device would report only acceleration. So, when the phone lies on the table, it would report 0 (neglecting the rotation of earth here :-)). Then, to take your phone up to your ear, you first accelerate it upward, then downward (to stop the movement). If this perfect accelerometer were reporting accelerations along a Z axis pointing up, it would thus report first a positive value, then 0, then a negative value (the other way around if the other convention is used). Now, lets suppose that this perfect accelerometer is not calibrated correctly, and offset every values by 900. It will then report first a value 900, then 900, then a value 900. This is what you get with the actual accelerometer (*). So, Z is actually pointing Up. (*): our accelerometer is not perfect, but not because of a random offset. It is not perfect because it has no mean to distinguish between an actual acceleration from the effect of gravity. This is not a defect and there is no need to send the phone back to OpenMoko :-) because it is just how nature work (http://en.wikipedia.org/wiki/File:Elevator_gravity.svg) The effect of gravity (downwards) is the same as an acceleration (upwards!!) The freerunner is really a great thing. First it remembers me the good old days when I was hacking open all the guts of my 8bits computer ; then it remembers me the good old physic classes ! :-) ___ Openmoko community mailing list community@lists.openmoko.org http://lists.openmoko.org/mailman/listinfo/community
Re: Accelerometer wiki page erroneous axes ?
On Wed, Apr 08, 2009 at 06:10:03PM +0200, ri...@happyleptic.org wrote: -[ Wed, Apr 08, 2009 at 04:26:02PM +0200, giacomo giotti mariani ] If you accelerate upwards your phone (with the screen pointing up) with an acceleration a you see a greater acceleration a_tot (along the z direction) that is the sum of gravity acceleration g and your imposed acceleration a (a_tot=a+g). Well, some confusion here. OK, let's imagine a _perfect_ accelerometer. This device would report only acceleration. So, when the phone lies on the table, it would report 0 (neglecting the rotation of earth here :-)). It does report zero (well, in the vicinity of zero), you just have to calculate it's difference towards the gravity vector (which is in the vicinity of zero). Rui -- Fnord. Today is Pungenday, the 25th day of Discord in the YOLD 3175 + No matter how much you do, you never do enough -- unknown + Whatever you do will be insignificant, | but it is very important that you do it -- Gandhi + So let's do it...? ___ Openmoko community mailing list community@lists.openmoko.org http://lists.openmoko.org/mailman/listinfo/community
Re: Accelerometer wiki page erroneous axes ?
Le 14342ième jour après Epoch, ri...@happyleptic.org écrivait: Well, some confusion here. Yes, probably because of mixing gravity, force, acceleration, inertia, etc... :) OK, let's imagine a _perfect_ accelerometer. This device would report only acceleration. So, when the phone lies on the table, it would report 0 (neglecting the rotation of earth here :-)). Please, don't stop earth rotation, because my weight will increase too much :( The _perfect_ accelerometer is measuring 2 forces: The gravity, and the table reaction against the phone fall. These 2 forces are in opposite directions, then accel value is 0. If you shift left (or right) your _perfect_ accelerometer, it will (probably) indicate a X acceleration value. But only if it has no mass so no inertia :) The Freerunner accelerometers are able to compare these 2 forces, and so it compares gravity vs reaction, or force vs inertia. Now, lets suppose that this perfect accelerometer is not calibrated correctly, and offset every values by 900. It will then report first a value 900, then 900, then a value 900. Your idea works only for the accelerometer Z, in case of the phone is left horizontaly, screen up. X and Y accelerometers don't have the calibration error in this case. And if your phone is falling, the accel values will turn to zero, even in the earth gravity field. If you will try this, please consider using a pillow (for example) to avoid crash ;) (*): our accelerometer is not perfect, but not because of a random offset. It is not perfect because it has no mean to distinguish between an actual acceleration from the effect of gravity. This is not a defect and there is no need to send the phone back to OpenMoko :-) I agree ;) because it is just how nature work (http://en.wikipedia.org/wiki/File:Elevator_gravity.svg) The effect of gravity (downwards) is the same as an acceleration (upwards!!) Not the same, but we can't make any differences just looking at the trajectory. ___ Openmoko community mailing list community@lists.openmoko.org http://lists.openmoko.org/mailman/listinfo/community
