Post by thread:[EE]: Gyro interfacing

I currently have some sample SiliconSensing CRS02 rate gyros that I would like to use to make a heading-hold autopilot for my airplane. Since the gyro is a rate device, how would I employ an accelerometer such as the ADXL202 to compensate for drift? I understand that this is how many heading-hold gyros work in model aircraft.... Thanks, Bob -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 >I currently have some sample SiliconSensing CRS02 rate gyros that I >would like to >use to make a heading-hold autopilot for my airplane. > Since the gyro is a rate device, >how would I employ an >accelerometer such as the ADXL202 to compensate for >drift? I >understand that this is how many heading-hold gyros work in model >aircraft.... You could use a GPS with heading output, and not bother with the accelerometers :D - --BJM -----BEGIN PGP SIGNATURE----- Version: PGPfreeware 6.5.8 for non-commercial use <http://www.pgp.com> iQA/AwUBPTRG8wVk8xtQuK+BEQLyIQCglqn4vqqZG7emw2uxaBc3KupHBHgAoMba Xs7MZiAC8+0IXZ81UM72+z3L =pK+n -----END PGP SIGNATURE----- -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

I've been tinkering with the ADLX202's recently. Dropping them seems to give them vertigo, ie 0-g, as expected. How would you work around the vertigo problem? The R/C model autopilots use the light or heat gradient at the horizon. Fine as long as you're not flying indoors. Derek Cowburn spam_OUTderekTakeThisOuTcowburn.us, http://www.cowburn.us 908-735-2706 (home) 908-797-3399 (mobile) {Original Message removed}

No acceleromeetrs used on HH gyros. Just some math. groups.google.com/groups?q=heading+hold+gyros&hl=en&lr=&ie=UTF- 8&selm=3A393E9C.E1FAFBB6%40vcu.edu&rnum=1 > I currently have some sample SiliconSensing CRS02 rate gyros that I = > would like to use to make a heading-hold autopilot for my airplane. = > Since the gyro is a rate device, how would I employ an accelerometer = > such as the ADXL202 to compensate for drift? I understand that this is = > how many heading-hold gyros work in model aircraft.... =20 Thanks, Bob > -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

Hi Bob, In general, there is no way to use accelerometers to compensate for drift. You really need some kind of drift-free angle reference (like a horizon sensor). You might be wondering, then, what good are gyros if they need some additional source of angle information? Gyros are nice because they can give you a fast response, so that if your other, drift-free angle sensor is slow, you can combine it with the gyro to give a fast acting, drift-free angle measurement. You can only use gyros by themselves if you don't care about the drift (either it is small enough to be tolerable or you only want to resist quick angle changes, like heading hold). That being said, if you can make certain assumptions about the way your aircraft will fly, then it may be possible to use accelerometers, along with some math and some cleverness, to remove the drift from your gyros. For example, if you know that the long-term average lateral acceleration of the plane will be zero, and the same is true of its vertical acceleration (i.e., it is flying straight and level on average), then the long-term average of the accelerometers will correctly indicate the direction of the gravity vector. You have to be careful in doing this, though, because if you choose, say, a period of 10 seconds to be your "long term average", and someone does a maneuver that lasts a significant fraction of 10 seconds, your results could be way off. Sean At 09:56 AM 7/17/2002 -0500, you wrote: >Yes, but this article doesn't address drift, and I'm wondering if somehow >I could use an accelerometer to compensate for it. > >Bob > > > {Original Message removed}

If you see zero G acceleration on all three axis of an accelerometer setup, you can assume you are accelerating toward ground at 9.8m/s^2 or that your sensors are busted. It isn't vertigo. You just need to move your baseline - the object is not accelerating if there is 0 g on two axis and 1 g on a third axis. The adxl202 only has two accelerometers though, so it is limited in what you can measure. -Adam Derek Cowburn wrote: {Quote hidden}>I've been tinkering with the ADLX202's recently. Dropping them seems to >give them vertigo, ie 0-g, as expected. How would you work around the >vertigo problem? > >The R/C model autopilots use the light or heat gradient at the horizon. >Fine as long as you're not flying indoors. > >Derek Cowburn >.....derekKILLspam@spam@cowburn.us, http://www.cowburn.us >908-735-2706 (home) >908-797-3399 (mobile) > > >{Original Message removed}

The math involved in making a rate gyro to hold the heading are based on the integration (discrete summation) of the gyro signal. This integration will, however, magnify gyro errors (like temperature drifting). The bad news are that gyros like yours, MEMS based, have significant temperature drift (this is not a problem of your gyro; it is common in some MEMS sensors). You have a couple of possible approaches. You can monitorize the gyro temp, and compensate for it in your maths, or force a constant temperature... not good ideas. The other solution is to use sensors that do not need integration to get _position_ instead of _speed_. This way, the possible temperature (and other) drifts will produce _only_ absolute and constant position errors. However it will be hard (but not impossible) to compensate rate gyro errors with linear accelerometers. But you can use other kind of absolute position sensors, like magnetometers (i.e. Honeywell 1001, 1002, 2003). With a magnetometer you can measure the magnetic earth field, and know for sure the heading of your craft. Using 3 ortogonally oriented magnetometers you can hold heading and pitch of the plane easily. Sofisticated virtual reality devices use three rate gyros, three linear accelerometers and three magnetometers to estimate position, orientation and speed of objects inmersed in virtual worlds. The maths behind this are heavy, however. You can find a detailed description of such devices in the work (PhD) of Eric R. Bachmann and others: http://www.movesinstitute.org/~bachmann/research.htm Hope this helps, Alvaro Deibe. > Yes, but this article doesn't address drift, and I'm wondering if somehow I could use an accelerometer to compensate for it. > > Bob > -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

I have heard there are some piezo based accelerometers that don't see acceleration due to gravity. Could the output of these be combined with the ADXL type to get both gravitational and momentum elements? {Quote hidden}> If you see zero G acceleration on all three axis of an accelerometer > setup, you can assume you are accelerating toward ground at 9.8m/s^2 or > that your sensors are busted. > > It isn't vertigo. You just need to move your baseline - the object is not > accelerating if there is 0 g on two axis and 1 g on a third axis. > > The adxl202 only has two accelerometers though, so it is limited in what > you can measure. > > -Adam -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

Hi Mike, No accelerometers see gravity itself. They all can only see forces that are applied to the accelerometer through its mounting to the aircraft (and gravity is the only force that is not applied through the mounting). You can show through basic physics that it is impossible to measure gravity directly (i.e., without some knowledge of the acceleration of the craft). When you "measure" gravity with static accelerometers, you are using the knowledge that the accelerometers are not accelerating and you are really measuring the force you are using to hold the accelerometer up against gravity. Sean At 09:00 AM 7/18/2002 -0500, Mike DeMetz wrote: {Quote hidden}> I have heard there are some piezo based accelerometers that don't see > acceleration >due to gravity. Could the output of these be combined with the ADXL type >to get both >gravitational and momentum elements? > > > If you see zero G acceleration on all three axis of an accelerometer > > setup, you can assume you are accelerating toward ground at 9.8m/s^2 or > > that your sensors are busted. > > > > It isn't vertigo. You just need to move your baseline - the object is not > > accelerating if there is 0 g on two axis and 1 g on a third axis. > > > > The adxl202 only has two accelerometers though, so it is limited in what > > you can measure. > > > > -Adam > >-- >http://www.piclist.com hint: The PICList is archived three different >ways. See http://www.piclist.com/#archives for details. ------------------------------------------- Introducing NetZero Long Distance Unlimited Long Distance only $29.95/ month! Sign Up Today! http://www.netzerolongdistance.com -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

> I have heard there are some piezo based accelerometers that don't see acceleration > due to gravity. They are only for use in perpetual motion machines or other applications where the laws of physics don't apply. You can try making one by from a normal accellerometer by waving a dead fish over it during a full moon while muttering the proper incantation. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

Thanks to all that have replied. I have found out that a new autopilot on the market for experimental aircraft use a magnetometer internally to compensate for drift when a gps signal is not present (most autopilots slave to your navigation equipment such as gps). Basically they reset bias calculation when the heading is at a steady-state. I'm trying to figure out how to make sense of the data coming from the piezo gyro. It is very simple, its output is 2.5v when there is no motion, and goes up or down from that at a rate of 20mV/deg/sec. Once the aircraft stops rolling it will go back to 2.5v since it just outputs rate. I will be sampling that with the 16F877's internal AD converter, but now I need to come up with the right algorithm that will take a measurement at fixed time intervals, and keep track of the bank angle, but at the same time ignoring vibration. Any ideas? I also found a schematic for a circuit that will compensate the gyro's output for drift, but I'm not quite understanding how it works. The link is: http://www.e-insite.net/ednmag/index.asp?layout=article&articleId=CA46375&pu. Can anyone explain this to me a little better? Help is much appreciated. Thanks, Bob > {Original Message removed}

On Thu, 18 Jul 2002, Olin Lathrop wrote: >> I have heard there are some piezo based accelerometers that don't see >acceleration >> due to gravity. > >They are only for use in perpetual motion machines or other applications >where the laws of physics don't apply. > >You can try making one by from a normal accellerometer by waving a dead fish >over it during a full moon while muttering the proper incantation. That might work but I think he really meant rate sensors. Those will not sense gravity but if they are good enough they will sense earth rotation. Peter -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 > That might work but I think he really meant rate sensors. Those > will not sense gravity but if they are good enough they will sense > earth rotation. Or just take the derivative of the output of an accelerometer... In software or hardware, whichever works... - --Brendan -----BEGIN PGP SIGNATURE----- Version: PGPfreeware 6.5.8 for non-commercial use <http://www.pgp.com> iQA/AwUBPTcRCwVk8xtQuK+BEQLtZACfVPPJvF6dDdnvI3A7GgofYur29HoAnR3N J5I9Q3HxUtRTocDQmok41VL+ =dXt2 -----END PGP SIGNATURE----- -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

On Thu, 18 Jul 2002, Brendan Moran wrote: >-----BEGIN PGP SIGNED MESSAGE----- >Hash: SHA1 > >> That might work but I think he really meant rate sensors. Those >> will not sense gravity but if they are good enough they will sense >> earth rotation. > >Or just take the derivative of the output of an accelerometer... In >software or hardware, whichever works... ?! the derivative of the output of an accelerometer give the rate of change of acceleration. This may be useful in a rocket setting. What were you trying to say ? Peter -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 > On Thu, 18 Jul 2002, Brendan Moran wrote: > ?! the derivative of the output of an accelerometer give the rate > of change of acceleration. This may be useful in a rocket setting. > What were you trying to say ? I thought it was simple enough. Someone said "That might work but I think he really meant rate sensors." so I said "Or just take the derivative of the output of an accelerometer... In software or hardware, whichever works..." It made sense to me. Or did he mean velocity sensors? I don't know. - --Brendan -----BEGIN PGP SIGNATURE----- Version: PGPfreeware 6.5.8 for non-commercial use <http://www.pgp.com> iQA/AwUBPTclJgVk8xtQuK+BEQIaxQCghU948EpwcHbz94jdbpjW/FJkcfoAnirF 7szrT2psPT8xsEHTIDJsRjny =i7sh -----END PGP SIGNATURE----- -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.