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4hv.org :: Forums :: General Science and Electronics
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PID + servo PWM... how to implement !?

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Patrick
Mon Jul 09 2012, 08:55PM
Patrick Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Steve Conner wrote ...

Well, you hope servos are fast compared to the vehicle dynamics, but from the video (yay!) the tilt rotor looks pretty skittish. I think the wooden pole needs fastened on tighter.
yeah, it wobbled pretty bad a few times there, and my cardboard tube landing skid broke off too. im currently laying up fiberglass to solve these problems.


Steve Conner wrote ...

I guess the way the system works is this: The Kalman filter spits out a 6-dimensional estimate of the vehicle position. (3 translation, 3 rotation.) Or maybe it works with velocities or accelerations, I don't know.

A 6-D vector of the desired position (or velocity or acceleration) is derived from the RC control inputs.

Then 6 PID controllers are used to derive 6 error signals, one for each dimension, which get mixed down in some sort of transformation matrix to drive the servos.

Things are complicated (or maybe simplified? :)) if the craft doesn't have control authority in every dimension. So for instance, a copter doesn't have independent control over translation in the horizontal plane. If it wants to translate, it first has to tilt itself (rotate) so that the downward thrust vector points partly sideways. So rotation and translation are all mixed together. I have no idea how this is handled by an autopilot.
yes, the tilting to gain a motion, worries me.
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AndrewM
Mon Jul 09 2012, 10:06PM
AndrewM Registered Member #49 Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362
Steve Conner wrote ...

Well, you hope servos are fast compared to the vehicle dynamics, but from the video (yay!) the tilt rotor looks pretty skittish. I think the wooden pole needs fastened on tighter.

I guess the way the system works is this: The Kalman filter spits out a 6-dimensional estimate of the vehicle position. (3 translation, 3 rotation.) Or maybe it works with velocities or accelerations, I don't know.

A 6-D vector of the desired position (or velocity or acceleration) is derived from the RC control inputs.

Then 6 PID controllers are used to derive 6 error signals, one for each dimension, which get mixed down in some sort of transformation matrix to drive the servos.

Things are complicated (or maybe simplified? :)) if the craft doesn't have control authority in every dimension. So for instance, a copter doesn't have independent control over translation in the horizontal plane. If it wants to translate, it first has to tilt itself (rotate) so that the downward thrust vector points partly sideways. So rotation and translation are all mixed together. I have no idea how this is handled by an autopilot.

To my eye the helicopter example is no different than the inverted pendulum controller that virtually every undergraduate makes in Controls lab - you are balancing the pendulum upwards, but to translate it you have to let it tilt. No?


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Patrick
Mon Jul 09 2012, 10:23PM
Patrick Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
AndrewM wrote ...

Steve Conner wrote ...

Well, you hope servos are fast compared to the vehicle dynamics, but from the video (yay!) the tilt rotor looks pretty skittish. I think the wooden pole needs fastened on tighter.

I guess the way the system works is this: The Kalman filter spits out a 6-dimensional estimate of the vehicle position. (3 translation, 3 rotation.) Or maybe it works with velocities or accelerations, I don't know.

A 6-D vector of the desired position (or velocity or acceleration) is derived from the RC control inputs.

Then 6 PID controllers are used to derive 6 error signals, one for each dimension, which get mixed down in some sort of transformation matrix to drive the servos.

Things are complicated (or maybe simplified? :)) if the craft doesn't have control authority in every dimension. So for instance, a copter doesn't have independent control over translation in the horizontal plane. If it wants to translate, it first has to tilt itself (rotate) so that the downward thrust vector points partly sideways. So rotation and translation are all mixed together. I have no idea how this is handled by an autopilot.

To my eye the helicopter example is no different than the inverted pendulum controller that virtually every undergraduate makes in Controls lab - you are balancing the pendulum upwards, but to translate it you have to let it tilt. No?



yes! and once it tilts(the props) it also tilts the gyros and accels(tilts the main body), which scares me of a feedback loop setting in too an uncontrollable situtuation.
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