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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
hey everybody my brain is not able to wrap around this problem. i need to go from angles and velocities to RC servo PWMs, so am i not understanding the implemantation of a pid?
is it possible for a pid to not be aware of the pwm, and only the result? ordoes the output need to be "normalized" somehow to be complatiable with the rc pwm?
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
What variable are you trying to control? My understanding was that servomotors are already controlling their shaft position based on the input pulse time so there's no need for a PID loop. If you're trying to control velocity I'm not sure you can with a servomotor because you don't have feedback from the shaft sensor.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Mattski wrote ...
What variable are you trying to control? My understanding was that servomotors are already controlling their shaft position based on the input pulse time so there's no need for a PID loop. If you're trying to control velocity I'm not sure you can with a servomotor because you don't have feedback from the shaft sensor.
well its my tilt rotor fans and angular position im trying to control with a pid + complementary filter.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
You probably don;t need PID to control the fan angle, just set the PWM to to required position and let the servo get there. If that's too fast, ramp the PWM request to the servo.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
It's the magic of negative feedback. The "I" term of the PID will make the output go to whatever value it takes to reduce the error to zero.
Say your PID was implemented with 16-bit integers, then the output would have a range of -32768 to 32767. You just need to adapt this to the range of the PWM module on the microcontroller, which might be 0 to 1023, but not all of that range may correspond to valid inputs for a RC servo. So some scaling and maybe saturation might be needed, and as soon as you do saturation you should probably implement anti-windup for the I term too.
If your PID is implemented in floating point, you may want to think twice before using it in an embedded application. And if you don't understand the difference between fixed and floating point, you should probably think twice before trying to write an embedded application.
RC model servos already have a PID loop inside to control the motor so that shaft position (sensed by a pot inside the servo) tracks the input pulse width.
Registered Member #49
Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362
The servos really aren't what he wants to control; he wants to control his vehicle's pitch/roll for which he gets feedback from his gyro and accels. Servos are fast compared to the vehicle dynamics, so generally you'd model them as "perfect"/instantaneous since, as Steve says, they have their own PID controller already.
Patrick, you don't need to do anything special. Develop a function and controller that generates the servo positions as output. Servo position-to-PWM is 1:1 so just translate the output with a simple transfer function.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
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.
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PID + servo PWM... how to implement !?
