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[DPRG] When to PID?
Subject: [DPRG] When to PID?
From: Kipton Moravec
kip at kdream.com
Date: Sat Mar 17 08:11:34 CDT 2007
O.K. then what you want to do you can not do with PID.
Kip
On Fri, 2007-03-16 at 19:13 -0500, David M Wilson wrote:
> Kip,
>
> Your feedback is useful - I'd like to respond to these points.
>
> >Again if you are controlling the robot correctly you should not request
> >a speed that the robot can not attain. Most people run the robot at
> >50-80% maximum speed to have margin for control.
>
> 'Correctly' is an opinion and a mindset that should be challenged on a
> regular basis - hope you agree.
>
> Why request a speed that can not be attained? The max speed that is
> requested can be maintained on ideal (smooth & low friction) surfaces. But
> other indoor surfaces aren't so great.
>
> Consistent speed errors provide good information that we can use for
> decision making. If the top speed or the speed to power ratio is low then
> the device could increase the minimum turning radius to adapt to the
> environment.
>
> Lowering the requested top speed to match the temporary limits of the device
> just reduces the accumulated error and doesn't change the power consumption.
> Since we are talking about underspeed and not stalled motors, backing off
> from max power only slows the device further and doesn't do much for battery
> life.
>
> Getting an exact motor speed will matter on many applications. It is a low
> priority on a bot powered by Lego motors. The focus should be on relative
> wheel speeds and on using the controller to expand the operational envelope
> of a limited device since there isn't much torque to spare.
>
>
> >Then you are accelerating too fast for your system.
>
> Grandma used to tell me the same thing. Bad for the system. Lot of fun
> however.
>
>
> >Running the motors full out is not controlling them. It is open loop,
> >not closed loop.
>
> This is inaccurate. Setting a power level and forgetting about it is open
> loop. Running with a set point at max power presents challenges and limits
> corrective actions but it doesn't earn an open loop classification. Here is
> an example. Take my car out to a wet parking lot, rev it to 4k, close your
> eyes, and pop the clutch. This appears be an open loop system but in fact
> the traction control system will be limiting wheel spin. Open your eyes and
> you've just activated another closed loop in the car/driver system.
>
>
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