|
[DPRG] When to PID?
Subject: [DPRG] When to PID?
From: David M Wilson
davidmw at tx.rr.com
Date: Fri Mar 16 19:13:18 CDT 2007
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.
More information about the DPRG mailing list
|
