Go to Post The more interesting arguments were around a related subject: whether it was pronounced "Wago tool" or "screwdriver". - Billfred [more]
Home
Go Back   Chief Delphi > Technical > Programming
CD-Media   CD-Spy  
portal register members calendar search Today's Posts Mark Forums Read FAQ rules

 
Closed Thread
Thread Tools Rate Thread Display Modes
  #16   Spotlight this post!  
Unread 26-01-2011, 14:04
AustinSchuh AustinSchuh is offline
Registered User
FRC #0971 (Spartan Robotics) #254 (The Cheesy Poofs)
Team Role: Engineer
 
Join Date: Feb 2005
Rookie Year: 1999
Location: Los Altos, CA
Posts: 803
AustinSchuh has a reputation beyond reputeAustinSchuh has a reputation beyond reputeAustinSchuh has a reputation beyond reputeAustinSchuh has a reputation beyond reputeAustinSchuh has a reputation beyond reputeAustinSchuh has a reputation beyond reputeAustinSchuh has a reputation beyond reputeAustinSchuh has a reputation beyond reputeAustinSchuh has a reputation beyond reputeAustinSchuh has a reputation beyond reputeAustinSchuh has a reputation beyond repute
Re: PID Loop Question

Quote:
Originally Posted by Ether View Post
I'll take a crack at answering my own question.

If the center of mass is not equidistant from all four wheels, then during conditions of acceleration the wheel(s) to which the CoM is closest will see a greater effective inertia. If there is no (or minimal) acceleration, the only effect would be an increase in rolling resistance which may or may not be significant depending on the flooring material and the wheels.
Sounds like a nice explanation. In addition, if you are at the traction limit, the wheels with more normal force on them will be able to contribute more to the acceleration of the robot, and should have a more aggressive controller to do that.
  #17   Spotlight this post!  
Unread 26-01-2011, 14:49
Jared Russell's Avatar
Jared Russell Jared Russell is offline
Taking a year (mostly) off
FRC #0254 (The Cheesy Poofs), FRC #0341 (Miss Daisy)
Team Role: Engineer
 
Join Date: Nov 2002
Rookie Year: 2001
Location: San Francisco, CA
Posts: 3,082
Jared Russell has a reputation beyond reputeJared Russell has a reputation beyond reputeJared Russell has a reputation beyond reputeJared Russell has a reputation beyond reputeJared Russell has a reputation beyond reputeJared Russell has a reputation beyond reputeJared Russell has a reputation beyond reputeJared Russell has a reputation beyond reputeJared Russell has a reputation beyond reputeJared Russell has a reputation beyond reputeJared Russell has a reputation beyond repute
Re: PID Loop Question

The topic of doing speed control via PID has come up several times in recent weeks. I have referred people to this thread: http://www.chiefdelphi.com/forums/sh...&highlight=PID

To quote myself:

Quote:
A couple things to keep in mind here:

The standard POSITION PID loop might look something like this:

output = Kp*e_pos + Ki*e_pos_sum + Kd*e_pos_delta;

Where output is the output.
e_pos is the error in position = desired_pos - actual_pos
e_pos_sum is the sum of the position errors = e_pos_sum + e_pos
e_pos_delta is the derivative of the error = e_pos - e_pos_last

This loop, if well tuned, should provide pretty good position control. But what about VELOCITY? Speed is the first derivative of position - so we could differentiate the position loop with respect to time to obtain a velocity controller.

I will use the D() operator to represent taking the time derivative.

D(output) = Kp*D(e_pos) + Ki*D(e_pos_sum) + Kd*D(e_pos_delta);

So far so good. What's the derivative of the output? Well, that's the change in output over time, so D(output) = output - output_last.

Whats the derivative of e_pos? Remember that e_pos itself is (desired_pos - actual_pos). It's derivative would simply replace "pos" with "vel".

e_vel = D(e_pos) = desired_vel - actual_vel.
e_vel_sum = D(e_pos_sum) = e_vel_sum + e_vel.
e_vel_delta = D(e_pos_delta) = e_vel - e_vel_last.

Putting it all together, you get:

output - output_last = Kp*e_vel + Ki*e_vel_sum + Kd*e_vel_delta;

Let's rearrange...

output = output_last + Kp*e_vel + Ki*e_vel_sum + Kd*e_vel_delta;

or...

output += Kp*e_vel + Ki*e_vel_sum + Kd*e_vel_delta;
(assuming output is global or static and persists between loop iterations)
The key is the "+=" in the final equation. E.g. your PID loop output should be the DELTA to the motor PWM. If you use the output directly, you essentially only took the time derivative of one side of the (well understood) position PID equation. You can still tune it to work, but Kp, Ki, and Kd no longer represent what you thought they did.

In simpler terms, position PID uses motor PWM (~= speed, the time derivative of position) as the quantity being output. Thus, velocity PID uses motor acceleration (the time derivative, or DELTA in motor PWM) as the quantity being output.

Last edited by Jared Russell : 26-01-2011 at 14:51.
  #18   Spotlight this post!  
Unread 26-01-2011, 16:54
Geek 2.0 Geek 2.0 is offline
Registered User
FRC #0107
Team Role: Programmer
 
Join Date: Sep 2009
Rookie Year: 2008
Location: Holland, MI
Posts: 120
Geek 2.0 will become famous soon enough
Re: PID Loop Question

So how would one go about tuning a speed-based PID? I mean, the thing is on the ground and moving, and if you have multiple PID loops going, each one will affect the others, right? It just seems like it would be a huge pain to tune. Someone have a method they wouldn't mind sharing?
  #19   Spotlight this post!  
Unread 26-01-2011, 18:08
Deetman Deetman is offline
Registered User
AKA: Kevin Dieterle
no team
 
Join Date: Apr 2004
Rookie Year: 2004
Location: Philadelphia, PA
Posts: 203
Deetman has a reputation beyond reputeDeetman has a reputation beyond reputeDeetman has a reputation beyond reputeDeetman has a reputation beyond reputeDeetman has a reputation beyond reputeDeetman has a reputation beyond reputeDeetman has a reputation beyond reputeDeetman has a reputation beyond reputeDeetman has a reputation beyond reputeDeetman has a reputation beyond reputeDeetman has a reputation beyond repute
Re: PID Loop Question

Quote:
Originally Posted by Geek 2.0 View Post
So how would one go about tuning a speed-based PID? I mean, the thing is on the ground and moving, and if you have multiple PID loops going, each one will affect the others, right? It just seems like it would be a huge pain to tune. Someone have a method they wouldn't mind sharing?
And there lies one problem with the application of PID control... Unless you have developed a mathematical model of the whole system, an analytical/simulation solution is fairly hard to come by.

Now, to specifically answer your question I'd probably do something like the following. Note that I haven't done this specifically for multiple loops so take it all with a grain of salt. I'm sure someone here has more experience in this regards as most of my experience with successful loops has come with a fully modeled system. Most importantly you are going to want telemetry data on the system's response as you are tuning. Without this you'll be taking a stab in the dark saying "I think that was better..".

1. Assuming multiple control loops, tune them using the same constants throughout. Unless the system each controller is controlling is vastly different, this should get you pretty close.

2. Once you've gotten pretty close with each controller, analyze your telemetry data and tune from that making only ONE change at a time and proceeding to test and analyze. Rinse and repeat as needed.

For example, lets say you have two control loops, one for the speed of the left drive wheels and one for the right drive wheels. You've tuned them using the same constants, however your telemetry (and probably physical observations) indicate that your right drive wheels speed up slightly slower than the left leading to the robot veering to the left before the right wheels catch up and straighten out. In this situation I would probably increase Kp slightly to improve the rise time.

Again, this is all in theory and assumes an understanding of manually tuning of a PID controller. If I remember correctly back in 2005 my team used a control loops on the speed of our drive wheels. We only used one controller for both however and achieved acceptable results. The robot was fairly equally balanced though so your mileage may vary. I'd definitely suggest trying to tune them the same first and then if that doesn't work, go for individual tuning of the loops.
__________________

FIRST Mid-Atlantic Volunteer (2012-present)
Team 1014 Alumni (2004-2005)
Team 1712 Mentor (2011-2015)
  #20   Spotlight this post!  
Unread 26-01-2011, 21:52
apalrd's Avatar
apalrd apalrd is offline
More Torque!
AKA: Andrew Palardy (Most people call me Palardy)
VRC #3333
Team Role: College Student
 
Join Date: Mar 2009
Rookie Year: 2009
Location: Auburn Hills, MI
Posts: 1,347
apalrd has a reputation beyond reputeapalrd has a reputation beyond reputeapalrd has a reputation beyond reputeapalrd has a reputation beyond reputeapalrd has a reputation beyond reputeapalrd has a reputation beyond reputeapalrd has a reputation beyond reputeapalrd has a reputation beyond reputeapalrd has a reputation beyond reputeapalrd has a reputation beyond reputeapalrd has a reputation beyond repute
Re: PID Loop Question

How I tuned the gain of our drivetrain, on a practice bot (to get the code right):

1. Run the robot in each direction to determine the maximum forward/reverse speed of the slower side (since that limits the forward/reverse speed)
2. Setup LabVIEW to graph the Sensor, Setpoint, Delta, and Output (basically just open the VI that shows the graph, and open the constants VI as well)
3. Set the gain to a known in-range number (in my case, that was 0.01)
4. Jack up the gain until the graphs show a reasonable rise time and minimum overshoot.
5. Decide if I need to write a gain scheduler (and I decided to, so I did write a linear gain scheduler)
6. Tune the gains again (go back to 4) for each end of the spectrum, and check that the performance is good in all zones
7. See how the extreme and precise response is, and decide what to do about those. I found that with an I only, the robot backs up slightly when stopping (integral windup), so I wrote some code to handle sign mismatches between setpoint and sensor differently.
8. Drive it again and tune, repeat until perfect.

On the chassis I tested (34 lbs chassis w/ everything but 1 front bumper and the battery, + 50lbs in weights) with 6" kit wheels and a 1-speed, I was fine with 1 set of constants. I assume I will need to tune High and Low separately, but that comes when the robot is done.
__________________
Kettering University - Computer Engineering
Kettering Motorsports
Williams International - Commercial Engines - Controls and Accessories
FRC 33 - The Killer Bees - 2009-2012 Student, 2013-2014 Advisor
VEX IQ 3333 - The Bumble Bees - 2014+ Mentor

"Sometimes, the elegant implementation is a function. Not a method. Not a class. Not a framework. Just a function." ~ John Carmack
Closed Thread


Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -5. The time now is 03:53.

The Chief Delphi Forums are sponsored by Innovation First International, Inc.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2017, Jelsoft Enterprises Ltd.
Copyright © Chief Delphi