FIRST is really a community, we only compete on the field. - InFlight [more]
 Chief Delphi Are we still using only 256 unique PWM steps?
 CD-Media CD-Spy
 portal register members calendar search Today's Posts Mark Forums Read FAQ rules

#16
08-07-2012, 05:08 PM
 Chris Hibner Eschewing Obfuscation Since 1990 AKA: Lars Kamen's Roadie FRC #0051 (Wings of Fire (on sabbatical)) Team Role: Engineer Join Date: May 2001 Rookie Year: 1997 Location: Canton, MI Posts: 1,506
Re: Are we still using only 256 unique PWM steps?

Quote:
 Originally Posted by Michael Hill The reason I mentioned that it would be nicer to have more resolution is well...put it this way. This year, the majority of teams (including 3138), had shooter wheels with a speed controller. If your max speed (that is full voltage to the motors/128 counts) is 4000 RPM, which is easily attainable, the finest you can change your motor speed by is ~30 RPM, which can be a heck of a difference. I'm sure there's got to be a way to use it as an unsigned value (that is 0-12V rather than -12-12V) to get 256 counts of resolution, but that would still only give you a 15 RPM resolution. While many teams figured out it didn't make a huge difference, it would be nice from a controls perspective to be able to control it down to a single RPM (which would be attainable with a 12-bit PWM signal)
You have the power to do this, with some custom software.

What you need is an output converter function (or sub-vi) that takes your desired 12-bit value and modulates the 8-bit PWM value. The modulation of the 8-bit PWM signal will increase your resolution. Here's an example of how to do it (get 12 bits from an 8-bit PWM):

1) Multiply the PWM output command (0.0 - 1.0 for your shooter motor that you should only command in one direction) by 127. You should end up with a PWM command with a whole part and a decimal part.

2) Take the decimal part and multiply by 16 and round to the nearest whole number. Call this number FractionalDuty. (Why 16? Because 12 bits has 16 time more resolution than 8 bits.)

3) Take the whole part of the number from step 1 and call it WholePWM.

3) In your fast loop, implement the following code:
Code:
```if (counter < FractionalDuty)
PWMOut = WholePWM + 1;
else
PWMOut = WholePWM;

counter++;
if (counter >= 16)
counter = 0;```
Finally, convert back to 0.0 to 1.0 by dividing by 127.

What this does is it adds a one-count duty cycle on top of the PWM signal. It does it by increasing the 8-bit PWM by one count for a portion of every 16 timer loops. For example, if your FractionalDuty is 4, then it will increase the PWM output by one count for 4 out of the 16 loops. This averages your PWM output to be an additional 1/4 count. This will increase the resolution of your output, providing that your system time constant is much larger than the frequency of your added one-count duty cycle.

If you implement the above code in a 10 ms loop, you are modulating the PWM signal over a period of 160 ms. Most shooters this year spun up in about 2 seconds, giving them a time constant of about 500 ms. That makes the ratio of time constant to control cycle about 3/1, which isn't great, but it's not too bad. You might see your shooter speed oscillate by 5 RPM or so.

Note that the above code is to get the 12-bits that you want. If you go to 11-bits, you only need eight 10 ms loops to modulate your PWM. That gives your control cycle to time constant ratio of about 6:1, which is getting pretty good.

Disclaimer: I'm typing this while I'm being distracted by something, so my math might be off a bit.
__________________
-
An ounce of perception is worth a pound of obscure.

Last edited by Chris Hibner : 08-07-2012 at 10:27 PM.
#17
08-08-2012, 06:36 AM
 Al Skierkiewicz Broadcast Eng/Chief Robot Inspector AKA: Big Al WFFA 2005 FRC #0111 (WildStang) Team Role: Engineer Join Date: Jun 2001 Rookie Year: 1996 Location: Wheeling, IL Posts: 11,077
Re: Are we still using only 256 unique PWM steps?

What we had resorted to this year was to bump the speed on our flywheel-like shooter wheel to keep it in the range we needed for a specific shooting solution. The software people can discuss this in greater detail. When needed we added a little power to the wheel motors to keep it in the range. We used a simple encoder for wheel speed feedback. The first time I heard it running, I thought it was broken.
__________________
Good Luck All. Learn something new, everyday!
Al
WB9UVJ
www.wildstang.org
________________________
Knowledge is power. Power UP!
#18
08-11-2012, 04:44 PM
 Tom Line Don't lay blame. Fix probems. FRC #1718 (The Fighting Pi) Team Role: Mentor Join Date: Jan 2007 Rookie Year: 1999 Location: Armada, Michigan Posts: 3,050
Re: Are we still using only 256 unique PWM steps?

Quote:
 Originally Posted by Michael Hill Ah, that makes a lot more sense. I was wondering how they would get by the 1.25 reference voltage of a LM317. I guess they COULD use LT3080s to get around that, but like I mentioned before, I haven't taken a Victor apart (mostly because they're not mine, and our team doesn't have the kind of resources to waste on me accidentally breaking a Victor). The reason I mentioned that it would be nicer to have more resolution is well...put it this way. This year, the majority of teams (including 3138), had shooter wheels with a speed controller. If your max speed (that is full voltage to the motors/128 counts) is 4000 RPM, which is easily attainable, the finest you can change your motor speed by is ~30 RPM, which can be a heck of a difference. I'm sure there's got to be a way to use it as an unsigned value (that is 0-12V rather than -12-12V) to get 256 counts of resolution, but that would still only give you a 15 RPM resolution. While many teams figured out it didn't make a huge difference, it would be nice from a controls perspective to be able to control it down to a single RPM (which would be attainable with a 12-bit PWM signal)
Chris tackled how to get in-between PWM values by playing with the output signal.

To go a step further, you are stuck thinking in purely electrical terms here. You need to start thinking about the system you are controlling mechanically, which is what ether was getting at by pointing out control with bang-bang. More specifically you need to think about the inertia of the system.

You can send minute increases and decreases to the motors powering the wheels MUCH faster than the intertia of the wheel system will allow it to react. That means through the use of PID, or Bang-Bang, you can achieve much better resolution that your electrical system analysis would suggest, because the intertia of your mechanical system serves to damp or average the response.

In conversations with other teams that I trust, I heard of some managing to control as tightly as +/- 5 RPM. Our final value was about +/-13 rpm by the time I got sick of playing with encoders and Jaguars and gave up.

Being able to control it to a 'single' rpm in the way that you suggest wouldn't work very well. It would take a very long time for the inertia of the system to respond to a tiny increase in voltage so that you could actually get to that rpm.

 Thread Tools Display Modes Rate This Thread Linear Mode Rate This Thread: 5 : Excellent 4 : Good 3 : Average 2 : Bad 1 : Terrible

 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 User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Announcements     User Announcements FIRST     General Forum         FIRST E-Mail Blast Archive     Rumor Mill     Career     Robot Showcase Technical     Technical Discussion     Robotics Education and Curriculum     Motors     Electrical         CAN     Programming         NI LabVIEW         C/C++         Java         Python     Control System         FRC Control System         Sensors     Pneumatics     Kit & Additional Hardware     CAD         Inventor         SolidWorks         Creo     IT / Communications         3D Animation and Competition         Website Design/Showcase         Videography and Photography         Computer Graphics     National Instruments LabVIEW and Data Acquisition         LabView and Data Acquisition Competition     Unsung FIRST Heroes     Awards         Chairman's Award     Rules/Strategy         Scouting         You Make The Call     Team Organization         Fundraising         Starting New Teams         Finding A Team         College Teams     Championship Event     Regional Competitions     District Events     Off-Season Events     Thanks and/or Congrats     FRC Game Design     OCCRA         OCCRA Q&A         OCCRA Programming Other     Chit-Chat         Games/Trivia             Fantasy FIRST     Car Nack's Corner     College & University Education     Dean Kamen's Inventions     FIRST-related Organizations         Western Region Robotics Forum         Southern California Regional Robotics Forum         The Blue Alliance             Video Archives     FIRST In the News...     FIRST Lego League         Lego Mindstorm Discussion     FIRST Tech Challenge     VEX         VEX Robotics Competition         VEX IQ     Televised Robotics     Math and Science         NASA Discussion ChiefDelphi.com Website     CD Forum Support     Extra Discussion

All times are GMT -5. The time now is 05:10 AM.

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

 -- English (12 hour) -- English (24 hour) Contact Us - Chief Delphi - Rules - Archive - Top