Go to Post Wow, my dad's sadistic nature is rubbing off on me. - Lil' Lavery [more]
Home
Go Back   Chief Delphi > Technical > Technical Discussion
CD-Media   CD-Spy  
portal register members calendar search Today's Posts Mark Forums Read FAQ rules

 
 
 
Thread Tools Rate Thread Display Modes
Prev Previous Post   Next Post Next
  #14   Spotlight this post!  
Unread 16-05-2009, 15:13
vamfun vamfun is offline
Mentor :Contol System Engineer
AKA: Chris
FRC #0599 (Robodox)
Team Role: Engineer
 
Join Date: Jan 2009
Rookie Year: 2003
Location: Van Nuys, California
Posts: 182
vamfun is a glorious beacon of lightvamfun is a glorious beacon of lightvamfun is a glorious beacon of lightvamfun is a glorious beacon of lightvamfun is a glorious beacon of lightvamfun is a glorious beacon of light
Send a message via AIM to vamfun
Re: Failed attempt to explain JAG linearity

Quote:
Originally Posted by vamfun View Post
It all hinges on the "off" PWM current. We have a low FET "on" and the rest "off". Any current flow must now pass through a protection diode. As long as the diode can conduct, the current is heading toward a steady state value of -Vemf/R (assuming positive current during the "on" PWM drive). If the L/R is short with respect to the PWM period, the current decays quickly and is zero for most of the off phase. So the average current for the whole pulse is determined by the area under the "on" pulse which = (12-Vemf)/R*duty. This would be the case for the slow Victors and we would expect a nonlinear response equal to my old coast formula.

For the JAG, the L/R is large relative to the PWM period. If the current cannot decay to zero during the "off" phase, it will continue to accumulate until the average current stabilizes. Since the current never gets below zero we can consider the problem as the superposition of a steady state Vemf and a pulsed 12v. And this leads us to the linear equation = (12*duty -Vemf)/R .

SOOOO...I think the two formulas are ok but now it would be due to the 15khz JAG vs the slow Victors. I didn't see this in my previous excel because I assumed the steady state current was heading towards zero in the "off" state instead of -Vemf/R.
OK, finished my TODO and have verified that linearity is indeed due to the higher frequency of the JAG. Based upon my quoted rationale we can show that

120Hz Victor v_emf = 12 - i_free*R/duty......................varies with 1/duty

15kHz Jag v_emf = 12*duty - i_free*R .....................varies with duty

The 2kHz Victor is very close to the Jag.


I wrote a LABVIEW simulation (see attch hbridge.vi) of a one sided Hbridge and the curves (see attch thumbnail) verify what we are seeing. As Al says, this is independent of coast or brake mode however, with the JAG, the "off" ckt is essentially the brake mode since the diode is always conducting..the only difference is for the voltage drop across the diode. This is why I did not see any non spike Vemf in the JAG waveform ..only duty cycle variations.

Feel free to run the program and vary the inductance to see its effects. I used a nominal 120uh which is close to what people have measured in the past. I think the max posted was 230uh.



I wonder what all this is worth? Since I have not seen this mentioned in any of the Hbridge tutorials maybe I'll publish a paper. Lets see...what should I call it. How about

"Special Theory of Nonlinearity: An Inconvient Truth of Low Hz Hbridges".

Maybe the good folks in Stockholm are reading ChiefDelphi threads
Well, it might be worth a minor entry to Wikipedia under Hbridge theory
or under alternative ways to spend time when Friday night TV is on.


Erick...put those beers on ice...assuming Al doesn't send me back to the showers again.

RIP thread ;
Attached Thumbnails
Click image for larger version

Name:	Motor Speed Graph.JPG
Views:	109
Size:	24.1 KB
ID:	7949  
Attached Files
File Type: vi Hbridge.vi (35.1 KB, 23 views)

Last edited by vamfun : 16-05-2009 at 20:25. Reason: Added jpg for those that dont have LABVIEW
 


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

Similar Threads
Thread Thread Starter Forum Replies Last Post
2009 Camera and Jag inventor models problem Creator Mat CAD 5 08-03-2009 14:05
1 Jag and 1 Victor not responding correctly. elbuo NI LabVIEW 4 17-02-2009 22:57
Joystick Linearity NinJA999 Control System 4 28-01-2007 18:49
pic: Yet another attempt at Photoshop Cody Carey Extra Discussion 1 08-04-2006 23:13
pic: Failed attempt at simulation CD47-Bot Extra Discussion 5 17-06-2004 11:03


All times are GMT -5. The time now is 04:22.

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