It is often less about what you say, and more about how you say it. - IKE [more]
 Chief Delphi Skid Drive Longitudinal Slip Percent
 CD-Media CD-Spy
 portal register members calendar search Today's Posts Mark Forums Read FAQ rules

#1
01-15-2018, 11:40 AM
 AndrewGazelka Registered User FRC #2502 (Talon Robotics) Team Role: Programmer Join Date: Jan 2018 Rookie Year: 2017 Location: Eden Prairie, MN Posts: 6
Skid Drive Longitudinal Slip Percent

I am attempting to use encoders for localization of a skid steer robot. Among others, I am using the paper Experimental Comparison of Skid Steering Vs. Explicit Steering for a Wheeled Mobile Robot to do this.

The paper states:

What I do not understand is why the formula isn't simply i=(1-R/r)*100, where R is the radius of the wheel. I must be understanding this wrong because I have seen this same formula in multiple papers; it doesn't make sense to base the formula off of dynamic variables when it can be based off of static ones. Can someone enlighten me?
#2
01-15-2018, 12:01 PM
 SenorZ Physics Teacher AKA: Tom Zook FRC #4276 (Surf City Vikings) Team Role: Teacher Join Date: Jan 2011 Rookie Year: 2011 Location: Huntington Beach, California Posts: 1,281
Re: Skid Drive Longitudinal Slip Percent

This is a ALL new to me, but if the equation is what you are thinking it should be, the i value would be 0... always.

The idea is that if the V value is larger than that of a "non slipping" wheel, then V/omega is larger than R, and you gain a slip percentage.
__________________
2013-present: FRC Team 4276, Surf City Vikings
2011-2012: FRC Team 3677, The Don Bots
#3
01-15-2018, 12:56 PM
 nickbrickmaster Not Allowed Near Power Tools AKA: Nick Schatz FRC #3184 (Blaze Robotics) Team Role: Leadership Join Date: Jan 2015 Rookie Year: 2014 Location: Eagan MN Posts: 358
Re: Skid Drive Longitudinal Slip Percent

I don't understand why you think the slip percentage has anything to do with the radius of the wheel. What is R and r in your proposed equation?

As for explaining the original equation:
It's a way of describing the slip percentage in terms of things that could be known, not really a way of keeping slipping to a minimum.

Let v (small v) be the linear speed of some point at the edge of the wheel. I like to think of it as the point where the wheel contacts the ground. From physics, v = rw, or wheel radius times angular speed of the wheel.

Thus the important part of the equation is V/v. Because our wheels are fixed, V (big V) is the speed of the robot. If V is 0 and v is nonzero, our wheels are moving but the robot isn't, and our slip will be 100% e.g. the wheels are just spinning. If V = v, then the speed of the robot is the same as that of the wheels.
__________________
I have approximate knowledge of many things.
Any man who must say "I am the king" is no true king.
2018 is the year of Minnesota on Einstein.
FRC 3184: 2014-, FTC 10648: 2015-2016
#4
01-16-2018, 12:20 PM
 IKE Not so Custom User Title AKA: Isaac Rife no team (N/A) Team Role: Mechanical Join Date: Jan 2008 Rookie Year: 2003 Location: Michigan Posts: 2,243
Re: Skid Drive Longitudinal Slip Percent

Quote:
 Originally Posted by AndrewGazelka I am attempting to use encoders for localization of a skid steer robot. Among others, I am using the paper Experimental Comparison of Skid Steering Vs. Explicit Steering for a Wheeled Mobile Robot to do this. The paper states: What I do not understand is why the formula isn't simply i=(1-R/r)*100, where R is the radius of the wheel. I must be understanding this wrong because I have seen this same formula in multiple papers; it doesn't make sense to base the formula off of dynamic variables when it can be based off of static ones. Can someone enlighten me?
It looks like you got a very reasonable answer above. If it still does not make sense, I would recommend checking out a book on Vehicle Dynamics like Gillespie or Wong as they will have additional info. Wong is probably better for this application as he has some additional examples and text on skid steer dynamics.

Thanks for the tip to this paper, I am pretty excited to read through it and see what the author has to say.

Based off the project you are doing, you might want to look into some of the setups from 2009. The slick surface had many teams making their own "follower" wheels to help measure slip and create homemade traction control and launch control algorithms.
#5
01-16-2018, 01:48 PM
 Oblarg Registered User AKA: Eli Barnett FRC #0449 (The Blair Robot Project) Team Role: Mentor Join Date: Mar 2009 Rookie Year: 2008 Location: Philadelphia, PA Posts: 1,798
Re: Skid Drive Longitudinal Slip Percent

Our team has done some analysis and found that, so long as linear accelerations are low enough to avoid slipping the wheels, wheel scrub in a FRC differential drive is well-approximated by using an "effective trackwidth" somewhat larger than the robot's geometrical trackwidth, with everything else treated as if there is no wheel slip of any sort. The "effective trackwidth" does not appear to depend on either linear or angular velocity of the robot.

To measure the "effective trackwidth," simply turn the robot in place by driving the wheels a known distance, and measure the resulting angular displacement.
__________________
"Mmmmm, chain grease and aluminum shavings..."
"The breakfast of champions!"

Member, FRC Team 449: 2007-2010
Drive Mechanics Lead, FRC Team 449: 2009-2010
Alumnus/Technical Mentor, FRC Team 449: 2010-Present
Lead Technical Mentor, FRC Team 4464: 2012-2015
Technical Mentor, FRC Team 5830: 2015-2016

FRC Drivetrain Characterization
#6
01-16-2018, 03:45 PM
 JamesCH95 Hardcore Dork AKA: JCH FRC #0095 (The Grasshoppers) Team Role: Engineer Join Date: Dec 2004 Rookie Year: 2001 Location: Enfield, NH Posts: 2,393
Re: Skid Drive Longitudinal Slip Percent

A possible explanation as to why the longitudinal slippage depends on torque is because the wheel and substrate deform as torque is applied. To take an extreme example, check out this top-fuel drag racer wheel:

https://i.imgur.com/tlkJh1Y.gifv

The wheel sits there at one resting radius, as soon as torque is applied you can see the wall of the tire ripple and reduce the effective radius of the tire.
__________________
Theory is a nice place, I'd like to go there one day, I hear everything works there.

Maturity is knowing you were an idiot, common sense is trying to not be an idiot, wisdom is knowing that you will still be an idiot.

 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 05:19 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