Y'know... I think I'd much rather have big balls than hang myself... - FotoPlasma [more]
 Chief Delphi Drive system calculations
 CD-Media CD-Spy
 portal register members calendar search Today's Posts Mark Forums Read FAQ rules

#1
09-02-2017, 10:24 AM
 JSG1111 Registered User AKA: Jimmy FRC #3146 Team Role: Mechanical Join Date: Sep 2017 Rookie Year: 2016 Location: Granby, CT Posts: 6
Drive system calculations

As of late I have been researching and trying to figure out calculations regarding the force required by a motor to move a robot that weighs 120 pounds. I understand torque and speed calculations and I can find how fast the wheels (6wd) will spin under no load but i'm not sure how to find how fast they will spin with 120lbs of material atop them.
#2
09-02-2017, 11:39 AM
 EricH New year, new team FRC #1197 (Torbots) Team Role: Engineer Join Date: Jan 2005 Rookie Year: 2003 Location: SoCal Posts: 20,636
Re: Drive system calculations

https://www.vexrobotics.com/vexpro/e...xamples-guides, click "Additional Examples and Guides", then click the bottom right one, "JVN's Mechanical Design Calculator". You should be able to work through that and come up with something.

Also, make sure to account for battery and bumpers--you want 150 lb or more for the robot.
__________________
Past teams:
2003-2007: FRC0330 BeachBots; 2008: FRC1135 Shmoebotics; 2012: FRC4046 Schroedinger's Dragons

"Rockets are tricky..."--Elon Musk

#3
09-02-2017, 07:25 PM
 GeeTwo Technical Director AKA: Gus Michel II FRC #3946 (Tiger Robotics) Team Role: Mentor Join Date: Jan 2014 Rookie Year: 2013 Location: Slidell, LA Posts: 4,644
Re: Drive system calculations

If you're more interested in theory than a canned answer:
• Top speed is affected by load, but not as much as I'd have thought before getting in to robots. The various FRC calculators, based on a number of measurements and observations, estimate a robot's top speed based on a percentage of the "free" (no load) motor speed, most typically 80% to 85%.
• The load will more significantly affect acceleration, especially at startup from a stationary position and in a "pushing match" where the robot is not moving (or only slowly) but applying a force against another robot, field element, or other resistant object. The calculators will take you through this, but approximately:
The motors each draw their "free current" with no benefit other than overcoming friction. Above that point, torque (and therefore applied force on the carpet) is proportional to the current, after multiplicative increases for gearing down and decreases for the wheel radius. Note that a 40A breaker can actually pass 50A for about an FRC match, and rather higher currents for short time periods. Also check out the various curves for your motors - the BAG and CIM style motors can draw stall current for a significant number of seconds before giving up the smoke - but may require cooling between matches, whereas air-cooled motors such as the 775pro cannot draw anywhere near stall for more than a couple of seconds, but can draw current up to the point that they can vent it essentially forever.

Vex has great measurements online for how much current is required to shut down each of the common FRC motors. (click on each motor for the info)
__________________

If you can't find time to do it right, how are you going to find time to do it over?
If you don't pass it on, it never happened.
Robots are great, but inspiration is the reason we're here.
Friends don't let friends use master links.
#4
09-02-2017, 07:34 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: 1,529
Re: Drive system calculations

Quote:
 Originally Posted by JSG1111 As of late I have been researching and trying to figure out calculations regarding the force required by a motor to move a robot that weighs 120 pounds. I understand torque and speed calculations and I can find how fast the wheels (6wd) will spin under no load but i'm not sure how to find how fast they will spin with 120lbs of material atop them.
In a frictionless system, the mass would not affect top speed at all, and only change how long it takes the robot to get there. Unfortunately, we don't build frictionless drive trains, and so this question boils down to "how much loss due to friction is there in a fully-loaded robot drive?" The answer to this is, unfortunately, "it's complicated" and your choices are basically to use an arbitrary fudge factor (most people use ~80%), or to test it empirically. Tools like the spreadsheet linked above do the former. The "correct" course of action is the latter.
__________________
"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

 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         Pro ENGINEER / 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     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 07:58 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