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Is there a difference between force and load weight? — Parallax Forums

Is there a difference between force and load weight?

rwgast_logicdesignrwgast_logicdesign Posts: 1,464
edited 2013-02-12 13:27 in Robotics
So I was just running some numbers on the the stingray, im basically just trying to make a quick refernce sheet for my self with all the numbers I would ever need for motor control. When I calculated the force(F=Torque/Wheel Radius) or (1.89=4.6lb-in/2.423) I get about 1.9lbs per wheel, but when I run all the sting ray numbers through the robot shop motor sizing tool, It tells me at a 0 degree angel I can carry 13lbs at a max speed of 6.59ft/sec with an acceleration of 6.59ft squared/sec. Now I know force is pushing power so according to my math the sting ray can push 3.8lbs with no angels involved, put wouldn't force also include weight pushing own on the robot?

This isnt all so important, I was actually trying to decide how many slices I wanted in my encoder discs, but somehow got sidetracked making a cheat sheet for important numbers and equations.

Comments

  • al1970al1970 Posts: 64
    edited 2013-02-09 23:07
    Hi:

    That is why you put things you want to move on wheels. Ex. if you put a 100 lbs of rocks in a cart, it may only take 10 lbs of force to move it. You would only need a 100 lbs of force if you wanted to lift the cart up.

    Al
  • rwgast_logicdesignrwgast_logicdesign Posts: 1,464
    edited 2013-02-09 23:34
    So force is not the same as load capacity? I would say its pretty safe to say no, from the reply given.

    How does one manually calculate, the amount of load weight, given a 0 degree angle and target acceleration value? I know calculators can do this well enough but id like to know how its done by hand.
  • John AbshierJohn Abshier Posts: 1,116
    edited 2013-02-10 07:51
    Take your robots mass (not its weight) and the desired acceleration. Force needed is F = mass x acceleration. This calculation ignores resistance. For a robot that we would build, one can ignore air resistance. Rolling resistance will depend on mass of robot, characteristics of wheels and of surface (easier to roll on concrete than loose sand). For an introduction see the Wikipedia article on rolling resistance.

    John Abshier
  • ercoerco Posts: 20,256
    edited 2013-02-10 14:03
    An ounce of testing is worth more than a pound of calculation.
  • Duane DegnDuane Degn Posts: 10,588
    edited 2013-02-12 09:26
    erco wrote: »
    An ounce of testing is worth more than a pound of calculation.

    Agreed.

    This topic has came up with your other bot.

    Obviously a robot can carry a lot of weight without any powered motors, if it's going down hill. Theoretically any motor that can overcome the friction of the robot's system could move any amount of weight on a level surface. You could use the torque to calculate possible acceleration and how steep of slope the robot could overcome but it's often just easier (with these relatively small robots) just to try it.
  • rwgast_logicdesignrwgast_logicdesign Posts: 1,464
    edited 2013-02-12 11:01
    I know this came up on my original design, which is really a place math needs to be done. If your building something from scratch you need at least a rough estimate of how to zise the motors, not so much an issue when buying a kit. The reason I brought this up is that before my math jived and the numbers were close to the calculator, but there is something wrong here.
  • Duane DegnDuane Degn Posts: 10,588
    edited 2013-02-12 13:10
    rwgast_logicdesign wrote: »
    I know this came up on my original design, which is really a place math needs to be done. If your building something from scratch you need at least a rough estimate of how to zise the motors, not so much an issue when buying a kit. The reason I brought this up is that before my math jived and the numbers were close to the calculator, but there is something wrong here.

    It is a good idea to "run the numbers".

    As mentioned the torque of the wheels don't really tell much about a robot's load carrying ability but it does tell you something about the robot's acceleration and hill climbing ability.

    I think your previous robot could have theoretically climbed a vertical wall (if it had enough traction). This new robot can climb a 54.4 degree hill if it had the required traction.

    Angle = arccosine(7.56 lb force from wheels / 13 lb force on robot from gravity)

    Force resisting robot motion from weight of robot = 13 lb * cos(angle of slope robot climbs)

    The above assumes no friction (which dosn't happen very often). I also used the words "from weight of robot", this is the gravitational force, there's also a resistance to acceleration caused by the mass even without gravity. (As you may have guessed, this can get complicated, hence the "try it" advice.)

    BTW, if you get a chance to take a physics class, I'd suggest you do. Lots of useful stuff to be learned in a physics class.
  • W9GFOW9GFO Posts: 4,010
    edited 2013-02-12 13:27
    Duane Degn wrote: »
    I think your previous robot could have theoretically climbed a vertical wall (if it had enough traction).

    There ought to be some 'rules of thumb' about this. For instance*, an all terrain outdoor robot should have enough torque to climb a vertical wall, a carpet running bot should be able to pull 40% of it's weight up the wall and a hard level surface bot, 15%.



    * all numbers pulled out of thin air
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