Go Down

### Topic: Mechanics: More of a physics question (Read 1 time)previous topic - next topic

#### Hasan999

##### Feb 27, 2011, 09:45 pmLast Edit: Feb 28, 2011, 01:40 pm by Hasan999 Reason: 1
Hi,

Nevermind guys, Don't answer ! ...this is just a 'part' of my design that is becoming impossible to illustrate and explain

Have a look at this example:

The two blue ones are servos. [the lower servo holds the lower brown Bar of length R, and the upper servo holds the upper Bar] ...and they have to lift the Mass M vertically.

Question is: Whether the torque of both the servos would 'help' each other lifting the mass? - OR - would each servo have to do same* amount of 'work' in order to lift the Mass, *that it would do even if the other servo was not present (a torque of T=FR)?

The reason I asked because, assuming both bars are ~horizontal, both servos seem to apply a torque of T = (Mg)R.Cos0 in order to lift the mass M, whereas at a good 45º angle, both servos seem to divide the 'required torque' among themselves ( T = ½MgR.Cos45 )?

Thanks !

#### jackrae

#1
##### Feb 27, 2011, 10:52 pm
Your basic design is seriously flawed in that the conversion from rotary to linear motion must allow for the change in effective length of the radius arms as they are rotated - and your design does not make such allowance.

For example, if the upper servo rotates, the only effect is to drive the mass about the pivot centre, with both horizontal and vertical vectors, - not a single vertical vector (up the cylinder).
Similarly, if the lower servo rotates, the effect is to rotate the upper servo rotates about the centre of rotation of the lower servo as per the upper servo.

However, if, and only if, you drive both servos simultaneously in opposite directions by exactly the same amount then your concept will change rotary to linear motion and drive the piston up the cylinder.  However, whilst this would be an interesting programming exercise, it is doomed to failure since any loss of synchronisation will result in lock-up of the drive train.

If you really wish to drive a piston up a cylinder (rotary to linear conversion) then you need only one drive (servo), located at the lowest pivot point and two freely pivoting knuckle joints.  The knuckle joints should be where you currently have the servos located.

jack

#### Hasan999

#2
##### Feb 28, 2011, 12:38 amLast Edit: Feb 28, 2011, 02:16 am by Hasan999 Reason: 1
Thanks for your reply jackrae, but (due to my confusing drawing earlier), you might not have understood the design as I wanted it to be. Now, to illustrate, I played around in 3D CAD, and used 'Paint' to produce a better 3D drawing:

> Mass is attached to Servo 1 bracket
> Servo 1 Link is attached to Servo 2 bracket
> Servo 2 Link is connected to a Free Joint

Also, assume that both servos will rotate at equal rotation/angle.

I desperately need an answer to the Torque question above (i.e. whether the two servos will lift the mass upwards by sharing the load? - especially when the links are almost horizontal)

Thanks !

#### jcarrr

#3
##### Feb 28, 2011, 04:00 am
is the mass laterally constrained as in the first picture or free to rotate about the pin joint as in the second as in the second?

#### jcarrr

#4
##### Feb 28, 2011, 04:11 am
In the first drawing, either servo could raise the mass, if the other was a pin joint, so they could both contribute to raising the mass.   You do the math.

The second one is probably doomed to fall over, absent a very capable control system, and it may not be possible to keep it erect and stable for even a moment.

#### zoomkat

#5
##### Feb 28, 2011, 05:00 am
Best to probably skip talking about servos and such an get back into tangental forces generated by a rotating shaft.
Google forum search: Use Google Search box in upper right side of this page.
Why I like my 2005 Rio Yellow Honda S2000  https://www.youtube.com/watch?v=pWjMvrkUqX0

#### BrIDo

#6
##### Feb 28, 2011, 12:07 pm
I don't see how the arm of the bottom, the one linking the 'free joint' to 'servo1' is ever going to be in anything other than a horizontal position - given there is a free joint there. There is nothing to provide a reaction force.

I would have thought you would need to have a servo located when you have the free joint and another at servo1, to achieve any vertical displacement.

#### jackrae

#7
##### Feb 28, 2011, 12:53 pm
Makes one wonder how a guy who calls himself a mechatronics engineer got this far
Is this a piece of university homework that he's expecting us to do or a question from someone who either doesn't understand the basics of mechanics or has failed to phrase his question correctly
We DO like to help but sometimes it's a bit of a struggle
jack

#### Hasan999

#8
##### Feb 28, 2011, 01:53 pmLast Edit: Feb 28, 2011, 01:56 pm by Hasan999 Reason: 1

In the first drawing, either servo could raise the mass, if the other was a pin joint, so they could both contribute to raising the mass.

Thanks

I don't see how the arm of the bottom, the one linking the 'free joint' to 'servo1' is ever going to be in anything other than a horizontal position - given there is a free joint there. There is nothing to provide a reaction force.

I have mentioned this many times above, the mass is constrained to move only vertically; what do you think servo 1 (along with servo 2) would do if the mass is constrained at that position (as shown in the first diagram @ first post) ?

Makes one wonder how a guy who calls himself a mechatronics engineer got this far

I am sorry, I should have mentioned that this question is for open-minded people only.

Is this a piece of university homework that he's expecting us to do or a question from someone who either doesn't understand the basics of mechanics or has failed to phrase his question correctly

You must be a moron to think someone would join Arduino forum for his homework questions.
I did fail to explain my design properly (and that is because it is only a part of the mechanism of my whole robot design), and I do admit it is my fault to over-estimate your intelligence assuming that you could figure out the possibilities and try to answer my question, as user: jcarrr did.

Go Up