how to attach two objects using force sensor

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Matthew Qi
Posts: 10
Joined: 31 Aug 2017, 02:11

how to attach two objects using force sensor

Post by Matthew Qi » 07 Sep 2017, 03:51

Hi guys,
I am building my own robot by V-REP importing meshes from SoildWorks, when I started the simulation, the robot broke up into pieces. So do I need to attach these objects together?

I found that in the tutorial that we could group them or using force sensors to attach them.

My problem is how could I attach two objects with force sensor when I do not know their position, in the BubbleRob tutorial we attach force sensor to the slider because we know its position.

Sorry for my poor English, and thanks a lot!

coppelia
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Posts: 6183
Joined: 14 Dec 2012, 00:25

Re: how to attach two objects using force sensor

Post by coppelia » 10 Sep 2017, 11:39

Hello,

first of all, if two parts are fixed relative to each other, and if they have the same material property (e.g. coeff. of friction), then group them! On the other hand, if two parts are fixed relative to each other, but do not have the same material property, then place a force sensor between then: attach a force sensor to the first shape, then attach the second shape to the force sensor. This can also be used if you need to measure the force/torque on the sensor.

If two shapes are attached via a force sensor, then make sure that they do not overlap (or at least do have exclusive local respondable masks), otherwise they will start to shake because of the contradictory constraints.

Make sure to carefully read this page, and that tutorial.

Cheers

Matthew Qi
Posts: 10
Joined: 31 Aug 2017, 02:11

Re: how to attach two objects using force sensor

Post by Matthew Qi » 12 Sep 2017, 03:16

coppelia wrote:Hello,

first of all, if two parts are fixed relative to each other, and if they have the same material property (e.g. coeff. of friction), then group them! On the other hand, if two parts are fixed relative to each other, but do not have the same material property, then place a force sensor between then: attach a force sensor to the first shape, then attach the second shape to the force sensor. This can also be used if you need to measure the force/torque on the sensor.

If two shapes are attached via a force sensor, then make sure that they do not overlap (or at least do have exclusive local respondable masks), otherwise they will start to shake because of the contradictory constraints.

Make sure to carefully read this page, and that tutorial.

Cheers
Hi,
Do you mean I can just attach two objects by add a force sensor, and then just attach a force sensor to the first shape, then attach the second shape to the force sensor as you said? I tried several times, the force sensor still remain at its original location, and when simulate, objects linked by force sensors disintegrated, however icon showed the force sensor was dynamically enabled, as you can see in the following picture.
Image

I failed to find more information about add & attach force sensors in manual, sorry to bother you again.

Thanks in advance.

coppelia
Site Admin
Posts: 6183
Joined: 14 Dec 2012, 00:25

Re: how to attach two objects using force sensor

Post by coppelia » 12 Sep 2017, 15:03

It seems you are having another problem in your scene, since your force/torque sensor appears to come apart (break apart). This usually happens if you have large mass or inertia ratios. Check the design considerations 7 and 8 here.

Cheers

Matthew Qi
Posts: 10
Joined: 31 Aug 2017, 02:11

Re: how to attach two objects using force sensor

Post by Matthew Qi » 13 Sep 2017, 03:03

Hi,
I adjust the mass and inertia ratios, the simulation result seems to be alright. So is it ok to leave force sensors in the position in scene as shown in the picture?
Image

And do we have any tips to set the mass and inertia ratios in order to make our robot perform normally?
Thanks a lot!

coppelia
Site Admin
Posts: 6183
Joined: 14 Dec 2012, 00:25

Re: how to attach two objects using force sensor

Post by coppelia » 13 Sep 2017, 08:33

I would put the force/torque sensor close to the point where the two shapes connect. For some physics engines the position of the connection makes a difference, for others it doesn't. Of course if at some point later you want to read-out the force/torque values of the sensor, the location of the force/torque sensor is very inportant (less if you just need to rigidly connect two shapes).

Adjusting the masses and inertias is with Bullet and ODE can be a guessing and trial-and-error game. With Vortex (and to a certain extent Newton), it is much less, since two two engines behave much better with realistic values (i.e. values taken from the real robot for instance).

Cheers

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