What stops grippers from crushing objects?

[Kelpy]

Presumably, some sort of code can be written, which tells the processor when the servo operating grippers comes up against resistance.
Is it an easy bit of code (bearing in mind I'm a complete noob)
Can someone advise me, ( or better still, share the code  )
Cheers.

[Razor Concepts]

Force/pressure sensors on the gripper

Motor current monitoring - if current gets high, motor is stalling and gripping something

[Joker94]

on many robot grippers, more often seen on the robots that i have been involved with at the uni use force sensors. they prefer this method over the motor monitoring method due to the fact that they find the information more usable/accurate.

for us robot hobbyists resistance sensors like

http://www.sparkfun.com/commerce/product_info.php?products_id=9375

and

http://www.sparkfun.com/commerce/product_info.php?products_id=9376

can be used to get the results you are after.

As for writing some code to incorporate these sensors into the system you would say something along the lines of, if over the resistance threshold pause the gripper movement until instructed to release. when programmed what would happen is when it senses that it has a firm grip on an object sop the contracting movement maintaining enough force to hold the object. the object would maintain in that position unless instructed otherwise by a sensor or human control.

cheers

Joker94

[Kelpy]

Many thanks for your replies, guys.
I think I will probably go with the force pads, as it seems to me, as a newbie, this should be the easiest solution.
In any case, as I'm using ordinary RC servos, I don't think they have feedback capabilities, do they?

[KurtEck]

I don't remember seeing anywhere here what gripper/processor or the like you using.  But as an example Lynxmotion sells both grippers as well as the FSR sensor.  They also have a tutorial(http://www.lynxmotion.com/images/html/build129.htm), that shows how they recommend mounting the FSR and possible ways of wiring it in (with either a pull-up or a pull-down resistor) and the differences it makes on the input.

You then use the Analog to Digital input for your processor to read the sensor.  There is code up on the Lynxmotion site as part of the Rover with Arm code (and probably else where).  What the code basically does, is when you give it input from the PS2 to close the gripper, it will continue to close the gripper, until either it gets to it's minimum or until the FSR returns a value that says the pressure exceeds...

And no most ordinary RC servos do not have any feedback mechanism.

Good Luck
Kurt

[madsci1016]

In any case, as I'm using ordinary RC servos, I don't think they have feedback capabilities, do they?

No external feedback, but a few minutes with a soldering iron and one of these and you can give it external feedback.

[Kelpy]

I don't remember seeing anywhere here what gripper/processor or the like you using. 

OK, I'll come clean.
This site has started me on this road to madness, and I do have the Axon 2, which I have used so far in a couple of projects - with a modicum of success. However, for my birthday, she who must be obeyed, bought me a Robonova, and it's this I am looking to put hands on. (I am in the process of making, by hand, some Melissa type hands for it)
Many thanks for the link, Kurt, - more studying needed.

No external feedback, but a few minutes with a soldering iron and one of these and you can give it external feedback.

Thanks for that madsci1016, but I think at my present level, it would be beyond me. Even the force sensors are going to give me a massive challenge.