I will be discussing over here various methods to hold objects , this is mostly used in pick and place comps.
1)USING ONLY MOTORS N DIRECTLY GRIPPING THEM TO ARMS.
Basic idea is to hold the object and also leave the object.We can use two simple dc geared motors of lower rpm n directly fix it to the bars as shown below
We used two dc geared motors of 10 RPM at the points A and B , fixed to the arms using gears at the two dots shown above A n B
RESULT:Couldnt sustain the force required to lift for a long period of time, so we changed our plan.
2)USING NUT BOLT MECHANISM:
This has been taken frm another site robotics for u which u can c in the link list on the rite side on my blog
This thing is self xplanatory when the motor starts rotating the nut keeps moving inside ,and hence the one of the arms fixed to it keeps moving inside . It gives gud result but was pretty to tough to implement for us , we gave try at it but werent much happy wth wat we had done so had to think some thing else which can be much simple .
3)SIMPLE GRIPPER USING SINGLE MOTOR:
We used some simple objects tat we could get hand on , so we used a a girder u can get this in any mechanical shop(he will charge pretty high if u tell u r purpose).This is how they luk like
The first pic is that of girders (which u can make them using aluminium and drilling holes in it)
and the second pic is that of our arm , we used a 10 rpm motor to pull the two strings.Use a spring in reverse of the strings to pull so tat it can even with draw objects when needed.This is how it goes finally
This is how it finally luks like dont use a high rpm motor thinking tat we can grip it fast , it will be out of control so be carefull of it , we could grip this to lift a 4.6kg objects which we tested in our colleges . This was all because we could use a low rpm motor.