I’m designing my own claw using just VEX starter kit parts. Currently a servomotor controls the two fingers. The claw remains closed by default and you open it by radio control and let it shut on any object you want to pick up. I noticed that when it is holding an object the servo motor hums as it tries to close all the way but can’t due to the object between the fingers.
Could this be bad for the servo?
Yes this is bad for the servo. This is what is called Stalling a servo or motor. In the long run the servo will heat up and eventually trip the thermal protecting circuit which will temporary shut off the servo until it cools down. Also in the long run the servo will lose strength.
If you are only holding the servos in that stall position for a couple of seconds at a time it shouldn’t be a problem.
Also keep in mind that this is the nature of utilizing servos or even motors in manipulators/grippers. The servos need to exert some force to keep the object in its grasp. The amount of force needed is going to have some proportionality to the weight and any force exerted on it due to movement.
You should also look at the mechanical design that the servo will be working within. Lets say the gripper is designed with a range of motion of 5" fully open and closes with a 1" clearance between the jaws. If the object you are gripping is 1.25 inches wide then the servo will not see a lot of load. On the other hand, if you are gripping an object that is 4" wide, then you most likely will see a lot of constant load on the servo.
So with that said, if you have a good design and are not gripping a heavy object that requires a lot of force to keep it within the jaws, the servo should not be loaded 100% all the time.
As an alternative, you can also accomodate some of the gripper opening issue by trimming the channel the servo is connected to. This is easily done on your transmitter (just read the manual). This way you may be able to minor adjustments to the size of the clearance between the jaws without redesigning your gripper.
One option when designing a claw is to couple the motor/servo to the fingers using springs or rubber bands. After the fingers make contact with the object, any further motion by the motor simply increases pressure, but won’t stall the motor/servo.
Of course, this limits your maximum gripping pressure to whatever the spring or rubber band can exert, but it also protects your motors, and it can give fairly fine pressure control.
Adding compressible foam (such as self-adhesive weather stripping) to the insides of the fingers is a nice compromise between driect-drive and spring-drive. You get a quarter-inch or so of grip range before stalling the motor, and you can still exert the full force of the motor if you need to:
http://web.mac.com/dean.reece/Vex/Improved_GRIPPER_files/Media/Closed%20Side/web.jpg
Cheers,
Thanks for all the help!
Wow thanks! That is a great idea!
yes, this is bad. if there were no thermal protectors, the coiled wire inside the motor would get so hot the insulation will burn off, destroying the motor. the only thing i would suggest would be to have a defaultly open claw and use a servo to close it.
Ha ha!
Last night I took the plunge and made a gearless claw. It now opens and closes using elastic bands pulled by a servomotor. There is also padding within the claws fingures to help with grip. This design works so well!
Would you be willing to share pictures? I’m sure there are many other designers who would benefit from your experiences.
-John
I would be glad to, I’ll try and take some tonight.
Ok, so when I had the camera in front of the claw I decided it needed a cleaner design before publishing. So I spent the last couple of nights taking it apart and designing the “elastic drive” to take advantage of leverage to create a more powerful grip. Will post pictures this weekend.
Hi
Well it is still in the testing stages but this claw can pick up quite heavy objects without stalling the servo-motor. Here are the pictures, tell me what you think:
Claw open:
https://vexforum.com/gallery/showimage.php?i=1646&c=
Claw closed:
https://vexforum.com/gallery/showimage.php?i=1641&c=
Claw resting:
https://vexforum.com/gallery/showimage.php?i=1631&c=
Very nice! I like the combination of foam on the finger and a rubber band to couple the servo. Also, the way you are pivoting the finger looks like a better design than the one I posted above (which uses the drive-gear axle as the pivot). I’ll probably borrow some aspects of your design if you don’t mind.
Thanks for posting the pics,
I’m glad you like it 
(it was really your idea)
Of course, take what you can. I think we should all learn from each other’s experiments. I really enjoy vex! Now I just have to convince my better half to buy me the tank tracks 