Vex Claw

I purchased the Vex Claw kit for my students to use. The students have installed it following the online directions. The claw is running on a 269 motor connected to a PIC controller. The claw will open and close about three times and then stop working completely. I told the student to wait a few minutes and try again or switch batteries, however the same thing (claw opens and closes three times then stalls) happens.
Any suggestions?

Is the little gear inside the claw catching on the spring? I’ve seen that in one of our kids robots. Fully open position is when we saw it.

Are you maybe over-opening or over-tightening the claw and kicking the motors into a tripped state from too much current?

It’s hard to stick a quad encoder on these claws so an IME might be what you need to ensure you don’t over open or over close. Potentiommeter is out of the question too.

hello ! … just to say the year 9’s that i mentor had a problem like this, basically they were using a 269 with a 12 tooth metal pinion gear attached onto one of the sides, basically it worked well and everything but after about a day or two they came to me saying it was not working, so i went across and for a test i replaced it with a 393 and it worked well, and has had no problems at all

if you can i suggest putting a 393 on it

—> with potentiometer, it can b done but is tricky, you have to fix one side on and use cable ties to stop it from shifting !

my adivce: use a 393 if you can, and if you can’t use an IME or pot as described above

Hope This Helps !

If you replace the motor with another 269 without turning off the robot does it begin working?

It may be that the PTC temperature controller in your motor is overheating. This mostly happens when the motor is being given power but is not actually moving (ie holding the claw closed.) If it gets too hot, the motor will throttle the power going to it.

I would personally suggest using a Servo because then you can just control the position without giving the motor constant power. You could also use the 393s because they have a bit more tolerance to this sort of thing.

i agree, because the servo has a built in encoder it would work well

Have you used a servo for this in the past? I tried, but did not have success, for two reasons:

  1. The servo is fairly weak. It doesn’t push hard enough against the spring in the claw to open anywhere near all the way open.
  2. The servo also has a ~180degree range of motion. The problem with this is that the VEX Claw kit has a considerable reduction on the motor shaft, which means that 180 degree range of motion is shortened even more.

I found a proportional control loop and potentiometer on one of the 3-wire motors to work perfectly. Your mileage may vary, of course.

//Andrew

Sorry I have never actually used the claw (well a stock one anyway) it just seemed like the logical part to use. I can see how it might be too weak though.

Most likely the motor is trying to open or close the claw past the point that it is physically able to go so the motor is trying to move something that can’t be moved and thus stalls.

We used to have this problem all the time. It is 99% probably the fact that the claw is being held closed at full 127 power and the motor is stressing out.

We added a provision to the programming that if it is held open/closed for greater than 2 seconds, the motor power will reduce to +/- 15 power (enough to hold the claw closed/open without stressing the motor). Then when the other button is pressed, it returns to full power.

Let me know if you have anymore questions.

Well, the problem might be on my side, too–it was an older servo, but I’d still be surprised to see it work really well. I don’t like the servos, though. :wink:

This is important, too. Doing this (or similar) made my demo-bot claw go from lasting a couple open/close cycles to lasting for probably an hour or more of strenuous use.

You know I used to dislike servos too because I found them to be really weak but at the same time if you put some thought into the design they actually can be very useful since they hold their position with very minimal effort from the programmer. The only big problem we’ve had has been breaking the internal gears, rather than strength issues, but that’s because we rarely use clutches :o

I would make sure that the motors are not trying to drive the claw closed at full power. I would think that this is overloading the PTC.