I’m assembling a 6-Axis arm. The rest of the arm went together fine and controls easily. The gripper is having a problem.
When I open/close the gripper by hand it seems to slide smoothly. When I open/close it with the servo, the assembly jams and then just grinds the servo gears. The gears were ground completely off on the original in the few seconds it took reach for the power plug. I replaced the servo thinking it was a manufacturing defect.
The replacement servo has the same problem. It gets halfway through closing the gripper and then jams. I’ve installed a quick power off switch, but the gears are already ground down on the second servo. After two servos, it’s getting expensive.
If I leave the passive half of the gear set off, the servo wil drive the active gear through the full range of motion. Any ideas on what’s going wrong or how I could troubleshoot it?
It sounds like you don’t have endpoints set up in the software to protect the servo gears. Micro servos usually have fairly fine gears inside them, in order to achieve the “micro” sizes that they do. As you have discovered, this often means that the drive train develops more torque than is necessary to strip out its own gears. It’s always prudent, when working with micro-sized servos, to be extra-careful with positions and limits so that this sort of thing doesn’t happen.
In regard to your specific instance:
Did you properly center the grip servo during construction? (not just physically, but with the software commands)
Are you opening and closing the gripper in small increments, or are you relying on the software’s default values to act as limits?
With micro servos, it’s important to use soft limits for the motion, rather than relying on physical stops, since you are likely to strip out the gears if you let it push too hard against an immovable object. I suppose that a metal gearset would add a bit more resilience, at the cost of greater expense and weight. I haven’t played with this solution though, so I can’t tell you for certain; for all I know, they’ll still strip out if pushed too hard, but they might take a little bit more abuse before doing so.
If the gripper is jamming at points other than the end points, then you need to hand rotate the servo while it is attached to the gripper to find where the binding is occurring (maybe easy with the gears already stripped). A little silicone grease may be needed on the gripper moving parts if a mechanical issue isn’t found. I agree with the idea that the gripper needs to have software limits if the servo will strip its gears at a stalled condition. One can slowly increment the servo position in both directions to find the gripper end points. If the servo gears do strip at stall, then some provision may be needed for sensing pressure on the gripper faces, as the servo may be in a stalled condition when gripping an object. Making the last small amount gripping movement at a slow speed might also be better on the servo gearing.
Thanks for the quick replies. I did finally get it to jam when manipulating it by hand. It seem like the passive gear can tilt so it’s not quite level with the active gear. The corner of one of the gear teeth catches. I’m thinking of taking some sandpaper to the tooth. Is there a good way to stabilize the gear?
I think it’s just badly engineered. There’s too much slop in the holes to let it work properly. I was able to file down the tooth to make it pass properly, but the length of the bolt holding it just lets the gear roam until it jams. Anyone have any recomendations of an alternate arm that may work? I don’t think I want to put any more time and money into this one.
Sounds like possibly the wrong bolt was used, the bolt is cross threaded and got tight too early, or some other probably easy to fix issue, if it is just bolt length. I don’t have an arm with a gripper, so just talking from ideas.
Thanks for the suggestions. I’m pretty sure I used the right bolt and it’s not cross threaded. The problem is that the machine cut hole is too loose. Muliply the maximum angle of the bolt under stress times the distance between the hole and the gear and it’s out of tollerance.
I built an additional brace to steady the passive gear. It seems to work, but the servo gears are too stripped now for me to be sure. I question whether it’s worth ordering another servo.
The machine vision and response end of it is the interesting part for me. It’s why I went with an off-the-shelf arm instead of designing one from scratch. To be honest I expected to spend my vacation working on edge detection algorithms instead of filing down aluminum by hand. If my algorithms are off and I close the gripper too tight after it’s fully functional will the gears just grind?
The rest of the arm seems to work well and moves with repeatability and agility. Is there anyone out there that has a gripper that works? Did I just get a bad piece or is this a universal problem?
I assume the below gripper is the one you are using. From looking at the assembly instructions (step 13), I would bet that either the wrong screw was used (3/4" steel instead of 5/8" nylon), or the two 4-40 nylon washers were not installed properly. Double check these items.
As to the servo gears stripping, the below from the assembly instructions has some info. It appears that the servo gears will strip at stall using 6v power. If you are using a 6v power supply for the servo, then you might want to put a diode in series in the power supply to the servo to drop the voltage to ~5.3v. This will reduce the gripping power of the servo, but may prevent the servo from generating sufficient torque to strip its gears. You can get a rectifier diode at Radio Shack.
The grippers use micro servos for the lightest gripper possible. With micro servos you will need to watch closely that the gripper is not commanded beyond a physically impossible position to get to.
There is a possibility that the gripper was not assembled properly. You can read through these forum pages and see there is no “universal problem” here. It’s easy to make a mistake in the assembly that will cause binding though. If you can send some close up images we can get it resolved.
