I am attempting to build a 6 axis robotic arm. I understand how to program the servos, I understand how to interface with them, I understand how to design the 3d printed parts for the robotic arm. The part I am still new at and having trouble with is purchasing the correct servos. I bought a ten pack of crappy sg90 servos from ebay, but they are giving me trouble. They cannot lift very much at all and their accuracy is bad to say the least. sending a command to the servo is turning out to be more of a guess than a command, and nothing below the elbow can support its own weight. I thought I could simply buy a slightly bigger servo for the elbow and a slightly bigger servo for the shoulder rotation and so on until I had a working arm with progressively stronger servos to lift the weight of the others, but I have run into a snag. Whereas the crappy servos were roughly 2 dollars each, I would be looking at more of a 20-40 dollar range for individual non crappy servos. I understand that the servos I already bought are incredibly crappy, but even so that seems like a bit of a jump to say the least. Am I just looking at some special expensive servos and there is a better option where I can get reasonable sized packs of decent servos hopefully that can lift a small arm (about 5kg/cm if I were to guess for the heaviest one) or are servos just this insanely expensive and even a small arm would cost over a hundred dollars? Sorry for the inexperienced question i just have no basis upon which to estimate how much servos should cost.
You get a 35kg/cm servo in standard form from aliexpress for ~ 20$, but they wont do what you want them to do. Just do the maths: to lift 1kg 25cm from the axis you need 25kg/cm stall momentum. To move it, you need ~ 5 times more. To stop it then it's moving down with gravity you need ~ 10 times more. And you need steel bearings, balanced, otherwise you'll just grind teeth.
Anyway, you can build a robot arm with 2 joints+rot. base, when you use light plstic parts and move the servos to the base.
zwieblum:
You get a 35kg/cm servo in standard form from aliexpress for ~ 20$, but they wont do what you want them to do. Just do the maths: to lift 1kg 25cm from the axis you need 25kg/cm stall momentum. To move it, you need ~ 5 times more. To stop it then it's moving down with gravity you need ~ 10 times more. And you need steel bearings, balanced, otherwise you'll just grind teeth.
Anyway, you can build a robot arm with 2 joints+rot. base, when you use light plstic parts and move the servos to the base.
I appreciate the alternative, but I am very set on 6 axis, 2 joints+rot. base is not really an option. What are my servo options for the full 6 axis?
Without some details of your arm (sizes and weights) and the torque values needed it's impossible to guess what servos you might need. Certainly cheap clone micro servos were never going to be useful unless the complete arm weighs less than about 5g and is no more than 5cm long. I'm guessing that isn't case.
Steve
conneranderson:
I appreciate the alternative, but I am very set on 6 axis, 2 joints+rot. base is not really an option. What are my servo options for the full 6 axis?
Something that mover a weight bigger than a fly: none.
slipstick:
Without some details of your arm (sizes and weights) and the torque values needed it's impossible to guess what servos you might need. Certainly cheap clone micro servos were never going to be useful unless the complete arm weighs less than about 5g and is no more than 5cm long. I'm guessing that isn't case.Steve
The version made with cheat crap servos was 20 cm long and weighed about 170 grams. I imagine those values would change with different servos though. above the elbow needs little modification, only better 9g servos, so no more than 1.2 kg/cm above that point. The elbow struggled slightly with faster speeds or any weight, so would likely need a bit more than that. At that point the problem becomes compounded because each servo would have to lift the weight of the servos ahead of it on the arm, so I do not really know how to estimate that other than by gut. This arm does not need to be extremely strong. I would say for the lowest servo i would need no more than 6-7 kg/cm, though that number is entirely a guesstimation.
zwieblum:
Something that mover a weight bigger than a fly: none.
I can easily believe that such servos would be out of my price range, or would be geared and and thus slow, but i have a very hard time believing that such servos do not exist. Even with crappy 9g servos, it can almost hold its weight even at the shoulder, so i really don't think that there are absolutely no option here.
I Just need to know whether servos really are as expensive as my search has suggested. The crap servos are 2 dollars each, servos capable of lifting twice as much are for some reason showing up online as over ten times that cost. I just want to know if these are some special servos and there are cheaper variants that do not cost an arm and a leg each, or if for some reason there is just an order of magnitude massive price increase past 9g servos.
I see, you have to make your own experience 
Alright so I did manage to find a solution. I dismantled the tiny servos, geared them to twice the torque half the speed using 3d printed parts, and put the pentameter on the reduced part. The servo now moves half as fast, but is now capable of supporting its own weight. It looks crappy, it is not neat, it is slower than I would like but it does function.
Pls search "MG996R" servo.
It is one step up by "SG90".
chrisknightley:
Pls search "MG996R" servo.
It is one step up by "SG90".
These look pretty good, but they might be a bit over kill. these servos provide 11kn/cm of torque, which is great but much bulkier than needed. If there are no intermediate servos in a reasonable price range then these work quite well, but is there anything in between the tiny 9g and the chungas that is the MG996R in a reasonable price range?
conneranderson:
These look pretty good, but they might be a bit over kill. these servos provide 11kn/cm of torque, which is great but much bulkier than needed. If there are no intermediate servos in a reasonable price range then these work quite well, but is there anything in between the tiny 9g and the chungas that is the MG996R in a reasonable price range?
Also, upon further investigation it seems these are only 90 degree servos, not 180, which is unfortunately not enough for this application.
conneranderson:
Also, upon further investigation it seems these are only 90 degree servos, not 180, which is unfortunately not enough for this application.
Incorrect.
It is 180 degree servos.
And if MG996R is overpowered, use SG5010.
However, I feel that the size does not change that much.
The more small servo and more powerful than SG90 is MG90S.
It's the same size as the SG90 and uses metal gear, but the torque doesn't increase that much.
The MG996R will definitely rotate 180 degrees.
Some sellers also sell 360 degree items.
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