Mechanics and Construction > Mechanics and Construction

Servos

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YAN-1:
Hi everyone. I'm attempting to build my first robotc arm and I need some help! I will be using servos for the joints and will control them using PIC microcontrollers. The arm I'm required to do has to have a shoulder, an elbow, a wrist, and a gripper. It should handle small loads like hold a cup of water and pour it. I've been looking at servos by Hitec and Futaba but I'm not sure what to choose! Please help me by directing me to torque specifications and prices. Thanks a lot.

this should be useful for determining required robot arm torques
http://www.societyofrobots.com/mechanics_statics.shtml

my personal recommendation is just get the most torque you can . . . but remember to also consider speed of the servo too
seen this yet?
http://www.societyofrobots.com/actuators_servos.shtml

lastly, have a look at the picture of the robot arm at the very bottom of this page
http://www.societyofrobots.com/materials_hdpe.shtml
i used a servo for the gripper, a servo for vertical motion, and a dc motor for rotation motion. the batteries are also used as counterweights.

one last thing good to mention, try to keep your weight as close to the base as possible. you want your arm as light as possible.

YAN-1:
Thanks a lot. Well I've been looking around and I found some servos by Hitec and Futaba. The high-torque ones are rated at around 200 oz-in. There is one that says 161 oz-in, which is around 11.6 Kg.cm. This means that it will be able to provide a torque equivalent to a mass of 11.6 Kg hanging from an arm of 1 cm, right? I know this will need some more detailed calculations, but I guess this will be enough. I mean if it is used for the shoulder and it is to carry another one just like it (the servo weights 134g) at the elbow and one at the wrist and a small gripper, it should be fine, right? Assuming of course that the dimensions of the arm are small enough so as not to produce high torques from the weights of the components. I would appreciate your advice and directions.

P.S. I have to build the arm so that the shoulder is the first joint and it emerges from a wall so it won't be exactly like that picture. The shoulder motor will have to carry the whole arm and the load.

Nichola V. Abdo

Yeap that is all correct. If after your more complex calculations are done and you want to just verify them, just send over the info.

Also, dont be limited to just one servo per joint. The base joint can have two servo's (one on each side) rotating the arm. This way you can either double the torque, or have two smaller (and cheaper) servos actuating it.

Lastly, for the base joint, consider using a worm gear. Worm gears work only one way, so that your motor can rotate the arm, but the weight of the arm cannot rotate itself. You would still require the same amount of torque, but you only need power to move the arm and not hold its position. The other possible benefit of worm gears is the high torque possible. Something to look into . . .

YAN-1:

--- Quote from: Admin on March 22, 2006, 01:27:24 PM ---Also, dont be limited to just one servo per joint. The base joint can have two servo's (one on each side) rotating the arm. This way you can either double the torque, or have two smaller (and cheaper) servos actuating it.

--- End quote ---

I just wanted to clarify one point. I'm not sure what you mean by 'base joint'. The arm won't be resting on a base like the one in the picture you showed me. It will resemble the shape of a human arm from the shoulder to the hand. So the 'base' is the shoulder joint which will be fixed to a vertical post or something and the arm will be hanging freely from there and will move to carry objects like the human arm. Do you think the shoulder will be able to carry the whole arm? Thanks a lot.