Mechanics and Construction > Mechanics and Construction

Robotic arm

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bdeuell:
for servo 2 the distance used to calculate torque would be cos(45deg)*15cm=10.6 kg*cm because you are limiting the range of motion. I suspect acceleration is of little concern (if I am interpreting your use correctly) you will be fallowing the sun with the arm. As Jwatte mentioned you do not want to operate the servo continuously at stall so you will need to use a larger servo to prevent damage due to overheating.

Servo 1 can be smaller than servo 2 but it is difficult to specify an exact torque without knowing the details of the design as much of the load may come from friction on this axis.

jwatte:
Ah, this is for a solar facing panel!

Why do you need the 10cm arm? Can't you just mount the panel on a bracket to the servo horn, say 2 cm out? If so, you'd need only 2 kg.cm holding torque, so about 10 kg.cm stall torque.

alehandro:
Yea it's for a solar facing panel..
It would be simpler like that but I'm doing a project that a professor gave me and it must be exactly like that!! Like the sketch I mean.. So like bdeuell said "for servo 2 the distance used to calculate torque would be cos(45deg)*15cm=10.6 kg*cm" is this my holding torque?? and then is this servo good for my case http://www.ebay.co.uk/itm/1-2-4x-Digital-MG996R-High-Torque-Servo-Metal-Gear-for-RC-Car-Truck-Boat-Model-/400669823345?pt=UK_ToysGames_RadioControlled_JN&var=&hash=item5d49c85171 because the stall torque is 15kg.cm. So stall torque is 15kg.cm so  the holding stall torque is: 15kg.cm*0.2=3kg.cm?? From what jwatte wrote me or am I understading it wrong?

bdeuell:
That servo would probably move your load but would quickly burn up. Your calculations are correct. From the estimation of the holding torque Jwatte provided you have a 3kg-cm  holding torque but you will need to hold at least 10.6kg-cm. While the servo is capable of providing this torque (because it is less than the stall torque) constantly supplying this torque will cause the servo to overheat.

jwatte:
Also, the amount of holding torque versus stall torque varies by motor and implementation -- better cooling increases the 0.2 factor.

I'd start looking at a 40 kg.cm servo, and measure it under actual maximum load (arm angled 45 degrees.) Use a contactless thermometer and measure how warm it gets. If it's less than 60 degrees C on the case after 1, 5, and 15 minutes, you're probably OK.

Also, if you're doing this for a project -- for how long does the load need to be at 45 degrees? You can get away with a weaker servo, if the time at max load is limited (a dozen seconds, say.) I'd look at a 20 kg.cm or higher servo, still.

These servos might do it:
http://www.robotshop.com/en/herkulex-drs-0201-robot-servo.html
http://www.trossenrobotics.com/dynamixel-mx-28-robot-actuator.aspx

These servos almost certainly would do it:
http://www.robotshop.com/en/herkulex-drs-0402-smart-robot-servo.html
http://www.trossenrobotics.com/p/mx-64t-dynamixel-robot-actuator.aspx (I use these for my walker robot)

Cheap crap that might get you through the assignment if the demo is short:
http://www.hobbypartz.com/33p-solarservo-d772.html (I use some of these for four-wheel steering on my rover)

An alternative would be to use a stepper motor (or brushed DC motor) with appropriate controller, and a limit switch to sense the "zero" position. That would probably be cheaper, but more fiddly to get to work.