View Full Version : An impossible robotic arm?

03-28-2015, 11:02 PM
Hello all,
I'm new to everything robotics but I've spent this entire day researching Trossen Servo motors but my research is inconclusive given my limited knowledge of things such as these so I've come to y'all for help. Please forgive my ignorance on the subject.

I'm looking into the possibility of building a robotic arm that can extend roughly 36 inches, remove a 3 pound object from, say, a shelf, and then lower that object to the floor. It doesn't necessarily have to lift the object vertically but it does have to have the ability to lower it to the floor. The Arm would also have to have the ability to grasp that same 3 pound object, bring it close to the Arms' body, and hold it there while waiting for the appropriate time to deliver the cargo to the floor.

My question is, will any of the servos that Trossen makes be able to bear the weight of such an object at the 36" distance required to perform that task and do it countless of millions of times?

Thank you for any insight you care to offer.

03-29-2015, 04:10 PM
No... Just No. What you are looking for is an industrial robotic arm, and nothing in the trossen shop qualifies for that except maybe the Dynamixel Pro and a cheaper/safer alternative to them would be large stepper motors intended for CNC routers (probotix, etc.).

Cheapest and most reliable option would probably be to build a SCARA arm with stepper motors and a leadscrew driven vertical lift. SCARA would allow you to grab an object from its side (instead of overhead like a gantry) and not worry about any servos/steppers needing to handle the 1728[oz-in] of torque from your 3[lbf] object dangling 36[in] away. All of that torque would be handled by ball/roller bearings in the arm and the frame of the lift structure, because the SCARA design has the motors rotating only in the horizontal plane to rotate/extend/retract the end effector to place it anywhere in the horizontal plane.

03-29-2015, 08:11 PM
3 feet of reach for 3 pounds is a lot of torque. To hold that without overload, you'd need a servo capable of 60 N.m of max torque (12.2 N.m of full-duty-cycle holding torque.)

Something intended for labs/medical is likely about right -- the really big arms for welding and such are likely too heavy.
The smallest Kuka arm perhaps? http://www.kuka-robotics.com/usa/en/products/industrial_robots/low/kr5_arc/start.htm
Or the IRB 1200 from ABB? Or, for good looks, the YuMi? http://new.abb.com/products/robotics/yumi

You should be able to find something on eBay for a few thousand dollars and up.
(Note: sometimes the arm is separate from the controller, and sometimes other parts, too, so read carefully to figure out what you need for a complete system.)
Something like this? http://www.ebay.com/itm/ABB-S3-IRB-1500-robot-and-controler-/201296808151?pt=LH_DefaultDomain_0&hash=item2ede3988d7

03-29-2015, 09:39 PM
The thing is, I have to hold the cost of the entire arm to under a thousand dollars. And a lot less for the prototype. For the prototype, what if I were to use a pulley system (like a block and tackle) that would halve the torque needed to move and hold the weight? I know it would slow down the whole process of moving the arm but speed is not necessarily an issue. At least not for the prototype. Or if there's such a thing as a gear reducer that could be incorporated into the servo or stepper would that work?

03-30-2015, 12:51 AM
Depending on the accuracy you are after, you could use cheap linear actuators to do this. They certainly have the torque - not sure about the millions of times. Here is a example costing only a couple of hundred bucks.



03-30-2015, 01:32 AM
The thing is, I have to hold the cost of the entire arm to under a thousand dollars. And a lot less for the prototype.
Who is trying to get you to build this arm? I ask because it sounds like you know next to nothing of anything involved in its design and manufacture, and someone is trying to get some sort of professional product for dangerously sooper cheap.

One could probably toss this at a 3rd year ME undergrad and get something workable in a week or so of them searching mcmaster (to spec parts; not so much for ordering motors or bearings unless in a rush and/or need parts of guaranteed quality), doing some CAD/CAM+FEM/FEA, and getting assists from a machinist and maybe an EE undergrad. Does not mean it would be usable as anything other than a proof of concept.

03-30-2015, 08:33 AM
I'm trying to get me to build this arm. And all I have is a clue, no other knowledge about robotics. I've had the idea for a very long time but can't seem to come up with a workable plan for parts, etc. to see it through and still maintain a low cost to compete with an already established machine that does the same thing but a whole different way. The robotic arm would simplify the process if I could just get it to work at a reasonable cost.

I'd need about ten of these arms to replace roughly half of the entire machine that's already in existence. That entire machine costs the consumer around $20,000.00. From what I've researched so far it seems my dream will have to remain just that. A dream, for I don't see how it's possible to compete for what available now and the costs involved.

I'll be investing my own money in this project and am only looking for a "proof of concept". Actually, I've already built a very crude prototype using a motor from a cordless drill, a home-made worm gear, arm, and gripper, and it worked but was crude as hell. I'm looking now to go a little further with my design and have a little more sophistication with readily available parts.

It has to have a moveable turret and at least a two axis arm. And in the three feet it has to perform it's function I only need to be within an eighth inch of accuracy.

I know this project is a long shot but I'm not ready to give up yet. I just wanted to pick y'alls brain before I gave up entirely.

03-30-2015, 02:44 PM
The robotic arm would simplify the process if I could just get it to work at a reasonable cost.

And a Tesla Model D would significantly simplify my daily commute, if I could just get it to work at a reasonable cost.
Perhaps the reason that the existing machine doesn't use robot arms, is that robot arms isn't the optimal solution?

However, if you want to get help brainstorming how to solve the problem you're having, you will need to share what the problem is.
If you want to get (free) ideas from helpful people online, you must make it simple for us to contribute, and we can't contribute if we don't know what it is.

Two axis arm, and gripper, with reasonable torque and precision, can be built using a few aluminum profiles and some linear actuators. They will run slow, but they will have the torque. Linear actuators, in turn, can be driven with H-bridge controllers or stepper motor controllers, depending on kind.
Try some actobotics parts and some linear actuators like these: http://www.trossenrobotics.com/p/figelli-L12-100mm-100-1-Linear-Actuator.aspx


03-30-2015, 10:51 PM
Well, my biggest problem was not knowing torque values which is why I queried this forum in the first place, as indicated in my initial post inquiring about Trossen Servos. I appreciate the information you've provided and thank you for looking at my posts in an effort to help...