Using Inventor for Arm torque calcs

[AdamHeard]

Rather than estimation and integrating the weight of an arm + whatever it is lifting to calculate what kind of gearing/motor to use... is there a feature in inventor that can give you those numbers? It'd be nice to simply put whatever object that needs to be lifted (with accurate weight) at the end of the arm and see the actual numbers.

[Ryank]

well yes and no. I believe what you want is in inventor, but you have to remember that unless you set your materials (of each part in the assembly) in the CAD, the thicknesses of the materials, and the exact dimensions of all parts of the arm you are going to use you could get wildly over or under estimated torque values (the default material is abs plastic or something I forget, but it's gray and bland). I think if you have access to autoCAD you can take an inventor file and do stress testing on the assembly under load too. But my helpfulness here is admittedly low because I don't actually remember how I used to use either of those functions.

[ChrisH]

The latest version of Inventor contains an application to do this. I've been working on learning it for about a month. I'm still not good at it. Oh and by the way I'm a licensed PE in Mechanical and I have some background in Finite Element ANalysis. SO I have some idea what I'm doing. The program seems quite powerful but, as with any analysis program, the way you set up the problem is critical to getting good results.

The Dynamic Analysis program can feed the forces it computes directly into the stress analysis section. I haven't done much with this yet. I'm still getting the dynamics down and there are some things I really don't like about the way they do the stress. The big thing is they give you the total number but don't break it down into components. There can be a really big difference between 50ksi compression and 50ksi tension.

Another problem is that critical components in the sign I'm working on are PVC which behaves in a non-linear fashion at high stress levels. I can't tell if the stress program handles that properly or not.

Not that I'm complaining. A seat of NASTRAN, (one of NASA's most overlooked revolutionary inventions and the program we use at work) will set you back tens of thousands of dollars a year. This doesn't count the cost of the person to drive it. An experienced stress person can set you back $100K per year,easy.

Getting a toy like this to play with for free? I'll gladly put up with the limitations. It is certainly far more capable than it needs to be for our purposes.

Has anyone else out there been playing with this stuff? Or am I the only one who has had the requesit curiousity?

[kramarczyk]

Originally Posted by ChrisH

Has anyone else out there been playing with this stuff? Or am I the only one who has had the requisite curiosity?

You are not alone. I played with the FEA package a little last year and I concluded that it was linear. I haven't seen anything that leads me to believe that a non-linear material is available.

I also played with the dynamic analysis this fall in trying to simulate the shock loads of reversing the drive train. That was a bit confusing as I couldn't figure out how to model the slack in the system. I hope to take another shot at it shortly to model our gripper/ejector. I'm planning to mount a string pot to a pneumatic cylinder to take displacement vs. time data with LabVIEW and then use that as the input for the dynamic simulation to see if we can optimize it further. We'll see how that pans out.

As far as the original poster is concerned, can you use the CG feature in Inventor to determine the mass and CG location of your 'arm' assembly. If x is the distance from the CG to you pivot as measured parallel to the ground then the minimum torque you need to generate is simply = mass * gravity * x. This torque divided by your motor torque gives you the gear ratio required. Note that if you use the motor's stall torque in the previous eqn then the ratio generated is what is required to stall the arm, not lift it. If you pick a motor torque at the point that you want it to operate, say 25% of stall, then you should be able to move the arm at the corresponding speed. That's a simple way to address the issue, hope it's helpful.

[JD Mather]

Originally Posted by kramarczyk

You are not alone. I played with the FEA package a little last year...

The FEA is different than the Dynamic Simulation that I think will be important in solving this particular problem.

[Molten]

Who needs such analysis for this? Just make it good and strong and you will be fine. Besides, My philosophy is that if it doesn't look like it might break then it won't. Also, even with stress analysis and exact figures for the computer will still not help anyone out. There is always the fluke accident and though I will agree that stress analysis is good for long-term projects like NASA, it is not that useful for a six-week project like FIRST. Unless you are already an expert in which case go ahead.

[Greg Needel]

Originally Posted by AdamHeard

Rather than estimation and integrating the weight of an arm + whatever it is lifting to calculate what kind of gearing/motor to use... is there a feature in inventor that can give you those numbers? It'd be nice to simply put whatever object that needs to be lifted (with accurate weight) at the end of the arm and see the actual numbers.

As continent as it may seem you will never beat the speed of just a simple paper and pencil, or you can use a spreadsheet/ mathcad document. But there are just too many issues setting up complex dynamic simulations to
make it worth the effort for something like this.

Originally Posted by Molten

Who needs such analysis for this? Just make it good and strong and you will be fine. Besides, My philosophy is that if it doesn't look like it might break then it won't. Also, even with stress analysis and exact figures for the computer will still not help anyone out. There is always the fluke accident and though I will agree that stress analysis is good for long-term projects like NASA, it is not that useful for a six-week project like FIRST. Unless you are already an expert in which case go ahead.

I don't agree with this at all. You need to be able to somewhat optimize your design otherwise you are going to end up with an arm that is to heavy and/or a gearing that won't be able to lift it. In many cases it is true that you can overlook doing a complete analysis but teams should always do some form of calculations to get a ball park of what will work. On another note, FIRST robot design is about teaching kids engineering methods, and just saying " make it good and strong" doesn't teach a student anything.



Now here is how simple it is to calculate the torque required on an arm. 1) draw a simple Free Body Diagram (look below)



The Force of the arm = How much your arm weighs total (lbs)

Force of the Ball = How much the ball weighs (lbs)

Arm Length (Al) = The distance from the arm pivot (A in this example) to the center of gravity of the arm. this can be a best guess. If your gripper is at the end then your center of gravity should be somewhere between the middle of your arm and the gripper.

Ball Length (BL) = The distance from the arm pivot (A in this example) to the center of gravity of the ball. (In some cases this can be beyond the end of your arm)

Plug and Chug....Thats really how simple it is. Your answer will be in torque units (ft-lbs, in-lbs, N-m, etc) depending on what units you use, convert as needed. Now remember this is the MINIMUM torque required to get your arm moving and you will need to add a factor of safety to this to make it usable, but ultimately this will give you a good ball park number to start from.

[Molten]

Originally Posted by Greg Needel

On another note, FIRST robot design is about teaching kids engineering methods, and just saying " make it good and strong" doesn't teach a student anything.

I did not intend that they should not learn it. I am just stating that for a six week project there is not time to learn such an advanced software command. Afterall, look at ChrisH for example. From the resume he posted, he is more experienced then most if not all of the students in FIRST. Also, he spent a month to learn this command. This is clearly not a command to try and learn within the six weeks that you are building the robot. I suggest to any teams out there trying to learn this technique and build a robot, that you focus on building the robot now, and learn about stress analysis during the off-season.

[AdamHeard]

Thanks Greg; That's pretty similar to what I've been doing, but I just learned a a few things from you to make it nicer.

I was just hoping for inventor to be able to do that calculation with accuracy as a way of checking my hand calculations.