cylinder vs slider for hydraulic actuator

[anthonyws]

I am having no luck in applying servomotors to mechanisms. I have done it in the past but either I Am missing some step or creo is not functioning. Using manufacturer supplied assemblies was my first thought for source of failures. so I manually redefined the piston to the cylinder. no luck. Then I made a super rudimentary piston/cylinder assy. nope. I can drag the piston and the limits work as intended in the model. However, in mechanism, I can only get a partial move then it fails with errors. 

error msg with 2 servos:  All position constraints can't be simultaneously satisfied. The following position constraints can't be satisfied:
1) Servo motor constraint for servo motor Motor 1 in model 62205K194_NO_THREADS_HYDRAULIC_ can't be satisfied.  2) Servo motor constraint for servo motor Motor 2 in model 62205K194_NO_THREADS_HYDRAULIC_ can't be satisfied.   

or with a single servo:

All position constraints can't be simultaneously satisfied. The following position constraints can't be satisfied:

1) Upper limit of the first translation axis in connection Connection_1 in model 62205K194_NO_THREADS_HYDRAULI_1 (component id path [5]) can't be satisfied. 

 2) Servo motor constraint for servo motor Motor 1 in model 62205K194_NO_THREADS_HYDRAULIC_ can't be satisfied.   

[vnamboodheri]

Hi anthonyws,

Thank you for your question. 
I’d like to recommend to bring more details and context to your initial inquiry. 

It also helps to have screenshot(s) to better understand what you are trying to do in your process. 

This will increase your chances to receive meaningful help from other Community members. 

Regards,

Community Moderation Team.

[anthonyws]

The cyl on the left is from Parker. many parts and features.  Since it was not functioning with the mechanism servomotor using cylinder assembly constraints, I tried slider constraints. This was the advice on many youtube tutorials. Apparently is is more reliable? Not sure why connecting a piston to a cylinder would have trouble using the "cylinder" constraint specifically designed for this. So after many attempts to get the motor to drive the piston out and back I decided there may be issues with the Parker model. I then made the simplified model assembly on the right and went through the process of putting the cylinder on the default constraint then adding the piston using cylinder constraint. Setting limits and testing with the drag function. All good (as before). Now I start mechanism in creo and attempt to assign motors to the cylinder assembly. The motor will drive the piston out most of the way then fail with the errors I listed above. Sometimes there was a " bobble" in the travel and sometimes it would stop part way. When  a second motor was added to derive it back in the opposite direction (home) it would not work. ever. when i set the times in the motor dialog as shown, nothing happens and the values reset to start/end for both motors. The manually entered values always revert to default. 

So this has me wondering if I missed a step. There are no meaningful warnings or suggested corrections.

I have done this many times before in creo and other software. It's not complicated. maybe there is a step that is assumed that I don't remember. I have followed creo tutorials from the learning conector as well as many top tier creo youtube channels. 

Some variations I have tried include using different references for the motion constraint. axis', piston face/cylinder blind end, rod end/cyl bore shell surface. I tried to use planes but they are unselectable. Any suggestions or ideas are most welcome, 

Cheers

[Pettersson]

Maybe try uploading your assembly? I made a quick cylinder in Creo 6 with a servo motor that drives it out and then back in. Try running the analysis in this one and see if it works for you. It works on my machine, so if it's not working on yours, something is wrong on your end. If it works on yours, try to see if there are any changes, or again, upload your own assembly so we can look at it.

[anthonyws]

It does work, out and back. thats good news. but I still get the errors. 

Funny, it is configured just like mine as far as I can tell. 

[Pettersson]

You've only attached the assembly file. We need the parts as well. Use the "Save a Backup" option, not "Save a Copy" to get all the necessary files.

Do you mean that when you run the analysis in the assembly I sent you, you get these error messages? But the analysis still works?

[anthonyws]

Sorry,

Here is the complete upload.

[Pettersson]

I'm off on a business trip in a few minutes, so I won't be able to take a look right now. Maybe someone else here can look at it. If not, I should be able to check it out perhaps on Friday.

[anthonyws]

Thank you so much for your time. Much appreciated. 

[pausob]

Hi @anthonyws 

Well, I tried @Pettersson cyl.zip model on my system, and it runs without a hitch (Creo 10.0.1)

I had a look at your model cyl_test.asm, and tbh - you have 16 different analyses defined in it and I don't have time to test them all...

I am thinking something in your setup is different than 'standard'.   Not sure where I should tell you to look, though - have you tweaked settings related to accuracy:

[anthonyws]

Ah, ok. I didn't realize it was storing each attempted analysis run. They are all just variations as I tweaked the settings to find a working combination. None of them worked so any one would yield a failure. 

I can delete them and re-run the analysis and upload a zip.

As for accuracy settings, I did make a change at one point at the recommendation of someone as a possible cause of failures. It didn't make any difference.

The change I made was to reduce it from default (0.0000001) to something like 0.000. Seemed reasonable. 

[pausob]

Ok, I tried your 2nd model (cyl_test_2.zip) and sure enough, the analysis fails.

So I looked and it seems to me that your cylinder is limited to only be able to travel 5.75":

yet the analysis is asking for it to go 8":

(because the motor on that connection is defined to have a constant velocity of 1"/sec...)

[anthonyws]

This is great. So I was led to believe/understand that the motor parameters (start/8/9/end) were times not stroke distance. And yes I edited the stroke from 6 to 5.75 inches to see if that made a difference. I had tried all sorts of numbers in the motor parameters dialog. I have matched the stroke to the times and the speed. If this is correct (motor params are distance and not simulation time) I will redo and see what happens. Thank you for your insight. 

[anthonyws]

Still no good. Motor drives the piston out and stops before retracting with the errors mentioned before. In fact I recall seeing a creo demo where the motor parameters were start/20/23/end and those numbers were frames or time and did not seem to relate to any dimensions of the actuators. I am stumped.

[pausob · Accepted solution]

No, the parameters in that analysis definition motor tab table are the times that the motors are 'active'

And your motor is defined as 1"/sec, so after 8 seconds, the piston will travel 8"... but it can only go 5.75" due to the limits!

Probably same thing happens when the "other motor" is turned on.

BTW, I do wonder why you are using 2 motors to simulate this, instead of using a simple table to specify the motion, but anyway: for sake of diagnosing your situation: If I redefine your analysis with these values: 

it works fine on my end.  Does it work for you?

[anthonyws]

Understood. Thanks. I got conflicting info from a tutorial that showed the times as a way to control/scale the frame rate. Its all very nebulous that there are no units called out ( I assume seconds based on this explanation for this scenario) and the ratio of stroke to time must match. How would this work with a variable speed like ramping I wonder? In all of my trial and error I am sure I must have tried 6 seconds for 6 inches. The 5.75 is an artifact of dozens of variations. Sorry for any confusion. 

I did try to use one motor for both directions with no luck. I've never heard of using a table for this even though I took all of the classes related to advanced assembly and simulations and mechanisms. Interesting. I will investigate.

So yes! I can get it out, back in and out again... but interestingly, I have to set the times for some moves at a few 1/10ths shorter time than stroke length. It is not clear why this happens so I have no clue when these "offsets" need to be used. Documentation makes no mention of this either. 

Thank you for this valuable insight as I now have some idea of how this works. I'm not sure how I've accomplished this all these years without knowing this. 

[pausob]

Well, no arguments from me about lack of documentation and the UI not being intuitive enough to figure out things on your own.

The software is getting better, and maybe you should submit feedback to PTC and get the mechanism module to be updated.

Good luck! - and try using the "table" function to generate the motion profile that you are trying to achieve.  This way one motor is required and also you have better way to visualize the motion.  For your example, try this and run the analysis:

[anthonyws]

Great reply. And fantastic example. Thank you. I will look at your example. I found the table tool. Now to learn what it all means. 

[anthonyws]

So, yes, it runs as expected, out and back, then stops with the message in the screenshot. But at least it runs. That is more than I could get from mine. It's weird. I've done this many times without issue. all sorts of mechanism motors and assembly constrains. rotary motors with cam/follower mechanisms, geneva mechanisms with cylinders cycling in and out. 

[anthonyws]

So I submitted a support request and the engineer that was working with this also had issues. We got the cylinder to run out and back when the rate multiplied by motor time matched the stroke distance but still errors. and the support person could not figure out how to make anything other than linear velocity profile. We tried a ramp function and it fails. My question regarding the ramp function was why the system can't do the math and prompt for a value in the coefficient variable field. This has been elevated to a higher support tier. I'll report back with the outcome.