System Performance

[JerryFlyGuy]

Just curious as to how close people's system perform's compared to what had been calculated. I'm particularly interested in forces and accel rates.

The reason I'm interested in all of this is because I'm starting to have second thoughts on my stepper system. I've read in past days of several people who built their system with steppers and then later 'upgraded' to servo's. I'm poised to buy both the stepper's and the drives in the next day or so and now, I'm wondering if I'm making the correct decision. I've already bought the power supply parts and lots of the actual machine parts so I'm in this for the long haul, however I'm worried that I won't get the performance I need [ even though its calculated that I should be able to get it fairly easily] and will have to move to servo's at a later date anyway. If that ends up being the case I probably should do it now.. instead of later. Pretty generic question I know.. but.. I can offer more accurate details if someone wants to look at the math... but no point in putting it all up here if its not needed..

Opinion's welcome!!
Jerry

[H500]

If you done your calcs using the motor's torque curve, then you're probably in good shape.

I believe most dissatisfation with steppers come from using the stall torque to select a motor.

[JerryFlyGuy]

Thanks H500, I've done exactly that. By using the supplied motor curves and assuming a slight in-efficentcy of the motor/drive and then also over designed by about twice as much as what I think I'm going to need. I'm hoping there is enough excess power to do what I need. Since I posted this earlier today, I've been thinking about it more and have pretty much convinced myself of the reality of going with steppers. In the long run I to will probably end up going w/ servo's simply for the closed loop control. However I do know that they are harder to set up and get running than steppers so its probably smart to just focus on getting the machine running, and then see if there are needed improvements before worrying about optimizing my first system. I've got to keep reminding myself that I've over built this in many ways and that if my math is correct [ and I've checked it several times over] then it has to be close and should work satisfactoraly [sp] for a first machine. I'm hoping that the structural side of things won't need tweaking as I don't think I can convince the wife to let me build a second one.. and scrap the first.. It pretty much has to work the first time.. or I'm in some pretty hot water. I think she's keeping a closer tally on the total costs than I am, and she's doing it in her head..... there is more to this than just my kicking her V-dub to the curb [$$$$]


Thanks again

Jerry

[Geof]

If you plan on doing something eventually the most sensible thing is to do it now; go with servoes from the start.

I know I am going to ruffle a lot of feathers with this but my view is that steppers are for toys; I have read your threads/posts and I do not think you are embarking on building a toy.

However if the V-dub owner is keeping track of things just in memory I think you are heading for trouble

[JerryFlyGuy]

No kidding.. I think she already has the noose tied.. and stashed some place already :frown:

I hear you on the stepper thing... but for now I think this is going to be the route I'll travel. I'll keep ya posted Geof!!

Jerry

[H500]

Servos are better, they're also quite expensive. Steppers seems to be the most cost effective solution for power levels up to about 150W. That's actually a fair amount of power.

[JerryFlyGuy]

Originally Posted by H500 Steppers seems to be the most cost effective solution for power levels up to about 150W. That's actually a fair amount of power.

So are you saying my 350w steppers aren't up to the job

Oh well I guess we'll just have to try them out.. and if it don't work.. then I'll get lynched..... Actually I sat back down again tonight and went through the entire math excercise again.. by hand this time.. and came out w/in spitting distance of where I was w/ my spread sheet. My other concern was the Step Hz out of the comp. but at 400ipm its only 21Khz so we're a long ways from maxing that out. If I was [could] max out the steppers at 45khz [ the max from Mach 3] I'd be.. well the axis would be... humming along at over 800ipm. I can just see the large hole in the back wall of my shop/garage from my 800lb gantry going cruising through.. hope the dog isn't in the back yard at the time....

Jerry [chained to the idea of steppers for now.. & screw the consequences ]

[Geof]

Your mental image of the gantry cruising out the back end of your garage brought to my mind a question; how do you stop steppers? With servoes if you cut the power they coast and friction eventually stops them or when you want to stop at a particular point quickly you just hit them with a severe reverse overload, up to the limit of your servo amps, for a very short time. But if you drive something heavy up to a decent speed with steppers and then stop providing steps they will coast, surely? Presumably you can give them reverse steps to bring them to a halt; but what happens to the relationship between your step count and actual position?

[H500]

Decelleration is done by slowing down the stepping pulses. As long as the retarding force don't exceed the motor's rating, no steps will be lost.

[H500]

Jerry, can you post the data for the motor you're using. I don't think I seen a 350w stepper before. What do they cost?

[JerryFlyGuy]

They are from Kelinginc.net I don't have to info right here but they are the 640oz-in variety. Now that I think about it, they probably aren't 350w. I'm going to run then at 60v and 6a so that was where I got that number. I can't remember what the exact power rating for them is and they've changed their website around and I can't remember the part no. of the ones I'm buying. When I get home I'll pass the numbers along.

Edit, I found the spec's [looked at the wrong part of the website]
They are 637oz in 6.3a and 2.3v/phase so thats only 15w??

Jerry

[H500]

Try to get the torque curve . The 15w is just the power wasted by the coil resistance. A motor that size will put out between 100-150w. Did you buy the motor yet?