Switching from VFD/Motor to Servo to reduce cycle time. (Siemens)(Video)

[AutomationTechBrian]

I have a newer customer that I've been working with one or two days a week, over the last couple of months. Their automation is Siemens Step7 Classic, so this has given me a great opportunity to build my Siemens skills. I've done a few small projects for them so far, but as I get more familiar with their cement block production, I'm seeing some possibilities for reducing cycle times. I noticed the conveyor turntable was creating a bottleneck for production, so I mentioned that to my customer. He confirmed that it was indeed the bottleneck, and they've tried other solutions to speed it up. They tried an air cylinder solution, but they couldn't control it enough to keep the different blocks from sliding on the boards. They currently use a VFD and motor to control the speed. I think the speed and ramps could be adjusted a little better than what they have. I'm still gathering details about the different products they make, so the resulting solution is applicable to all their products... except one that has a special speed setting. I don't think the prior automation guy knew about s-curve ramps (or "rounding" in the Siemens world). I'm also wondering about the need for more precise decel time when heavier products are being produced... I'm thinking they may need a brake resistor.

But thinking through the possibilities, probably the better solution might be a servo motor. I've yet to program a servo application, and it seems like this would be a great one to learn with. I'm in the very early stages of thinking about doing this, so it would be a good time to get some input from those with experience.

Here is a video of the turntable in action. I didn't take it specifically for this post, so it's only the functioning application, not the details like the motor and the gears. But you can certainly see the inefficiency. The cycle times vary for the different products, and the seconds are displayed on the main control screen on the HMI. I've noticed it hovers around 20 seconds, so a 2 sec decrease would be a significant increase in productivity. ...and a 4 second, even better! Not sure where the theoretical limit is yet, but I wouldn't be surprised if a 4 second would be reachable. ...And a 20 percent increase in productivity would easily make it possible to completely load their kiln in one day, which would be a worthy goal.

Check out the video on YouTube: https://youtu.be/2bRNREnMVXE

Let me know your thoughts.

 

[Peter Nachtwey]

I noticed that there are stops that the boardshit. That must move the blocks a little due to the deceleration jerk. First, I would avoid hitting the stops. I would find how high the deceleration rate can be without the boards slipping on the chains. A motion profile is required that minimizes acceleration and jerk. This would be one that accelerates and decelerates with not constant velocity segment. The down side of this profile is that the peak speed is higher. Can something be placed under the boards that would increase the friction? Or the chains? This would allow faster accelerations and decelerations. Overall the motion is quite slow. There should be a lot of room for improvement.


This is where the jerk limited minimal time profiles are handy.

 

[GaryS]

I don't see anything to be gained with a servo in a lot of jobs I have gotten away from servos in favor of Flux vector VFD's
you should be able to program out the 20 sec hover you have to be careful of the acc and dcc times with produce on because you could shift the product on the plate
I would try adjusting the dwell times acc and dcc times and even the travel speed
maybe a different speed forward and reverse
there are other things that could be adjusted to increase the production throughput speed

 

[AutomationTechBrian]

I should clarify some of the details:

-There are two motors on the turntable: the chains motor, and the rotation motor.
-The chains motor is just using a contactor... They don't have a problem with the boards bumping the end stops. It's the rotating that has caused issues when they tried the air cylinder solution.
-I just put a VFD on the supplying conveyor last week. It was using a contactor and a motor with a brake. The brake was gone, and it was coliding with the turntable, causing a delay of a second or two. I was shooting for a way to adjust the speed of the supplying conveyor so the brake was not needed... but I did put a brake resistor in for the times it needed to brake. I found that the turntable was *too* slow, and slowing the VFD down below 45 Hz created issues feeding the boards onto the turntable, as well as adding extra wear to the boards from the previous section feeding the boards at a different speed. If the turntable cycles faster, I could speed the feeding conveyor up to 60 Hz, and hopefully not have to slow down at all.
-The feeding conveyor is currently set at 45 Hz, which like I said is slow because of the turntable.
-Upstream, the mold/vibrator is limited by downstream components. It waits to release the molded blocks until it gets the appropriate "OK" bits. It could go faster than shown here.
-The focus is on the rotation, both there and back. That's where the problems have been with the blocks moving on the board. Before the blocks meet the second conveyor, the rotation speed changes to "slow" until a prox sensor senses the correct position to stop. I see probably two seconds of wasted time there. I'm thinking a precision move without sensors, with a higher peak speed and a smooth s-curve decel would do it better... thus the servo idea.
-I'll be looking at returning the turntable faster either way.

 

[JRW]

Brian
Maybe I missed it but what Siemens drive and power module is in the turn table?

 

[daba]

I watched your video, and my gut reaction was "how inefficient is that?!"

It's obvious the boards have to be turned 90 degrees, because of the asymmetric pattern of the blocks. And it's the turning that is the bottleneck, you say.

I am certain I have seen "pallet turners" you could use on the 90 degree bend of your line, and certain I have seen much faster operations, without "spitting the load off".


Just found this, the first section of the video may be useful ...

 

[mp91]

Lots of stuff that could be done before I'd go for a servo. Another trick I've seen is on the rotating platform, add an extra set of chain conveyor at 90 degrees to the current and put it on a pneumatic lifter. That could completely eliminate having to wait for the platform to spin back as you will load on to chain a, turn 90. Raise chain b, load, turn 90. Lower chain b, load, turn 90, etc. You would have to do some changes to prevent cable tangling which can be tricky but there are nice rotary unions for pneumatics.

There are also conveyor that can do that sort of turn but that need a bit more space and can be a little bit tricky in dirty environments like this.

 

[daba]

Lots of stuff that could be done before I'd go for a servo. Another trick I've seen is on the rotating platform, add an extra set of chain conveyor at 90 degrees to the current and put it on a pneumatic lifter. That could completely eliminate having to wait for the platform to spin back as you will load on to chain a, turn 90. Raise chain b, load, turn 90. Lower chain b, load, turn 90, etc. You would have to do some changes to prevent cable tangling which can be tricky but there are nice rotary unions for pneumatics.

There are also conveyor that can do that sort of turn but that need a bit more space and can be a little bit tricky in dirty environments like this.

I am 100% certain a decent mechanical handling company will have the perfect solution to this "not unique" scenario.

Buy it in, and get a warranty !!

 

[daba]

The wheel has already been invented, and so is the solution to your problem.

Put feelers out to conveyor companies to do the job right, it's not a difficult thing, and the right companies will already know the best solutions in your application.

Rolling your own is gonna work out more expensive in the long run, you'll be refining and changing things for months, whereas the experts will have done it before, and know how to do it successfully. Go and get quotes ....

 

[mp91]

I am 100% certain a decent mechanical handling company will have the perfect solution to this "not unique" scenario.

Buy it in, and get a warranty !!

But where is the fun in that...

 

[GaryS]

Your best option is to replace the turn table with a 90 degree turn roller conveyor
That way there is no stop start, no delay and they are not that expensive

If you can’t change the conveyor then you will have to make some small changes no single change will do it.
Put the table chain drive on a VFD a small decrease in the load / unload time - Increase the chain drive speed even just a few hertz would help
Install sensors in the table to detect when the plate is on the table far enough to allow the turn
And to know when the plate has cleared the table when unloading, to start the return
Increase the accelerate and decelerate rates for the return of the empty table
I am sure there are other things that can be done as well I would have to study on it a while

 

[daba]

But where is the fun in that...

Having "fun", and not achieving results equates to goofing !

No management wants to employ goofers

 

[daba]

It seems like many people are making what I would class as "uninformed" solutions.

My advice stands, get someone in to deliver a solution at a price you can afford to pay, it'll be cheaper overall, deliver the design spec., and give a performance guaranty !!

 

[mp91]

It seems like many people are making what I would class as "uninformed" solutions.

My advice stands, get someone in to deliver a solution at a price you can afford to pay, it'll be cheaper overall, deliver the design spec., and give a performance guaranty !!

I'm not so sure. It sounds like this 90 degree turn only becomes the bottleneck in some circumstances and so perhaps increasing it's throughput by only 10-20 percent will be enough. In that case, small control changes would surely be preferable to ripping the whole thing out and replacing with a new solution. I'm all for letting the professionals do what they do but when you look at the efficiency on the current set up I reckon the OP could get to where he needs to be with very little investment.

 

[RVaughan]

Looks like you can save a second and a half at least on the return rotation if you start it sooner.
I see the two limit switches on the turntable. Where are the part detection switches on the downstream conveyor?