Flying Cutoff Modification

[widelto]

My customer wants to add a DC controlled servo valve (0-10 VDC) to an existing and working flying cutoff (steel pipe mill), nowadays saw is move by a hydraulic cylinder that is driven by a two solenoid valve (Fwd/Rev).
We are using movable limit switch to signalize home end extended positions for the cylinder.
Customer installed a temposonic feedback unit inside the hydraulic cylinder (0-10 VDC).
They also provided us analog modules 1756-OF4 and a 1756-IF4 to be used as drive por servovalve and feedback respectively.
I would prefer to use 1756-HYD02 board, but i have no choice.
We are using a controllogix CPU processor & Rslogix 5000 V19.
Is there a way to include those boards as part of my motion group ??
Or do you have a technique to move cylinder back&forth other that using move instruction.
Is PID an option ?, I think standard PID is too slow to drive and control this servo valve.

 

[allscott]

It sounds like the customer had the right idea with the temposonic and servo valve and then cheaped out with the analog cards. This is precisely the kind of application the motion control card or a motion controller are designed for. Sure you can do it but it with analogs and a PID but it won't be ideal and it won't be easy. Your customer should be made aware of the advantages of doing this properly. Namely the speed of the cut and the life of the blade. Whether or not this is a friction saw or a carbide cut off blades are a big expense on a pipe mill (not to mention down time changing them) and maximizing blade life should be top priority.

I personally wouldn't attempt this with a PLC doing the motion but here is what I know about FCO's and steel pipe mills.

You probably want to control the speed of the cut into about 5 steps.

1- Fast as possible (with the control available) to the OD of the pipe.
2- Slow down while the blade cuts through the OD of the pipe
3- Speed up through the wall of the pipe to the back wall (least amount of teeth engaged during cutting).
4 Slow way down (less than step 2 to cut through the back wall), this is the hardest part of the cut since the most teeth are enganged.
5 Reverse as fast as possible to home.

I have installed a Delta RMC system for a FCO on a pipe mill and it worked well. This particular system had two blades so it was quite a bit more complicated than I described. The two blade system was designed specifically so no blade ever had to cut through the back wall.

My advice is to tell your customer to bite the bullet and either buy a motion controller or a motion card for the PLC.

I should also ad that just slapping in any old proportional valve isn't a great idea either. To do this properly you really want a high performance servo or proportional valve with very little deadband in the spool. This adds to the complexity. Have a look at the performance charts of the valve they selected, in my experience the flow rates in these aren't exactly linear which is going to add to the headache of trying to do this with a PLC. Low performance valves are designed with a large overlap in the spool presumably because they are cheaper to manufacture and so that things don't move with no signal applied. You will need to setup feed forwards to overcome this.

Hopefully Peter chimes in as he is the undisputed expert in this kind of thing.

 

[widelto]

Allscott: Thanks for your comments, I've been thinking of something similar to what your suggested (steps), but your explanation is excellent.
I'll wait until Peter chimes in , he is the one.

 

[Peter Nachtwey]

wilelto, i thought you had done flying cut offs before.
Don't you have to synchronize the speed between saw controlled by the hydraulic actuator and the pipe? There should be some sort of encoder that tells how fast the pipe is moving so the hydraulic actuator can match its speed. There is no mention of this. The speeds must be matched at intervals of the cut length so the pipe are all cut at some desired length instead of random lengths. There is no mention of this. The hydraulic actuator and pipes speed must be synchronized during the cut. If not the saw will stall or pull the pipe and there will not be a clean cut plus the pipe may damage the saw blade. After the saw comes back up the hydraulic actuator is free to move back to the home position. I usually don't send it back as fast as possible. I send it back only as fast but as smoothly as necessary.

Using a Temposonic rod is a good idea but NEVER us an analog Temposonic rod on an application where precision is required. Use SSI because it allows for much more precise control because positions can be differentiated to determine actual speeds and accelerations. This is very important in a flying cutoff saw application. Do you know about SSI Temposonic rods?

What allscott said about the valve is very important. It would be good to know the part number of the valve so I an determine if it is suitable or not. Is there an accumulator in the system? This is important too. Pressure compensate pumps react to pressure changes but servo valves work best if the system pressure is constant. The accumulator acts like a big hydraulic capacitor that keeps pressure constant instead of voltage.

Finally, don't even attempt to do this in a PLC. To do a flying-cutoff right requires a lot of math that would take you too long to figure out and implement. Anyone that is serious about selling motion controllers has a 'canned' flying-cutoff program that basically works and only needs a little tweaking to get it to run in your particular application. This saves a lot of time.

How fast are PLC analog cards now? In the past they had response times in the many of milliseconds and were definitely too slow to be used in a motion control application although there were suitable for temperature control.

Here is our example.
http://deltamotion.com/peter/Videos/FlyingShear/
Notice there are steps similar to what allscott mentioned. If you download RMCTools you can load the program and see the comments. The basics are already done.

BTW, our controller has a built in simulator so we can test the application before the machine is built. It minimizes startup time because you arrive on site with a program that is close to being done.

 

[widelto]

Peter, my cuttoff is working fine with no problem. This system is based on 1756-M02AE units. Servo drive and servo motor are used , mill speed is 60 meter/minute.
Now, the customer wants to changes just the blade control (saw), not the carriage.
We use a 150 HP motor coupled to a saw to cut pipes and that is working fine.
Basically he wants to change two solenoids hydraulic valve system used now by a servo valve in order to move the saw fwd/rev to make the cut.

 

[allscott]

Peter, my cuttoff is working fine with no problem. This system is based on 1756-M02AE units. Servo drive and servo motor are used , mill speed is 60 meter/minute.
Now, the customer wants to changes just the blade control (saw), not the carriage.
We use a 150 HP motor coupled to a saw to cut pipes and that is working fine.
Basically he wants to change two solenoids hydraulic valve system used now by a servo valve in order to move the saw fwd/rev to make the cut.

I assume you are clamping the carriage to the pipe during the cut? 150HP so must be a big friction blade?

 

[widelto]

Allscott: Yes, It's a huge friction blade. Cut accuracy is good +/- 3 mm in 6 meters. The customer wants to increase speed by positioning blade nearer to the pipe depending on pipe O.D.. Nowadays blade is at home position when home limit switch is reached, and go forward to cut until fwd limit switch is reached, then retracts to home.
Limit switches are movable, they want to use a pipe OD to set home and fwd limits for the blade, that's the reason for the servovalve and temposonic feedback unit.
My preferance would have been a 1756-hyd02 but you know.

 

[allscott]

Allscott: Yes, It's a huge friction blade. Cut accuracy is good +/- 3 mm in 6 meters. The customer wants to increase speed by positioning blade nearer to the pipe depending on pipe O.D.. Nowadays blade is at home position when home limit switch is reached, and go forward to cut until fwd limit switch is reached, then retracts to home.
Limit switches are movable, they want to use a pipe OD to set home and fwd limits for the blade, that's the reason for the servovalve and temposonic feedback unit.
My preferance would have been a 1756-hyd02 but you know.

I am intimately familiar with one of those. My office was pretty much above one for two years. Double hearing protection and you still can't here yourself think when it's cutting a 12" OD pipe.

No I don't miss it lol.