Problems(STEP7)

Hello all.

I'm working with step 7, and I'm updateing an old lathe...
For the main spindle wich rotates the semimanufactured product I have selected an MM420 inverter and a 2,2kW motor. This axis is allso equiped with an incremental encoder with the resolution of 1000 impulses per revolution.
For the X and Y axis's I have 2 300W POSMO's.

One of the functions of the lath is to make an M25X1,25 thread on the semimanufactured product.
I make the thread in 3 steps. The first step cut's the thread aproximetly 0.2 mm deap on the semimanufacured product, the second step makes the cutting depth aproximetly 0.45 mm dep, and the third the depth to 0.72mm.

I'm useing the incremetal encoder to start the traversing path of the X axis to traverse towards the product and cut the thread on to the semimanufactured product.
The first traversing taskt towards the product and cutting the thread starts when the counter of the incremental encoder reaches 10000 impulses(around 8000 impulses takes for the drive to accelerate to the reference frequency)

The second traversing task starts when the X and Y axis's are repositoned for the second traversing task and when the comparator reaches 18000 impulses from the incremental encoder.

The third traversing task starts again when the x and Y axis's are repositioned and when the comparator reaches 26000 impulses.

What I don't undersand is why sometimes the product come's out perfect with the perfect thread, but sometimes it come's out all wrong. The lathe doesn't start cutting the thread in the same position as the first cut was made.... How is this possible...???

At first I thought that the main spindle doesn't have constant rpm's, but that isn't the case... so it must be something wrong in the program... or something. Is it possible that the S7 station can't send the status word to the drive to start traversing in the same time period???

If you guys need to see the entire code tell me and I'l post it!
Any advice would be helpful...

Regards
Aleš

I have heard that for MM drives on Profibus, the control data should be "consistent", that means that you should use SFCs (dont remember exactly which ones) to access the data.
Maybe that is related to your problem.

Hi,

Few questions to clarify situation:
1 - where is encoder mounted (drive or spindle axis)
2 - how do you exchange signals with drives and encoder
3 - what is rpm of the main spindle
4 - what is your program scan time

1 - The encoder is mounted on the spindle axis!
2 - I exchange signlas via profibus with the drives. The incremental encoder is mounted on the first 3 digital inputs. I am useing S7-314 2DP station wich has an integrated count module. For the counting the impulses that come from the encoder I use SFB47 count block! I hope that this is what you ment.
3 - The rpm of the main spindle is around 2000 give or take a few... they are constant though.
4 - Shortest: 3ms
Curent/last: 9ms
Longest: 10ms

So if I got it right it seems, like you start traversing movements with command send over Profibus after counting pulses in PLC.
Then it is possible that time intervals are very different.
I'm afraid that "naked" S7 PLC isn't NC machine, and task you want to accomplish at that speed is not possible.

jacekd said:

So if I got it right it seems, like you start traversing movements with command send over Profibus after counting pulses in PLC.
Then it is possible that time intervals are very different.
I'm afraid that "naked" S7 PLC isn't NC machine, and task you want to accomplish at that speed is not possible.

Click to expand...

Jup I came to the same conclusion. I even asked Siemens about this problem and they gave me almoast the same answer.
I asked them if S7 station allways sends the control word in the same time period. And they replyed that it doesn't and that I would have to use a diferent profibuss setting. Can't remember wich one though... I'll look tomorow and post it.

Thank's for the efort jacked and Jesper!