Assume Nothing. ( Where did I recently here that?)
SLY said:
PERSONALY I DON'T LIKE TO CONTROL AXIS WITH CLX AXIS CONTROL CARD
IT'S THE OLD WAY TO DO IT
GET INTELLIGENT SERVO DRIVE LIKE THE DKC03 FROM INDRAMAT
AND SERCOS INTERFACE OR DEVICENET THEN YOU DON'T HAVE TO WORRY ABOUT SCAN TIME BECAUSE THE DRIVE HANDLE ALL THE MOTION ITSELF
ALL YOU HAVE TO DO IS GIVE THE DRIVE A SETPOINT AND WAIT FOR THE MOVEMENT TO BE COMPLETE
What do you think the PLC is doing when it commands an axis to move from one point to another?
Your intelligent drive controller may do a fine job at controlling one axis but how does it coordinate 16? Obviously there must be some master smarts some where beside the drive controller. In Rockwell's case it is in the Control Logix.
DeviceNet is fine for small applications but to get all the data the master controller near as fast or as synchronously as over the Control Logix back plane. The Control Logix is sort of like a sercos in the way information is synchoronously transmitted from the master controller to the axis controllers. The only issue I see between sercos and cards in the the control logix back plane is the 'field bus' issue. Not a performance issue.
The control logix has a MAM ( motoin axis move ) command that starts the motion and has a move complete ( I forget what it is really called ) when it is done. That is simple.
Steve, I don't like to play 20 questions but:
1. Have you calculated how long each motion should take? Provide Info.
2. Have you done a timing diagram? ( post .jpg or .bmp)
If you don't have one, go to the hall of shame.
3. Are you getting errors? Which?
4. How can you be off by 2 seconds and not see something obvious?
5. What is the RPI for the motion controllers? Isn't that just as important as the scan time?
6. What are the parameter you have for each of the MAM blocks? If the accelerations are too low then you may never get to the velocity you set and all the time is spent accelerating and decelerating.
7. Rockwell has table that show what percent of the CPU time each axis will take at different RPIs. So how much time does your CPU have left for ladder execution?
Get the execution time for each step of the cycle. Use the real time clock to save the time away at the beginning of each move and at the end of each move. From this you can build your timing diagram and compare it with your desired timing diagram. Then you can TELL us where the problem is.
Don't you think this information matters? I don't think that the CPU being overloaded would slow done the cycle time by another 2 seconds. The motion profiles should always take the same time.
Keith is right about the 16 axes being controlled from just one cpu.
16 axes will use up most of your cpu time. I just think you should find the real problem before spending money.