interrupt question fx3u

[plcdp 11]

Hello, I have a question regarding interrupts, or interrupt pointers. From what I understand from the programming manual, an interrupt can only be triggered by an input. (X0-X5) My problem is this: I am working on an blow molding machine with a reciprocating screw. The position of this screw is monitored through an LVDT wired to the analog input module (fx2n-8ad), and set by the operator through an HMI. So the start/stop positions of of the entire shot are based on compare functions, i.e. M8000 ---//---/-----[<= D1006 D56]_____(M15) / /-------[>= D1006 D57]_____(M16) M15 being screw forward bit, M16 being screw recovered bit, D1006 being actual screw position from the LVDT, D56 being the screw forward setpoint, and D57 being the screw recovered setpoint. The problem I have is that my scan time is around 35ms, I get 15 inches of shot in .8 to 1.0 sec. depending on weight, so 35 ms of scan time computes to a possible fluctuation of shot length of .5 to .6 in, (Which, by the way I am seeing just that). I am trying to figure out a way to have these functions on an interrupt, but would like to keep the control through the transducer, so that the above program would be the same with the exception of the outputs would be interrupts. Any suggestions would be greatly appreciated. Thank you, plcdp

[Crossbow 296]

One recommendation I would make is to clean up the code some. Are there things you can run in subroutines instead of in the main PLC scan? 35ms for an FX3U seems high. How large is the program? Unfortunately with the FX you don't get multiple programs like the Q Series, so you can't turn on and off entire sections of code. But you could use the CJ instructions to skip unnecessary sections of the code to try and maximize the scan time.

[panic mode 246]

i was trying interrupts on FX3U few weeks ago. most of the interrupts are input driven (directly or through high speed counter). one can also specify time interval - unfortunately it can be only 10ms or slower (10-99ms if i remember correctly). i wonder why this has to be so slow (in many cases this is actually slower than scan time - it defeats the purpose of interrupt). should have been 1ms....

[plcdp 11]

Hello, Thanks for the response...my program is 3320 steps. Does 35ms still seem high for that size program? I put in an input interrupt today using a proxy, and my fluctuation is gone. However this was just to prove that scan time was indeed the problem. I would like not to have to use the interrupt and keep my control through the LVDT. So I'm interested to know what you think the average scan time should be for a program this size. Thank You plcdp

[Crossbow 296]

Scan time isn't dependent on the size of the program. It's dependent on the commands used. In the back of the programming manual is a list of commands and the time required to process them. 3K is a small program. FX3U holds up to 64k. FX2N went as high as 8K.

[panic mode 246]

actually one can turn entire sections of code on and off. just move pieces of code in subroutines and control calls (via pointer P).

[plcdp 11]

Where can I find out what size the program is? Does it say somewhere, or do I need to figure it out based on the list in the back of the programming manual? Thanks again... plcdp

[kaare_t 155]

It's not the size (steps) of the program, it's the commands/intructions you use, and you'll have to check them in the back of the manual. Basically, some instructions take up a lot more time than other. One example is TO/FROM instructions (which you probably use); they take up pretty much time in the FX-series..... And also note that the instruction takes up some time, and in addition you'll have to add an amount of time for each device (word) you transfer. Do you use GX Developer, or GX IEC Developer? -If you use GX IEC Developer you could control the different code sections by using Task-Interrupt.

[plcdp 11]

I am using GX Developer FX, as that is the only brand Mitsubishi PLC's we use. Thanks, plcdp

[Crossbow 296]

The size of the program is indicated in the title bar when in the ladder window. But as we mentioned, the commands used each take a particular amount of time.