Weirdest thing...

[plcdp 11]

Hello, I just had something strange happen that I was hoping someone could give me an explination on. I have a Q series PLC controlling a plastic molding machine. I have a Q62DAN using a 0-10 volt output to a proportional valve controlling a clamp. This machine has been running without modifications for 2+ years. This morning the clamp speed became very inconsistant. The maintenance staff got me involved after swapping out the valve. I measured the output voltage and found it to be inconsistant proportional to the speed the clamp was moving. I replaced the analog output card and got the same result. I hooked up the laptop and monitored while opening and closing the clamp. I am using the RAMP command on both the open and close side of the clamp. What I saw was the ramp not fully ramping every time. If, for instance, it was ramping from 0 to 2000, sometimes it would undershoot the full ramp. Originally in the code the ramp was set to happen over 150 scans. (Machine scan time is 1ms). I changed this to 50 scans and everything started working??? The ramp on the open side is also set to 150 scans, and it is working fine. The total time it takes for the clamp to go from full open to full close is upwards of 2 seconds. I don't see how the change I made could make any difference. Any insight would be appreciated. -Dave

[plcdp 11]

Bueller?.......Bueller?....................Bueller?

[kaare_t 155]

I would guess it's a machine/hardware fault. Probably by some wear and tear of the moulding machine. It will not help to replace the valve if the cylinder/actuator is not moving free like it should. Probably the cylinder/actuator is not moving freely on the "in-process" but moves smoothly on the "out-process".... I do agree with you on your last statement but I would definetively start looking for machine hardware problems. The mechanics like to blame the PLC and saying that "something strange" has happened to the PLC and the program is suddenly doing something different, but as we all know; the program either works or don't work - and they NEVER "suddenly change"... and definetively not after 2 years.... I would start by checking the machine itself, and it's hardware.

[plcdp 11]

I was thinking the same thing originally, but could a mechanical problem stop the ramp function from doing its thing inside the PLC? I would think, regardless of mechanical resistance, if I tell the PLC to ramp from 0 to 2000 over 150 scans, it would do just that even if the outcome on the mechanical wasn't correct. Am I wrong on this assumption? -Dave

[kaare_t 155]

I might have misunderstood your explanation, when you say that it undershoots the ramp, do you mean it undershoots mechanical, or in the program? Also: Do you have any endpoint sensors or similar that causes the program to stop or interrupt the ramping function?

[plcdp 11]

I mean that it undershoots the ramp in the PLC program, and subsequently, mechanically. It seems that mechanically it is doing exactly what it is being told to do by the PLC. No pointers or interrupts in the code either. The machine has been fine since I dropped the number of scans from 150 to 50. I have identical machines running with this program still set to ramp for 150 scans that work fine. -Dave

[kaare_t 155]

Well, if you have an identical machine, and the machine in question have worked over a period of time it is surely a mechanical problem somewhere. PLC programs don't suddenly change so there has to be something external on the machine...

[Matt 0]

Have you any comms going on?