Koyo question

Hello all,

We have a machine in the field that has had an interesting issue. It uses a Koyo D0-06DR. The issue is that at atleast one time the power in the facility flickered at a high rate and when this happens there a few limit switches that drop out and then come back on which inturn increments a counter. When the counters are incremented postions of flow heads are lost and cause major issues. I have tried several different progamming methods with no luck. If some on wants more info on the programing let me know and maybe we can solve it that way. But, one suggestion that was brought to me was using a capacitor so the PLC does not see the flicker. The PLC runs of 110V AC. Any and all help would be great.

Thanks,
Micheal

why not use a UPS on your system? This would at least keep your power on.

UPS is the easiest way.
Your 24 DC supply is dropping out before the PLC.
The PLC 'saw' the inputs flicker, so it was running.
A capacitor on the 24 DC line would work. To do it in software, add timers to all the related inputs. 500 ms or less should be fine, depending on your application. To fully buffer, you need 2 timers for each input. In this case, just a delay-off should suffice.

OK......so what is a UPS? If I use the cap on the 24V side what size would I use or what would the equation be to find the correct size.

thanks a mil guys
Micheal

UPS = Uninterpretable Power Supply

Here's a link to APC's line of industrial UPS systems:
http://www.apc.com/products/family/index.cfm?id=175

About the time I posted, I realized....DUH!!!!! It seems a cap for the 24V Input would work the best. I just need an idea of what size to hold the charge for maybe a second. I will also look into the logic side, it is in a medical device so the extra protection of both might be the best way to go.

Thanks again,
Micheal

marksji said:

UPS = Uninterpretable Power Supply

Click to expand...

Those are the foreign ones...

try "Uninterruptable"

since it is a piece of medical equipment, if it is critical equipment, then go with the UPS. If not critical, then use timers in the software & the cap.

try this link for info on the cap you may need...
http://www.faradnet.com/

Gerry said:

Those are the foreign ones...

try "Uninterruptable"

Click to expand...

Stupid spell check

I agree about the UPS and ...

You probably already thought of this, but as far as improving your code, can you use a debounce timer on each of the limit switches to filter out rapid cycling? I use debounce timers a lot with hard limit switches. It's quite common to see contact bounce as two off to on transitions (2 counts) instead of 1.

You need to calculate the drop from 24 volts to 20 or so - the dropout point of the sensors. Try 1000 to 2000 ufd to start. Then drop the power to the 24DC and watch for dropout. I doubt if you need a full second. I'm sure the PLC was dead before that. You just need to keep the sensors active longer than the PLC. Again, that's why a UPS is better. A UPS for this should be less that 100 bucks or so.
Switch-Mode power supplies detect shorts on the output and shutdown. If you get too big with your cap, it may shutdown during power-up. Then you would need a resistor+diode, or go with a linear supply.

I vote aganst the cap and for the UPS. Two reasons being is 1) when you really want the power off, the 24vdc will still be live for a shot time; and 2) the power bump may cause other problems that the UPS will help to resolve also. So the "professional" solutioin get the UPS. Just make sure you follow the installiation instructions. Exp about the part about wiring it for disconnect when you turn the power disconnect off.

BTW, it's not gennerally accepted practice to try to program your way around an issue such as power bumps. It's really better to cure the problem then just the symptoms.

I agree with elevmike and others about the UPS, but that's not enough. A serious problem exists with this system.
"When the counters are incremented positions of flow heads are lost and cause major issues".
It's assumed that the sensors die before the PLC. A UPS won't fix this. An EMO would still cause the same problem. If this is the case, then a cap or other solution is needed to make sure the PLC stops before the 24DC is lost. If a simple glitch on a sensor causes a major failure, then digital filtering is needed on the inputs.

I suspect the opposite. It's more likely the PLC is scanning before the sensors are stable. In this case, the PLC should delay startup until sensors are stable.

More work and testing is needed. Whatever the problem, it's going to take most, if not all of the solutions mentioned. A cap or UPS won't be enough.

The counters get out of sequence with a fault - power glitch in this case. With proper design, that should never happen. The machine should never be allowed to restart in an unknown condition.
The power failure and/or first scan bits should be used to reinitialize to a known condition.

Hardware solutions will only mask the lack of 'fault-tolerance' in the program.

I would presume an intentional power up or power down would be accounted for (should be anyway). In anycase the UPS should provide power for the entire panel (both the PLC and sensors).

The real problem is the power supply, & this should be the real focus of any problem solving. Kick it back to Sparkey.

My two cents, since I answer this question several times a week...

I go with Elevmike. A UPS is the proper solution if the power is indeed flickering. Assuming that this is the case, the PLC is being subjected to short term brownouts. This is always bad in that semiconductor devices do not perform well under brownout conditions. (True of any semiconductor device, not just PLCs.)

Brownouts and spikes will severely shorten the life of the device. Either fix the problem where the power is being interrupted or if that is not possible use a UPS to help out.