Erratic behaviour

the plc and the automation system consisting of motors and cylinders work fine most of the time. however, there's still erratic behaviour by the cylinders. when a certain action is to be done by the cylinder, the cylinder may not be actuated. why is this so? in addition, the sensor light on the smc d-a73 limit switch is not on even though it works perfectly. how can i make the light turn on?
regards

Ken,
You just asked the equivalent of "My car did not start this morning. What is wrong with it?" We need more information about the system. Most likely you would need to post the part of the program that is related to the problem.

As for the D-A73 limit switch you may need to simply reverse the wires to make the LED (light emitting Diode ) turn on. As for your other problems try checking for double coils, Check the voltage of the output when it turns on to make sure the contacts aren't welded. Your description is vague. You will need to explain more and maybe post your ladder for more help

Further questions

hi guys, i'm a newbie to all these and really appreciate all the help so far. however, some questions still linger in my mind. What does welding of contacts mean? what about double coil? what does it mean that the input or output is 1A @ 24VDC? does it mean the sensors and proximity switch used must be of 24VDC? must refer to the specs right? Sorry for asking lots of silly questions but think that's the way to learn..

Welding of contacts means like it sounds. The contacts of a device (limit switch, pushbutton, relay, etc.) literally melt (weld) together due to a current higher than it is rated for. The device will be stuck in the "ON" position.

Double coil- I am not sure, but in the other thread they were referring to using the same coil more than once in a program. This can cause unpredictable behavior. This term is a little confusing since many of us PLC people control pneumatic and hydraulic valves. In that world a double coil valve refers to one that can be pulled in two directions. One should never energize both at the same time as they are pulling against each other.

For you third question- Yes, they are the specs. If it says only 24VDC, that is what must be used. Some things will be rated for AC or DC but different current rating. The 1A in your example means that you should not exceed that rating.

Example do not put a solenoid rated at 1.5 amps on a 1A output.

Is this a real production system that you're working on?

Please don't take this the wrong way, but by the nature of the questions you're asking, I sincerely hope that there is someone else with more experience working with you.

There is nothing wrong with asking those questions, but you should already know the answers before tackling a control project on a production machine all by yourself. The consequences of guessing wrong could be damage to the machine, waste of a lot of material, or even somebody getting injured or killed.

Your problem could be the limit switches.
I have had a few similar problems in the past, and we tracked them down to this cause.
I found that these type of limit switch can be a bit difficult to set up, and are succeptable to bumps and vibration, and often work loose and flap around in the breeze giving erratic signals.
Of late, this has been quite annoying, so I tend to rip them out wherever I find them. I replace them with inductive proximity switches attached to the actual equipment being moved by the cylinder. This tends to be quite robust and also guards against linkages breaking or working loose.

Hope this helps,

Doug

PS, I agree with Steve Bailey, as a newbie, make sure you have someone experienced looking over your shoulder.