New to PLC programming hope someone can help!

I am new to programming plcs and this is by far the most helpful site I have found online. I hope that you guys can understand what I'm asking and help me out! Or at least put me in a smarter direction.


I'm curious if there is a way I can have a timer start when a sensor is activated, that is not dependent on the sensor itself being on.

I have a motor stop when it's overheat sensor activates, it then turns on a timer so that it doesn't start up again for XX minutes. My issue is if it cools down before the XX minutes are up it stops the timer, and everything starts again. Is there a way I can have the sensor activate, start the timer, and than not turn back on until both the timer and sensor have turned off, without the timer shutting off due to the sensor turning off?

Sure, have the sensor latch an internal bit when it detects an overheat condition. Use the internal bit to enable the timer and have the timer's done bit unlatch the internal bit if the sensor has cooled down. Prevent the motor from running or starting while the internal bit is true. If you want, you can also delay enabling the timer until the sensor has cooled.

Welcome to the site
As you are trying to work this out I have a hint only for you

If you press a start button how do you keep a motor output on
that is your cryptic answer


either for PLC's or think of hard wiring

Just adding a little to what Ian has said, depending on the PLC type being used, there maybe some timer bits you can use. ie TIMER.DN or TIMER.TT

Mark.

A good question !

What you're describing is generally called "seal-in" logic. It applies to many of the elementary logic circuits used in industrial programming.

As my colleagues have described, in the ladder logic rung that starts the Cool Down Timer, you put a branch around the Overheat sensor input that starts the timer and put a contact onto that branch that represents the "Timer In Progress" status of the Timer.

That causes the timer to start when the motor overheats, and keep running until the timer is complete no matter what happens to the overheat sensor.

Use both the Overheat Timer input and the "Cool Down Timer" status bit to prevent starting of the motor.

What PLC platform are you using ? The "Timer In Progress" status bit syntax varies between PLC operating systems.

Wow that was awesome, I wasn't really expecting a response that fast.


After thinking about that, I think that's what I was trying before was just doing it incorrectly let me take a look at it try it and I'll get back to you guys.

Ian, your cryptic answer was exactly what I was thinking from the beginning, I was using, set and res, but I guess I just wasn't doing it right. Glad to know I was on the right track.

For the record I'm using a Keyence KV16AR, and their supplied ladder builder software.

To add some more confusion to this, I might suggest that you be careful in how you do this (I am NOT familiar with Keyence ladder builder software).

In my experience, operators are very resourceful and can figure out a way around anything that you didn't plan for. In the scheme of using a 'timer in progress bit' (after the sensor turns off), then a quick power down and back up of the PLC will render that unusable (at least in all the software that I've dealt with).

Sometimes in the plc world, there are bits that retain their state (unless you've programmed an initize routine on a first scan) after a cycle of power. Even though the timer won't conitnue timing, the "lockout portion" of the rung could possible remember that the last condition was an overheat and then restart the timer again.

Just something to consider.

the Set and reset will work
But not at the same time with the same device.

again in hardwire can you rely on a set reset off the same device?

Still Cryptic - sorry

Alright this is what I decided to do, I have the temp sensor come on and "Set" an internal bit 1010, once the sensor turns off, I have an nc input of the sensor, as well as the internal bit being on required to start a 15 minute timer. So, the timer won't start until the sensor has come on and then turned off. Which is ideally what I want so that there is never a time where the timer will end and the sensor will still be on. In the idea of the power cycling, if if turns on and off the temp sensor will stop the motor anyway because it's still overheated. If its during its timer cycle. Then it will just over heat faster and start the cycle again.

BTW guys sorry my response took so long. I had to finish up some other projects while at work and this took a backburner. You guys are really helpful.

If you are realy having a motor stop on overheating, then you might want to look at the entire process. I am assuming that you are not talking about the overloads in the starter circuit, but about the winding RTDs (or similar). If so, your motor life will be greatly affected if you are constantly overheating it. And since these are usually in larger motors, you might want to look at the reason why the motor is overheating. If you are talking about some process sensor that stops the motor, then that is different.

No! That would be bad! The motor is turning a gear that is turning a cam to fatigue a part, I don't want the cam overheating because its messing up the bearing thats aiding in fatigue. So thats what the sensor is checking for overheating.

Sorry for the confusion.

So is your sensor a temperature switch?

If it is a therocouple or something that the PLC can read the actual temperature, then just write some logic to have the temperature below a certain value before you allow the restart. That might be a better way as you can verify the process is ready to start again instead of below the overtemp limit for xx minutes.

Its basically like a relay, when the temperature goes above a certain threshold it gives a load, which is what I'm reading, I can't actually pull the temperature off it.