ON/OFF with only 1 PLC input

Is there any safety issues to try to use only 1 input to turn ON and OFF a device?
If there is no problem, this will save the numbers of Inputs required in my project. What is the best way to write the code?
Thank you for the help.

mapizarro said:

Is there any safety issues to try to use only 1 input to turn ON and OFF a device?
If there is no problem, this will save the numbers of Inputs required in my project. What is the best way to write the code? Thank you for the help.

Click to expand...

Sorry crystal ball broke and ESP batteries wore out.
Have no idea of what the machine is
therefore
no idea of how to write code.

Tell us what the machine is and what it does and we may be able to help.

Dan Bentler

The project I have in mind is to control a few conveyors lines.
The operator will have a control panel with only 1 button for each conveyor. He will once to start and once again (the same buttom) to stop. Once the output is ON the conveyor will run.
As feedback, the operator will see a light and also he can visually see the actual belt running.
I have a Micrlogix 1000 and I want to maximize the use of the minimal numbers of inputs that I have.

mapizarro said:

The project I have in mind is to control a few conveyors lines.
The operator will have a control panel with only 1 button for each conveyor. He will once to start and once again (the same buttom) to stop. Once the output is ON the conveyor will run.
As feedback, the operator will see a light and also he can visually see the actual belt running.
I have a Micrlogix 1000 and I want to maximize the use of the minimal numbers of inputs that I have.

Click to expand...

OK so you want to use PLC to
control multiple conveyers
with on off switch at each conveyer

What HP are the motors driving conveyers
Why do you need to use PLC

Can you control each conveyer with suitably sized simple toggle switch ie up for on -- down for off?

Depending on your layout and what you are doing I would also consider E stops at each end and maybe in middle of conveyer.
Dan Bentler

mapizarro said:

Is there any safety issues to try to use only 1 input to turn ON and OFF a [conveyor]?

Click to expand...

Yes, probably. What happens if the conveyor needs to stop and the off button is pushed twice? In a panic situation operators tend to slap buttons several times. We cannot evaluate safety for you and the safety issues are your responsibility to properly engineer the system. This is not a time for taking short cuts - you could be legally and civilly liable.

What is the best way to write the code?

Click to expand...

What you are asking for is flip flop or toggle logic. To learn about flip flop logic use the forum search feature to look for threads on the subject, its a common topic here.

The only marginally safe way that I can see is this:

Any short momentary press of the button turns it OFF.

If the button is held for 2 - 3 seconds , then it turns ON.

This helps alleviate the "panic" press problem, but does not eliminate it.

To me, the small cost of an another pushbutton outweighs the potential costs of an accident/injury. Money talks, even with the moral aspects aside.

First of all, if there are any safety issues, this shouldn't be handled through a standard PLC in the first place... I would have an e-stop, that kills power to the outputs, and use Starr's implementation of 2-3 seconds to start, and momentary off... You will also want a latch so that when you transition from run to stop, holding down the button won't restart the conveyor after 2-3 seconds.

But it is still dependent on the machine... are these small conveyors or something that can carry a person into some horrible machine found in action movies, carrying the hero to his certain death.... what are the consequences if you tell the machine to stop, and it doesn't?

-MUR

The only good answer is: YES, there are ALWAYS safety issues. The thing to do is to evaluate the severity. With the info given here by the OP we cannot do this.

Kind regards,

Brijm said:

First of all, if there are any safety issues, this shouldn't be handled through a standard PLC in the first place... I would have an e-stop, that kills power to the outputs, and use Starr's implementation of 2-3 seconds to start, and momentary off... You will also want a latch so that when you transition from run to stop, holding down the button won't restart the conveyor after 2-3 seconds.

But it is still dependent on the machine... are these small conveyors or something that can carry a person into some horrible machine found in action movies, carrying the hero to his certain death.... what are the consequences if you tell the machine to stop, and it doesn't? -MUR

Click to expand...

This is one of many reasons I ask for description of machine.

Larger conveyers CAN carry you. There was a case in Oregon where a guy fell into a conveyer feeding a chipper using an airplane type propeller. Took both legs to the knees before people came to pull him out.
There was no safety type device to keep a person from getting conveyed in or some kind of emergency trip switch (this was back in 80's or so).

I have never liked the one shot single switch control. I prefer the green start and red stop switches. Most people recognize those in an instant. After reading this post where someone said people tend to press a switch repeatedly I can now REALLY see the danger. I will never install one.

E stops on larger equipment are VITAL and should be prominantly located. I also have seen most E stops capable of starting the machine - mostly in worksites where production is more important than safety. I dont like this and have argued against it but cannot overcome the "high cost" or the Production God is waiting arguments. Codes? they dont care about codes.

Dan Bentler

To use just one input from switch to start / stop equipment such as a conveyor I use a two position switch labelled STOP / RUN. This way the physical position of the switch makes it clear what is intended.

I use a "conventional" pseudo start button, pseudo stop button type logic with the normally closed stop latching in the run output. This way other interlocks can stop the conveyor. If this happens then when the interlock is cleared the operator must switch the switch to STOP and then back to RUN to restart the conveyor.

John Gaunts description describes how I would do this, it only uses one input.

You really need a separate hardwired emergency stop circuit, with emergency stop buttons at strategic points, or possibly even a pull cord along the full length of the conveyors. The auxillary contact on the E stop relay can then be used to signal the PLC to unlatch all the conveyors, so that each switch has to be toggled to stop and back to run to restart.

Again to repeat what has already been said here you need to assess the hazards and design an emergency stop system that is suitable. My description is just about the minimum I would consider in any application, if the hazards demand it further measures will be required.

(Note I have said 'hardwired', there are safety plc available, but I am assuming cost rules this out from your earlier comments)

If you use only one butten to start/stop a motor it could happen that you are not able to stop the motor because a wire is broken. To start the motor you should use a NO-contact, to stop the motor you should use a NC-contact.
So when a wire is broken at the start-button you are not able to start the motor and if a wire is broken at the stop-button the motor will be switched off automatically.