Turn the SAME motor On/Off from two seperate controls - Logic Help Needed?

I know this should be pretty simple, but I can't seem to get my head around it today. What I'm fixing to do is to be able to control a motor from two independent controls.

#1 HMI ON / OFF Pushbuttons on screen

#2 Field ON / OFF Switches.



These two controls the SAME motor. But I want it to be independent. In other words, Start-up should happen if either ON is pressed and should be stopped if either OFF is pressed. Here comes the part I can't seem to figure out. If #1 fails (i.e BAD HMI SCREEN), I still want to be able to start/stop the motor independent of the logic for HMI push buttons, and if the field switches fail, I want to be able to start/stop using the HMI PB's independent of the Field switch logic. In my logic above, If field switch OFF fails in a open condition, I will never be able to start the motor from my screen's ON Button.Can someone guide me in the right direction?

On a side note- Do I need to use one-shots [ONS] for my field switches?

MANY THANKS!

Does this do what you want?

Thanks Bob,

That might do the trick. But there's a chance if one of the XIC MOTOR FAIL latches fails in ON condition, it would prevent me from starting the motor. Is'nt it a possibilty? or am I way over thinking on this one?

Thanks again for your help.

I think this will work, however it will require that your HMI buttons be momentary and that both the field start and stop be normally open switches. If your HMI buttons are latched, you could add an unlatch for them in parallel with the one shotted outputs.

How about this?

HMI_Stop Field_Stop    HMI_Start          OL         Motor

--]/[------] [------+----] [-----+--------]/[---------( )----

                    |            |

                    | Field_Start|

                    +----] [-----+

                    |            |

                    |   Motor    |

                    +----] [-----+

This is an execercise that in my humble opinion will develop bad habits. IF the FIELD start switch fails to work then it would be ok to start from an HMI but if the STOP switch is bad it should never be allowed to start.

Here is a picture of code for what you want to do BUT I REITERATE it is a BAD way to build a system.



I would always use the Stop buttons in series with an alarm condition warning me if Field Stop was "BAD" then repair it immediately. You have to also worry about the FIELD Start button physically latching, that can cause problems too.

I REITERATE THIS IS NOT A GOOD METHOD.

This is pretty simple, really. And RS is absolutely right- for the love of god don't program this so that the field STOP switch is in parallel with anything. That is really unsafe and will get somebody hurt. Similarly, the field stop switch should always be NC; if you use an NO, and the wire is pulled off or the switch is broken, the motor can't be stopped either.

All switches, HMI and field, are momentary. If you use a selector switch in the field, then yeah you will not be able to override from the HMI. If you use maintained switches in the HMI, same reasoning. Plus, there's no need for maintained switches in any case for this.

Aside from that, ONS and latches and all that are only going to confuse somebody (you, maybe!) down the road. A motor start/stop is about the most basic circuit there is; don't make it any more complicated than you have to.

I know little but then again I know what I know.

I know that what Gerry offered is a proper method; albeit the OL may be part of the hardwired ckt and not in the logic. Paul T offered basically the same thing.
THERE ARE MANY WAYS to "skin a cat" per se, for some they have to see a wrong way before they fully grasp the proper ways.

AS I MENTIONED above, what I offered the OP will give him his desired results BUT is NOT A PROPER METHOD i.e. is something that should never be used in a REAL WORLD APLLICATION.

Think about the what if's.

Another thing to keep in mind when using a field stop button is what will happen if the wire from the stop pushbutton to PLC input card breaks or opens. If you are using the N.C. contact on the pushbutton and XIC (Examine if Closed) instruction in the PLC, the motor will stop if wire opens. But using the N.O. contact on stop pushbutton and the XIO instruction in PLC will prevent the motor from stopping if the wire to PLC input becomes open due to wire breaking or a blown fuse.

KIm

I personally dislike this method also. You always need to know the STOP ckt is complete, whether hardwired or an input to a PLC. The first time I ran into N.O. stop switches it freaked me out.

I would rather suffer from many inconvenient STOP's then from one situation WHERE A STOP BUTTON WILL NOT WORK WHEN NEEDED.

Oops! Sorry... drew the diagram right before a meeting. Yep... since you want the field switch in this case to be NC, it would require an XIC in the PLC ladder (not an XIO like I drew). The other way that it can be done is to buffer the switch input such that the buffered bit is flipped, then use the buffered bit as an XIO in the logic. That's in fact what I'd normally do; I almost always buffer IO unless it's a really simple system.

Thanks for catching that guys... did not intend to imply that the field STOP should be NO!

Good Place for a Hand-Off-Auto?

I prefer having a seperate HOA switch which will override the HMI and let you start it at the motor. This way the controls are separate, yet you have the capability to control from either location.

I agree that the field stop should be normally closed for all the reasons already stated. Gerry's diagram shows the way this type of motor control would normally be handled. I do not believe RSDoran's solution will work because both the HMI and field stop buttons would have to be pressed at the same time to stop the motor.

But, the original question was how to continue to operate the motor from one location if the hardware in the other location failed. So far, everyone has suggested the use of a normally closed stop button so that the motor will stop if the circuit is broken. Nobody has mentioned what happens if a normally closed start button gets shorted. It does happen. An when it does, the only way to stop the motor is to hold the stop button in. But at least you can stop it, which would not be possible with a broken stop circuit that was designed with a normally open switch.

The following logic will work even if one of the inputs, field or HMI, gets stuck on or off. You could use a normally closed stop button and replace the XIC in rung 3 with an XIO and the motor would stop once if the circuit were broken but it would not stop a second time if the motor were restarted.



Again, I agree that the best way is to use a normally closed stop switch in the field and repair it if it breaks. But the original question was about logic.

I'm going to chime in on this one also, Gerrys mehtod is the only one that should be used, the system should be corrected to be able to implement that solution.

lbeit the OL may be part of the hardwired ckt and not in the logic

Click to expand...

You could also do both here, have in logic and hardwired, that way, the control signal is turned off when the o/l occurs, and won't start when the o/l is reset.

It all depends on the type of application, but as has been mentioned, safety should be the priority, ALWAYS!

So far, I like Danny Michael's solution. Although, as usual, many valid issues have been addressed.

For the record, I think any logic with latches should be trashed. What happens if the power is cycled? If there is not a normally closed "control power on" bit in the unlatch rung, who knows what trouble will occur.

After all this discussion of "What if this contact welds?" or "What if this wire is shorted?" or "What if this wire is broken?" etc., etc., why not use the redundancy that is offered in a "safety relay" and use redundant contacts from each pushbutton?