Auto/Manual Selector switch... one input or two?

[pimpim32]

We are using 3 position spring return for manual - auto

Reason is because customers want to switch from manual to auto from 2-3 locations.
Auto and manual state are just latching till any condition goes off.

 

[averytc]

We use 3-position HOA switches with nothing fed back to the PLC. Auto allows PLC control, Hand engages the motor starter, and Off is off. I did one recent project where Hand & Auto fed back to inputs. The electricians disliked it so much; all new projects are being done the old way... Given the choice I would feed everything back to the PLC with spring return switches.

 

[PeterW]

We use 3-position HOA switches with nothing fed back to the PLC. Auto allows PLC control, Hand engages the motor starter, and Off is off. I did one recent project where Hand & Auto fed back to inputs. The electricians disliked it so much; all new projects are being done the old way... Given the choice I would feed everything back to the PLC with spring return switches.

I don't like that to be honest. How does the PLC know that it is in Auto? If you start a sequence and it is selected in auto, does it Alarm as it sees no movement/feedback??

If it sits in auto thinking its OK, what happens when you select Auto? Jump into life without a start??

No, I don't thinkl I would do that personally.

 

[rsdoran]

I was wondering the same thing? How do you have an HOA and no input to the PLC?

Y'all getting a little knee jerky with the SAFETY thing!

OFF is OFF, Manual is manual, and AUTO is automatic, never shall they meet.

The switches are for operator control, not maintenance. Manual should be there for the operator to "jog" products or "manually" adjust the line as needed...IT is not meant as a safety feature and the operators should be TAUGHT that.

Y'all get too KNEE JERKY with the SAFETY thing, an HOA should never be thought of as a safety issue, it is plain and simple machine control, nothing more.

IF and when maintenance occurs, this could be an operator clearing a jam etc, then all forces should be de-energized and locked out before accessing the machine. The "controls" have nothing to do with this.

DO NOT get SILLY and confuse control with access and safety.

TEACH, TEACH, ......do not talk about it, TEACH IT to those that NEED to know.

 

[averytc]

I don't like it either, but it's not my decision to make.

HOAs are only used on motors for pumps and things or that sort. Each device can be put in auto or manual on the HMI, but will only function if the HOA switch is in auto. If the sequence is started and feedback is not detected, the sequence is aborted or set to manual if applicable, and both the device and sequence alarm. Sequences will only start if all devices are in auto on the HMI.

 

[averytc]

I'm afraid I don't see the problem, maybe I've done a poor job explaining, but there is NO knee jerking here. The lack of an input only prevents the program from seeing the selection of the switch if in auto or off. If a sequence tries to start and a device fails, the sequence is stopped and will only restart from operator input. How is that unsafe?

 

[elevmike]

ON/OFF are diffent, (or should be), then AUTO/MANUAL. They are two different issues. Generally when the input is ON the machine is in AUTO. With the input OFF the machine is in MANUAL. That's my take on it..

 

[rsdoran]

avery, my post was not directed to you, just to anyone in general per se.

Norml, in this post http://www.plctalk.net/qanda/showpost.php?p=190163&postcount=15 actually said the same thing I did in a better way.

Lock out/tag out is the only dependable method of ensuring safety regardless of what the control system is doing.

An HOA or any selector switch used in that type capacity is machine control, nothing more, has nothing to do with safety. Whether the machine is in Manual, Auto, or OFF noone should ENTER the machine until it is de-energized and locked out, does not matter what kind of machine either.

THAT is the #1 SAFETY RULE.

 

[averytc]

I agree ON/OFF is different, and when something is on it is usually in Auto, but I don't follow Off being Manual?

 

[rsdoran]

I agree ON/OFF is different, and when something is on it is usually in Auto, but I don't follow Off being Manual?

I THINK, which in my old age is difficult, that the poster has a 2 position switch marked AUTO and MANUAL, off (in this case) is a separate thing...I ASSUME, off in this case, would be OFF whether AUTO or MANUAL.

 

[averytc]

RS, I agree that Lock Out/Tag and proper machine garding is the only way to insure safety.

 

[DJM]

This thread started off as a simple question regarding preferences of a single input for Auto/Man or one for each. Our machines have both, depending on what was designed into the machine at its inception. I prefer the single input for its simplicity. Our machines do not start automatically on the initial "control power on". The machine cycle has to be initiated by a "cycle start" pb. The manual portion is just that; we give the operator the ability to manually start and stop devices from the control panel.

 

[jstolaruk]

Slightly off-topic, but a couple of years ago I was working on some assembly presses for Delphi that were headed to Poland. The controls specification called out that in order for the machine to be in auto or manual, a selector switch had to positioned correctly AND that auto or manual had to be selected on the HMI. Both physical and HMI selections had to agree for the modes to be valid. And if they didn't agree the machine had to fault like as if something terrible happened.

 

[Jimmie_Ohio]

Hand - Off - Auto has been around for a long time for good reasons.

Auto = Allow automatic operation from PLC etc. Add a contact block to tell the PLC that AUTO mode is enabled. "Not In Auto" works fine and only one PLC input is needed.

Hand = Allow manual operation without PLC intervention. If the PLC fries, some functionality should be allowed for jogging, maintenance, setup etc. Having an input to the PLC that tells it Manual mode is enabled does not serve any purpose if you assume that the PLC is junk. Adding a contact block in Hand mode is handy for changing the speed reference for a drive - for example, changing speed control from the PLC to a speed pot.

Off = Off period and no machine movement should be expected. This is not the same as lock out/tag out.

Lock out/tag out is the only dependable method of ensuring safety regardless of what the control system is doing.

I couldn't agree more!

 

[rsdoran]

I agree the original post was about preference for an input, where a PLC is concerned a NOT input is just as good as a TRUE input. When discussing Auto or Manual does not matter if it sticks in one position or another, STOP should still STOP it. It definitely should be easy to understand if in manual and runs in auto where the first place to look would be.

My point was this is not a safety issue, it is a control perspective, and does not matter AS LONG AS YOU TEACH THE OPERATORS TO UNDERSTAND WHAT IS HAPPENING. TEACH TEACH TEACH, you went to school to learn, now pass some of it on.