MOA or HOA or Other

What Do You Call Them?

  • FOA (Force Off Auto)

    Votes: 0 0.0%
  • OOA (On Off Auto)

    Votes: 0 0.0%

  • Total voters
    42
I wouldnt say MOA or HOA represents a pump etc but rather the operating mode (state) of that device

I would agree MOA and HOA are interchangeable
 
I wouldnt say MOA or HOA represents a pump etc but rather the operating mode (state) of that device

I would agree MOA and HOA are interchangeable
Well, most if not all of the examples I have ever seen also incorporate alarming and feedback from that device if applicable for that device anyway. Like an air valve, is just on/off, but if it is an Electric Motor, a Metering pump, or even a linear slide, they will have inputs for device status. And use that for alarming and control.

Yeah, that is a lot more than the core function of an HOA, but the term was coined back in the days of relay logic. Automation has gotten a lot more complicated since then.
 
I chose "Other":
A-O-I. Device must stop when transitioning between manual and auto.
O-A-I. Device must not stop when transitioning between manual and auto.
With:
I = On
O = Off
A = Auto
 
Auto-PLC controlled based on process conditions (start/stop based on level etc)
Manual-PLC controlled by an operator from SCADA HMI. Interlocked to levels etc
Hand-Operator controlled from push buttons on MCC. No control available from PLC
 
Here’s the other part to the question:
Does Hand (Manual) have any safeties or interlocks involved? Or does it permit the output to operate no matter what?

I have seen this turn into some unfortunate outcomes. We had a customer who made it clear we were stupid for asking that question and made it clear that “MANUAL MEANS MANUAL, DAMMIT!!” among other insults......so that’s what he got. After operators manually ran 3 or 4 finished diesel engines off of conveyors onto the floor he reconsidered his choice.
 
For us "manual" means manual without process interlocks. There are interlocks within the device in questions own functioniality. Manual mode will not override a critical prox. But manual mode does not safeguard against something stupid on a higher level, such as running parts of a conveyor or overfilling a silo etc.

But then, manual start requires that a keyswitch in the machine is turned to enable manual start. And the key is only given to maintenance personnel of a certain acuity (i.e not stopid).

edit: Btw the stop button ("O") is always active. If pressed it will stop the device no matter what. If the device was running in auto mode and the local stop button is pressed, then the device stops and an alarm on the HMI is triggered.
 
Last edited:
It depends on the line/machine.

Sometimes Hand/O/A is appropriate, sometimes Jog/ or Manual/

Even have seen Inch/O/A for a stamping press. Another press had Jog/Setup/Auto (Setup = Single revolution, Auto = Continuous)
 
Here’s the other part to the question:
Does Hand (Manual) have any safeties or interlocks involved? Or does it permit the output to operate no matter what?
We had that dilemma in the past, too. We ended up having an "Interlocks Enabled/Disabled" button as an option for the manual mode. Sometimes you need to run some tests without the interlocks.
The Auto mode had all the interlocks enabled and more.
 
Well, most if not all of the examples I have ever seen also incorporate alarming and feedback from that device if applicable for that device anyway. Like an air valve, is just on/off, but if it is an Electric Motor, a Metering pump, or even a linear slide, they will have inputs for device status. And use that for alarming and control.

Yeah, that is a lot more than the core function of an HOA, but the term was coined back in the days of relay logic. Automation has gotten a lot more complicated since then.

Hi Colonel
What i meant was this is simply the operating mode (state) of the device. So the device will have other properties such as configuration, status, command and alarms. To me a HOA is simply the process used to select a single device / equipment module into a hand/ off/ auto state
 
Functionally, does anyone see a difference between HOA & MOA?

No. "Manual" literally means "by hand".

HOA is the most common template I've encountered for such a selector switch, but HOA can mean different things in different applications. I've seen systems in which a local HOA switch simply energizes one of two different PLC inputs to override interlocks in the PLC logic when switched to Manual mode. I've also seen systems that have separate, hardwired circuits when in Manual mode, and the PLC output doesn't even operate the motor starter unless the selector switch is in Auto mode.

As others have already mentioned, a 3 position selector switch can have momentary or maintained states in either position. So, it gets complicated.
 

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