Thread: Manual/Auto
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Old July 27th, 2002, 08:45 AM   #5
Terry Woods
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United States

Terry Woods is offline
 
Join Date: Apr 2002
Posts: 3,170
This can be a tough question to handle.

The correct answer is... "It depends".

I've always had an affinity for the idea that "STOP means STOP", and "MANUAL means MANUAL".

If you press "STOP" to stop something, wouldn't you expect that "something" to stop - NOW? (Don't get this confused with "STOP after 5-Sec Delay".)

You might have a control situation where you need to stop the system in a particular sequence with specific delays between steps.

Scenario:
Hopper-1 -> Auger -> Conveyor -> Air-Lock -> Blow-Pipe -> Hopper-2

The Auger moves material from the Hopper-1 to the Conveyor.
The Conveyor moves material to the Air-Lock.
The rotating door in the Air-Lock moves material into the Blow-Pipe.

A Blower, connected at the Air-Lock, moves material, UP, 60-Feet (at a 45-Degree angle), through the Blow-Pipe, to Hopper-2.

Now, let's assume that the system is running. There is material in the Hopper, Auger, Conveyor, Air-Lock and the Blow-Pipe.

What happens if "STOP means STOP" and you press the "STOP" button?

If the Blower were to turn OFF immediately, you could have a whole lot of material sliding back down the pipe toward the Air-Lock. The Blower probably will NOT be able to move that material on re-start! You would be plugged!

So, you need to have the system do an orderly shut-down.
Pressing "STOP" should...
  1. Turn OFF the Auger.
  2. After a delay (conveyor is empty), Turn OFF the Conveyor.
    (This assumes that material should not be left idle on the conveyor)
  3. After a delay (Air-Lock is empty), Turn OFF the Air-Lock.
  4. After a delay (Blow-Pipe is empty), Turn OFF the Blower.
When the system finally comes to a stop, it is empty! Having the system empty makes it much easier to re-start.

This sounds entirely reasonable - and so it is!

BUT, there is that nagging concern that "STOP" should mean STOP!

This could be accomplished by means of an E-STOP. But then, an E-STOP type stop might be too extreme if Life & Limb are NOT in danger.

To handler this, we use the concept of "MODES"; AUTO-MODE and MANUAL-MODE.

By system-definition, "STOP" in "AUTO-MODE" means...
"perform an orderly shut-down sequence on the system".

Under normal production conditions, the system should be running in "AUTO-MODE". This mode should also allow for any Automated Control Signals to have an influence on its operation... such as "STOP" from a remote location (the destination Hopper is Full).

Also, by system-definition, "START" in "MANUAL-MODE" means...
"START-NOW"!

and "STOP" in "MANUAL-MODE" means...
"STOP-NOW"!

"START" what? "STOP" what?

The nature of this particular system requires a start-up sequence and a shut-down sequence. You can not have a "START" button that simply turns everything ON at once. By the same token, you can't have a "STOP" button that turns everything OFF at once.

So... in this case, "MANUAL means MANUAL" means that the individual devices in the system are under separate control. There is an individual START/STOP control for...
  • the Auger,
  • the Conveyor,
  • the Air-Lock, and
  • the Blower
While in "MANUAL-MODE", each device is controlled by its own START/STOP controller (Start/Stop buttons, relays and Aux Contacts).

In actuality, I have both a Full AUTOMATIC Control and a Full MANUAL Control working side-by-side. Neither interferes with the other. But only one has control at any given time. The particular mode is activated by means of a Selector Switch (AUTO/MAN).

While the system is operating in AUTO-MODE, the Manual Control buttons have no effect on the system.

Likewise, while the system is in MANUAL-MODE, the Auto Control buttons have no effect on the system.

If, for any reason, the PLC were to get weird, the operator could switch over to MANUAL-MODE and perform all of the Automatic operations manually. The PLC would essentially be Off-Line.

MANUAL-MODE also provides specific control for particular devices for maintenance purposes. Sometimes, while troubleshooting, it's real nice to be able to exercise just a single part of a system.

So... that is my view of AUTO-MODE and MANUAL-MODE in this particular system. This system uses more hardware, but since the system is really kinda small, the benefits far out-weigh the expense. As systems get larger, the expenses for this scheme become more questionable.

Now, as far as switching from one mode to the other...

In this case, I use the following maxim...
"AUTO is AUTO" and "MANUAL is MANUAL" and never the twain shall meet!

This means, if the system is running in AUTO and the operator switches to MANUAL, he had better know what he is doing! The system will STOP-NOW!

If the system is running in MANUAL and the operator switches to AUTO, then the reaction of the system depends on the situation. If all devices are ON and all conditions are proper, then the system will simply switch over to AUTO-MODE without skipping a beat.

If the system needs to be synchronized with other systems, then it might be necessary for the system to be stopped and restarted using the Start-Up Sequence. If so, then the program would force a "STOP-NOW" followed by the "Start-Up Sequence"... depends.

If synchronization is not required, and any one of the "earlier" devices is OFF, then the system will turn OFF any ON-devices that are after the particular OFF-device. The system will then proceed with the AUTO-Start sequence from that point.

This system, as I have described it, exists on my site.

It's a matter of developing a reasonable and consistent design philosophy. Sometimes operators want to be able to do this or that at any time that they wish... you need to determine the benefit/risk ratio. If the ratio comes in at greater than 1, then there is probably reason to consider their wish - otherwise, don't do it!
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