controling conveyer belt

[leitmotif]

Lancie

That is a neat setup. One question

On rung 5 you have a contact labeled S:4/9 "alternate slow"
Is the purpose of this to alternate the horn (1 sec on and one sec off?)
Never saw this - looks right handy - need to learn about it.

Looks like Rockwell Logix - is it? OK OK two questions

Dan Bentler

 

[Lancie1]

Dan,
Yes it is Rockwell Logix, and S:4 is the word location of the free-running clock, and you can use any of its bits for timing. S:4/4 is fast flash, S:4/7 is slower, and S:4/9 is more slow, and so on.

Terry, if you study my circuit more closely, you will see that you cannot reverse the motor without encountering the 10-second delay. This should be enough time for most conveyors to stop. The 10-second timer is used for TWO purposes in this logic. I always try to use each timer for a many functions as possible. If I can get two or more uses from one timer, I think that I am writing more efficient logic. Perhaps I should have labeled it "STARTUP WARNING & MOTOR REVERSING DELAY TIMER"?

 

[Werner]

I noticed that even with the interlock it is not safe. When the software activates both forward and revers in the same plc cycle. Both relais start pulling in. Resulting in an overload in the motor protection. The interlock works when one relay is already in (or fused in the on position) and the second relay is activated later.

 

[Lancie1]

Werner, what are you saying? That both forward and reverse starters can be activated at the same time in the above ladder logic? I don't see how, given that I have TWO sets of interlocks to prevent this, on rungs o & 1, and 3 & 4. Even if these fail, the Reversing motor starter (if it is a NEMA-rated starter) will also have an interlock that prevents both from being activated.

On rung 5 you have a contact labeled S:4/9 "alternate slow"
Is the purpose of this to alternate the horn (1 sec on and one sec off?)
Never saw this - looks right handy - need to learn about it.

This brings up an interesting topic: How do you handle audible alarms? One time long ago (before PLCs were in wide use), I was working in a chemical plant control room. The shift operator was complaining about an alarm horn that was going off on a nuisance trip. A control damper limit switch was stuck, and the horn kept sounding. The operator would push the Alarm Acknowledge button, the horn would quit for 15 seconds, then it would take off again, about 100 decibels. Finally the operator shouted, "It's either me or that horn! Either I am leaving or that horn is!" Then he picked up a hammer and knocked the horn off the wall.

Since that event, I have been cautious in the use of horns. There are many events that must have an alarm horn. When I use a horn, I never allow it to go continuously, except in a catastrophic event. I alternate, 1 second on, 3 seconds off. This is much easier on the nerves, and more acceptable to every operator that I have talked with.

Also, I use 4 alarm levels, unless otherwise required by the customer:

(1) Not serious, but you need to know about it eventually. Alarm is blinking light only.

(2) There is a problem and some action is required. Alarm is intermittant horn and light for 3 minutes, or until acknowledged, then blinking light until problem is resolved.

(3) There is a serious problem, and action is required immediately. Alarm is fast intermittant horn and light for 10 minutes, or until acknowledged, then blinking light until problem is resolved.

(4) This plant will explode soon, and you should run! Alarm is continuous horn until it blows. This level is rarely used. I seen it at the Eastman Composition 4 explosives plant up in Northeast Tennessee.

 

[leitmotif]

Please sir may I have more??

Lancie
Regarding

Dan,
Yes it is Rockwell Logix, and S:4 is the word location of the free-running clock, and you can use any of its bits for timing. S:4/4 is fast flash, S:4/7 is slower, and S:4/9 is more slow, and so on.

OK is this the 24 hr clock or some other function?? Is it common to all Allen Bradley units (Micrologix, PLC SLC etc) or just specific units?

Dan

 

[Lancie1]

Dan,

I just looked at a SLC 5/01, and it has the clock. Every PLC must have some type of clock to control the microprocessor. This is not the same "clock" that keeps up with current time and date. In RSLogix, go to "Project", "Controller", "Processor Status" (corrected by LM), then "Main. The second line shows the "Free-Running Clock, as Allen-Bradley calls it. It has 16 bits, the first rightmost 4 change very rapidly, the next 4 more slowly, and so on. You can also look in the S2 Data File, with Radix set to "Binary", and address S:4, to find this Time Base Word.

 

[leitmotif]

Lancie

Ok sounds like it is common to all units. Next Tue (class nite) will go looing for it and learn about it.

Thanks

Dan

 

[Lancie1]

Dan,

ERROR CORRECTION! I screwed up. The correct path to the RSLogix clock is: "Project", "Controller", "Processor Status", then "Main. I bet you would have found it anyway, though!

The S:4 bits are useful 1-bit timers for timing things that repeat on and off, aren't critical, or will not ever need adjusting from another device or HMI screen.

 

[Terry Woods]

Jeez...

Basic? Has anyone heard of Basics?

This is a simple issue.

It doesn't matter what PLC you are using... the Basics are the Basics!

When it comes to reversing a motor, the motor MUST be reasonably stopped before applying the reverse voltage. Why should you assume that there is a Mechanical Interlock, even if there is one? Instead, ASSUME that it takes time to stop the motor. Then, and only then, apply the reverse voltage!

If there is an Interlock, great! Even still, you should ASSUME that it takes at least some time for the motor to stop!

If you apply reversing voltage before the motor has a chance to stop then you will run into an over-load condition.

By doing so, you will eliminate the chance of Fwd/Rev at the same time.

Of course, by logicically inter-locking the Fwd/Rev signal you also prevent the Fwd/Rev from happening at the same time... however, is the motor stopped (or near stopped) when the opposite direction signal is applied?

 

[monkeyhead]

I noticed that even with the interlock it is not safe. When the software activates both forward and revers in the same plc cycle. Both relais start pulling in.

not in lancie's logic... if the motor was stopped and both buttons were turned on at exactly the same time, they would cancel each other out the cycle after his run forward and reverse bits were energized.

Of course, by logicically inter-locking the Fwd/Rev signal you also prevent the Fwd/Rev from happening at the same time... however, is the motor stopped (or near stopped) when the opposite direction signal is applied?

are you talking about lancie's logic here? he provides a ten second timer before the motor can start up and the only way to change directions is to stop the motor and then press the button for the other direction.

 

[Lancie1]

monkeyhead,
I believe Terry is pullling our legs here. He is not normally THAT dense...

I suppose it depends on which direction you think of the logic. You can put in a 10 second delay between a START command and the startup of any motor. That takes care of the coast-down time for any motor that is STOPPING during that 10-second period, and prevents the overload that Terry is so worried about. If it takes longer than 10 seconds for the conveyor to stop, then just increase the timer to whatever. BUT the above logic also takes care of the Startup Warning Delay. One timer, two functions. That is my approach.

Conversely, you can start a timer when a STOP command is given, then wait some time for a motor to stop before allowing a START. Then use a second 10-second timer to prevent the motors from starting until the warning horn is sounded, in case someone has got their hands in the conveyor gearbox. This seems inefficient to me, but then as the old folk saying goes, "different strokes for different folks".

 

[PhilipW]

Mind you if you go back to Post#2 I stated:

Both will work just fine, although I prefer Method 2 as it is a little more rigorous because there is no possible invalid state.

and you avoid the interlock issue altgether.

 

[Lancie1]

PhilipW,

Yes, you did a great job of defining the bit states. I believe that you are thinking about variable speed drives. For a VFD, your two bits, Drive On/Off and Drive Fwd/Rev, would certainly allow control of the drive. You would not even need a timer for most motor drives, as the reversing function automatically takes care of not applying reverse current until the back emf drops to a low level. However, Panidani's original question did not mention any type of drive. How would you translate your two bit states into ladder logic for a Forward/Reverse motor starter? In Post 4, Pandiani asked this:

1. How hardwired interlock is build in practice? (I'm still student and pretty much learn PLC programming by myself)
2. Should I use timer to make pause when reversing?

Unless Pandiani is using STL or some other assembler-type language, then we are on the right track when we explain how the ladder logic will look in practice, using typical start/stop pushbuttons, a forward/reverse motor starter, and a timer.

 

[PhilipW]

Is ok...I was just being a smarta$$.

 

[JesperMP]

..It shorted L1 to L3 and let the smoke out of a main fuse in our power distibution room. 1/3 of production was shut down for an hour co$ting thou$and$. We interlock in the PLC as a general practice, and electrically and mechanically by plant standard. It's cheap insurance.

Forgive me for being a little bit anal here.
But thats the wrong medicine you are taking !
You experienced a local shortcircuit phase-to-phase, and your shortcircuit protection failed to contain it to the locality where it occurred.
Adding mechanical interlocks will not remedy that for the next time it happens. Mechanical interlocks can help but not guarantee against shortcircuits.

In stead, you should check your overload/shortcircuit protection system with regards to selectivity and shortcircuit breaking capacity.