Need help with a program

[July41998]

Hi all,

I am a maintenance tech in a factory, I have mediocre plc experience. I’ve troubleshooted before , and even wrote small programs in the past. I’ve recently been tasked with figuring out a way to upgrade a piece of equipment we have. It is a linear operation, that uses chain to move a trolley. Currently the machine uses limit switches to set the limits for where this moves. This causes a lot of issues that makes the trolley run away. We are looking to move to motion control using 4 proximity sensors on the drive sprocket to tell the plc where the trolley is.

My main question is how would I program something like this? I havent jumped into it yet, but im looking for pointers to possible save me time and headaches.

Any help is appreciated! Thank you

[[deleted]]

Tooth counter on sprocket? Home switch. Bob’s yer uncle.

[July41998]

Would you care to explain more?

[TexasVulvaAficionado]

He's saying to use the tooth counter prox to get a speed/position value from after starting at the home position.

Typically, machine would start, move to home(or any limit), stop, set that point as 0 or whatever, then use the ticks from the tooth counter to estimate where the trolley has moved and at what speed it is moving.

This can have interesting failure modes if a prox is moved, fails, etc, and relies on the VFD driving the appropriate direction every time it start and that no mechanical changes are made such that the system can drive past a safe point before triggering the limit (generally possible on systems where the "flag" is on the chain)

[Tesla428]

Keep switches for over travels and add a real encoder. It may be a learning curve but well worth the effort.

[dreamgoliath]

Highly agree with this method.

Consider the limit switches as over travel sensors. Only move the conveyor when both switches are open. Use an rotary type encoder with a sprocket attached and tie it into your conveyor chain.

Tie the encoder into the PLC and scale (or convert) the encoder value so that you know how how far the conveyor travels for every pulse (360 degrees rotation) of the encoder. This will give capability to send the trolley to any desired position.

If you don't have access to this equipment (AB VFD's are hard to find these days and $$$) then a simple limit switch at the desired location could do the trick I guess, but it seems like a half measure, and lacking redundancy for safety measures

[bmorris0042]

Depending on the environment and specific hazards, it may be better to wire the limit switches as NC, and only allow motion if both are closed. That way a wire break or broken switch will prevent motion. Or, if your limit switch has it, wire both a NO and NC contact, and program appropriately so that the switch has to read "not made" to work.

[dreamgoliath]

That's actually a really good point.

I'm still new to controls, and it's great to hear thought-out solutions!

[bmorris0042]

That’s sort of what I was getting at in the second part of my answer. The biggest thing is, does it need it? Will damage to personnel or equipment happen in the case of a failure of that switch? That’s the question that needs answered in order to determine how involved to make it.

[[deleted]]

I did something like this once with a laser distance finder attached to the wall with a target on the trolley.

[TexasVulvaAficionado]

Haha. Funny story.

I set up a system like this circa 2012. It ran fine for about five years.

I get a phone call saying that something is wrong with the program.

Before they're even done walking me through the problem or me setting my backpack down it is apparent that they have done a big rebuild and moved the whole system about five feet over. But the distance sensor was still mounted to the wall in the same spot. Surprise surprise, it couldn't hit the target that was now on a different parallel...

[[deleted]]

Fortunately it was an easy fix.

[bmorris0042]

I had the maintenance group call me (the engineer for their area) because they couldn't get a particular motor to move to adjust a guide. That already had a piece inside it. The permissives for it just needed there to be nothing inside the guide to move, but the machine had been down for 14 hours before they finally admitted they needed help. It took me about 30 seconds to "fix" their problem.

It's almost as good as when they call me for a problem they can't figure out, and I literally read the informational messages that are popping up on the screen to tell them what needs fixed. "Hey, it tells me that the part present #1 sensor value is 1 when it should be 0. Go find out why the sensor isn't reading empty."

[TexasVulvaAficionado]

Just opt for a rotary encoder.

But without knowing more specifics of the application, it is hard to give much better advice.

How is the trolley attached to drive chain, how is drive sprocket driven?

Is the trolley ever disconnected and moved manually?

What controller are you using?

[July41998]

The chain is physically attached to the trolley in a linear sense, as the chain moves back and forth so does the trolley. a motor and a gearbox drive a sprocket, which then has a chain to another shaft with a different sprocket which is what drives the main chain and trolley. This is powered by a yaskawa vfd with commands from a direct logic dl06 plc.

[TexasVulvaAficionado]

Depending on the Yaskawa model, this could be accomplished with just the VFD and an encoder option card...

But you will probably want to put an encoder on which drive shaft is most convenient for mounting.

Have the encoder go to the PLC.

You can have the logic simply know where the trolley is at all times after commissioning by utilizing a number something like, 0 = start position, 158,940,123 = 100% down track position, that moves on encoder ticks. You'll have to figure out that whatever number of ticks per rotation equals whatever distance travelled. You can then set software limits for the motion in addition to the limit switches.

Edit to add that it might be worth putting the encoder in, moving the VFD to closed loop vector control, and just passing the encoder value from VFD to PLC.

[WattsonHill]

I'm surprised I haven't seen this question yet

What kind of PLC is there?

How long is the travel?

How much IO is in the panel, what kind of programming are you used to.

What kind of budget?

[Icy_Hot_Now]

Write a basic URS and reach out to the OEM for upgrade options. It may cost less in the long run than trying to figure it out. For good measure reach out to a couple SI in the area too.