Automation of a Coating Process

[eaglewood]

Hello people,

I've got what seems to me to be a pretty straightforward automation project I'd like to run past you, since it's my first motion control project.

It’s been my experience that there are very often surprises even in relatively simple projects such as this one. Sometimes those surprises can bite. I prefer to not get bit.

That said, here goes.

Automation of a Coating Process

Existing manual state of process:

Several chemical bath containers are aligned in a row on the ground floor. An operator picks up a basket of work pieces, lowers the basket into the first container, lets the pieces bathe. After a specified time, the operator lifts the basket and moves it to the next container. After this same basic operation happens several times with various durations of bathing, the coating process is complete.

This is a low-volume operation. Quality is more important than throughput speed. In fact, speed is not an issue at all. It could take all day to complete the coating process and the owner would be pleased. I say this because when I heard of the project, I immediately started thinking of various ways to maximize throughput. Not necessary. Not now, probably not ever. What matters is quality.

My plan is to have a monorail running above the bathing stations. An electric hoist with motorized trolly will replace the operator (who will be reassigned, not fired). The hoist will pick up a basket place it in the first bath, wait, raise the basket, move to the next station, bathe the work pieces, etc.

The work pieces, which are fragile, cannot be jarred. All stops, starts, lifting, and lowering must be executed smoothly. Also, the baskets must not swing and hit the sides of the containers, passersby noggins.

I’ve completed process control projects before---heaters, pumps, valves, etc.--- but I’ve never dealt with motion control. This seems like a very feasible starter project.

I am about ready to order a gantry crane with electric hoist and motorized trolley. It will come with a pendant for control, but I don’t want to use it. It seems feasible to me to chop off the pendant and run the wires to a control panel. I will program the motion of the hoist and trolley using mostly a PLC, but I figure I also should use a VFD to attain the required smooth operation.

Here are what seem to me the needed components other than the monorail crane, trolley, and hoist:

-Limit switches mounted somewhere on the I-beam to let the PLC know when the trolley has arrived at the drop target

-VFD to control motor speeds

-PLC, relays, push buttons, and other basic components I’ve worked with plenty

My tendency is to just get rolling on this. But I wanted to run it past any of you who may give a pointer or two.

Anyway, please let me know if you see anything that stands out as a possible issue. I’m working alone, and as I said, this will be my first motion control project.

Thanks in advance.

 

[Robb B]

Don't chop off the pendant cable, keep it connected, just parallel some wires back to your controller and tuck the pendant cable up in the rafters somewhere convenient. You'll want it later when something breaks and the customer reassigns the reassigned worker back to the quality production line.

I'm not sure motion control is absolutely necessary here, a ramped VFD with limit switches could probably substitute for the motion axis. Sort of a 6 of one half a dozen the other scenario, just throwing ideas out there.

 

[parky]

I agree with Rob, use VFD's I do suggest two limits on each station one for slowdown the other for stop (you can use alarm timers so if the trolley does not reach the slow limit within a set time alarm & gracefully stop same for the stop limit) done this on latex dipping machines as well as other dipping systems so pretty easy, if you use an HMI for operator control, you could have an engineering page for time setpoints for alarms, speeds etc. just remember to include dead end limits on the trolley on the infeed & outfeed better to be sure than ram the trolley into hard end stops.
Not sure of the complete process but think about if some products only need less dips so a recipe function might be in order, the recipes could contain things like dip tanks enabled or disabled, dip times probably not much else.

 

[arpus4KM]

Aside from the advice given, you should add an encoder with a rack and pinion track system to the gantry so the precise location of each bin can be determined, this is typically how crane encoder locations are kept due to crane wheels being metal and having a tendency to skid and slip which would throw off positions otherwise.

 

[eaglewood]

Thank you all for the advice. Just the sort of info I was looking for.

Not sure I'll go with the encoder, rack and pinion. Is that something I could add as a later improvement if slippage causes issues? Or is it going to be much harder to add on later and must be decided at the outset?

Maybe I'm mistaken, but I'm not too concerned with slippage with the low speeds and all the limits, along with the good advice about the alarm timeouts. It SEEMS that's sufficient.

Then again, learning encoders would be valuable experience. Is it the PLC that reads the data from the encoders? or is it something else?

 

[MikeyN]

We have a system similar to what Parky suggested. The only thing I would add to this is one more limit switch to be a home position for the hoist. Ours set the position of the hoist every time it reaches home so if a hoist loses its position because of a broken flag, the operator simply takes the hoist back home and the position is reset. The hoist can then be taken back to where it should be in cycle, set back to auto and the cycle takes off again. We indicate on our HMIs the current location of the hoists, and their destinations both with integers and pictures of the hoist above the tanks so the operators can fix these issues without our help.

Monitor the cycling of your limit switches so if a switch is hit out of sequence, the hoist knows that it missed something and needs to be reset. This should prevent the hoist from trying to skip a tank or worse, trying to drop product in between tanks because it thinks the slow down prox is the on-station prox.

 

[jaden]

How about the coating process chemicals? Any corrosive elements that might have fumes to affect limit switches, etc?
Cabling decisions: Do you need Festoons, cord reels, or ?

 

[parky]

I think the festoons etc. would already be in place, however, it might be productive to go through the system & check if these need replacing, it is common that after a long period of operation that core breakages can occur.
The home position limit is something that I assume you would already have catered for, as this is the pick up point for the components. so for example,
Let's assume 5 dip tanks, home position, offload position, left & right overtravel limits.
So assume slow & stop limits on all but the overtravel this requires a total of 16 limits, 2 for each dip tank (slow & stop), 2 for each Home & offload, 1 off for each overtravel.
One thing that we do not know is have the motors got brakes, this can become a problem with VFD's, I have seen where the brake is removed & use the VFD for braking, however, at low speeds probably not needed, VFD's are pretty good at slowing & position just a matter of getting the distance/ramp rate & speeds right between the slow position & stop.

 

[GaryS]

This would be a nice little interesting project to work on
Just for reference I have done more than a few projects like this
The fact that you haven’t ordered the hoist yet works in you favor. First when you order the hoist you want to use a “No Load Brake Hoist” to get the control you want. Please don’t confuse the load brake with the motor brake they are different.
Next I would not use any of the tank position switches they are expensive to install and a high maintence item that can give you nothing but headaches
I would use a laser measurement system for position feedback. That way when you are setting up the positions you just need to plug a number into the processor and you can adjust it on the fly when necessary.
As for the crane vfd’s I would use Magnetek crane drives. In this case I would use Flux vector drives on both the hoist and the trolley ( Noload brake hoist drive) these at designed for crane duty not just a label put on a standard drive, they do require an encoder on the motor but they will allow you to control the motor speed down to 0 including suspend the load on the motor and yes I have done this many times. Also don’t forget to order the correct braking resister for each drive to give you the needed braking to control the load.
Because the primary control of this system is from the plc I would swap out the pendent controls with a small radio control for those times when the operator or maintence needs to control the crane. That way the operator can have full control and still remain in a safe location. Each station would have a preset position you just need to tell the control what station to go to.
Sa for the hoist position hear again the hook position will be from the encoder feedback on the hoist motor. As not here with cable hoist’s the cable will stretch a little as time and load effect it so I have found I can correct for this by using the first hoist travel limit switch to reset the hoist position every time it is activated the cable drum on the hoist will give you a constant travel distance. You should also use the hoist travel limit to ensure that the hoist id fully raised before the trolley can move.
If this system were setup correctly I can see it running for many years with very little operator interaction
There other things to consider when doing a system like this you need to go over you requirements carefully

 

[Puddle]

The work pieces, which are fragile, cannot be jarred. All stops, starts, lifting, and lowering must be executed smoothly. Also, the baskets must not swing and hit the sides of the containers, passersby noggins.

That's going to make it tricky from a safety point of view. Are there light curtains in the area to keep the aforementioned noggins out of reach? And if an emergency stop is activated, will the system still perform a slow deceleration?

 

[cardosocea]

That's going to make it tricky from a safety point of view. Are there light curtains in the area to keep the aforementioned noggins out of reach? And if an emergency stop is activated, will the system still perform a slow deceleration?

I agree, this looks like a nice thing to automate until you reach the safety point of view and all the barriers needed in place.

 

[parky]

Don't think it's a problem safety comes first, a few broken parts is better than a dead employee. So, a risk assessment is a must, put in place all safeties, i.e. engineer out as much as can be practically done, if this proves inpracticable then procedures need to be written & staff trained to gain access. This goes without saying.

 

[cardosocea]

Definitely. I meant more around the cost (though I'm imagining the current setup) and managing the change in process with the operator.

 

[eaglewood]

How about the coating process chemicals? Any corrosive elements that might have fumes to affect limit switches, etc?
Cabling decisions: Do you need Festoons, cord reels, or ?

Thanks for the questions, Jaden.

A corrosive 50% Nitric acid bath will be present which will emit corrosive fumes. However, that bath is to be ventilated. The chemical cleaner Chemeon 4000 will also be present but it is only a mildly acidic liquid. No corrosive fumes expected.

Also, this line will be installed at my place of work, so I can monitor any unexpected corrosion and take action if necessary.

I will need festoons and/or cord reels, but I am just starting to look into those now. Not sure exactly what will be best. I'm also going to have to figure out how to mount the limits on the I-beam and trolley. These sort of details I have no experience with, but I'm pretty sure I can figure them out.

Any input/food for thought appreciated!

 

[eaglewood]

I think the festoons etc. would already be in place, however, it might be productive to go through the system & check if these need replacing, it is common that after a long period of operation that core breakages can occur.
The home position limit is something that I assume you would already have catered for, as this is the pick up point for the components. so for example,
Let's assume 5 dip tanks, home position, offload position, left & right overtravel limits.
So assume slow & stop limits on all but the overtravel this requires a total of 16 limits, 2 for each dip tank (slow & stop), 2 for each Home & offload, 1 off for each overtravel.
One thing that we do not know is have the motors got brakes, this can become a problem with VFD's, I have seen where the brake is removed & use the VFD for braking, however, at low speeds probably not needed, VFD's are pretty good at slowing & position just a matter of getting the distance/ramp rate & speeds right between the slow position & stop.

Parky, thanks for all the help. I have not purchased the hoist and trolley yet, so I can order the motors without breaks. Or at least the trolley motor. I'm not sure if it's a good idea to have no break on the hoist motor, though as I mentioned, loads will be light. (Less than 100lbs.)