Drives control theory. What do you think

OK I am new school guy. I like networking because we can get a lot of data from the drives.
So I have been talking to several guys about how they would do some simple drive controls. I have 2 drives that need to do some ratio control. A small mixing system 75% of this at 25% of that.

I can use discrete and/or analog, old school and reliable. Hopefully old relay Joe the maintenance guy can troubleshoot it. I loose a lot of diagnostics.

I can also use networking pick your poison (Modbus RTU, Ethernet IP, DeviceNet,etc....) Yes I am fully aware stop signals concerns over networked drives. 99% of the time my systems are using safe off drives so the process stop is over network but the safety stop is over hardwire. Almost impossible for old relay Joe to troubleshoot a bad drive. Process troubleshooting becomes so much easier because I can show him the exact fault and a possible reason for it.

So what is your preference and why.

I have zero concerns with control over the network.

Companies wouldn't release products like Safety over Ethernet/Ethercat/etc if they had qualms about it not working occasionally. That's liability and that's their a$$.

There are so many benefits to networking drives - time for Joe Relay to graduate to Joe Transistor.

Personally - If a machine can tell Old Relay Joe exactly where the problem is and how to fix it. then it is the best thing.

In saying that - OLD JOE wont UNDERSTAND the info as to how to fix it - more than likely he will take out the VSD and replace it with a contactor

I might add that @ 56 there aint many 'Old Joes' left.
it will generally be Building Contractor Fred

We're running some drives that we had to convert to hardwire control, because of response time issues, and all the other drive <> plc communication is still working. So you can probably have both if you feel it's worth the cost.

And my preference is something the end user can understand for the basic controls. Which is ladder logic and hardwired controls for the majority of end users.

I am new school also and very much prefer networking the drive signals. The drives with networking and built in STO have come down a lot and now I don't see much cost difference between having networking and not.

If I am doing a system that is start/stop and takes 2 preset speeds that never change then I will immediately change to network control. If you start thinking about adding an analog card it is, at least for my systems, almost always the same cost or cheaper to use networking.

I have gotten push back in the past from employers preferring analog signals and saying "nobody understands how to troubleshoot the communications." I think that argument is total BS when I hear it now because most places that say that don't put emphasis on troubleshooting skills, reading manuals, etc. These places typically don't have people that know how to troubleshoot the analog signals either, much less have the ability to read the drive diagnostic parameters, or looks up fault codes in the manual. What could be simpler for 1 solid light for cable connected to both ends, and 1 fast blinking light for communications active.

Anything you can do to reduce wiring, reduce programming by maintenance, change speeds without extra devices, and provide more layman's term information to the operators and maintenance guys the better.

Why not both?

I'm not saying that analog signals are better than comms, across the board. But the luster of communications over 485 (or ethernet) is dulled after your 12th install where you don't have authority to fix the comms wire routing or type(shielded vs unshielded).

And as the analog type are the lowest common denominator troubleshooting wise, nearly anyone can be walked through testing them out.

So I often preferred feedback over comms wire and control signals as analog.

The obvious advantage to a networked drive is the information, but the critical part here is presenting that information to relay Joe. If you're going to network a drive, I think you've pretty much committed to using an OIT. Otherwise, what's the point? All that extra information isn't doing anyone any good when it's hidden in the drive and you need a laptop with special programming software to get at it.

So you'll spend a lot of extra time on the front end with extra programming. A-B takes care of this a bit with their drive faceplates, so that's nice.

So I agree, using a networked drive is better, but ONLY if you invest the time and effort into making that information easily available to someone who doesn't have a laptop.

With ethernet-enabled drives, the drive manufacturer typically have a utility to setup or troubleshoot the drive that can be installed on a PC.
Have it installed ready to use on a HMI PC in the plant. So you dont necessarily have to make your homegrown user interface in your HMI.
The information you get from this utility is better for the maint, much better than a red blinking light.

Even better, with remote access to the site you can from your home office support your customer without having to travel to the site.

We use networked drives these days, and we started with making a user interface in our own hmi, but gave up on it because it is a huge task and the utility is free and better anyway. Also, if you change the drive for a different model, or there is a new generation drives you can start all over.

I run into this issue almost daily as people resist the movement toward networking of automation. But the reality is as Iant alluded to earlier; guys like Relay Joe will not know how to troubleshoot a VFD whether it is hard wired or networked, so he will likely attempt to fix an issue by replacing it, and then "smoke testing" the replacement, maybe more than once. Sure, he can troubleshoot the 2 wire or 3 wire run command if it is hard wired, but really, how many times is a problem with a drive going to be in that part of the circuit? If something stops working, the reasons are usually more complex than that, where network diagnostics and historical data will prove very useful.

Although still out there, "Relay Joes" are making themselves relevant only to cheapskate factories who think they can remain competitive by not spending any money in keeping up with the times. The factories are ultimately doomed to failure, at which time Relay Joe cannot find another job because he is useless to the modernized survivors. So designing to satisfy that guy who refuses to keep his skills relevant would be like car designers at the turn of the last century using leather reigns to control the cars because old people were used to a horse pulling their buggy and might not be able to understand a steering wheel, throttle or brake pedal.

We have moved exclusively to networked drives. The three big reasons for us are:

1) Build effort
2) Control availability
3) Command resolution

Being able to fully wire the control section of a drive by pressing and hearing the satisfying "click" of an RJ-45 connector beats the snot out of connecting 4 analog cables and 5 discrete conductors to get the drive to do just the minimum of what we want the drive it to.

In addition, you end up with access to a whole host of control-side information and commands that would be hard to get any other way. Need motor position? I can get that. Need the current value of the I^2t accumulator? Yep, I can get that. Want to be able to change the process loop or velocity loop PI parameters on the fly? No problemo. Please note, this is completely outside of the scope of diagnostics and troubleshooting.

This is the plc deciding what the drive needs to do to satisfy process conditions based on machine conditions.
And then there is command resolution. What good is it to purchase a drive with 0.001% steady state speed accuracy (or 1 part in 100000) when the best I can hope to do with analog commands is 1 part in 32767 assuming I REALLY babysit the physical wiring to keep noise out?

We have a reasonably sophisticated customer set so we generally don't need to worry about plant support people who don't understand device networking. If we were selling to one-man shows running machines out of their garages my position may be different. Then again, at that point I probably wouldn't need the benefits that device networking brings to the table so it may not be my first choice.

Keith

Sorry for the long delay. I was on the road today.

The diagnostics is why I usually go with comms.

I just wanted to spark a few ideas and see what others where doing. I believe in stimulating new ideas and thinking. I have the TV on alot when I want background noise. I don't watch it I just have it on. I do a lot of
As a few of you all know I an a small (2 man) system intergrator. So being small we don't have the 8, 10,20 or 100 other people in the office to bounce ideas off. That is where this site comes in. Thanks for all of your replies

Keith you know I cant even think about sophisticated*controls.

As a guy that has to maintain stuff long after you integrators are gone I prefer old school analog unless there is a big advantage to going on a bus for a few reasons.

1. Simple. There are a lot of old school Joes and they aren't stupid. The most intelligent electrical person I have ever worked with hated anything that went on a bus and I understand why. He can troubleshoot it with a meter and not a laptop for one.

2. Replacement. When a drive dies with analog control I can replace it with any drive. When a siemens masterdrive for instance on profibus dies I am kind of screwed. I need to find another drive that will run on profibus and I probably am going to need a siemens for everything to work. Meanwhile my machine is down and it is costing my company thousands of dollars waiting for parts and waiting for me to configure the bus. If it's analog I just run to stores and find another drive, anybodies drive, hook it up and go.

3. Obsolescence. Analog is never going away.

This is just my opinion as a guy who has had to fix stuff for 15 years now. I understand the allure of the latest bus for the guy that has to design it and with wiring costs it is definitely cheaper. But unless there is a really good reason to do otherwise I still insist on analog controls when specing a machine.

When you OEM guys are designing a machine please keep in mind that some poor ******* 10 years from now is going to have to figure out how to fix it, you may have moved on to another job and your company might have gone broke.

This week I am dealing with a masterdrive attached to a servo, we managed to cook both the drive and the motor. The dam thing doesn't need servo control to begin with, it could have been controlled with a contactor. So now my company is out 4K for a drive and probably 3k for a motor. The delivery for the motor is 6 weeks. Because the drive is on profibus it's not a simple process to put something else in. If it was at least an analog drive and a vector motor I could get it running fairly quickly.

Here is another example, I have a line that has "digital control". This is circa 1992. The drives have 8 bit 24V digital input cards in them for the speed reference. So there are 8 digital outputs from the PLC going to the drive. Some genius in germany around 1990 figured this was a good idea somehow!? The drives have been excellent but changing them over to something else is a hassle because I'm sure the same German Engineer decided this was a stupid idea and nobody does it anymore.

allscott said:

1. Simple. There are a lot of old school Joes and they aren't stupid. The most intelligent electrical person I have ever worked with hated anything that went on a bus and I understand why. He can troubleshoot it with a meter and not a laptop for one.

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How do you troubleshoot the problems with the VFD for all the problems that does not have to do with the analog or digital control signals ?

One thing, intermittent problems may be the hardest to troubleshoot. But the current VFDs help with that because they store the alarms in internal buffer. So you can investigate the faults even if they are not present at the moment, and even if the important fault is not the last one.

allscott said:

...

2. Replacement. When a drive dies with analog control I can replace it with any drive. When a siemens masterdrive for instance on profibus dies I am kind of screwed. I need to find another drive that will run on profibus and I probably am going to need a siemens for everything to work. Meanwhile my machine is down and it is costing my company thousands of dollars waiting for parts and waiting for me to configure the bus. If it's analog I just run to stores and find another drive, anybodies drive, hook it up and go.

3. Obsolescence. Analog is never going away.

This is just my opinion as a guy who has had to fix stuff for 15 years now. I understand the allure of the latest bus for the guy that has to design it and with wiring costs it is definitely cheaper. But unless there is a really good reason to do otherwise I still insist on analog controls when specing a machine.

Click to expand...

I agree with these two points. Simple and standardized interfacing that is guaranteed to be inter-brand compatible. This versatility alone just paid for the cost of all those analog points. That is the end of the argument.

Unless you work in a plant where downtime costs very little and you always have the exact spare drive you need, analog wins.


So, in my opinion, analog is superior, but network drives are cool for the lazy too. Why go get a megger and take apart a box when you can look at the HMI and read "VFD 424 Ground Fault"?

I like both. Use networking for the brains, feeding the laziness of the best maintenance tech while analog controls and spacious panel designs making him a super hero when upgrading when (not if) obsolescence starts creating threats to the temperature of his break room seat. If you have hardwired safety, start stop and speed control then do make sure the drive will run without a blip when you unplug the network cable or communications are lost and do accurately report communication status to the HMI. Do NOT just report "Drive fault"... I got a panel with 75 patch cables, ethernet switches, modern PC HMI and all I get is "drive fault". I could'a done that with one digital input and the fault relay...If you got it, use it to the fullest.

It's one or the other:

a) I'll just swap it out for ABB, give me an hour and we'll be running.

b) We're going to have to put an engineer on this for at least 2 days to be able to upgrade, and we need 4 hours downtime just to rearrange to panel.