HMI vs. Pushbutton Control

Hello,

I was discussing some of the changes we're making to our conveyor system (for education) with one of my coworkers today. There are 5 stations, each with a separate PLC and HMI (and each station has an E-Stop button). Our plan was to use the HMI at each station as the sole means of control. My coworker was of the opinion that we need to have at least extra start/stop pushbuttons at each station for the case that an HMI loses communication with the PLC, so that the station can be stopped. I thought I had read somewhere that it is common practice to use a lifeline signal to the PLC from the HMI, which if it goes low could be used to trigger a fault in the program and stop the station safely. In the worst case, we could also stop via the E-stop button.

What is your opinion about the best and most common way to do this? Are hard pushbuttons really necessary? Seems like old school thinking to me, but I may be totally off base, so would really appreciate any input. Thanks.

For that application I would say that a physical E-Stop that stops all motion, located at each station, would be sufficient. That way you could have either one E-Stop loop throughout the system, or separate E-Stop circuits for each station so you could report which station has the E-Stop tripped.

Each physical button would be an added expense [button(s), enclosure, wiring, conduit, terminals, schematic pages, etc.] and a few years down the road when troubleshooting & repairs come into play, less physical items are easier to trace.

If you need to operate the controls very frequently, you may be better served by real pushbuttons. Real pushbuttons will wear out but be simpler to replace and less costly than a touch screen with a spot worn out on it's screen. They also will be more responsive for things like jog or inching types of controls. There is no reason you cannot duplicate the hardwired buttons on the HMI so that you have a backup method of control if a button gets worn out.

As for the rope safety or cable operated "lifeline" switches, they should be part of the e-stop circuit and operate independent of the PLC. It is common practice to monitor the auxiliary contacts from them or from the safety relay with a PLC.

Thanks for the responses.

The E-Stop setup has been described on this board, basically it's chained through all the stations and will shutdown the entire system if any E-stop is pressed.

The question was more specifically related to the HMI losing communication though. I see the point about an HMI getting worn down, that makes a lot of sense. But what about the case where the equipment runs automatically and generally doesnt need to be constantly started and stopped? My comment about the HMI lifeline was referring not to the e-stop setup but to a constantly high signal from the HMI to the PLC in the software that lets the PLC know that the HMI is still hooked up. But maybe there is aready some built in plc function to check communications (siemens s7-1500).

With regard to loss of hmi comms, do you see the need for redundant pushbuttons? Is this a common industry practice?

None on last three machines

Jieve said:

Thanks for the responses.

The E-Stop setup has been described on this board, basically it's chained through all the stations and will shutdown the entire system if any E-stop is pressed.

The question was more specifically related to the HMI losing communication though. I see the point about an HMI getting worn down, that makes a lot of sense. But what about the case where the equipment runs automatically and generally doesnt need to be constantly started and stopped? My comment about the HMI lifeline was referring not to the e-stop setup but to a constantly high signal from the HMI to the PLC in the software that lets the PLC know that the HMI is still hooked up. But maybe there is aready some built in plc function to check communications (siemens s7-1500).

With regard to loss of hmi comms, do you see the need for redundant pushbuttons? Is this a common industry practice?

Click to expand...

The last 3 machines we got in have just hardwired stops no stop commands on HMI's. Yes hankshaking works to shut a whole or part of a system down if HMI and PLC lose comms. I have always liked hard wired stops: starts, stops, selectors resets, can be on the HMI, but hardwired stops are very nice and safe that way someone does not have to change screens to get to a stop button.

Jieve said:

Thanks for the responses.

The E-Stop setup has been described on this board, basically it's chained through all the stations and will shutdown the entire system if any E-stop is pressed.

The question was more specifically related to the HMI losing communication though. I see the point about an HMI getting worn down, that makes a lot of sense. But what about the case where the equipment runs automatically and generally doesnt need to be constantly started and stopped? My comment about the HMI lifeline was referring not to the e-stop setup but to a constantly high signal from the HMI to the PLC in the software that lets the PLC know that the HMI is still hooked up. But maybe there is aready some built in plc function to check communications (siemens s7-1500).

With regard to loss of hmi comms, do you see the need for redundant pushbuttons? Is this a common industry practice?

Click to expand...

If you are using a Siemens panel as well, take a look at the Coordination Area Pointer. It tells the PLC what the status of the HMI is, including a 1hz heartbeat bit.

Are the operators pushing the start and stop buttons dozens of times a day or just a few? If the former, then you're going to want push buttons. They hold up to repeated, heavy use much butter than HMIs do. I've seen OEMs who will use an opto-touch button in applications where the operator is using it for every machine cycle. No moving parts so it will last a very long time.

If this is a situation where the operators are just turning on the conveyors at the beginning of the shift, wear and tear wouldn't be so much of a concern, then. But buttons that are used often should be physical push buttons.

If that's not a concern, then I would say there's no need for physical start and stop buttons. You could set up a watchdog/handshake type setup depending on your HMI.

For you people who have problems with worn out touch screens you are still in the dark ages.
The latest touch screens are much better built and have the same touch screen technology as your smart phone.
If yours don't check out some that do, like the QuickPanel+

I agree that the only 'hard' buttons needed are the E-Stops.

RussB said:

For you people who have problems with worn out touch screens you are still in the dark ages.
The latest touch screens are much better built and have the same touch screen technology as your smart phone.
If yours don't check out some that do, like the QuickPanel+

I agree that the only 'hard' buttons needed are the E-Stops.

Click to expand...

I'll second this comment. HMI input failure is only a problem with hard keys around the screen. I've never seen a modern touchscreen fail because it was worn out from too many touches. They break eventually, sure, but I've never heard of it being because the touch sensor wore out from too many touches. The HMI is probably less likely to fail than the remote IO connected to the push buttons.

The main situation I've seen where touchscreens are a bad idea is in really messy environments. If metal cutting fluids or oil are going everywhere, then a touchscreen might not be able to tell what is a touch and what is oil. Also, some types of screens have very specific expectations on what counts as a finger, and gloves often don't qualify.

Thanks everyone for the responses.

If you are using a Siemens panel as well, take a look at the Coordination Area Pointer. It tells the PLC what the status of the HMI is, including a 1hz heartbeat bit.

Click to expand...

Heartbeat, lifeline - I knew it was something medical. The panels and PLCs are Siemens, I'll check that out.

mk42 said:

I'll second this comment. HMI input failure is only a problem with hard keys around the screen. I've never seen a modern touchscreen fail because it was worn out from too many touches. They break eventually, sure, but I've never heard of it being because the touch sensor wore out from too many touches. The HMI is probably less likely to fail than the remote IO connected to the push buttons.

The main situation I've seen where touchscreens are a bad idea is in really messy environments. If metal cutting fluids or oil are going everywhere, then a touchscreen might not be able to tell what is a touch and what is oil. Also, some types of screens have very specific expectations on what counts as a finger, and gloves often don't qualify.

Click to expand...

It all depends where these are installed. Put them in a clean room and they'll last a long time. Put them in a concrete plant, or some other high abrasive location where the operators are using gloves covered in grit and they will wear out. Though in some of those cases it is probably the outside edge of the screen getting too scuffed up to read clearly through, and the touch elements still being okay...

Jieve said:

Thanks everyone for the responses.



Heartbeat, lifeline - I knew it was something medical. The panels and PLCs are Siemens, I'll check that out.

Click to expand...

You did say lifeline from HMI, so I should not have assumed a rope safety.

Yeah, I wouldn't worry about touches wearing out the screen. Sand, grit, metal & glass dust are far worse in wearing out the screen.

You can check the specs(user manual); most touch screens are rated for 1 million touches. Those are the screens with a "plastic" resistive touch screen.

The GE QuickPanel+ has a glass touch screen surface; like your smart phone. Those are rated for a lot more touches. 50 million?

arlenjacobs said:

...Sand, grit, metal & glass dust are far worse in wearing out the screen..

Click to expand...

I had to replace a 2 month old Panelview Plus on a feedline that failed. The OEM wanted the unit back to find out why it failed. Seems at the factory they cut the floor for a pit for a new machine & the shards of concrete dust got pressed into the screen & ended up shorting out the touchscreen.

NASA was considering putting a touchscreen HMI on a new version of a lunar rover until they decided the moon dust was just as abrasive. (Until they were told not to come back to the moon)