GX developer/GT designer 3: how do I limit a setpoint entered value?

We have a water scavenge pump. The pump is switched on/off by the operator entering high & low setpoints. Operator has brought to my attention that a low setpoint of zero could be entered (causing damage to the pump), or a high setpoint of e.g. 999mm ( which would cause damage to the test equipment).
Is there a way to limit this value to restrict the inputs to "safe" values, e.g. 250mm (low) and say 800mm (high)?
Wasn't sure if this needs doing in the code eg do a limit check, set a M-bit, then use the M-bit to display a message on the HMI...or do the check in GT designer on the relevant 'numerical input' dialogue.

Look at attached pictures.

All you do is click on input case then add with the + key a limit (this will probably default to 0 <= $W <= 100 change the values by pressing the EXP ley just type in the values & accept.
Note: on the runtime screen the keyboard displays the limits & is outside it will show an error message.

Brilliant, thanks chaps.
I will give it a try and report back.

Update... That works fine, thanks.

I just realised that the operator could set the low setpoint to a value greater than the high setpoint. So I need to add a further check in the high setpoint set-up somehow.

I'm assuming you have two set-points just set the low setpoint say between 30-50 & the high one between 50 & 100 or whatever your limits are.

parky said:

I'm assuming you have two set-points just set the low setpoint say between 30-50 & the high one between 50 & 100 or whatever your limits are.

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As pragmatic as ever, great idea. Thanks

It sounds like we have switched from operator-manual on/off overrides to operator-manual limit overrides. I was wondering if the project might end up there.

But what happens if the operator sets the override low limit equal to or above the override high limit?

You could leave the HMI alone, and instead set up, in the PLC logic, the Start/Stop circuit pattern (using Seal-ins or Set/Reset) to effectively ignore level limits that are not a safe range, but start and/or stop the pump only from legal operator-input override values. That way, you don't have to bother with logic in the HMI, which has more limited logic capabilities than the PLC. E.g. see below, which ensures the pump is always running above 800, always stopped below 240, and there are always at least 100 units of level difference between the effective high and low limits, whether they are set by either the default 800/240 or the operator overrides.

Mas01 said:

Operator has brought to my attention ...

<later post:> I just realised that the operator could ...

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It sounds like you have a well-run facility, with communication, and everybody, including operators, thinking "What if?"

drbitboy said:

It sounds like we have switched from operator-manual on/off overrides to operator-manual limit overrides. I was wondering if the project might end up there.

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My understanding is that the manual override is a temporary thing as we're still in the development stage of the project. I guess it makes the operator more comfortable having this option, even though in auto-mode the pump is behaving exactly as expected.

I ALWAYS put in the PLC a check of any new value entered to make sure it is within allowable limits even if a limit is set in the HMI.


I also NEVER have the HMI directly write to the checked value (IE- Minn_Level) - it writes to an Entry tag (Minn_Level_HMI_Entry) that the PLC then checks to see if it is within limits, uses the new value if OK or resets the new value to the old value after ignoring it.


There is a guy on 3rd shift we'll just call Bubba with a PanelView+ in his locker he can program in a couple minutes to change any value or toggle any bit that shouldn't be. I'm serious - actually had this in a shop. My way he would have to change the Minn_Level AND Minn_Level_HMI_Entry values on the same scan or the PLC would overwrite his entry. As far as toggling bits I created a "Back-Up" tag I latched or unlatched and if the bit changed without the PLC code it got changed right back, again he would have had to toggle both tags on the same scan.

I_Automation said:

I ALWAYS put in the PLC a check of any new value entered to make sure it is within allowable limits even if a limit is set in the HMI. ...

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+1

In addition to the safety issue, as noted earlier it is usually easier to have the PLC do consistency and sanity checking.

I_Automation said:

I ALWAYS put in the PLC a check of any new value entered to make sure it is within allowable limits even if a limit is set in the HMI.


I also NEVER have the HMI directly write to the checked value (IE- Minn_Level) - it writes to an Entry tag (Minn_Level_HMI_Entry) that the PLC then checks to see if it is within limits, uses the new value if OK or resets the new value to the old value after ignoring it.


There is a guy on 3rd shift we'll just call Bubba with a PanelView+ in his locker he can program in a couple minutes to change any value or toggle any bit that shouldn't be. I'm serious - actually had this in a shop. My way he would have to change the Minn_Level AND Minn_Level_HMI_Entry values on the same scan or the PLC would overwrite his entry. As far as toggling bits I created a "Back-Up" tag I latched or unlatched and if the bit changed without the PLC code it got changed right back, again he would have had to toggle both tags on the same scan.

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Interesting, thanks. I suppose this "belt and braces" approach is a good one.

Mas01 said:

Interesting, thanks. I suppose this "belt and braces" approach is a good one.

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YOU have to make your machines "Idiot-Proof"


If any operator figures out how to force or override something it is going to be ALL on YOU, not them.



Once I was told I was underestimating the quality of idiots HR was hiring.

Well, I've been using HMI's for over 30 years of all descriptions some of the early did not have limits so yes put checks in the PLC code, however, in all those years, any with limits never had a single one where you could put a value outside the limits or heard of anyone that has managed it as it will not write the variable if not within the limits, in saying that, I have seen where some registers got mangled data but this turned out to be from a Scada system that did not write to the registers used for limit set-points from an HMI indeed they were not even in the same area, only found out that it was the Scada coms that managed it due to the HMI die, the HMI was on a log delivery time (the system was run from a sub panel with buttons so did not need the HMI only for information & set-points were rarely change), in the meantime the problem occurred again without the HMI, turned out to be the Scada, never found the problem but it was modified by the supplier (not me) to be read data only as the other functionality was not required.