Sensor fault

gibson · Jan 30, 2013

Hello everyone,

I am using Siemens S7-1200 PLC for my application. In which i am using many Limit switches, IR sensors and Solenoid valves. If any of the sensor failed to work during Process then how can i detect which one of them is faulty?

Also i want to display Faulty sensor status on HMI.

david90 · Jan 30, 2013

gibson said:
Hello everyone,

I am using Siemens S7-1200 PLC for my application. In which i am using many Limit switches, IR sensors and Solenoid valves. If any of the sensor failed to work during Process then how can i detect which one of them is faulty? Also i want to display Faulty sensor status on HMI.[/quote

What's your application? From my experience, you can sometime do fault detection by using a timer. For example, let say your solenoid valve lets water flow through a pipe and you have a flow sensor. You can use a timer to allow the flow sensor an X number of second to turn on after you open your solenoid valve. You know there is a problem with your solenoid valve if you open it and then the timer times out without the flow sensor detecting any water flow.

adfox · Jan 30, 2013

Use multiple instance FB called in another FB... quick and easy.

Does sensor fault = Command and no feedback in time OR No Command and feedback ?

gibson · Jan 30, 2013

Thanks for your suggestions. But my query is how can I program my PLC to detect the error without getting any feedback? How can I do it with the help of timers?

adfox · Jan 30, 2013

Maybe like this ?

Anirban Hazra · Jan 30, 2013

gibson said:
How can I do it with the help of timers?

I think David90 has explained it perfectly well with an example how to detect sensor fault with Timers when a Process occurs in sequential steps and there are time gaps between those steps which are more or less within a known limit.But in case you can't figure out how to do the following part :

david90 said:
You can use a timer to allow the flow sensor an X number of second to turn on after you open your solenoid valve.

then here's a rough idea how to do it using David's example

Start the timer(retentive) when Solenoid Valve is opened
Set the preset value a bit greater than the maximum time estimated by you it should take the flow sensor to get activated after opening of the valve.
Make the flow sensor reset the timer when it turns on within stipulated time.
Make sure that when machine stop is pressed the timer gets suspended otherwise whenever machine is stopped due to some reason with a product between two consecutive sensors the fault timers will time out and set the fault bits erroneously.The timers should continue only when machine start is pressed again.
Use the DN bit of the timer to latch/set a bit(retentive) and this bit in turn can be used in HMI to indicate the fault
Use another bit to unlatch/reset the fault bit.In the HMI you can link this reset bit to a Button saying Fault Repaired.

Hope this extremely rough idea clears up your doubt.

PeterW · Jan 30, 2013

Are you talking about the limit switches on valves (open/closed) and what do the IR sensors do.

maybe helpful to give us a clue what the sensor is detecting and how.

James Mcquade · Jan 30, 2013

gibson,

break your task down into little pieces.

for example, you have an air cylinder moving a slide and a limit switch at each end. when you energize the valve to extend the slide, start a timer (3 sec for example), when the timer times out and you are still on the retracted limit switch, create a fault for that limit switch. if you are off the limit switch and have not made the extend limit switch, create a fault, the slide is hung or the limit switch is bad. if both limit switches are on, start a 1 sec timer and create a fault.

your faults can go into a file for messaging onto a display scroll type screen.

i think this should give you a good starting point.

i had a project that had over 300 such faults and i was over whelmed until i looked at it piece by piece.

regards,
james

Tharon · Jan 30, 2013

James Mcquade said:
I had a project that had over 300 such faults and I was over whelmed until I looked at it piece by piece.

How did it feel when you realized that the end users didn't even look at your well laid out, 300 alarms, and instead just called you for help when the machine wasn't working.

(Or am I the only one that has to deal with that?

)

James Mcquade · Jan 30, 2013

Tharon,

it was the oem's grafcet program that we couldn't work with.
when the machine quit, the display just said fault, no details.

i worked with the maintenance staff and we went over everything with a fine tooth comb.

295 hours of downtime was reduced to less than 60 hours in one year. we detailed everything. we even put i/o screens on the panelview.

everyone loved it, except the production workers.

regards,
james

Eric Nelson · Jan 30, 2013

James Mcquade said:
break your task down into little pieces.

for example, you have an air cylinder moving a slide and a limit switch at each end. when you energize the valve to extend the slide, start a timer (3 sec for example), when the timer times out and you are still on the retracted limit switch, create a fault for that limit switch. if you are off the limit switch and have not made the extend limit switch, create a fault, the slide is hung or the limit switch is bad. if both limit switches are on, start a 1 sec timer and create a fault.

Glad to know I'm not alone! There is no reason why a machine can't tell you exactly what the problem is. If you need to drag out a laptop to figure out why a machine doesn't work, then you're not using the PLC to your advantage.

I explained some of my methods in this thread...

http://www.plctalk.net/qanda/showthread.php?p=50235#post50235

-Eric

gibson · Jan 31, 2013

Thank you everyone for your responses :site:

My application is of Automated car parking system. We are using hydraulic pump to lift the car pallets and solenoid valves are provided to each pallet to control vertical movement of pallets. Limit switches are used to make sure that pallet has moved to its desired position or not. IR sensor are used at the Entrance of parking lot to detect any physical object presence during pallet movement operation for safety purpose.

Anirban Hazra · Jan 31, 2013

gibson said:
We are using hydraulic pump to lift the car pallets and solenoid valves are provided to each pallet to control vertical movement of pallets. Limit switches are used to make sure that pallet has moved to its desired position or not.

For the above part just replace "flow sensor" in my explanation with "limit switch"

gibson said:
IR sensor are used at the Entrance of parking lot to detect any physical object presence during pallet movement operation for safety purpose.

Since it is for safety purpose I would suggest using 2 IR Sensors to ensure that if one fails the other makes up for it.An even better thing to do would be ,in addition to the 2 IR sensors,to also place a Load cell or weight sensor under a platform in the approach path of the vehicles. This will ensure that even if the IR sensors fail to work properly due to some optical disturbance(sudden change in ambient light,etc.. ) in the environment or due to different reflective properties of surface of car bodies,etc.. the weight sensors will still give proper readings in those conditions and safety will not be jeopardized.

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