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Need suggestions for a cycling life test

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Hello all.  We are conducting a life test and wanted to use Labview to cycle frypot burners and a water circuit.  We are at a snag on how to code the cycling and wanted some suggestions.  Here is our situation:

 

1.  Activate the burners to heat oil until 350°F.

2.  At 350°F, turn off the burners and activate the water circuit to cool down the oil.

3.  At 315°F, turn off the water and activate the burners again.

 

Steps 2 and 3 repeat indefinitely.

 

We thought we could use Case Statements to handle this, but we don't see how to handle step 2 if we use this method.  We thought of the following:

 

Case 1:  . . . 350 <this would activate the burners until 350°F>

Case 2: 315..350 <this would activate the water circuit>

Case 3?:  . . .315 <this case would be another burner activation to keep the burners going if the oil temperature gets below 315.  Would this even be needed?>

 

As you can see, we have an issue here in that the water circuit would turn on while the burners are going using the logic we have now.  The main problem is getting past the startup--heating from room temperature to 350.  If we were just looping between 315 and 350, we could work with this, but that initial heatup is confusing us as to what to do.

 

Is there another way to do this besides Case Statements?

 

Thanks.

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Accepted by Dhouston

This is begging for a state machine architecture.

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State Machine.  Yes, we have worked with that.  We will go that route.  Thanks.

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This is also begging for out of the loop overtemperature controls to disconnect the power mains to the heating elements in case the temperature exceeds a given safety sepoint. If the software controlling the heaters locks up the relay that supplies power to the heaters fails in the closed position things could get ugly depending on heater capacity.

 

-AK2DM

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"It’s the questions that drive us.”
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Yes, we have plans in place for overtemperature.  That is a crucial issue with all our life tests.  We plan to implement Watchdog to help with this too.  We have had way too many incidences of the controlling PCs going down and the Labview chassis locking closed.  Runaway heating is not fun.

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Definitely use external hardware controls like ak2dm said.  Watchdogs help as well.

 

For more certain reliability, you may want to run this on a cRIO instead of a PC.  Code based on the real-time controller of a RIO, or on the FPGA part should give you far more reliability than an ordinary PC.

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We do have cRIO, but we are still trying to learn it and get it to work.  FPGA coding is daunting.  We have  made some attempts at using cRIO and are having quite a bit of trouble with the coding.  Once we understand it better, we will definitely make use of it.

 

Well, we are working with Application Engineers on this, so hopefully we can make strides with it.

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