Analog treshold trigger is not able to react to the set value

Hello,

Anyone knows why analog treshold trigger is not able to react to the set value ( -120) ?

I have attached the diagram. In simulation mode- If I slide the analog input through the entire range, the trigger is not able to change to ''positive'' , even if the set threshold value has been exceeded.

It only change to positive if I change value by clicking through the Input range manually by mouse,  passing point -120. This way it works.

I am trying to built a FBD which monitors 0-4V output from frequency drive, and when a certain voltage is exceeded the latching relay will stay on until RESET.

Thanks, George

Ok, first off all -Thanks .

It works when I change the range as you said to:

ON: -200  

OFF:-120 

Every time I pull the input range slider from min to max, the blocks changes to positive. So my problem is solved.

The way I had it and I still do not understand why it does not work is:

ON: -121 and

OFF : -120

The block should generate a positive signal at the moment when I slide the input range  from max. to min.. Because I pass the ON point, therefore a short positive ''pulse''  which should activate the latching relay.

But it does not. Unless I manually change input value ( simulation ) by clicking  down on arrows which change the value of analog signal and pass the switching point that way.

Depend on the resolution and the roundings of the Logo it may be, after a value of -118 the next value is -122 or other direction.

Look at the help of the analog threshold trigger, there are two different ways to use it. If you take -120 for On and Off both, then with exceeding the trigger will go on and stay on, until the value get back to -120 or lower. This version is good enough for your RS block. If a value fluctuate, then you need also a hysteresis, e.g. switching on with -120 and switching off with -130 The difference is the hysteresis, but you need then On => Off.

I mostly use the analog differential trigger with On = my value and Delta = -10, where delta is the hysteresis. Then you only need one setpoint, because delta is a fix value in an application.

Regards, Scorp

Scorp,

That is a nice explanation, it is a new thing to me that such a narrow ON/OFF range ca be missed in the LOGO processing.  Thanks for the alternative suggestions. 

I will test both- differential trigger, and also threshold trigger set ON and OFF to the same value.. Will see what works best.

Thanks, George