====== Ramp with a constant time of change ======

{{:en:mervis-ide:35-help:b85.png}}

===== Block name =====

B85_VARIABLESLEWRATELIMIT

===== ST call =====

<code st-iec>
PROGRAM TEST B85
   VAR
 REAL1,REAL2 : REAL;
 FB : LIB.CORE.V1_0.B85_VARIABLESLEWRATELIMIT;
   END_VAR
 FB(IN:=REAL1, TDOWN:=T#3S ,TUP:=T#3S,OUT=>REAL2);    
END_PROGRAM
</code>

===== Library =====

LIB\CORE

===== Version =====

V1.0

===== Description =====

The block prevents the signal from rapid changes. The output change is recalculated to travel over a constant time of signal rise (//Tup//)  or fall (//Tdown//).

===== Inputs =====

|Input|Type|Description |Default value|
|IN   |REAL|Input signal|0            |
|TDOWN|TIME|Fall time   |30s          |
|TUP  |TIME|Rise time   |30s          |

===== Outputs =====

|Output|Type|Description                      |
|OUT   |REAL|Filtered output signal           |
|BS    |BOOL|Sign of the output signal        |
|NBS   |BOOL|Negated sign of the output signal|

===== Function =====

The block converts the input //in// to the output //out// with time delay as follows: every falling change at //in// causes a linear decline //out=in// in time of //Tdown//. The same is for a rising signal at //in:// it causes linear rise of //y// so that //out=in// in //Tup// time, see Fig. below. Each signal travel direction is thus controlled separately.

If //Tdown=0// then any falling edge of //in// is transferred immediately to //out//, accordingly for the rising signal and //Tup=0//. At program start, the input //in// is copied to the output.

The output //sign// indicates the sign of the output signal //out//. If //y>= 0// then //sign=true//, otherwise //sign=false//.

===== Function in case of wrong setting =====

If //Tdown<= 0// then //Tdown=0//, if //Tup<= 0// then //Tup=0//.

{{:en:mervis-ide:35-help:b85_1.png}}

B85 Situation for //Tup=3s//. Note that the time of change is equal to //Tup// for any change of the input //in//.

===== Application example =====

{{:en:mervis-ide:35-help:b85_example.png}}

The block is used for slow opening of valves at steam heat exchangers. The falling time is 0 so that the valve shuts down immediatelly according to the PI controller, while the rising time may be several minutes for the valve to open slowly even if the controller gives fast output response. This is to prevent rapid changes of pressure and temperature in the exchanger piping.