====== Ramp with a constant slope ======

{{:en:mervis-ide:35-help:b84.png}}

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

B84_SLEW_RATE_LIMIT

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

<code st-iec>
PROGRAM TEST B84
   VAR
 REAL1,REAL2 : REAL;
 FB : LIB.CORE.V1_0.B84_SLEW_RATE_LIMIT;
 END_VAR
 FB(IN:=REAL1, TDOWN:=2000 ,TUP:=2000,OUT=>REAL2);    
END_PROGRAM
</code>

===== Library =====

LIB\CORE

===== Version =====

V1.0

===== Description =====

The block prevents the signal from rapid changes, the maximum slew rate is defined by the //tup// and //tdown// times which define the time for the signal to change from 0 to 100 and back. There are separate parameters for signal rising and falling.

===== Inputs =====

|Input|Type|Description                                  |Default value|
|IN   |REAL|Input signal                                 |0            |
|TDOWN|TIME|Falling time for change from 100 to 0, in ms.|30s          |
|TUP  |TIME|Rising time for change from 0 to 100, in ms  |30s          |

===== Outputs =====

|Output|Type|Description              |
|OUT   |REAL|Processed output signal  |
|SIGN  |BOOL|Sign of the output signal|

===== Function =====

The block converts the input //in// to output //out// and limits the maximum velocity of change of the output signal //out.// The slope is defined by parameters //Tdown// and //Tup//, separately for rising and falling signal. //Tdown// is time necessary for change of //y// from //100// to //0//, while //Tup// is travel time from //0// to //100//. When the input //in// is changing its value more slowly than this,  then //out=in//, otherwise the output follows the input with maximum speed of change defined by //Tdown// and //Tup//.

If //Tdown=0// then the falling signal is copied to the output immediately. If //Tup=0// then the rising signal is copied to the output immediately. 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:b84_1.png}}

**Figure 1**: B84 Situation for //Tup=3s//. The slew rate is const.

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

{{:en:mervis-ide:35-help:b84_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.