Examples

The following examples show different expressions that can be used in an Expression Tag.

Sum of two tags:

//Returns the sum of the value of tags a and b
$.output= $.input.a.value + $.input.b.value;

Set a value based on a condition:

 //If the value of a is above the threshold, it will return 1,
//otherwise it will return 0
if ($.input.a.value > threshold) $.output =1;
else $.output = 0;

Increment a global counter based on a condition:

//First, the global variable counter needs to exist. The counter //then needs to be incremented when the condition is set to true //(a.value must be even)
if ($.input.a.value % 2 === 0) counter++; 
//Return counter so that it is saved to the tag;
$.output=counter;

Get total RAM for this machine:

//os.totalmem() returns the total memory of the machine in
//bytes, divided twice by 1024 to obtain MB.
$.output = os.totalmem()/1024/1024;

Get free RAM for this machine:

//os.freemem() returns the free memory of the machine in
//bytes, divided twice by 1024 to obtain MB.
$.output = os.freemem()/1024/1024;

Get hostname:

//os.hostname() returns the hostname as a string
$.output = os.hostname();

Get uptime:

//os.uptime() returns the host machine in seconds as a number
$.output =  os.uptime();

Get memory usage of Derived Tags:

//process.memoryUsage() returns an object with memory 
//information. The rss property holds the total memory usage
//of the process (in this case DerivedTags), in bytes
$.output =  process.memoryUsage().rss/1024/1024;

Assigning a quality based on a condition:

//If the value of the tag is above 100, we set the quality
//to 16 (BAD_SENSOR_FAILURE)
if ($.input.a.value > 100) $.output = {value: -1, quality:16}
else $.output = {value: a.value, quality:192};

Redundant tags

In this example, the DerivedTags module is used to provide an output tag after evaluating the quality of two redundant tags coming from two redundant nodes linked to a central node.

Figure 12: Main source node linked to the central node

Figure 13: Redundant source node linked to the central node

Figure 14: Central node

• Step 1: Add a new module in the “Modules” section, give it a name, and select “DerivedTags” in the “Module type” field.

Figure 15: Adding module

• Step 2: Configure the Logger and API for the DerivedTags module. In this example, the default configuration settings have been left unedited, since (in most cases) this is a valid configuration option.
• Step 3: Configure the module. In this example, the default configuration settings have been left unedited.

Figure 16: Module configuration

• Step 4: Configure the tag Source section. In this example, Mode is set to Expression tag and Trigger is set to Periodic. The Aliases created are “Amplitude”, which has the Input tag /SITE01/LINE01/MACHINE01/AMPLITUDE of the main node, and “AmplitudeBis”, which has the Input tag /SITE01Bis/LINE01/MACHINE01/AMPLITUDE of the redundant node. Figure N°16 shows the JavaScript written in Expression.

Figure 17: Tag configuration

Figure 18: Java script Code

Result:

Figure 19: Redundant tag

Aggregated tag

In this example, the DerivedTags module will be used to output the result of an aggregation of a source tag in a tag value over a period of time.Figure 20: The tag “AP” is used for aggregation in this example

• Step 1: Add a new module in the “Modules” section, give it a name, and select “DerivedTags” in the “Module type” field.

Figure 21: Adding module

• Step 2: Configure the Logger and API for the DerivedTags module. In this example, the default configuration settings have been left unedited, since (in most cases) this is a valid configuration option.
• Step 3: Configure the module. In this example, the default configuration settings have been left unedited.

Figure 22: Module configuration

• Step 4: Configure the tag Source section. In this example, Mode is set on Aggregated tag, Aggregation method is set on Average, and the Source Tag is /000_PLANT/010_ZONE/010_MTR/AP.

Figure 23: Tag configuration

Result:Figure 24: Aggregated tag