Modifying Operator Properties

Every operator block has a list of user-definable properties. These properties determine the characteristics and behavior of each operator. For instance, the 'Initial State' property of the PID operator determines the startup mode---automatic or manual. The Moving Average operator has a property that sets the number of signal samples to consider in the computation of the mean. And, a property of the Multiplication operator establishes the number of input signals that the operator can process. The properties of every operator are described in detail in the Operator instruction bulletin (publication PDG-TR-2011/13).

Design Pad G4 organizes the properties of each operator object with individual property sheets (see Figure 2.2 for an example). The property sheet contains a description of the operator and a two-column table of object parameters. Each row in the table contains the name of the property (left column) and the corresponding text, numeric, or boolean value assigned to that property (right column). To view the property sheet of an object, activate the Selection Tool, position the cursor on the object, and double-click the left mouse button.

With this brief introduction to operator properties, you are ready to resume the process of creating the schema of Figure 2.1. Activate the property sheet for the Analog Input object (using the Selection Tool double-click the left mouse button on the object). Recalling that this schema was designed to regulate a thermal process, name the object 'SAM RTD' and set the scale property to "Hardware Defined". This scale property indicates that the input's signal range is defined by the hardware configuration. Section 4.10 describes how hardware resources are configured. See the Design Pad Operators Reference Manual (instruction bulletin PDG-TR-2011/13) for more information on Analog Input operator properties.

Continuing with the sample schema design, insert an Analog Constant operator from the Operators  Signal Reference menu, and position it just above and to the right of the Analog Input object. This Constant object will serve as the set-point signal for the PID Controller block. Connect the Constant object to the SP (set-point) input of the PID Controller block. Next, retrieve the property sheet for this object and change the operator name to "Set-Point" and value to 220. With this arrangement, the PID Controller block will generate a control signal aimed at maintaining the temperature at 220 degrees.

You are now prepared to complete the schema illustrated in Figure 2.1. Search the Operators menu for all of the function blocks and arrange them as shown in the figure. Double-click on each block to view its property sheet and become familiar with how you can customize the schema (refer to publication ) for a full description of each operator and associated properties). Make assumptions about the flow process you are controlling and adjust the operator properties accordingly.