Introduction to Design Pad G4

Design Pad G4 is a graphical programming environment for Fairmount Automation Programmable Automation Controllers---including the Chameleon family of modular hybrid controller products, the FAC-2100 multi-loop digital process controllers, and programmable valve embedded controllers. Design Pad G4 makes programming control schemes for the target hardware as easy as connecting functional operator blocks together. Over 120 such operators are provided, including controller blocks, signal conditioning functions, comparison blocks, math operations, logic functions, data converters, diagnostic and data logging blocks, networking operators, and hardware components. The hardware blocks provide access to the available controller hardware, including a variety of analog I/O, digital I/O, displays, buttons, communication channels, timers, and built-in test information.

Design Pad G4 provides the control engineer with complete design flexibility. Each operator has various user-configurable properties that define its behavior. There are virtually no restrictions on how the functional operators can be interconnected. Design Pad G4 gives engineers the power to create their own operators and re-use them again and again. It also provides state-transition diagramming functionality to conveniently define machine behavior and implement automation sequences.

Figure 2.1. A typical Design Pad G4 "program": a schema diagram of a simple PID control strategy.

Perhaps the best way to illustrate the power and flexibility of Design Pad G4 is by example. A typical Design Pad "program" implementing a simple PID control strategy to regulate a thermal process is shown in Figure 2.1. In the control schema, a PID operator (the large block at the center of the figure) acts to maintain the process variable measured by the Analog Input block at a set point of 220 degrees. In the diagram, an Analog Input operator is connected to the Process Variable----input of the PID operator. Also connected to the PID operator are three Constant operators setting the PID operator's proportional gain (unitless), derivative time (in minutes), and integral time (in minutes) to 1.25, 0.0005, and 0.1, respectively.

The schema diagram of Figure 2.1 further specifies that the process value is to be displayed on the controller faceplate, as shown by the connection between the Analog Input operator and the Numeric Display operator. The boolean (blue) input of the Numeric Display operator determines if the value to be displayed should remain steady or flash on and off (flashing is a useful means to draw attention to a fault condition, for example). In the schema, the Flashing input of the Numeric Display block is connected to the output of a Self-Diagnostic operator (e.g., Built-In Test) that is configured to scan for an RTD measurement fault. If a fault (e.g., short-circuit) is detected, the Self-Diagnostic output will be HIGH and the Numeric Display will flash on and off.

The Analog Input operator is also connected to Greater-Than and Less-Than blocks that compare the process variable to alarm limits. The comparator blocks in turn feed LED blocks that indicate if the process value is within range (solid green) or out of bounds (flashing red). The output of each comparator block is connected directly to an LED block's Red (middle) and Flash (bottom) input pins, and through a NOT gate to an LED block's Green (top) input pin. For example, if the process measurement were less than 120 degrees, the Less-Than block would evaluate to TRUE and trigger its corresponding LED block to flash red, while the Greater-Than block would evaluate to FALSE and set its corresponding LED block to solid green.

The outputs of the comparator blocks are connected to an OR gate that in turn drives a Relay hardware operator. If the process is out of range, the relay activates. (This relay could be tied to an audible alarm that would alert plant personnel that unsafe operating conditions have been reached.)

The output M of the PID block (the control signal) is fed to a Numeric Display operator and an Analog Output with Feedback (AOwFB) hardware block. The AOwFB operator is associated with the physical hardware that is wired to the valve actuator used to regulate the physical process. In addition, the hardware is internally wired to feed the generated output signal back to itself. The AOwFB operator compares the desired output it is to generate to the feedback signal it receives; if it detects a sustained difference between the two, it declares an error condition (its error output goes HIGH). This is a useful arrangement for fault-detection.

The control schema in Figure 2.1 also includes three broadcast operators used to transmit various signals over a communications network. These values could be used by other networked controllers or displayed on a remote Human Machine Interface (HMI). In the schema, two digital broadcasters are used to signal I/O faults (from the Self Diagnostic operator and the Analog Output with Feedback operator), and one analog broadcaster is used to transmit the process variable (measured by the Analog Input block).

Figure 2.2. Property sheet for the Analog Output with Feedback operator of Figure 2.1.

All of the operators in the schema diagram of Figure 2.1 have user-definable properties. For example, Figure 2.2 illustrates the property sheet for the Analog Output with Feedback operator. It shows that the desired output must deviate from the feedback signal by more than 1 Volt for a period of 5 seconds for the error condition to be triggered. (These are defined by the "Max Allowable FB Deviation (volts)" property and the "Set Error Hysteresis Time (sec)" property.) If an error condition is indeed detected, the signals must be within 1V for 30 seconds in order for the error to be cleared (as specified by the "Clear Error Hysteresis Time (sec)" property). As shown in Figure 2.2, the schema also limits the output range to be within 0-100% (as defined by the "Aout Maximum Value" and "Aout Minimum Value" properties).

The preceding description of the schema shown in Figure 2.1 refers to hardware elements in generic terms. For instance, the narrative indicates that the temperature process variable measurement is provided by an Analog Input block. But it does not indicate which specific analog input hardware resource is tied to the generic Analog Input operator. In other words, it does not specify what the physical signal source for the Analog Input block is (i.e., which hardware module and input channel the corresponding sensor is physically wired to). The operator's property sheet, shown in Figure 2.3, does not provide a means to link the operator to a hardware resource. In fact, a schema document in and of itself provides no such mechanism. Instead, in Design Pad G4, the association between generic hardware operators included in a schema document and specific hardware resources found in a controller are maintained in a separate document called the Hardware Interface file. Segregating the control algorithm from hardware associations promotes program re-use, since the same schema document can be used again in again in a variety of hardware configurations. If the control algorithm must be revised, only one file needs to be changed: the single schema document.

Figure 2.3. Property sheet for the Analog Input operator of Figure 2.1.


Figure 2.4. Hardware Interface document associated with schema of Figure 2.1.


Figure 2.5. Schema document of Figure 2.1 linked to PCM-1 module in hardware interface document of Figure 2.4*.*

A sample hardware interface file which references the schema document of Figure 2.1 is shown in Figure 2.4. The figure shows a hardware configuration consisting of four Chameleon modules: an AC Power Module (ACP-1), a Process Control Module (PCM-1), a Discrete Automation Module (DAM-1), and a Network Interface Module (NIM-1). A different color is assigned to each module's title bar to easily identify associations between the module, schema documents, and hardware resources. Figure 2.5 shows the same schema document of Figure 2.1 now linked to the PCM-1 module. Note that the background color in the Figure 2.5 schema view matches the PCM-1 module's title bar color shown in Figure 2.4. Note also that in the Figure 2.1 view of the schema document, all of the hardware operators have a white background and a generic label, indicating that they have not been linked to any specific hardware resources. In the Figure 2.5 view of the same schema document, the hardware operators have now been linked to specific hardware resources. Those associations are reflected by labels that specify the hardware resource and by a background color that identifies which module provides that hardware resource. For example, the background color of the Relay operator is yellow to reflect that the hardware resource is physically housed in the DAM-1 module. In addition, the operator is now labeled "Rly1" to indicate that DAM-1 output channel 1 is linked to the operator. (The DAM-1 module has 8 relay outputs.). Similarly, the Analog Input operator is colored green and labeled "TOP" to indicate that it is linked the top numeric display of the PCM-1 module. The three broadcast operators in the schema have a pink background color to indicate that they are linked to the NIM-1 module; the signal values will be transmitted over the RS-485 network that the NIM-1 module is attached to. The examples cited above are of hardware resources that are only available in the selected modules. That is, the Analog Input operator could only be linked to the PCM-1 module since neither the DAM-1 nor the NIM-1 modules have analog input resources. Likewise, only the DAM-1 module provides relay outputs, and only the NIM-1 module provides networking connectivity. However, both the PCM-1 and DAM-1 modules provide LED indicator resources. In the schema of Figure 2.5, both LED operators have been linked to the PCM-1 module (as indicated by their green background), but they could have been linked to the DAM-1 LED resources.

Other than the ACP-1, all modules in Figure 2.4 are capable of executing the control schema of Figure 2.1. In this example, the schema document was linked to the PCM-1 module. But it could have been linked to either the DAM-1 or NIM-1 for execution in one of those modules.

The control schema shown in Figure 2.1 and corresponding hardware configuration shown in Figure 2.4 represent the first step in the controller programming process. The next step is to download the controller configuration into the device. Design Pad G4 has built-in functionality that provides two-way communication between a personal computer and Fairmount Automation's controller hardware. Once you have completed your schema design and hardware configuration, simply connect your computer to the controller. Chameleon products have a built-in infra-red interface for wireless line-of-sight connectivity with a PC's IRDA port. If the PC does not have an IRDA port, serial port to IRDA adapter cables are readily available. Since many modern PCs and laptops do not have serial ports or irda ports, Fairmount Automation recommends this USB to serial adapter: http://www.usconverters.com/index.php?main_page=product_info&cPath=67&products_id=290

Choose the Program Hardware item on the Communications menu to download the programming to your modules.