Trend Monitoring for Motion Control - LS Electric XGB PLC from AutomationDirect

The XGB PLC system offers several ways to test and observe the motion functions being executed. One of the most useful is the trend monitoring feature. This feature allows you to trend many important attributes of the motion system over time. While trend monitoring is available in both XG5000 and XG-PM, when using pulse train motion control, it is most useful to trend in XG5000. We have the “XEM Motion Training” project file open in XG5000 and are online with the XEM CPU. This file is available to download from the XGB interactive guide at AutomationDirect.com and is designed to allow us to simulate a 3-axis motion system without having to have any physical hardware attached to the CPU. We have already powered up, turned the servos on (which are figurative in this example), and homed the system. We see these bits showing true since we are in monitoring mode. To start trend monitoring we can go to “Monitor>Trend Monitoring” or click on the icon of a graph in the tool bar. This opens the “Trend Monitoring” tab in the main project window. The tab has three areas for data monitoring. The upper-left corner displays the bit graph, which is useful for monitoring Boolean data from the project. The lower left displays the trend graph, which is useful for monitoring numerical data. The right side displays the XY graph, which can display two data points at the same time for comparison. Because the bit graph is straightforward, and the same principles apply to it as to the Trend and XY graphs we won’t cover it in detail. Our project has several motion profiles programmed in it. We will run the circular profile and observe the X and Y position data to see our movements as they execute. First, we need to add the data we want to monitor to the graphs. Let’s look at the trend graph first. Our program uses Axis 1 for the X axis, and Axis 2 for the Y axis. We don’t need to see the other graphs, so let’s right-click in the trend monitoring window and unselect “View Bit Graph,” and “Display XY Graph.” This unselects these two graphs and removes them from the window. We can right-click again and select “Trend Setting.” This will open the “Trend Setup” pop-up window. In this window we can add variables to the Bit Graph, Trend Graph, and XY Graph as well as modify the trend storage and refresh settings. As we look across the top of the window, we see the display and keep sample and time fields. These fields give us the ability to specify how many total data points we want to display at a given time and how many we want to have captured overall. If we change the sample value or time value, the other automatically updates to match. The “Frequency” drop-down lets us select how often we want the trend monitor to record a data point. 200ms is the highest frequency possible, and we have the option to wait as long as 5 seconds between data points on the low end. Let’s leave these values at the default. We will go to the “Trend Graph” tab and add the position system flags for Axis 1 and Axis 2. If we were using local program variables, we would need to select the program, and then double-click on the variable name to access the local variables. Since we are using a global flag variable, we can just double-click on the variable name, and it will open the “Select Variable” window. We can click the radio button for “Flag” and then select “Positioning” from the Flag View drop down menu. This will display all the positioning flags in the PLC. If we scroll down, we can find the flag variable “_POS_1_CurPos.” This variable will display the first axis’s current position in real time. We can select “OK” and then add the “_POS_2_CurPos” and select “OK” twice to add these variables to the trend graph. As soon as we add these variables, we see trending go active because we have the trend monitor window loaded with valid data and are already in monitor mode. To stop trending we need to go out of monitor mode. We will also need to reset the trend back to the default. To do this we can right-click anywhere in the trend monitoring window and select “Graph Initialization.” We see that the view has now been reset. If we go back into monitor mode, and trigger our circular interpolation move we can see the trend graph show the position values change as the circles are drawn. As we watch the two positions change, we see the lines following the changes, and the graph automatically scales the Y axis to display the data. Once the move has been completed, we can go out of monitoring mode, and then scroll through the moves and watch the two positions change relative to each other and time. If we want to see a specific value at a specific point, we need to use the cursor. We can right-click in the trend graph and select “View Cursor.” This will then allow us to see each value at any point by clicking. We can click and drag to see the variable values change at different times. The trend graph shows us each value versus time, but often we wish to see the actual pattern made by the changing values. We can use the XY graph to display the Axis 1 value versus the Axis 2 value. Let’s reset the trend monitoring, and then right-click and select the XY graph, and unselect the trend graph. We can add variables to the XY graph the same way we did with the trend graph. However, this time we will go to the “XY Graph” tab and add the same two flag variables we trended previously. If we restart monitoring, and trigger the circular interpolation again, we can watch the circles being drawn in the position data of Axis 1 and 2. This will give us a good idea of what the actual servos would do if we had a real motion system attached to this PLC. As we see here, the Trend Monitoring tool is a very useful way to monitor, troubleshoot and preview values, patterns, motion, and much more in the XEM CPU.