Control Loop Page

To access the Control Loop Page:
In the Project pane, expand the Modules folder, double-click the CPU module and click Control Loop.

Use this page to select a control loop time for the RMC and to monitor the loop time usage.

Selected Control Loop

Set this value to the desired control loop time. This is the time the controller takes to execute one loop of internal code. It is the rate at which the RMC can respond to inputs. This value can be set to any of the options in the Loop Time box. However, the RMC may not be able to achieve a selected loop time with all configurations.

The Maximum Actual Loop Time value shown in the Loop Time Usage Statistics section (see below) can be used to help determine whether a selected loop time is achievable. RMCTools must be online with the controller for these values to be updated.

For a list of the loop times supported by various modules, see the Loop Time topic.

High Control Loop Utilization

The High Control Loop Utilization does the following:

Allows selecting faster loop times given the number of control axes defined in the controller
Allows downloading user programming with estimated worst-case processing times that exceed the allocated time.

Selecting Faster Loop Times

As described in detail in the Loop Time topic, it is important that the actual loop processing time does not frequently or excessively exceed the selected loop time. For each selected loop time, there are a maximum number of axes for which the actual loop processing time is never expected to exceed the selected loop time, given a full load of motion commands, plots, user programs, and communications.

It is rare for any application to have a full load of motion commands, plots, user programs, and communications. Therefore, in many cases, it is acceptable to choose a loop time that, given the number of axes, may be exceeded by a full load, because the full load will likely not occur. In these cases, selecting that loop time requires setting the High Control Loop Utilization check box and verifying that the application does not exceed the loop time excessively, as described in the Loop Time topic.

Allow Downloading of User Programs that May Exceed the Allocated Time

As described in details in the Program Capacity and Time Usage topic, RMCTools calculates the worst-case scenario of how much time of any control loop may be taken up by program calculations, which includes the user programs and the Program Triggers. If this calculated time exceeds the portion of the loop time that is allocated to user programming, RMCTools prevents downloading the programming.

In many applications, the calculated worst-case scenario is unlikely to occur. Therefore, in many cases, it is acceptable to create user programming for which the calculated worst-case scenario exceeds the allocated time. This requires setting the High Control Loop Utilization check box and verifying that the application does not exceed the loop time excessively, as described in the Loop Time topic. The user should also consider the methods described in the Program Capacity and Time Usage topic to modify the programming to use less time.

Loop Time Usage Statistics

RMCTools must be online with the controller for these values to be updated.

Last Loop Time Used

This is the portion of the last lop time that was used for all the processing and calculations the RMC needs to do. The remaining loop time is unused. It is important that this loop time never exceed the selected loop time, or control errors may result.

Maximum Loop Time Used

This is the value of the longest loop time of the controller since this value was last cleared. If this value is greater than the selected Loop Time, then the controller is not able to achieve the selected loop time with the current configuration. Select a longer loop time, or change the controller configuration.

For tips on how to reduce the loop time usage, see the Loop Time topic.

Plotting Loop Time

To clearly visualize the loop time usage of the RMC, you can add the loop time usage registers to a plot. For details, see Plotting the Loop Time.