How to adjust sensor dwell time to avoid chatter
A Control Design reader writes: In applications using industrial Ethernet, what is the general rule for sensor dwell time to avoid chatter?
Answers
Data stream traffic
Every device has an inherent dwell time for data collection, but the effects of dwell time can vary, depending on the type of data stream traffic that the application is producing and how the data is being transmitted.
While dwell times do raise concerns with efficiency and accuracy due to chatter in industrial systems, dwell time can also have an adverse effect and serve as one method for stability analysis and stabilization in digital control systems to help ensure the mean length of all queues in the network remain bounded.
Industrial Ethernet provides the capability to utilize advanced software for analytics and diagnostics as another means of improving, and the speed of communication allows operators to observe any errors due to system or environmental interference. This additional speed in transmission is a good measure for calculating the average dwell time and, in return, using this value as a reference point for increasing accuracy and reducing concerns of data loss and distortions.
Andrew Barco / director of automation products and solutions / Weidmuller / www.weidmulller.com
Communication cycle and detection interval timing
This is a great question because it highlights some of the challenges faced with configuring an industrial Ethernet network. With older industrial Ethernet networks sensor dwell time (holding time) needs to be factored into the network communication cycle time in order to avoid losing data as the PLC completes an operation cycle. Imagine a sensor turning ON/OFF within one communication cycle. If the communication cycle is greater than the sensor ON time, the PLC may miss detect the sensor ON status. This has the potential of resulting in inferior part quality or incomplete motion. The sensor should be on more than the communication cycle. In addition, the detection interval cannot be less than the communication cycle. The responsibility for setting the correct timing is dependent on the user.
With modern industrial Ethernet networks, this is no longer required. New time-sensitive networks allow users to timestamp data from external sensors meaning that regardless of the communication cycle time of the PLC, it can detect the timing exactly when the sensor is turned on, even if it is turned on at the middle of a communication cycle. This timestamping ensures that when the data is received and processed by the PLC it can be used effectively as the operation cycle time is completed, regardless of timing. Modern industrial Ethernet networks are focused on enhancing system performance and allowing users to configure robust and secure networks while reducing the overall required effort from the user.
Dan Zachacki / senior product marketing engineer / Mitsubishi Electric / www.mitsubishielectric.com
