I came across an email that the ISA Honors & Awards Committee has selected the paper, Improving PID Control with Unreliable Communications, for its excellence in documentation award. Emerson’s Deji Chen, Mark Nixon, Terry Blevins, Willy Wojsznis and the University of Texas, Department of Computer Sciences’ Jianping Song and Aloysius K. Mok wrote the paper.
The paper examines PID control in a wireless network where intermittent loss of communications is likely to happen. It identifies the poor dynamic response of standard PID algorithms in this loss of communications scenario. The team proposed an enhanced PID algorithm to improve dynamic response under these conditions.
One of the technical challenges is that the 2.4 GHz spectrum defined by IEEE 802.15.4 is also used by Wi-Fi and Bluetooth devices. Also, some electrical devices found in industry generate noise in this frequency band. Thus, at times it is expected that a transmission will be corrupted. To help minimize the impact of these other devices on communications, the Time Synchronized Mesh Protocol (TSMP) selected for wireless HART uses frequency hopping. Even so, at times it is expected that multiple transmissions of a measurement used in control or multiple communications of control actions to an actuator may be lost.
Terry describes how the loss of communications can cause the PID loop to continue executing and wind up due to the reset action. This reset action can be disruptive to the control of the loop. And, if the derivative (the D in PID) action is used, the loss and resumption of the control measurement signal can cause a spike, again bumping the control of the loop.
The Emerson and UT technologists worked through a solution to minimize the impact of this loss of communications. Terry sums up the change:
However, by modifying the reset and derivative calculation to account for the time since the last measurement update, then it is possible to minimize the impact of loosing multiple measurement transmissions.
If you want to look at the math behind this innovation, check out the overview presentation, PID for Unreliable Communications, given at ISA 2006.
Congratulations to the team for their contribution to furthering the advancement of wireless technologies in process automation!