As noted in my December 4th posting, the Hart Communication Foundation has adopted the IEEE 802.15.4 physical layer for wireless HART. 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. Thus, a few years ago we started looking at the control requirements under these conditions. In particular we examined the behavior during communication loss and after communications are re-established.
When the control measurement is lost, a standard PID may be expected to continue executing and thus could windup because of reset action. This condition might be addressed by changing the actual mode of the PID to manual on detection of a measurement loss. However, with either approach, the reset action taken by the PID under this condition will be disruptive to the control. If derivative action is utilized in the PID, then the abrupt transition in the measured value on recover of transmission may cause a spike in output since the derivate contribution is normally calculated based on the period of execution. 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.
The loss of multiple transmissions from the PID to an actuator may also disrupt loop operation. A standard PID under these conditions would continue to takes control action even though these actions have not reached the actuator. Thus, under these conditions, the reset action would wind up and when communications are re-establish you would expect to see a significant bump in the process. However, by using feedback from the actuator in the reset calculation , as defined by the Fieldbus Foundation, then windup under this condition may be avoided.
Details on the PID modifications to account for loss of the control measurement or the path to the actuator are described in detail in a paper that we presented at ISA2006, “Improving PID Control with Unreliable Communications“. An overview of this work is provided in the following presentation:
In this presentation, the performance of a standard PID is compared to a modified PID. The modified PID uses actuator feedback and the time since last good communication in the reset and rate calculations. The modified PID provides a significant control improvement over the standard PID for the conditions that were considered in these tests.