Table 1 Control Performance Wired vs. Wireless
The reliability of WirelessHART device communication has been well established. Even so, in the event of loss communication, the expected control behavior is of interest. The simulation environment was used to compare the behavior of the PIDPlus for loss of communications to a PID with a wired transmitter where the measurement value is frozen for a period of time. The response observed when the measurement was lost during a setpoint change is shown below.
Figure 8 - Response for measurement loss – during recovery for a setpoint change
The response observed when the measurement is lost after a process disturbance is shown below.
Figure 9 – Response for measurement loss – during disturbance recovery
The conventional PID provides poor dynamic response when the measurement is lost. As illustrated by these tests, the PIDPlus provides improved dynamic response under these conditions.
During the development of the PIDPlus, the control performance using WirelessHART communications was tested extensively using a simulation environment. Before this capability was incorporated in DeltaV, the performance was also verified in several field trials where the PIDPlus was used for control using WirelessHART transmitters. Two of the field trial installations were on the following processes:
- Disposable bioreactor – pH and temperature control
- Stripper Column –Column pressure, steam flow control
The performance documented at these field sites for control using the PIDPlus and WirelessHART transmitters was shown to be comparable to that achieved using wired transmitters and conventional PID for control.
Broadley James Bioreactor Control
Broadley James manufactures bioreactors and control systems (based on DeltaV) that are used in the life sciences industry. To explore the use of WirelessHART transmitters in bioreactor control, a portable Hyclone 100-liter disposable bioreactor was instrumented with a Rosemount WirelessHART gateway and transmitters for measurement and control of pH and temperature. A WirelessHART transmitter was also installed to monitor pressure. The installation is shown below.
Figure 10 – Hyclone disposable bioreactor at Broadley James
At this site, the WirelessHART pH transmitter was configured for window communication, and the temperature transmitter was configured for continuous communications. The temperature control provided by PIDPlus and the WirelessHART transmitter for a batch run using mammalian cell culture is shown below.
Very precise pH control was achieved using PIDPlus and window communications as illustrated below.
Separations Research Program, University of Texas at Austin
The Separations Research Program was established at the J.J. Pickle Research Campus in 1984. This cooperative industry/university program performs fundamental research of interest to chemical, biotechnological, petroleum refining, gas processing, pharmaceutical and food companies. CO2 removal from stack gas is a focus project for which WirelessHART transmitters were installed for pressure and steam flow control on a stripper column. A picture of the process is shown below.
Figure 11 –Bioreactor temperature control using PIDPlus and a WirelessHART transmitter.
Figure 12 – Bioreactor pH control using PIDPlus and WirelessHART transmitter.
Figure 13 – Stripper and absorber column at UT
Standard WirelessHART pressure and flow transmitters were installed to demonstrate and test control using WirelessHART and the PIDPlus. The control was modified to allow the operator to switch between control using WirelessHART and PIDPlus and the wired transmitters and PID. The stripper column pressure control is shown below for each manner of control.
The same dynamic control response was observed for SP changes as illustrated above. Original plant PID tuning was used for both wired and wireless control. The control performance for wireless vs. wired control is summarized in the following table.
Comparable control as measured by IAE was achieved using WirelessHART measurements and PIDPlus vs. control with wired measurements and PID. However, the number of measurement samples with WirelessHART vs. wired transmitter was reduced by a factor of 10 times for flow control and 6 times for pressure control – accounting for differences in test duration.