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Addressing Control Applications Using Wireless
Devices
Addressing Control Applications Using Wireless
DevicesTerry Blevins – Principal Technologist
Mark Nixon – Manager, Future Architecture
PresentersPresenters
Terry Blevins
Mark Nixon
AgendaAgenda
Background on WirelessHART Wirelesss Impact on Control Modified PID for Wireless Measurements Performance Comparison to Wired Transmitter Addressing Lost Communications Test results Conclusion
Control over WirelessControl over Wireless
For some applications it is desirable to utilize wireless networks for control– Cost reduction– New wireless measurements meets installation
requirements
WirelessHART– Designed to support monitoring and control applications– Use of wireless measurements in closed loop control
automation may improve process operations.
Wireless ArchitectureWireless ArchitectureWireless ArchitectureWireless Architecture
Operations Asset Management
Plant Network
Control Network
Plant Network
Wireless Standards:
802.11
802.16
802.11
802.16
Field Network 802.15.4
Wireless HART*Field Devices
Control Network
Asset Tracking
Security
Mobile Worker
Self-Organizing Networks
Focus:Focus:
6
FieldDevice
FieldDevice
FieldDevice
RouterDevice
RouterDevice
FieldDevice
FieldDevice
FieldDevice
GatewayDevice
Plant Automation Network
Plant Automation Application Host
Wireless HART
Handheld
Network Manager
WirelessHART Network TopologyWirelessHART Network Topology
Wireless Field Devices– Relatively simple - Obeys Network Manager– All devices are full-function (e.g., must route)
Adapters– Provide access to existing HART-enabled
Field Devices– Fully Documented, well defined requirements
Gateway and Access Points – Allows access to WirelessHART Network
from the Process Automation Network– Gateways can offer multiple Access Points for
increased Bandwidth and Reliability– Caches measurement and control values– Directly Supports WirelessHART Adapters– Seamless access fro existing HART
Applications Network Manager
– Manages communication bandwidth and routing
– Redundant Network Managers supported – Often embedded in Gateway– Critical to performance of the network
Handheld– Supports direct communication to field device– For security, one hop only communication
Gateway - Network Management Gateway - Network Management
Establishment of routes– The Network Manager is responsible for the creation of
routes that can be used by plant automation hosts, gateways, other devices, and the Network Manager itself to perform communications with the application layer in network devices.
– The Network Manager continually collects data from devices on the health of connections and traffic patterns and uses this information to adjust routing and scheduling.
7
Gateway - Network Management (Cont)Gateway - Network Management (Cont)
Scheduling communications– The Network Manager is responsible for the establishment
of communication schedule that the user layer application of a network device may use to transfer process data, alerts, diagnostics and other traffic to the gateway for access by the plant automation host.
– For network devices that are actuators, interlocks, or any device that affects the process, the Network Manager is responsible for the establishments of scheduled communication that the plant automation host may use to send setpoints and outputs to the user layer application in field devices.
8
Example - Link ScheduleExample - Link Schedule
9
•The top portion shows the overall
slot allocations subdivided into
channels.
•The bottom portion shows the
transmit and receive slots for each
device
Challenge – Control Using WirelessChallenge – Control Using Wireless
To reduce transmitter power consumption, it is desirable to minimize how often a measurement value is communicated.
To avoid the restrictions of synchronizing the measurement value with the control, most multi-loop controller in use today are designed to over-sample the measurement by a factor of 2-10X.
Also, to minimize control variation, the typical rule of thumb is that feedback control should be executed 4X to 10X times faster that the process response time, process time constant plus process delay.
Thus, to satisfy these requirements, the measurement value in a wired system is often sampled much faster that the process responds
Traditional Approach – Over Sampling of MeasurementTraditional Approach – Over Sampling of Measurement
Control Execution
63% of Change
Process Output
Process Input
Deadtime (TD )
O
I
New Measurement Available
Time Constant ( )
Fieldbus Foundation Approach - Synchronizing Measurement and Control ExecutionFieldbus Foundation Approach - Synchronizing Measurement and Control Execution
The approach taken in Foundation Fieldbus devices is to eliminate the need to over sample the measurement by synchronizing measurement with control execution.
If the traditional approach is taken in scheduling control 4-10X faster than the process response, then the power consumption associated with the transmission of the measurement value each time communications are scheduled may be excessive for all but the slowest types of process.
However, slowing down the control execution to reduce the power consumption associated with communication may increase control variability when the process is characterized by frequent unmeasured disturbances.
WirelessHART SolutionWirelessHART Solution
Power consumption is minimized by transmitting the measurement value using the following rule:1. The transmitter will periodically sample the measurement 4-
10x faster than the process response time.
2. The new value will be communicated as scheduled only if the magnitude of the difference between the new measurement value and the last communicated measurement value is greater that a specified resolution or if the time since the last communication exceeds a refresh time
3. Thus, the measurement is communicated only as often as required to allow control action to correct for unmeasured disturbances or changes in operation point.
Impact of Wireless Communication on Control ImplementationImpact of Wireless Communication on Control Implementation
The underlying assumption in traditional control design is that the PID is executed on a periodic basis.
When the measurement is not updated on a periodic basis, then the calculated reset action may not be appropriate.
If control execution is only executed when a new measurement is communicated, then this could delay control response to setpoint changes and feedforward action on measured disturbances.
To provide best control when a measurement is not updated on periodic basis, the PID may be restructured to reflect the reset contribute for the expected process response since the last measurement update.
PID ENHANCEMENT FOR WIRELESS TRANSMISSIONPID ENHANCEMENT FOR WIRELESS TRANSMISSION
Restructuring the PID for WirelessRestructuring the PID for Wireless
The positive feedback network used to create the reset contribution may be modified to accommodate non-periodic measurement update. Specifically, the filter used in this network can be modified to have the following behavior:– Maintain the last calculated filter output until a new
measurement is communicated.– When a new measurement is received, calculate the new
filter output based on the last controller output and the elapsed time since a new measurement value was communicated.
Filter CalculationFilter Calculation
To account for the process response, the filter output may be calculated in the following manner when a new measurement is received.
edcommunicatwasvaluenewasincetimeElapsedT
executionlastoutputControllerO
tmeasuremennewlastafteroutputfilterexecutionlastoutputFilterF
outputfilterNewFwhere
eFOFF
N
N
N
T
T
NNNNReset
1
1
111 1
THE CLOSED LOOP RESPONSE OF MODIFIED PI CONTROLLERTHE CLOSED LOOP RESPONSE OF MODIFIED PI CONTROLLER
CONTROL PERFORMANCE DIFFERENCECONTROL PERFORMANCE DIFFERENCE
Communications transmissions are reduced by over 96 % when the rules for wireless communication are followed.
The impact of non-periodic measurement updates on control performance is minimized through the use of the modified PI algorithm for wireless communication.
PID Performance for Lost CommunicationsPID Performance for Lost Communications
The traditional PID algorithms provides poor dynamic response in the case of lost communications.
Further modifications of the PID algorithm may be made to improve the dynamic response under these conditions
When there is no communication lost, the new PID block acts exactly the same as a traditional PID block.
Modified PID for Wireless — PIDPLUSModified PID for Wireless — PIDPLUS
Note: Controller output in the equation above is based on the actuator position feedback supplied by
BKCAL_IN
Integral Contribution – Calculated only on arrival of new measurementIntegral Contribution – Calculated only on arrival of new measurement
Rate Contribution – Calculated on arrival of new measurementRate Contribution – Calculated on arrival of new measurement
Experimental SetupExperimental Setup
PIDPlus ImplementationPIDPlus Implementation
PIDPlus ExpressionPIDPlus Expression
Experimental ScenariosExperimental Scenarios
Measurements Lost– Setpoint Change– Process Disturbance
Actuator Commands Lost– Setpoint Change– Process Disturbance
Communication Loss
Traditional PID
Modified PID
Measurement Communication Loss – During Setpoint ChangeMeasurement Communication Loss – During Setpoint Change
Communication Loss
Modified PID
Traditional PID
Measurement Communication Loss – During Process DisturbanceMeasurement Communication Loss – During Process Disturbance
Communication Loss
Modified PID
OUT
PV & SP
SP
Traditional PID
Actuator Communication Loss – During Setpoint ChangeActuator Communication Loss – During Setpoint Change
Communication Loss
Traditional PID
Modified PID
OUT
PV & SPSP
Actuator Communication Loss – During Process DisturbanceActuator Communication Loss – During Process Disturbance
Results in numbersResults in numbers
Unreliable Inputs: Setpoint Change
Unreliable Inputs: Process Disturbance
Unreliable Outputs: Setpoint Change
Unreliable Outputs: Process Disturbance
PID 372 366 196 388
PIDPLUS 169 333 190 267
Scenarios
PIDs
IAE
Business Results AchievedBusiness Results Achieved
WirelessHART transmitters may allow previously unavailable measurements to be made that enable close loop control to be implemented to improve plant operation.
SummarySummary
WirelessHART measurements may be used in closed loop control applications.
Standard PID doesn’t perform well using non-period sample updates provided in a wireless environment
The performance of PIDPLUS in a wireless control networks is comparably to PID using wired measurements
PIDPLUS handles recovery after loss of communications.
ReferencesReferences
Mark Nixon, Deji Chen, Terry Blevins, and Aloysius K. Mok, “Meeting Control Performance over a Wireless Mesh Network”, The 4th Annual IEEE Conference on Automation Science and Engineering (CASE 2008), August 23-26, 2008,, Washington DC, USA.
Chen, Nixon, Blevins, Wojsznis, Song, Mok “Improving PID Control under Wireless Environments”, ISA EXPO2006, Houston, TX
Chen, Nixon, Aneweer, Mok, Shepard, Blevins, McMillan “Similarity-based Traffic Reduction to Increase Battery Life in a Wireless Process Control Network”, ISA EXPO2005, Houston, TX
Where to Learn MoreWhere to Learn More
Visit the Smart Wireless theater in the exhibit hall– Features 15 minute live demos of process, plant and business
applications and showcases new wireless products and solutions Attend session 399: Smart Wireless Vision, Opportunities and
Solutions– Presented by Dave Imming, Bob Karschnia, and Dan Carlson from
Emerson Process Management– Gives an overview of Smart Wireless vision across all Emerson
Process divisions– Covers strategy for the wireless field networks and wireless plant
networks Attend any of the other 30 wireless sessions at Exchange Visit www.plantwebuniversity.com and take the 20+ wireless
courses available online Contact your local Emerson representative Visit www.emersonprocess.com/smartwireless
Where To Get More InformationWhere To Get More Information
Smart Wireless Presentations at Emerson Exchange
Websites– emersonprocess.com/smartwireless/– PlantwebUniversity.com– http://www.hartcomm2.org/index.html
Rosemount Wireless Specialist