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:

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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.

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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