Addressing Control Applications Using Wireless Devices

1
Addressing Control Applications Using Wireless
Devices

• Terry Blevins Principal Technologist
• Mark Nixon Manager, Future Architecture

2
Presenters

• Terry Blevins
• Mark Nixon

3
Agenda

• Background on WirelessHART
• Wirelesss Impact on Control
• Modified PID for Wireless Measurements
• Performance Comparison to Wired Transmitter
• Addressing Lost Communications
• Test results
• Conclusion

4
Control 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.

5
Wireless Architecture
Wireless Standards
802.11 802.16
Plant Network
Plant Network
Operations
Asset Management
802.11 802.16
Control Network
Control Network
Focus
Field Network 802.15.4Wireless HART
Field Devices
Self-Organizing Networks
6
WirelessHART 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

Network Manager
6
7
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
8
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
9
Example - Link Schedule

• The top portion shows the overall slot
  allocations subdivided into channels.

• The bottom portion shows the transmit and receive
  slots for each device

9
10
Challenge 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

11
Traditional Approach Over Sampling of
Measurement
Process Output
63 of Change
O
Time Constant ( )
Deadtime (TD )
Process Input
I
Control Execution
New Measurement Available
12
Fieldbus 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.

13
WirelessHART Solution

• Power consumption is minimized by
  transmitting the measurement value using the
  following rule
• The transmitter will periodically sample the
  measurement 4-10x faster than the process
  response time.
• 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
• Thus, the measurement is communicated only as
  often as required to allow control action to
  correct for unmeasured disturbances or changes in
  operation point.

14
Impact 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.

15
PID ENHANCEMENT FOR WIRELESS TRANSMISSION
16
Restructuring 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.

17
Filter Calculation

• To account for the process response, the filter
  output may be calculated in the following manner
  when a new measurement is received.

18
THE CLOSED LOOP RESPONSE OF MODIFIED PI CONTROLLER
19
CONTROL 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.

20
PID 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.

21
Modified PID for Wireless PIDPLUS
22
Integral Contribution Calculated only on
arrival of new measurement
Note Controller output in the equation above is
based on the actuator position feedback supplied
by BKCAL_IN
23
Rate Contribution Calculated on arrival of new
measurement
24
Experimental Setup
25
PIDPlus Implementation
26
PIDPlus Expression
27
Experimental Scenarios

• Measurements Lost
• Setpoint Change
• Process Disturbance
• Actuator Commands Lost
• Setpoint Change
• Process Disturbance

28
Measurement Communication Loss During Setpoint
Change
Modified PID
Traditional PID
Communication Loss
29
Measurement Communication Loss During Process
Disturbance
30
Actuator Communication Loss During Setpoint
Change
Traditional PID
31
Actuator Communication Loss During Process
Disturbance
32
Results 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
IAE
PIDs
33
Business Results Achieved

• WirelessHART transmitters may allow previously
  unavailable measurements to be made that enable
  close loop control to be implemented to improve
  plant operation.

34
Summary

• WirelessHART measurements may be used in closed
  loop control applications.
• Standard PID doesnt 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.

35
References

• 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

36
Where 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

37
Where 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