On the Performance of the PIDPLUS Controller in  Wireless Control Systems Ossi Kaltiokallio, Lasse Eriksson, Maurizio Bocc...
Outline <ul><li></li></ul><ul><li>Motivation </li></ul><ul><li>PIDPLUS controller </li></ul><ul><li>PIDPLUS performance an...
<ul><li>Industrial process control has strict real-time and reliability requirements </li></ul><ul><li>Wireless Networked ...
PID Controller under Packets Losses  <ul><li></li></ul><ul><li>Spikes in the control signal </li></ul><ul><ul><li>Caused b...
PIDPLUS Controller  <ul><li></li></ul><ul><ul><li>Integral and derivative parts updated only at the arrival of new packets...
Controller Performance Measures  <ul><li></li></ul><ul><li>Control performance: Integral of Square Error (ISE) </li></ul><...
PIDPLUS Performance Analysis  <ul><li></li></ul><ul><li>18th Mediterranean Conference on Control and Automation (MED’10) <...
PIDPLUS Performance Analysis  <ul><li></li></ul><ul><li>18th Mediterranean Conference on Control and Automation (MED’10) <...
PIDPLUS vs PID  <ul><li></li></ul><ul><li>18th Mediterranean Conference on Control and Automation (MED’10) </li></ul><ul><...
Case Study – Ball Balancing System <ul><li></li></ul><ul><li>18th Mediterranean Conference on Control and Automation (MED’...
Implementation <ul><li></li></ul><ul><li>18th Mediterranean Conference on Control and Automation (MED’10) </li></ul><ul><l...
Implementation <ul><li></li></ul><ul><li>18th Mediterranean Conference on Control and Automation (MED’10) </li></ul><ul><l...
Implementation <ul><li></li></ul><ul><li>18th Mediterranean Conference on Control and Automation (MED’10) </li></ul><ul><l...
Wired vs Wireless <ul><li></li></ul><ul><li>18th Mediterranean Conference on Control and Automation (MED’10) </li></ul><ul...
Conclusions <ul><li></li></ul><ul><li>18th Mediterranean Conference on Control and Automation (MED’10) </li></ul><ul><li>M...
Questions and Answers <ul><li>Maurizio Bocca </li></ul><ul><li>[email_address] </li></ul><ul><li>http://autsys.tkk.fi/Maur...
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On the Performance of the PIDPLUS Controller in Wireless Control Systems

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On the Performance of the PIDPLUS Controller in Wireless Control Systems

  1. 1. On the Performance of the PIDPLUS Controller in Wireless Control Systems Ossi Kaltiokallio, Lasse Eriksson, Maurizio Bocca Department of Automation and Systems Technology Aalto University School of Science and Technology Helsinki, Finland <ul><li>18th Mediterranean Conference on Control and Automation (MED’10) </li></ul><ul><li>Marrakech, Morocco, 24.6.2010 </li></ul>
  2. 2. Outline <ul><li></li></ul><ul><li>Motivation </li></ul><ul><li>PIDPLUS controller </li></ul><ul><li>PIDPLUS performance analysis </li></ul><ul><li>Event-based PIDPLUS vs time-based PID </li></ul><ul><li>Case study: ball balancing system </li></ul><ul><li>Implementation and performance analysis </li></ul><ul><li>Conclusions </li></ul><ul><li>18th Mediterranean Conference on Control and Automation (MED’10) </li></ul><ul><li>Marrakech, Morocco, 24.6.2010 </li></ul>
  3. 3. <ul><li>Industrial process control has strict real-time and reliability requirements </li></ul><ul><li>Wireless Networked Control Systems (WiNCS) are prone to: </li></ul><ul><ul><li>Delays (time-varying, often random), due to e.g. MAC layer policy, MCUs scheduling, low-quality clocks, routing, etc... </li></ul></ul><ul><ul><li>Packets losses , due to e.g. multipath fading, distortion, scattering, interferences (e.g. WLAN-ZigBee) </li></ul></ul><ul><li>Wireless nodes have limited resources in terms of: </li></ul><ul><ul><li>Computational power </li></ul></ul><ul><ul><li>Available memory space </li></ul></ul><ul><ul><li>Radio communication capabilities </li></ul></ul>Motivation <ul><li></li></ul><ul><li>Computationally light-weight controllers robust to the typical unreliabilities of WiNCS </li></ul><ul><li>18th Mediterranean Conference on Control and Automation (MED’10) </li></ul><ul><li>Marrakech, Morocco, 24.6.2010 </li></ul>
  4. 4. PID Controller under Packets Losses <ul><li></li></ul><ul><li>Spikes in the control signal </li></ul><ul><ul><li>Caused by the derivative term when the communication is re-established after a period of disconnectivity </li></ul></ul><ul><ul><li>Bumps at the actuator(s) </li></ul></ul><ul><li>Windup </li></ul><ul><ul><ul><li>While measurement packets are lost, the error signal remains constant, and the integral terms keeps on integrating the error over time </li></ul></ul></ul><ul><ul><li>No information about actuator(s) saturation </li></ul></ul><ul><ul><ul><li>Can lead to failure of the anti-windup functions </li></ul></ul></ul><ul><li>18th Mediterranean Conference on Control and Automation (MED’10) </li></ul><ul><li>Marrakech, Morocco, 24.6.2010 </li></ul>
  5. 5. PIDPLUS Controller <ul><li></li></ul><ul><ul><li>Integral and derivative parts updated only at the arrival of new packets </li></ul></ul><ul><ul><li>The actuator holds the last communicated value </li></ul></ul><ul><ul><li>Integrator replaced by a filter: </li></ul></ul><ul><li>O (k-1): previous realized actuator value </li></ul><ul><li> T : time interval between the two last received measurement packets </li></ul><ul><li>18th Mediterranean Conference on Control and Automation (MED’10) </li></ul><ul><li>Marrakech, Morocco, 24.6.2010 </li></ul>
  6. 6. Controller Performance Measures <ul><li></li></ul><ul><li>Control performance: Integral of Square Error (ISE) </li></ul><ul><li>Robustness: jitter margin </li></ul><ul><ul><ul><li>to communication delays and packets losses </li></ul></ul></ul><ul><ul><ul><li>For a closed-loop system with process G , controller C , and a variable time-delay  , the control loop is stable if: </li></ul></ul></ul><ul><ul><ul><li> max defines how much delays and packets losses the system can tolerate </li></ul></ul></ul><ul><li>18th Mediterranean Conference on Control and Automation (MED’10) </li></ul><ul><li>Marrakech, Morocco, 24.6.2010 </li></ul>
  7. 7. PIDPLUS Performance Analysis <ul><li></li></ul><ul><li>18th Mediterranean Conference on Control and Automation (MED’10) </li></ul><ul><li>Marrakech, Morocco, 24.6.2010 </li></ul><ul><li>Effect of packets losses on the control performance </li></ul><ul><li>K p : process gain </li></ul><ul><li>L : delay </li></ul><ul><li>T : time constant </li></ul><ul><li>K p =T=L=1 </li></ul><ul><li>(5%) </li></ul><ul><li>(95%) </li></ul><ul><li>1% settling time = 6.7 s </li></ul><ul><li>1% settling time = 39.3 s </li></ul>
  8. 8. PIDPLUS Performance Analysis <ul><li></li></ul><ul><li>18th Mediterranean Conference on Control and Automation (MED’10) </li></ul><ul><li>Marrakech, Morocco, 24.6.2010 </li></ul><ul><li>Effect of packets losses on the control performance </li></ul><ul><ul><ul><li>To double the cost function: </li></ul></ul></ul><ul><ul><ul><li>P loss needs to be ≥ 85% in one of the two links </li></ul></ul></ul><ul><ul><ul><li>P loss needs to be ≥ 75% in both the links simultaneously </li></ul></ul></ul><ul><li>PIDPLUS controller is resiliant to packets losses </li></ul>
  9. 9. PIDPLUS vs PID <ul><li></li></ul><ul><li>18th Mediterranean Conference on Control and Automation (MED’10) </li></ul><ul><li>Marrakech, Morocco, 24.6.2010 </li></ul><ul><li>3 process types </li></ul><ul><li>( K p = L = 1 constant): </li></ul><ul><ul><li>Delay-dominant ( T =0.4) </li></ul></ul><ul><ul><li>Balanced lag and delay ( T =1) </li></ul></ul><ul><ul><li>Lag dominated ( T =10) </li></ul></ul><ul><li>Balanced lag and delay: </li></ul><ul><li>DRAW </li></ul><ul><li>Delay-dominant: </li></ul><ul><li>PID WINS </li></ul><ul><li>Lag dominated: </li></ul><ul><li>PIDPLUS WINS </li></ul>
  10. 10. Case Study – Ball Balancing System <ul><li></li></ul><ul><li>18th Mediterranean Conference on Control and Automation (MED’10) </li></ul><ul><li>Marrakech, Morocco, 24.6.2010 </li></ul><ul><li>h (sample time) = 0.05 s </li></ul>
  11. 11. Implementation <ul><li></li></ul><ul><li>18th Mediterranean Conference on Control and Automation (MED’10) </li></ul><ul><li>Marrakech, Morocco, 24.6.2010 </li></ul><ul><li>MSP430 MCU </li></ul><ul><li>CC2420 ZigBee 802.15.4 compatible radio </li></ul><ul><ul><li>Data rate: 250 kbps (theoretical) </li></ul></ul><ul><ul><li>Frequency band: 2.4 GHz </li></ul></ul><ul><li>FreeRTOS OS </li></ul>
  12. 12. Implementation <ul><li></li></ul><ul><li>18th Mediterranean Conference on Control and Automation (MED’10) </li></ul><ul><li>Marrakech, Morocco, 24.6.2010 </li></ul><ul><ul><ul><li>N3 (sensor) : </li></ul></ul></ul><ul><ul><ul><li>Reads cart and ball positions (ADC) </li></ul></ul></ul><ul><ul><ul><li>Peak filtering </li></ul></ul></ul><ul><ul><ul><li>Transmits to controller the filtered measurements on ch. 26 </li></ul></ul></ul><ul><ul><ul><li>Waits h = 0.05 s </li></ul></ul></ul><ul><ul><ul><li>N3.5 (controller) : </li></ul></ul></ul><ul><ul><ul><li>Receives the packet from the sensor node and evaluates  T </li></ul></ul></ul><ul><ul><ul><li>Computes the control value </li></ul></ul></ul><ul><ul><ul><li>Transmits the control value to actuator using ch. 11 </li></ul></ul></ul><ul><ul><ul><li>Waits for an ACK packet from the actuator </li></ul></ul></ul><ul><ul><ul><li>N4 (actuator) : </li></ul></ul></ul><ul><ul><ul><li>Receives the control value </li></ul></ul></ul><ul><ul><ul><li>Sets speed and direction of the motor (DACs) </li></ul></ul></ul><ul><ul><ul><li>Transmits an ACK message back to the controller </li></ul></ul></ul>
  13. 13. Implementation <ul><li></li></ul><ul><li>18th Mediterranean Conference on Control and Automation (MED’10) </li></ul><ul><li>Marrakech, Morocco, 24.6.2010 </li></ul><ul><ul><ul><li>Floating point computations and radio transmissions consume most of the time of execution </li></ul></ul></ul><ul><ul><ul><li>Total cycle time = 36 ms </li></ul></ul></ul>
  14. 14. Wired vs Wireless <ul><li></li></ul><ul><li>18th Mediterranean Conference on Control and Automation (MED’10) </li></ul><ul><li>Marrakech, Morocco, 24.6.2010 </li></ul><ul><li>Both the systems stabilize the ball around the equilibrium </li></ul><ul><li>Higher variation of the results in the wireless system </li></ul><ul><li>The wired system outperforms the wireless one: </li></ul><ul><ul><li>Packet drops and varying delays </li></ul></ul><ul><ul><li>Lower accuracy of the sensing and actuating components </li></ul></ul>
  15. 15. Conclusions <ul><li></li></ul><ul><li>18th Mediterranean Conference on Control and Automation (MED’10) </li></ul><ul><li>Marrakech, Morocco, 24.6.2010 </li></ul><ul><li>The use of Wireless Sensor Networks in wireless automation introduces significant benefits... </li></ul><ul><ul><ul><li>Reduced cabling (meaning reduced costs!) </li></ul></ul></ul><ul><ul><ul><li>Ease of installation </li></ul></ul></ul><ul><ul><ul><li>Improved monitoring capability </li></ul></ul></ul><ul><li>...However, the use of WSNs for real-time closed-loop WiNCS is certainly challenging... </li></ul><ul><ul><ul><li>Limited resources of the nodes (computational power, memory space, radio communication capabilities, battery life) </li></ul></ul></ul><ul><li>In the paper, the event-based PIDPLUS controller was shown to be resiliant to delays and packets losses </li></ul><ul><ul><li>Future work will consider the establishment of tuning rules for the PIDPLUS controller: </li></ul></ul><ul><ul><ul><li>e.g. adaptive controller tuning under varying network quality-of-service </li></ul></ul></ul>
  16. 16. Questions and Answers <ul><li>Maurizio Bocca </li></ul><ul><li>[email_address] </li></ul><ul><li>http://autsys.tkk.fi/MaurizioBocca </li></ul><ul><li>18th Mediterranean Conference on Control and Automation (MED’10) </li></ul><ul><li>Marrakech, Morocco, 24.6.2010 </li></ul>

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