EEB5213 / EAB4233 Plant Process Control Systems Digital Implementation of PID Controller

Objectives At the end of this lecture, students should be able to: identify digital application in process control computation and signal transmission program PID algorithm in digital controllers select a proper execution rate for a digital PID controller

Digital Feedback Control

Digital Control Digital control employs a distributed control network (DCS). D/A

Benefits of Distributed Control FeatureEffect on process control Calculations performed in parallel by multiple processors. Calculations are performed faster. Limited number of controller calculations performed by a single processor. Control system is more reliable since a processor failure affects only few control loops. Control calculations and interfacing to process are independent of other devices connected to LAN. Control is more reliable since failure of other devices does not immediately affect a control processor. Small amount of equipment required for the minimum system. Only the equipment required must be purchased and the system can be easily expanded. Each type of processor can have different hardware and software. Hardware and software can be tailored to specific applications like control, monitoring, operator console and general data processing.

Digital Control The techniques presented will be applicable for digital sampling and control calculation. Transmission can be either digital or electronic. Periodically, the measurement is sampled and a calculation is performed.

Digital Control - Sampling

The last sampled value is kept constant between control executions by using zero-order hold.

Digital Control - Sampling The red line is the continuous approximation of the signal after the sample & hold. This shows that the effect is to introduce a “dead time” of about  t/2.

Digital Control Performance What is the effect of digital execution of the PID controller on tuning and performance ?

Digital Control Performance

Digital Control Tuning Guideline for selecting execution time To prevent degradation of control loop performance, select a controller execution time / sampling period of Note: Typical sampling period for chemical process control is 1/3 – ½ sec. Much faster is possible, if needed.

Digital Control Tuning Steps to tune a PID digital controller using open-loop tuning methods: 1. Obtain process model. 2. Determine the sampling period or maximum execution period,  t  0.05(  +  ). 3. Recalculate new dead time,  ’ =  +  t/2. 4. Applying the new dead time,  ’, calculate PID tuning constants using any of the open-loop tuning methods used for analog PID controller. 5. Implement and fine-tune as needed.

Digital Control Tuning Let’s apply this guideline for the three-tank mixer with a long sampling period = 15 min.

Digital Control Tuning The performance is about as good as possible with the very long sampling time ! Would you fine-tune further ? Tuning from chart K C = 26 T I = 13 T D = 0.8 IAE increased from 12.2 to 20+

Discrete PID Controller The measured CV is sampled, giving values of CV 1, CV 2, CV 3, , CV N. How would we estimate each mode of the PID controller in discrete equations ? Proportional Integral Derivative ? ? ?

Discrete PID Controller The measured CV is sampled, giving values of CV 1, CV 2, CV 3, , CV N. Proportional Integral

Discrete PID Controller The measured CV is sampled, giving values of CV 1, CV 2, CV 3, , CV N. Derivative

Discrete PID Controller – Bumpless Transfer Bumpless transfer: No change to the MV when controller first executed Initialization / Bias

Types of Discrete PID Algorithm Positional form or full-position algorithm of digital PID calculates the actual controller output to the final element at each execution.

Types of Discrete PID Algorithm Velocity form of digital PID calculates the change in the controller output at every execution.

Digital Control If the PID controller performs no better in its digital form, why did we spend decades in engineering time and billions of dollars converting the world’s process control to digital? Complex controllers - Improved performance can be achieved with algorithms that optimize the path to the set point, every controller execution! Process monitoring – We have digital history of measurements for: a) Troubleshooting b) Calculation of process performance indicators eg. reactor yield, energy efficiency per kg of product etc. c) Excellent graphical display

What Next ? Next Lecture : Practical Implementation Issues for PID Controller (Marlin, Chapter 12)