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Keeping in Control PID Controller

Modern manufacturing plants require increasingly complex processes to produce finished products. Raw materials are subjected to a myriad of process steps—and each process step must be under control. That control must be efficient, safe, and reliable.

The process control work horse is the three term local loop PID controller. This was first described and functionally analyzed by Nicholas Minorsky in his 1922 paper “Directional Stability of Automatically Steered Bodies”, and in its many guises this is still the mainstay control philosophy.

However, the derivation of the PID constants required for optimal setting of the three term controller is still a major problem in the control technician/engineers work load.

The “Handbook of PI and PID Controller Rules” by Aidan O`Dwyer may be seen as a sound basis for manual tuning of a process loop. A large number of different imperial/graphical derivation methods are available, ranging from the Ziegler-Nichols system developed in 1942, the Cohen-Coon system developed in 1953, the Minimum Error-step Load Change developed by J.P. Gerry in 1998, and others are available to the plant control engineer. These rely on an understanding of the plant transfer function to select the appropriate model and settings.

The automated setting of the three parameters P I and D by the controller are a boon to most control systems, and relatively easy to implement, but is this automated method better than manual tuning, or even leaving the controller in manual?

As recently as January 2009, an article by Greg Baker (Baker, 1/1/2009), cited that more than 30% of control loops were operated manually, and this was based on the 1993 article by (David B. Ender) indicating little change over the 15 years. In the feedback to the article Mr Baker replied,

“Until a current and comprehensive study on the root causes of underperforming PID loops exists, we may only agree the relative magnitude of underperformance, due to incorrect PID configuration, is not insignificant. We’d hope it has improved since the time of the source information, 15 years ago.”

It may be somewhat surprising to know that the Automatic Setting v. Manual Setting v. Manual Control debate has been ongoing for years, and shows no sign of a resolution

It is beyond the scope of the writer to verify this information In the United Kingdom, but the FP7 research grant frame work in February 2010 is calling for research into modelling and control of intensified process systems based on data indicating the production processes of many chemicals and biopharmaceuticals are not fully optimised.

All the algorithms/tuning methods have benefits and drawbacks, and the drawbacks, unrealised benefits, complexity, may be why a number of process loops remain in a manual state.

In all cases of plant loop tuning, the plant has to be disturbed and the resultant effects monitored and observed, so that an estimation of the plant transfer function type can be made, and this, as the writer knows from the past , can be a daunting experience.

Adaptive Control Systems

Fundamental process understanding, coupled with appropriate modelling and development of robust on-line monitoring, is necessary to identify which are the critical process parameters that require monitoring and control, for an individual plant system.

Unitronics, an Israeli based manufacture of process control PLC/HMI systems, proprietary auto tune PID algorithm removes the burden of plant tuning in a novel and unique way, and this knowledge and experience is available in the latest generation of Unistream controllers software, the Unilogic™  Studio.

The New Unistream™

A revolution in ladder programming, the tag based all-in-one: ladder, HMI, Hardware configuration and communication programming environment, Unilogic™, enables the programmer to control complex processes in any factory wide control project as easy as 1-2-3.

For the Auto tune PID function, the key objective was to develop an easy to use production control system and elements using knowledge-based engineering concepts.  Based on the proven Ziegler–Nichols heuristic method of tuning a PID controller, with propriety modifications by Unitronics based on real world results, Ku (the Ultimate gain) and the oscillation period Tu (the plant oscillation period) are used to set the P, I, and D gains. These gains apply to the ideal, parallel form of the PID controller. When applied to the standard PID form, the integral and derivative time parameters Ti and Td are only dependent on the Plant oscillation period Tu, which includes any plant non-linearity and time lags.

The Ziegler–Nichols tuning creates a “quarter wave decay” to give PID loops with best disturbance rejection performance. The Ziegler–Nichols method has some disadvantages, as this setting typically does not give very good command tracking performance. Also, the Ziegler–Nichols method creates an aggressive gain and overshoot, and most applications require the PID system to minimize or eliminate overshoot. Using the step function, bounded system, the Unitronics propriety algorithm produces a practical and industry accepted PID system for a wide range of plant types.  .

The auto tune facility requires five (5) inputs from the control engineer for the initial auto tune run, Set point (SP), Process variable (PV) low limit, high limit, controlled variable (CV) low limit and high limit.

The user can set the number of stages the auto tune applies in the plant recognition algorithm. The default setting is three, but can be decreased to a single cycle, an improvement on the vision range which is 3 to 8 cycles.

The higher the number of stages, the more accurate the derived plant model will be, and better PID control system, but a longer tuning profile is required.

With this minimum information, the PLC controller is connect to the control loop and during the auto tune process the plant reaction to the proprietary stimulus algorithm, is measured and compared in controlled iterations.

PID control, as a mathematical and logical algorithm, does not care if the real output is a valve, electric heater, gas burner or pump, but the process control engineer does.

For ease of implementation in automated PID loops in a dynamic or Non-Steady State (NSS) scenario, the Unitronics  PID algorithm is a functional block, within the ladder control network, and there are a number of output options to enable the process control Engineer to relate the CV to the physical process.   Analogue output or time proportional digital outputs giving pulses with dynamic duty cycle via High Speed Output or Pulse Width Modulation including an option to drive directly motorized proportional valves via two Digital Outputs for open and close control.

Industrial adaptive control

The Unitronics  Unistream™ enables manufacturers and system integrators to add a 10.4” or 7” 65K TFT colour touch-screen, and full PLC functionality with multiple auto-tuned PID  loops, open loop stepper/motor control, screen displays, data, colour trend graphs, and attention-grabbing alarm screens.

Industrial adaptive control can now rely on a single device: the Unistream™, with 2,048 I/O options, and additional I/O via the Remote I/O expansion so you never run out of I/O.

USB programming port, 2 x USB Host, 2 x Ethernet, Can Bus, RS485 and in addition, data logging, trends, Micro SD card, USB flash drive and MP3 player. The Unilogic™ has built-in recipe capability, tents, actions (alarms), and UniApps™ an expanding library of value added PLC embedded apps while maintaining a low budget, reduced programming time, and minimum wiring.

The controller is fast; using High performance – 800MHz, 32 Bit processor with Graphic accelerator with a bit operation time of 0.13 μsec, making it a natural for ‘rapid response’ applications such as packaging machines.

Communication options include TCP/IP Ethernet, cellular, and protocols such as MODBUS, DF1 slave, J1939, CAN-open. In addition, the Unistream™ can be ‘taught’ to communicate via almost any device-based protocol using the 3rd party message composer.

As is normal with Unitronics, all the software is free to download.

The loop tuning debate will continue, but as auto tuning algorithms reach maturity, industry will accept the reliability and ease of tuning and, one hopes, that in the next 15 years the reported percentage of process control loop set to manual will be significantly reduced.

The complete range of Unitronics products can be obtained from Alldrives & Controls,

References

Baker, G. (1/1/2009). Is Automated PID Tuning Dependable? Retrieved Febuary 8, 2010, from http://www.controleng.com: http://www.controleng.com/article/274738-Is_Automated_PID_Tuning_Dependable_.php

David B. Ender. (n.d.). www.protuner.com/perform.pdf. Retrieved Febuary 8, 2010, from www.protuner.com: www.protuner.com/perform.pdf

Summary
Article Name
White Paper on PID Controllers.
Description
Modern manufacturing plants require increasingly complex processes to produce finished products. Raw materials are subjected to a myriad of process steps—and each process step must be under control. That control must be efficient, safe, and reliable
Author
Publisher Name
AllDrives and Controls Ltd
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