Unitronics In control

Are you in Control?

In modern manufacturing plants, the myriad of processes that the raw materials are subjected to, with increasing complexity to form the finished product, are many. Each process step required has to be under control, and that the control is efficient, safe and reliable.

The processes control work horse, the three term local loop PID controller, was first described analytically by Nicholas Minorsky in his 1922 paper “Directional stability of automatically steered bodies” and in its many guises 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 1942, Cohen-Coon, developed 1953, Minimum error-step load change Gerry developed 1998,and others are available to the plant control engineer and 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), sited that more than 30% of control loops where operating in manual, and this was based on the 1993 article by (David B. Ender) indicating little change over the last 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 or ending.

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 has developed a proprietary auto tune PID algorithm to remove the burden of plant tuning in a novel and unique way.

The New Vision V1040

V1040The key objective was to develop an easy to use production control system and elements using knowledge-based engineering concepts.  Based on a step function bounded system, the auto tune facility requires five (5) inputs for the initial auto tune run, Set point (SP), Process variable(PV) low limit, process variable high limit, controlled variable(CV) low limit and controlled variable high limit. The CV may be analogue or a time –proportional variable.

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 increased to a maximum of eight.

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.

The Unitronics  Vision1040™ enables manufacturers and system integrators to add a 10.4” 65K SVGA colour touch-screen equipped with 9 programmable function key display, and full PLC functionality with 24 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 Vision1040™, with 1,024 I/O options, a USB programming port, data logging, and built-in recipe capability while maintaining a low budget, reduced programming time, and minimum wiring.

The controller is fast; scanning 1K of a typical application in 9 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 Vision1040™ can be ‘taught’ to communicate via almost any device-based protocol.

Is Automated PID Tuning Dependable?

To make automation simple and efficient, Unitronics’ R&D has field-tested PID Server to provide a fast, easy loop tuning pc based system that is linked to the PLC controller.

The PID Server PC utility enables the control engineer to auto-tune PID loops for both the Vision and M90/91 controller series. Although it is installed as part of the VisiLogic/U90 Setup, PID Server runs independently of other Unitronics software.

The Unitronics controllers with auto tune have been in the market over 4 years with successful results. Access to the forum reveals a wide range of applications and high loop counts, in some cases over 40 PID loops for one controller, indicating a high acceptance, high reliability and ease of use.

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

The vision of Unitronics is to continue to refine the control algorithm, and to fine-tune the PID Server to suit a broad range of PID applications.

Inserted in the software is a ‘Tell Us’ feature. Clicking ‘Tell us’ will create an email with an attached copy of your auto-tune and PID process, and if possible, before sending  the email, take a moment to put the details of the application in the body of the email so that all users benefit from the synergy of improvements in the algorithm.

As seen in the image, the desired set point (blue) is tracked by the process variable (red) due to the controlled variable (green). Each of the individual control variable parameters, P,I and D can be observed in the green trace, P + D for the step change followed by the integral action to reduce the set point error to zero.

With the Server PID software the control engineer can review the results of the auto tune system.

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,

Email Sales@alldrivesandcontrols.co.uk

References

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

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