PID control algorithm top or measurement reliability

Paul Boughton

Coorous Mohtadi looks at the evolution of general purpose temperature controllers.

In the analogue control market, there are three distinct yet related segments where key analogue process variables are controlled.

These are: At the base there is the simple stand-alone systems where majority of control is performed with on-off schemes. Almost all applications involve temperature. At the top we have the sophisticated and complex petrochemical and chemical as well as many electro mechanical systems where the variables are highly interactive. Cascade, ratio, predictive or other advanced control schemes need to be used and solutions are arrived at by expert integrators and project teams. In the middle, where the largest segment of the market resides, general purpose stand-alone controllers are typically employed.

The industry standard PID control algorithm in its various incarnations continues to be the most reliable technology for the general purpose control of analogue measurements.

Temperature loops form over 80percent of this market segment. Other common variables such as flow, pressure and level are also controlled using the same strategies. In this sector the user expects a combination of ease of use, good user interface, high performance with high stability, accuracy and repeatability.

Ease of use starts with attention to detail regarding all aspects of a controller. These days where size of panels is shrinking progressively, it is critical that instruments are as shallow as possible. Flat membrane keys and high IP rating are also extremely important especially in applications in food and beverage industries. Small details such as size and orientation of screw terminals to allow quicker wiring of panels, as well as meeting all the relevant international standards without resorting to external components, play a significant role in the decisions made by the panel builders, OEMs and nd-users.

PID control is typically used to deal with two categories of applications. The PID parameters can be either adjusted to provide the best disturbance response or to provide the best response to changes in set-point. It is difficult to tune for both, as they require different tuning rules. Sometimes, as in many autoclaves, extruders and furnaces, a compromise performance is tolerated by the users. For these applications a two degree of freedom control is absolutely essential so that set-point change response and disturbance response can be controlled separately. In this way it is no longer necessary to obtain a compromise solution but one can obtain the best solution for both cases.

It is also common place to expect an auto-tune feature, where on-off (or other cyclic) test signals are used to identify some characteristic feature of the process, combined with self-tune, where the PID performance is continuously monitored and tuning parameters are optimally adjusted at appropriate times. In this way, the new software technology allows the average operator to make best use of the controller with minimal amount of interference. It empowers the operator to obtain the best performance and yield out of his process without having to become an expert in the control technology. These tuning strategies have been available in industrial controllers for over two decades. By employing some of the advanced strategies used in highly demanding petrochemical industries, now even 1/16DIN temperature controllers are able to claim to possess robust tuning features, usually only available in more sophisticated DCS systems.

Use of transmissive LCD displays with two or three colours, which can change under program control or alarm conditions, open a new vista for control of simple machines and processes. Simple colour change from green to orange and red can signify changes in process conditions very visibly and clearly -- where in the past combinations of lamps and LEDs had to be used. This in turn reduces the amount of unnecessary clutter on a control panel.

Of particular importance is the ease of use, configuration and operation of standalone controllers. Simple to use Windows based software usually complements well-structured user interface menus and scroll lists, which allow the user access to the most commonly used parameters. Effective password security, parameter masking and key protection, allows safe yet powerful use of temperature control equipment.

Clear diagnostics and alarm functions such as thermocouple shorts and breaks, RTD failures, heater burnout and SSR short circuit alarms as well as normal operating currents, etc. allow the users to establish a proper scheme for maintenance and fault finding in processes. Modern schemes of detecting loop breaks without dedicated sensors complement the sensor-based methods and result in powerful instruments.

Last and by no means least is open communications. A range of communication platforms are required ranging from tried and tested serial Modbus to DeviceNet, Profibus, Ethernet and wireless technologies. Many of these are at present achieved through economic use of industrial gateways, which in conjunction with function block technology yield a smart yet simple platform for getting the data from the shop floor to the board room and vice versa.

The requirements of the general purpose control market sector appear to be too taxing for any one supplier or product to suit many applications. The breadth of product experience and deep understanding of the market demands have allowed Omron to design the E5_N series of controllers to meet these requirements in an evolutionary and progressive manner. The series has 2-PID, Omron's two degree of freedom algorithm, which has been tried and tested for many years.

In addition, the new robust tuning based on predictive control has been proven to be invaluable in some more demanding applications. The diagnostics such as three-phase heater alarm and loop break alarm simplify the fault finding task significantly.

Furthermore, the E5_N family of controllers is easy to use, and parameter masking offers the possibility of customised menus. Finally, the PV colour change features coupled with the quality of the display and the IP66 rating make the E5_N series the leaders in their class of controllers.

Coorous Mohtadi Is Technical Manager Industrial Components, Omron Europe, Hoofddorp, The Netherlands. www.eu.omron.com

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