9.3 Control Properties

9.3 Control Properties

Self-operated regulators are usually designed as proportional controllers. The control behavior of a P-controller is essentially determined by the proportional-action coefficient (former term: proportional band) as well as by the adjusted operating point.

To describe the correlations as application-oriented and clear as possible, the following explanation is based on the example of the pressure reducing valve, as in the chapters above. With respect to control engineering, these statements are applicable to any other self-operated regulator with proportional control action.

The fundamentals of control engineering (see L102EN, for instance) teach us that if steady-state errors are to be kept as small as possible, a proportional-action coefficient as high as possible (or a small proportional band) is required. In the vicinity of an operating point, is calculated from the manipulated variable y and the error e:

In pressure reducing valves, it must therefore be achieved that small pressure changes create great travel adjustments which in turn create great value changes:

- Large travel adjustments are created if the spring stiffness cf is as small as possible and the actuator diaphragm area is large.

- The change of the value is related to the contour of the plug and the value. At the same travel, if the gradient of the control characteristic is high and/or the value is high, the value changes are bigger than with a flat characteristic and/or small value.

If the valve is sized for a high proportional-action coefficient, i.e. small system deviation, the following equipment is required:

- soft spring, large actuator area, and high value, i.e. overdimensional in this case.

Example: With respect to spring and actuator these requirements are best met by a self-operated regulator with the lowest set point range. For a set point of 1.0bar, therefore, a set point range of 0.2 to 1.2bar must be selected, and not the version ranging from 0.8 to 2.5bar.

Note: As described on page 11, a control valve requires the following equipment for high positioning forces:

- stiff spring, small actuator area or long spring range.

If the required device shall exhibit high positioning forces while system deviations are to remain small, contradictory requirements must be fulfilled in the sizing of spring and actuator area. There are only two solutions to this problem:

- Realization of small system deviations via high values.

- Compensation for high positioning forces via soft, though sufficiently long springs.

Both possibilities are restricted in their application. While extremely long springs result in complex and expensive units with large dimensions, the use of an oversized value is restricted due to physical limitations: during positioning, the actuator must overcome the static and the sliding friction which is created along the guide and seal of the plug and actuator stem. If these frictional forces as well as the additional forces required to close the valve are taken into consideration, the result is the actual manipulated reaction as illustrated in Fig. 9, and not the ideal characteristic shown in Fig. 7.

Fig. 9: Characteristic of a pressure reducing valve

The hysteresis created by the static friction limits the control accuracy that can be reached. This error cannot be compensated for by using a higher value to increase . Although this would reduce the stationary system deviation , the hysteresis in the control characteristic will remain (Fig. 10).

Fig. 10: System devitation with different proportional-action coefficient

Therefore, an oversized value involves the risk that the system begins to oscillate: on the one hand, the accurate adjustment of the value will become more difficult due to the hysteresis; on the other hand, already small system deviations will then result in extremely big position changes.

Due to the described correlations and when control demands are high, it will always be desirable to reduce the positioning forces as much as possible by using pressure balancing systems and, at the same time, selecting the version with the smallest set point range.

By following these sizing principles - balancing bellows, soft springs, large actuator diaphragm and, if required high values - the system deviation in self-operated regulators can be kept at a minimum. However, proportional-action coefficients that are too high, especially when realized in combination with oversized values, involve the risk that the control loop starts to oscillate.