PID controller integrated

Dipl.-Ing. Joachim Gräfer

Due to those intelligent control devices a large-scale cable routing is not required. But also for new installations being coupled via fieldbus systems, the transfer of functions to the field level is useful for reasons of unloading the bus. In the following contribution the intelligent control actuator series Contrac is described, into which in addition to the actual position controller among others a PID process controller is integrated optionally.

A control engineering process is generally influenced by changing the mass or energy supply via a final control element. This is effected e.g. by a valve in connection with the intelligent linear actuator Contrac of the LME and/or LSB series or via a control damper in connection with the intelligent part-turn actuator Contrac of the PME and/or RHB series (Fig. 1 and 2).

In almost all automated processes of control engineering it is necessary to keep the different physical variables, such as temperature, pressure or mass flow, constantly on a specified value or to adjust them to the process requirements.

For this purpose, a control is required which can maintain or adjust the amount even in case of an influence by external disturbance variables.

A usual control algorithm is described by the PID controller:

Y(t) = -Kp * [ (x – w) +

– 1/Tn (x – w) dt + Tv d/dt (x – w) ]

with w = command variable, x = controlled variable, Kp = gain of the P controller, Tn = integral action time of the I controller and Tv = derivative action time; device constant of the PD controller.

Due to this, it is possible to react on changes of a specified output variable (P) without a constant deviation (I) and without derivation (D).

In case of interferences of the control system, the task of modulating control actuators, being integrated in control loops, is to transmit the positioning commands of the controller to a final control element (e.g. valve, damper, etc.) in such a way that the control deviation remains as small as possible (Fig. 3). For the continuous positioning of the final control element, the preceding controller generates the position setpoint. This setpoint is compared to the actual position value of the current position which is fed back by a position transmitter. The resulting differential signal is used as variable for triggering the actuator motor. The triggering is effected as long as the differential signal is balanced. Due to this principle, the actuator continuously follows the position signal even for very small deviations and is permanently in operation. Reactive process forces on the final control element, which are opposed continuously, also cause a deviation of the actual position value.

For reasons of an intended simplification (costs and efforts), even today the controller is frequently integrated into the process control level as device or as software module as shown in figure 4a. Additional cable, installation as well as commissioning and testing efforts in spite of important costs often are not specifically included in the consideration of the efforts. When including all expenses, however, there is no cost saving in this regard.

Also for the separate installation of a controller unit on site (Fig. 4b), nevertheless three devices are still required: transmitter, controller and actuator. Although the installation and testing effort compared to the PID controller version in the process control system is reduced since the distances to the control room are inapplicable, now as before separate additional instruments and power supply units are required.

By means of the controller integration in the actuator (Fig. 4c), it is possible to include the superimposed controller function, which so far has been processed in the existing control system or in a separate controller, in the actuator directly in the real control loop on site. Due to this, apart from the controller module as implemented software or as separate instrument also the cabling effort to the process control system is not required. Furthermore, a possibly existing bus system in case of a bus coupling is not loaded additionally by the controller parameters.

Moreover, there might be further reasons to close the control loop directly on site:

• a not existing process control system,

• a required quick control (for an analog imposition) and/or

• an intended cost reduction (omission of instruments and components).

The PID controller is an optional part of the Contrac actuators. Embedded in the electronic unit, as far as required, in this case it is integrated in the software and available for applications where the control loop shall not be closed only via the process control system but directly in the field, on site.

The configuration, parameter setting and operation of the integrated controller is possible via a graphic screen operating surface. The transmitter power supply is effected from the existing transmitter supply source of the Contrac actuator.

A decision for integrating the process controller into the actuator certainly depends on several factors. Apart from the present plant philosophy and future planning requirements, there are further criteria, not being considered here, which have to be taken into account in this connection.

However, the following advantages result from the consideration of a controller integrated in the actuator compared to a PID control in the process control system in favour of the integrated version:

• improvement of the computing performance of the control system; in case of a direct analog imposition the controller integrated in the actuator is faster,

• the control loop remains operable even in case of higher-level failures of subsystems,

• facilitated commissioning by the transfer of the controller function into the field level and consequently a definite local assignment.

The advantages of a controller integrated in the actuator compared to a control loop installation with separate PID controller can be outlined as follows:

• the control loop consists of only two devices (actuator and transmitter),

• the controller as instrument to be installed and the necessary additional instruments, such as for example power packs and supply units are not required

• unimportant cabling and installation effort,

• reduced probable errors and

• lower test and commissioning effort.

Hartmann & Braun

Fax: ++49/571/830-1860

Further information cpp 205