Control is the direction of an operation, e.g. to ensure the correct operation of a machine, [...] as opposed to the straightforward compensation of a malfunction - the regulation process. [Lexikon, Lingen Verlag, Volume 17] The essential function of a control process is the permanent measurement of different process parameters.

The most important prerequisite for achieving the controlled implementation of a process is the measurement of critical process parameters. Instead of critical process parameters, quality-relevant variables (that influence product quality) are also referred to. Sensors are used to measure the different variables such as temperature, pressure, humidity, speed, etc. When selecting the sensors, it is particularly important to specify the following at the outset (see chapter 1 Design qualification and figure 4.C-1).

Quality-relevant measuring points must be calibrated regularly. In addition to the actual measuring sensors, cables and evaluation units are also integrated into the measuring circuit. All components in the measuring circuit have built-in tolerances which influence the measurements so the entire measuring circuit must always be taken into account during calibration. (See chapter 4.G Calibration.) A list of all quality-relevant measuring points is required for the qualification, and the necessary documentation must be included in the specified delivery scope of the facility supplier.

Straightforward processes may be controlled manually provided the measured values are visible. Manual control is no longer feasible as soon as two or more elements influence the process. At this point automatic control loops take over and keep the parameters within the specified limits. The qualification of critical control loops is an important part of the function check for control systems in pharmaceutical machines. The most important aspects of this check are the input/output tests and review of the circuit diagrams.

To enhance product safety, facilities with several control loops are operated via plant controls. In former times facilities were controlled by contactors. Today microprocessors or programmable logic control (PLC) are used. Operating and monitoring systems are used to enable the user to visualise and operate these control systems - often via a computer. These cases are referred to as computerised systems, as in: "A system including the input of data, electronic processing and the output of information to be used either for reporting or automatic control." [EU GMP Guideline, Glossary] (see chapter chapter 9 Computer Validation).

Furthermore, many plant controls are designed for networking with higher-level process control systems. These kinds of configurations make "paper-free" operations feasible and may be realised in individual cases. However, the degree of automation should be carefully defined while considering the required validation expenditure. Considerable costs are generated by the initial validation, and more so for the revalidation of more complex computerised systems. Moreover, it must be considered that isolated defects may lead to failure of the entire system with attendant down times and revalidation times.

In practice it is useful to differentiate between plant controls which require a conventional qualification and computerised systems which require a computer validation. The EU GMP Guideline, appendix 11.2. grants the following flexibility: "The extent of validation necessary will depend on a number of factors [...]". In the case of standard machines with microprocessors or standard PLC components, a simplified computer validation should be carried out while considering the cost-benefit equation. The expenditure required for the validation of computerised systems increases with the degree of software customisation, as it does for systems that are not as widely used. Straightforward standard machines with non-variable microprocessor controls may be validated with relatively little effort.

Several basic GMP features that should be available with every control are listed below: