Process and apparatus for the control of a process variable when an indefinite measuring signal is encountered

A process for controlling a variable value, wherein a measuring signal value is a constant nonlinear function of said variable value so as to define a characteristic curve having an extreme value which divides said curve into a monotonically ascending section and a monotonically descending section, said process comprising the steps of:

• producing said measuring signal value as a function of said variable value,
• providing a control signal value corresponding to a set value of said variable value and lying on one of said ascending and descending sections of said curve,
• comparing said control signal value to said measuring signal value and, in the event that a differential value exists between said control signal value and said measuring signal value, said differential value is delivered to a signal generating means which generates and supplies a correcting signal to a control element which changes said variable value in a manner causing said measuring signal value to approach said control signal value,
• said differential value is delivered to said signal generating means by:
• delivering said differential value to operate a first signal generator of said signal generator means when said differential value lies within a predetermined range, whereupon said first signal generator generates said correcting signal, and
• delivering said differential value to operate a second signal generator of said signal generating means when said differential signal lies outside of said range, wherein said second signal generator generates said correcting signal.