Positive safety control device

A positive-safety control device for producing action when the value of a signal delivered in the form of direct-current voltage by a detector is located outside a range limited by two adjustable values, namely the value of voltage at which the device delivers a direct-current output voltage having the logic value +1, said device being such as to deliver the output logic value 0 in respect of any other value of the signal outside said range of values or in the event of any failure of an element of the circuit, wherein said device comprises:

• a stabilized supply which delivers a periodic rectangular-wave voltage of constant amplitude,
• a rectifier connected to a secondary winding of an electrical isolation transformer whose primary winding is supplied with the voltage delivered by the supply said rectifier being such as to deliver a direct-current voltage,
• a detector which is supplied with direct-current voltage through the rectifier and delivers a direct-current voltage having an amplitude Ve which is proportional to the lvalue of the signal to be measured,
• two generators for producing direct-current voltages of opposite sign Vd and Vref, the inputs of said generators being supplied with the voltage induced in the secondary winding of the transformer
• an adjustable voltage divider which is connected to the secondary winding of the electrical isolation transformer and delivers a periodic rectangular-wave voltage having an amplitude Vs,
• means for comparing the voltage Ve with the algebraic sum of three voltages consisting of the two direct-current voltages having amplitudes Vd and Vref and the periodic rectangular-wave voltage Vs and for delivering an output voltage S which is zero if the voltage Ve is not comprised within the range (Vd + Vref, Vd + Vref + Vs) and equal to +1 if the output voltage is comprised within said range,
• means for electrically isolating, amplifying and rectifying the output voltage S.