Patent GB-1226770-A (GB)
IPC: H03K5/15
Filed: 1968-06-19
Abstract: 1,226,770. Electrical tests. ELECTROFACT N.V. June 19, 1968 [June 19, 1967], No.29119/68. Heading G1U. [Also in Division H3] Apparatus for cyclically scanning a plurality of sensors and testing whether the value of a quantity derived from each lies inside a predetermined limit comprises a self-stepping scanner which connects the sensors in turn to a common testing device, the testing device being arranged to stop the stepping action if the measured value lies outside the limit or if a fault occurs in the scanning and testing apparatus, and to give a warning if the measured value lies outside the limit or if the stepping action ceases. In one embodiment the voltage at each sensor is measured and compared with upper and lower limits preset for that sensor. Alternatively the voltage may be tested against, say, two upper limits which respectively represent deviations of a minor and more serious nature from its normal value. When a limit is exceeded the fact may be recorded and stepping automatically resumed. Scanning Circuit The scanner may be a stepping switch or the scanner circuit of Fig. 1. This circuit comprises a ring of units 1, 2..., one for each sensor, each including a relay 4 which, when energized, connects the respective sensor (not shown) to the testing circuit. On energization of relay 4 of unit 1 a positive voltage on lead C starts to charge the capacitor 9<SP>1</SP> of unit 2 through contact 7 and resistor 8<SP>1</SP>. When the capacitor voltage reaches a sufficient value thyristor 3<SP>1</SP> is fired and conducts through resistor 5<SP>1</SP>, relay coil 4<SP>1</SP> and diode 6<SP>1</SP>. Through the connection L relay 4 of unit 1 is now deenergized, and at the same time relay 4<SP>1</SP> is energized, thus connecting lead C to the next unit and also, by means of contact 11<SP>1</SP>, switching off thyristor 3<SP>1</SP>. Diodes 12, 13, 14 supply lamps to indicate which unit is operative at any given time. To give warning should the stepping action cease, diodes 18, 18<SP>1</SP> connect the charging circuits alternately to leads X, Y. Under normal operation these leads therefore carry repetitive pulses which are used (Fig.2) to keep capacitors 21, 22 continuously charged. If the stepping ceases these capacitors will, after a short time, have discharged sufficiently to switch off the normally-saturated transistors 27, 28 thereby de-energizing the coil of relay 29 and sounding an alarm. Voltage Testing and Indicating Circuits For each sensor two potential dividers 35, 36, Fig. 4 (not shown) are provided, which, on energization of the associated relay 4, are connected between supply leads E, F to provide upper and lower limit voltages. The voltage at the sensor is amplified and compared with these limits, and if outside them the upper or lower limit relay 62, 63 Fig. 5 (not shown) is operated, which cuts off the voltage normally applied to the lead C, thereby, after a short time de-energizing relay 29. To store an indication that a limit has been exceeded the register of Fig. 6 may be used, comprising a pair of relays 89, 99 for each sensor, connected into circuit in turn by a contact 77 of the corresponding selector relay 4. Capacitor 84 is normally charged through Zener diode 85, but if relay 29 becomes de-energized because, say, the upper limit has been exceeded, the capacitor is disconnected from the supply and discharged through diode 88, relay 89 and the appropriate contacts 78-83 of relays 62, 63, 29. The momentary energization of relay 89 is sufficient to operate its contacts 90-92, thereby connecting it through diode 96 to a +80V supply so that it remains energized, indicating that a limit has been exceeded. If on the next cycle it is found that the limit is no longer exceeded, the still-closed contact 91 will enable capacitor 98 to be charged through resistor 103 from a +160V supply, which will result in the deenergization of relay 89. Relay 99 functions in the same way as relay 89 for the lower limit. When the above register is used, de-energization of relay 29 owing to the operation of a limit relay can be arranged to reapply the voltage to the lead C, thus automatically restarting the scanning. Detecting Faults in the Scanning and Testing Apparatus Faults in the scanning or testing circuits result in the application of a positive voltage to one of the inputs 48, 49, 61, Fig. 3, of an OR gate. Transistors 32, 33 are thereby switched on, bringing lead B to a low potential and cutting off transistor 70, thus de-energizing relay 71. Lead B supplies the voltage to the lead C, so that a fault automatically stops the stepping action. Relay 71 indicates that a fault has occurred. The faults which operate the OR gate are: (i) Failure of thyristor 3 in the scanning circuit to switch off. The thyristor cathodes are connected to earth through resistor 30 so that prolonged conduction of a thyristor applies a voltage to diode 49. (ii) Open circuit between the sensor and the amplifier. The circuit connecting the sensor to the amplifier has a low impedance at high frequencies. The secondary 55, 56 of transformer T is connected in this circuit so as to present to the oscillator 58 a low impedance, but on occurrence of an opencircuit fault a high impedance is presented, resulting in the charging of capacitor 60 through rectifier 59 and consequent triggering of the gate. (iii) Failure in the circuit of the limit setting potential dividers. If at any instant no potential dividers are connected to the supply E, F no current flows through diode 46 and as a result a voltage is applied to diode 48. Detecting faults in the Register Circuit Faults in the register circuit are indicated by relay 104, Fig. 6, which is normally energized through diode 85. Should the stepping cease, relay 104 will be de-energized unless contacts 80, 78, 79, 77 and 90 or 93 have operated correctly, that is, unless all the relays in the register are functioning correctly. De-energization of relay 104 activates an alarm and stops further stepping.