Patent GB-773982-A (GB)
Title: An improved proximity switch
Inventor: ELMS GEOFFREY
IPC: G01F23/26
Filed: 1954-03-10
Abstract: 773,982. Proximity switches. ELCONTROL, Ltd. March 10, 1955 [March 10, 1954], No. 7000/54. Class 37. In a capacity-operated proximity switch of the kind in which a R.F. oscillator has its state of oscillation or non-oscillation determined by the nearness of an external object to a sensing electrode 1 connected to the oscillator circuit 21, the R.F. oscillator controlling the A energization of an electromagnetic relay - hav- 2 ing contacts A1, A2 for controlling an external circuit, the electrode and the R.F. circuits concerned are mounted as a single unit 21 for location at the desired sensing position and the oscillator is connected to the relay energizing and power supply circuits 23 through a flexible low-impedance power cable 22 which does not carry any R.F. current. In the embodiment illustrated, the oscillator circuit or sensing unit 21, mounted in a separate unit at the location adjacent the level of a substance to be detected, comprises a sensing electrode 1 projecting through and insulated from a short metal sleeve 2 which is earthed, is connected to the anode 3 of a pentode VI the external circuit of which is arranged in the manner of a Colpitts oscillator such that a change in the external anodecathode capacity is operative to control the degree of feedback in the oscillator. The H.T. terminal 12, earth line terminal 15, heater terminal 17, and measuring circuit terminals 19, 20 of the sensing unit 21 are connected by a flexible low-impedance power cable 22 to corresponding terminals 121, 151, 171, 191 and 20<SP>1</SP> of a remote relay and power supply unit 23. The A unit 23 comprises an electromagnetic relay - 2 having a pair of normally closed contacts A1 and a pair of normally open contacts A2, a power supply transformer 24 and a double triode V2, A V3. The relay - is connected to the anode 25 2 of the side V2 of the double triode, the cathode of which is connected to the H.T. terminal 12<SP>1</SP>. A The other side of the relay - is connected to 2 one end 26 of a potential divider 27 the other end 28 of which is connected to one part 29 of a secondary winding of the transformer 24. The other part 30 of this winding is connected between the end 26 of the potential divider and the H.T. terminal 12<SP>1</SP>. Across the potential divider 27 is connected a reservoir condenser 31. The slider 32 of the potential divider 27 is connected through a current limiting resistance 33 to the control grid 34 of the side V3 of the double triode. The anode 35 of this triode V3 is connected to the H.T. end of the winding 30 through an anode load 36, and to the control grid 37 of the triode V2 2 through a high resistance 38. In operation, the capacity of the probe 1 to earth forms, with the control grid-cathode network 8, 9, 10 a potential divider across the inductance 6 and the R.F. voltage across the network 8, 9, 10 is applied between grid 4 and the cathode 7 of pentode VI to determine the state of oscillation or non-oscillation of the pentode VI according to the capacity to earth of the probe. Normally in the state of non- oscillation, with no object near probe 1, the pentode VI draws current from the divided secondary 29, 30 of the power supply transformer 24 and a voltage appears across the potential divider 27 which charges the reservoir condenser 31 so that the end 28 of the potential divider is positive to the end 26. If the slider 32 is set towards the negative end 26 of the potential divider so that the triode V3 is cut off so long as the pentode V1 is not oscillating, its anode 35 is at H.T. potential, since the grid 37 of the other triode is D.C. coupled to it, the grid 37 and cathode 39 are at the same potential and V2 passes sufficient current to energize the A relay -. If now the capacity to earth of the 2 probe 1 is increased by the approach of an object, the pentode VI begins to oscillate and draws less current from the winding 29, 30. The D.C. voltage drop across the potential divider 27 thus decreases until the triode V3 becomes conducting and cuts off the other triode A V2. The relay - is de-energized, the triode 2 V2 being held cut off by the potential maintained across the condenser 40.