INDUCTIVE VIBRATION PICKUP APPARATUS

• US 3,707,671 A
• Filed: 05/01/1970
• Issued: 12/26/1972
• Est. Priority Date: 05/01/1970
• Status: Expired due to Term

• Current Assignee: Kenneth E. Hays, Lloyd D. Penn, Robert S. Morrow
• Original Assignee: Kenneth E. Hays, Lloyd D. Penn, Robert S. Morrow
• Inventors: Morrow, Robert S., Penn, Lloyd D., Hays, Kenneth E.
• Primary Examiner(s): Smith, Alfred E.
• Application Number:
  US05/033,673
• Time in Patent Office: 970 Days
• Field of Search: 324/34,40,41 331/65,109 336/179
• US Class Current: 324/224
• CPC Class Codes: H03K 17/14 Modifications for compensating variations of physical values, e.g. of temperature
• CPC Class Codes: H03K 17/9505 Constructional details
• CPC Class Codes: H03K 17/9525 controlled by an oscillatory signal H03K17/9537 takes precedence
• CPC Class Codes: H03K 17/9547 with variable amplitude

Abstract

Electrical sensing apparatus such as proximity switches and vibration pickups of the type in which the distance between an inductor in the tank circuit of an oscillator and a metallic object in the field of the inductor is indicated by the amplitude at the output of the oscillator. The apparatus incorporates means adapted to compensate the output and/or sensitivity of the oscillator for temperature variations in either the inductor or the oscillator circuitry. Temperature compensation is achieved by means of thermistors, one of which is in series with the inductor and the other of which is in the oscillator network.

Claims

1. In vibration pickup apparatus, the combination of an electrical oscillator including a tuned circuit having at least one inductive and capacitive element therein, said inductive element being in the form of a coil of wire at one end of a probe adapted to be placed in close proximity to a body whose vibrations are to be sensed whereby the amplitude of the oscillations produced by said oscillator will be a function of the spacing between said coil and said body in the field of said coil, a metallic tubular element surrounding said coil and forming with said coil an autotransformer arrangement, said coil of wire having a positive temperature coefficient of resistance, and a thermistor having a negative temperature coefficient of resistance connected in series with said coil in the tuned circuit, said thermistor being carried within said probe closely adjacent said coil whereby the coil, the surrounding tubular element and the thermistor will be subjected to the same temperature and the total impedance of the coil and thermistoR as presented to the oscillator will be substantially temperature invariant.

2. The electrical sensing apparatus of claim 1 including at least one resistor connected in parallel with said thermistor whereby the combined resistance of the parallel combination will vary linearly with temperature.

3. The electrical sensing apparatus of claim 1 wherein the series combination of said coil and thermistor is connected in parallel with a second inductor in said tuned circuit.

4. The combination of claim 1 wherein said coil of wire is wound around a bobbin of insulating material disposed adjacent to one end of said metallic tubular element, the thermistor being carried within a bore in the bobbin.

5. The combination of claim 1 wherein said oscillator includes a transistor having said tuned circuit connected between its emitter and collector, and a second thermistor included in the base bias network for said transistor.

6. The combination of claim 5 wherein said second thermistor also has a negative temperature coefficient of resistance.

7. The combination of claim 5 wherein said base bias network comprises a plurality of impedance elements connected in series between the opposite terminals of a source of driving potential for said oscillator, one of said impedance elements comprising said second thermistor, and a connection between the junction of two of said series-connected impedance elements and the base of said transistor.

8. In vibration pickup apparatus, the combination of an electrical oscillator including a tuned circuit having at least one inductive and capacitive element therein, said inductive element being in the form of a coil of wire at one end of a probe adapted to be placed in close proximity to a body whose vibrations are to be sensed whereby the amplitude of the oscillations produced by said oscillator will be a function of the spacing between said coil and said body in the field of said coil, said coil of wire having a positive temperature coefficient of resistance, a thermistor having a negative temperature coefficient of resistance connected in series with said coil in the tuned circuit, the series combination of said coil and thermistor being connected in parallel with a second inductor in said tuned circuit, at least one resistor connected in parallel with said thermistor whereby the combined resistance of the parallel combination will vary linearly with temperature, said probe being generally tubular in configuration and said coil of wire being wound around a bobbin of insulating material disposed adjacent one end of said tubular probe, the thermistor being carried within a bore in the bobbin, said oscillator including a transistor having said tuned circuit connected between its emitter and collector, a base bias network for said transistor, said base bias network comprising a plurality of impedance elements connected in series between opposite terminals of a source of driving potential for said oscillator, one of said impedance elements comprising a second thermistor having a negative temperature coefficient of resistance, and a connection between the junction of two of said series-connected impedance elements and the base of said transistor.