Capacitive distance sensor

• US 6,320,394 B1
• Filed: 02/05/1998
• Issued: 11/20/2001
• Est. Priority Date: 02/14/1996
• Status: Expired due to Term

• Current Assignee: Apple Inc.
• Original Assignee: STMicroelectronics SRL (STMicroelectronics NV)
• Inventors: Tartagni, Marco
• Primary Examiner(s): BROWN, GLENN WARNER
• Application Number:
  US09/019,496
• Time in Patent Office: 1,384 Days
• Field of Search: 324/661, 324/662, 324/671, 324/676, 324/686, 324/687
• US Class Current: 324/671
• CPC Class Codes: G01B 7/004 for measuring coordinates of points
• CPC Class Codes: G01B 7/023 for measuring distance between sensor and object G01B7/082 and G01B7/102 take precedence
• CPC Class Codes: G01B 7/34 for measuring roughness or irregularity of surfaces
• CPC Class Codes: G06V 40/1306 non-optical, e.g. ultrasonic or capacitive sensing

Abstract

A distance sensor has a capacitive element in turn having a first armature which is positioned facing a second armature whose distance is to be measured. In the case of fingerprinting, the second armature is defined directly by the skin surface of the finger being printed. The sensor comprises an inverting amplifier, between the input and output of which the capacitive element is connected to form a negative feedback branch. By supplying an electric charge step to the input of the inverting amplifier, a voltage step directly proportional to the distance being measured is obtained at the output.

Claims

1. A capacitive presence sensor comprising:

a first plate positioned for facing an object for determining a distance to be measured therebetween and forming a feedback capacitive element;

a second plate positioned for facing the object, the second plate and first plate being the feedback capacitive element;

an amplifier having an input and an output, wherein said feedback capacitive element is connected between said input and said output of said amplifier, and

a negative feedback branch from the output to the input of the amplifier that includes the feedback capacitive element, the amplifier providing a voltage step output in response to a voltage step input, the output having an amplitude indicative of the presence of the object adjacent the plates.

2. The sensor of claim 1, wherein said amplifier comprises an inverting amplifier.

3. The sensor of claim 1, further comprising a layer of insulating material contacting an outer surface of said first plate, such that the insulating material is positioned between the first plate and the object.

4. The sensor of claim 1, further comprising:

a logic circuit connected to said input of said amplifier for generating an electric charge variation; and

output detecting means for detecting a voltage step at said output of said amplifier.

5. The sensor of claim 4, wherein said logic circuit comprises a reference voltage source for generating a voltage step;

and a capacitive element interposed between said voltage source and said input of said amplifying means.

6. The sensor of claim 1, further comprising a switching element connected between said input and said output of said amplifying means.

7. A distance sensor device comprising:

input voltage source for providing a step voltage;

output lines; and

an array of distance detecting cells selectively connected to the input voltage source and to the output lines, wherein said cells each comprise a capacitive distance sensor that includes;

a first plate positioned for facing an object;

a second plate positioned for facing the object, the first and second plate positioned for determining a distance to be measured to the object and forming a feedback capacitive element; and

an amplifier having an input terminal and an output terminal with said feedback capacitive element being connected between said input and said output of said amplifier;

a negative feedback branch, and an output signal producing a voltage step in response to the input voltage source providing a step voltage, the output signal having an amplitude proportional to the distance between the plates and the object.

8. The sensor device of claim 7, further comprising:

logic means connected to the input of said amplifier of each detecting cell, said logic means supplying said inputs with an electric charge variation; and

output detecting means for detecting a voltage step at the output terminal of said amplifier of each detecting cell.

9. The sensor device of claim 8, wherein said logic means comprises a reference voltage source and capacitive element coupled to each respective cell, each said distance detecting cell having a respective input connected to said reference voltage source via said capacitive element.

10. The sensor device of claim 9, wherein said input voltage source comprises a horizontal scanning circuit, and a vertical scanning circuit;

and

said output lines are connected to an output buffer element.

11. The sensor device of claim 9, wherein said reference voltage source comprises means for generating a reference voltage step supplied in parallel to said distance detecting cells and further including horizontal and vertical scanning means for sequentially enabling said distance detecting cells.

12. The sensor device of claim 7, further comprising a second plate positioned facing said object such that the second plate is substantially parallel to the object, said first plate being connected to said input of said amplifying means and said second plate being connected to said output of said amplifying means.

13. A method comprising:

placing an object on a dielectric layer positioned over a sensor cell, the sensor cell having a first plate connected to an input terminal of an amplifier, and a second plate connected to an output terminal of the amplifier, the first and second plates forming a first capacitive element in a a capacitive negative feedback branch of said amplifier;

applying an electric charge variation to the input terminal of said amplifier; and

detecting a voltage step at an output of said amplifier, said voltage step having an amplitude that is indicative of the distance between said dielectric layer and said object.

14. The method of claim 13, wherein said step of applying an electric charge variation comprises the steps of applying a first reference voltage to a second capacitive element, and then applying, with a step variation, a second reference voltage higher than said first reference voltage to the second capacitive element.

15. The method of claim 14, wherein said step of applying an electric charge variation is preceded by an initializing step comprising:

the steps of;

closing a reset switch to connect the input and output of said amplifier to each other; and

opening said reset switch prior to applying the electric charge variation.

16. The method of claim 13, wherein said step of applying an electric charge variation comprises the steps of applying a first reference voltage to a second capacitive element, and then applying, with a step variation, a second reference voltage lower than said first reference voltage to the second capacitive element.

22. The sensor according to claim 16 further including:

an insulation layer positioned over the first and second plates with the first and second plates being in the same plane under the insulation layer.

23. The sensor according to claim 16 further including:

a step voltage source connected to the input terminal;

a voltage sensor connected to the output terminal for sensing a change in the output voltage when a step voltage is applied to the input terminal.

17. A circuit comprising:

an amplifier having an input and an output;

a negative feedback capacitor circuit coupled from the output of the amplifier to the input of the amplifier; and

a dielectric layer being a part of the negative feedback capacitor circuit, and positioned for providing a negative feedback signal from the output of the amplifier to the input of the amplifier that is modified based on the relative location of an object adjacent the dielectric layer.

18. The circuit according to claim 17, further including:

a reset switch directly coupling the output of the amplifier to the input of the amplifier, such that when the switch is closed, the output and input are at the same voltage and when the switch is open, the output and input can be at different voltages.

19. The circuit according to claim 17 further including:

wherein the feedback circuit comprises a first capacitor plate coupled to the input of the amplifier; and

a second plate of a capacitor coupled to the output of the amplifier.

20. The circuit according to claim 17 further including:

a step voltage circuit to apply a step voltage to the input of the amplifier.

21. A capacitive sensor comprising:

an amplifier having a negative input terminal and an output terminal;

a first plate positioned for facing a location for receiving an object, the first plate being coupled to the input terminal;

a second plate positioned for facing the location for receiving the object, the second plate being coupled to the output terminal; and

a negative feedback branch from the output terminal of the amplifier to the negative input terminal of the amplifier to provide a negative feedback branch, a capacitive element being in the negative feedback branch that includes the first plate, the second plate and the object if the object is present.