Optical reflectance proximity sensor

• US 7,486,386 B1
• Filed: 09/21/2007
• Issued: 02/03/2009
• Est. Priority Date: 09/21/2007
• Status: Active Grant

• Current Assignee: Silicon Laboratories Incorporated
• Original Assignee: Silison Laboratories Incorporated
• Inventors: Holcombe, Wayne Thomas, Svajda, Miroslav
• Primary Examiner(s): Tarcza; Thomas H
• Assistant Examiner(s): Ratcliffe; Luke D
• Application Number:
  US11/859,403
• Time in Patent Office: 501 Days
• Field of Search: 356 301- 285
• US Class Current: 356/4.01
• CPC Class Codes: G01C 3/08 Use of electric radiation detectors
• CPC Class Codes: G01S 17/04 Systems determining the presence of a target
• CPC Class Codes: G01S 7/4815 using multiple transmitters
• CPC Class Codes: G01S 7/497 Means for monitoring or calibrating
• CPC Class Codes: H03K 17/941 using an optical detector H03K17/968 takes precedence
• CPC Class Codes: H03K 2217/94026 Automatic threshold calibration; e.g. threshold automatically adapts to ambient conditions or follows variation of input

Abstract

A method for calibrating an optical reflectance proximity sensor and then measuring proximity in a repeating cycle or on demand, the sensor including one or more wavelength transmitting diodes, one or more wavelength receiving diodes, an ambient correction circuit, and a comparator circuit, and further teaching steps for powering on the sensor, canceling the ambient signal during a calibration period, transmitting wavelengths to and receiving reflectance from an object in the path of the transmitted wavelengths, and measuring a reflectance pulse width and comparing the value to a preset value to determine proximity.

Claims

1. A proximity sensor comprising:

one or more wavelength transmitting light-emitting diodes;

one or more wavelength receiving photodiodes;

a photocurrent receiver connected to the photodiode and consisting of a transimpedance amplifier;

an ambient correction circuit for calibrating DC ambient in a signal generated at the receiving photodiode(s) immediately prior to a reflectance measurement, the circuit constructed as a gyrator from a transconductance amplifier, a capacitor which stores the voltage analog of the DC correction current, and a voltage to current circuit, MOS transistor, for producing the correction current ambient signal at a known rate, the period above a fixed receive-detect threshold required to cancel the DC ambient used to produce a pulse width proportional to the DC ambient signal, the same gyrator circuit used during a subsequent proximity cycle to convert an increase in input signal due to reflection above a fixed receive detect threshold into a pulse width proportional to the reflectance signal;

a pulse width comparator circuit for comparing a pulse width signal proportional to the reflected signal against a preset pulse width value to detect proximity;

a transmitter connected to light-emitting diode(s), which emit light pulses that are reflected by an object in proximity and received at the photodiode; and

a controller system that controls the these elements by continuously alternating a DC calibration cycle with a proximity measurement cycle or on demand first performs the DC calibration cycle followed by the proximity measurement cycle.

2. The proximity sensor of claim 1 further comprising a feedback blanking switch, which disconnects the signal path between the comparator and the transimpedance amplifier on the leading edge of a transmit signal for a small portion of the transmit pulse duration.

3. The sensor of claim 1, wherein at least one of the one or more transmitting diodes is a visible-light emitting diode and at least one of the one or more receiving diodes is a visible photodiode.

4. The sensor of claim 1, wherein at least one of the one or more transmitting diodes is an infrared light emitting diode and at least one or more of the receiving diodes is an infrared receiver.

5. The sensor of claim 1 further comprising:

a noise peak detector of the received signal, which provides dynamic adjustment of the receive detect threshold to a peak value above transient ambient spikes to reduce spurious detection.

6. The sensor of claim 5, wherein at least one of the one or more transmitting diodes is a visible-light or infrared emitting diode and at least one of the one or more receiving diodes is a visible-light or infrared photodiode.

7. The sensor of claim 1 further comprising:

an adjustable delay clock; and

a data latching circuit;

characterized in that a clock signal is input into the data latching circuit to latch the output of the comparator circuit, the delay clock advanced or retarded to adjust proximity detection sensitivity.

8. The sensor of claim 7, wherein at least one of the one or more transmitting diodes is a visible-light or infrared emitting diode and at least one of the one or more receiving diodes is a visible-light or infrared photodiode.

9. The sensor of one of claims 1, 5, or 7, wherein the output pulse width of the comparator circuit is proportional to the DC ambient background signal level.

10. The sensor of one of claims 1, 5, or 7, wherein the output pulse width of the comparator is converted to a digital value by a counter driven by a frequency source.

11. The sensor of one of claims 1, 5, or 7, wherein there are more than one transmitting diode, and a single receiving diode, the transmitting diodes geometrically arraigned about the receiving diode at different distances from the receiving diode.

12. The sensor of claim 11, wherein the transmitting diodes reflectance contribution levels are compared to determine proximity according to a mathematical formula.

13. The sensor of claim 12 wherein comparison of each transmitting diodes reflectance contribution determines direction of motion of an object moving in the sensor field.

21. The sensor of claim 1 integrated in a form of a monolithic integrated circuit combining proximity detection function and optical communication function including but not limited to IrDA and consumer remote control.

22. The sensor of claim 21 where the same gyrator circuit used in the optical communication function to remove DC ambient from the high gain receiver is re-configured to provide the gyrator to produce pulse widths for DC ambient measurement and proximity measurement, wherein the same optical photodiode, high gain receiver, bias and interface circuits are reconfigured for use in DC ambient and proximity mode, and optionally the same transmitter and LED are also used for proximity mode, not DC ambient measurement.

23. The sensor of claim 21, which uses optical communication mode to program or control it'"'"'s DC ambient or proximity mode and other operating parameters;

such as sensitivity, proximity sample rate, etc.

14. A method for calibrating a proximity sensor and then measuring proximity in a repeating cycle or on demand, the sensor including one or more wavelength transmitting diodes, one or more wavelength receiving diodes, an ambient correction circuit, and a comparator circuit comprising the steps of:

(a) powering on the sensor;

(b) canceling the ambient signal during a calibration period by slewing the correction current at a fixed rate by charging a capacitor at a fixed rate until the receive comparator falls below detection. The voltage on the capacitor determines the correction current. The pulse width of the period required for the correction to completely cancel the input DC ambient represents the DC ambient level;

(c) in the proximity measurement cycle, using the method in (b) to convert the increase in reflection signal above the DC ambient into a reflectance pulse width;

(d) transmitting wavelengths to and receiving reflectance from an object in the path of the transmitted wavelengths;

(e) measuring a reflectance pulse width and comparing the width value to a preset width value to determine proximity; and

(f) powering down the sensor to a standby mode.

15. The method of claim 14, wherein in step (a), power up is performed in response to an internal or external event.

16. The method of one of claims 14 or 15, wherein the sensor further includes a peak detector for establishing a peak value above transient ambient noise spikes and correspondingly adjusts the receive detection threshold value of the comparator to above the value of the noise spikes.

17. The method of one of claims 14 or 15, wherein the sensor further includes an adjustable delay clock, and a data latching circuit for storing a sampled proximity measurement between cycles.

18. The method of one of claims 14 or 15, wherein there are more than one transmitting diode and in step (c), each of the transmitting diodes is caused to transmit separately from the other diodes.

19. The method of one of claims 14 or 15, wherein in step (d), the proximity determination controls an on/off switch.

20. The method of one of claims 14 or 15, wherein in step (d), the proximity determination is analyzed to provide additional information about direction and or speed of movement of an object sensed.