Tachometer sensor using a lens system to a in sensing the rotational speed of an object

• US 4,880,966 A
• Filed: 04/08/1988
• Issued: 11/14/1989
• Est. Priority Date: 04/08/1988
• Status: Expired due to Fees

• Current Assignee: Global Associates, Ltd.
• Original Assignee: Scientific-Atlanta Incorporated (Cisco Systems, Inc.)
• Inventors: Goodrich, Jeffrey A., Poe, Charles M. Jr., Prsha, Jeffrey A.
• Primary Examiner(s): Nelms, David C.
• Assistant Examiner(s): Allen, Stephone B.
• Application Number:
  US07/179,644
• Time in Patent Office: 585 Days
• Field of Search: 250/231 R, 250/231 SE, 250/237 G, 324/175, 341/13
• US Class Current: 356/28
• CPC Class Codes: G01P 3/486 delivered by photo-electric detectors

Abstract

An optical tachometer sensor for providing output relating to rotational speed of an object. The sensor illuminates a reflective mark on the object with modulated light and focuses that light via a receiving lens arrangement onto a photoelectric sensor which provides signals to conditioning circuitry for controlling output. Lenses in the sensor also form a Galilean telescope or a terrestrial telescope to allow a user to boresight the sensor by viewing the reflective marker through the sensor. A beam splitter or "hot mirror" reverses the optical path of modulated rays generated by the device for forcusing on the photoelectric sensor, while allowing visible light to pass and be aligned by a divergent ocular lens. In the terrestrial telescope embodiment, a convergent inverting lens and ocular lens provide the image for the user and the photoelectric sensor may be located at the focal point of the objective lens or the inverting lens.

Claims

1. A tachometer sensor for sensing rotational speed of an object, comprising:

(a) a light emitting device for illuminating the object;

(b) at least one reflective mark on the object for reflecting light rays emitted by the light emitting device;

(c) an objective lens located in the path of the light rays emitted by the light emitting device and reflected by the mark;

(d) a beam splitter located in the path of light rays reflected by the objective lens and oriented substantially perpendicular to that ray-path, for reflecting a first portion of the rays in a direction substantially opposite and parallel to that in which the first portion of rays strikes the beam splitter and for allowing a second portion of the rays to pass through;

(e) a photoelectric sensor located at substantially the focal point of the first portion of the light rays reflected by the objective lens and reflected by the beam splitting means;

(f) an ocular lens located in the path of the second portion of light rays reflected by the objective lens and penetrating the beam splitting means, for aligning the second portion of rays so that one using the sensor can view the mark on the rotating object through the sensor in order to align the sensor;

(g) driver means for supplying power to the light emitting device; and

(h) circuit means for conditioning and providing for output the signals produced by the photoelectric sensor.

2. A device according to claim 1 in which the beam splitting device is a hot mirror.

3. A device according to claim 1 in which the light emitting device is an infrared emitting diode which emits the first portion of light rays.

4. A device according to claim 1 in which the photoelectric sensor is a phototransistor.

5. A device according to claim 1 in which the circuit means contains a peak detector for providing feedback signals to control output of the driver means.

6. A device according to claim 1 in which the light emitting device, objective lens, beam splitting means, photoelectric sensor and ocular lens are located coaxially to one another.

7. A device according to claim 1 which the driver means modulates power to the light emitting device.

8. A device according to claim 2 in which the objective lens is a convergent lens whose focal length substantially equals the distance from the objective lens to the beam splitting device plus the distance from the beam splitting device to the photoelectric sensor.

9. A device according to claim 3 in which the ocular lens is a divergent lens.

10. A device according to claim 8 in which the circuit means contains filters for reducing sensitivity of the tachometer sensor to reflections from the mark modulated at frequencies below that of the signal provided by the driver means.

11. A device according to claim 8 in which an oscillator circuit modulates the driver means and feeds a synchronous demodulator in the circuit means, which synchronous demodulator rejects signals from the photoelectric sensor corresponding to light received by the photoelectric sensor not emitted by the light emitting device.

12. A tachometer sensor for sensing rotational speed of an object, comprising:

(a) a light emitting device for illuminating the object;

(b) at least one reflective mark on the object for reflecting light rays emitted by the light emitting device;

(c) an objective lens located in the path of the light rays emitted by the light emitting device and reflected by the mark, which objective lens has a focal length f;

(d) an inverting lens having a first focal length f_a and a second focal length f_b, located at a distance substantially f_a +f from the objective lens for inverting and refocusing light rays focused by the objective lens;

(e) a photoelectric sensor located substantially at the focal point f_b from the inverting lens;

(f) an ocular lens having a focal length f_o located at a distance substantially f_b +f_o from the inverting lens for aligning rays focused by the inverting lens so that one using the sensor can view the mark on the rotating object through the sensor in order to align the sensor;

(g) driver means for supplying power to the light emitting device; and

(h) circuit means for conditioning and providing for output the signal produced by the photoelectric sensor.

13. A device according to claim 12 in which the light emitting device, objective lens, inverting lens, photoelectric sensor and ocular lens are located coaxially to one another.

14. A device according to claim 12 in which the objective lens, inverting lens and ocular lens are convergent lenses and form a terrestrial telescope.

15. A device according to claim 12 in which the light emitting device is an infrared emitting diode.

16. A device according to claim 12 in which the photoelectric sensor is a phototransistor.

17. A device according to claim 12 in which the circuit means contains a peak detector for providing feedback signals to control output of the power supply means.

18. A device according to claim 12 in which the driver means modulates power to the light emitting device.

19. A device according to claim 18 in which the circuit means contains filters for reducing sensitivity of the tachometer sensor to reflections from the mark modulated at frequencies below that of the signal provided by the driver means.

20. A device according to claim 18 in which an oscillator circuit modulates the driver means and feeds a synchronous demodulator in the circuit means, which synchronous demodulator rejects signals from the photoelectric sensor corresponding to light received by the photoelectric sensor not emitted by the light emitting device.