Laser vibrometer for vibration measurements

• US 5,883,715 A
• Filed: 02/19/1997
• Issued: 03/16/1999
• Est. Priority Date: 06/20/1995
• Status: Expired due to Term

• Current Assignee: Robert Bosch GmbH
• Original Assignee: Robert Bosch GmbH
• Inventors: Drabarek, Pawel, Van Keulen, Michael, Steinlechner, Siegbert
• Primary Examiner(s): KIM, ROBERT H
• Application Number:
  US08/793,140
• Time in Patent Office: 755 Days
• Field of Search: 356/28.5, 356/345, 356/349, 356/351, 356/361, 356/357
• US Class Current: 356/487
• CPC Class Codes: G01H 9/00 Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means

Abstract

A laser diode (1) of a laser vibrometer emits light which is guided onto a polarization beam splitter (3) by a lens 2. A portion of the beam is guided through directly to an imaging lens (4) and a second portion is deflected onto a mirror (5). Quarter-wave plates (6) and (7) are arranged between the polarization beam splitter and respectively the lens or the mirror. The beam which is reflected on the mirror is guided completely through the polarization beam splitter onto a photodetector unit (8). The beam reflected by the object (9) to be measured, which swings in the direction (14), is also deflected to the photodetector unit. The output signal of the photodetector unit, the interference signal generated from the superposition of the two reflected beams, is supplied to an electronic evaluation equipment (10). The latter is also supplied with a signal of a current modulator (11) which, on the other hand, supplies the laser diode with modulated injection current via line (12). The distance between the imaging lens and the object to be measured forms an optical detour (13) which is considerably lengthened compared to the reference arm.

Claims

1. A laser vibrometer for vibration measurements which operates according to the principle of the heterodyne interferometer, comprising in combination:

means including a laser diode for producing a parallel beam of light radiation;

a Michelson interferometer including means for receiving the parallel beam and splitting same into first partial beam in a reference arm and a second partial beam in a measuring arm, with the first partial beam being reflected from a mirror disposed at a fixed position and the second partial beam being reflected from an object to be measured disposed at a known distance, and for combining the reflected partial beams to form an interference pattern in an interference arm, with one of the reference and measuring arms being considerably longer than the other by approximately 20 cm to approximately 100 cm;

means for modulating the frequency of the laser produced radiation to attain a necessary frequency shift (Δ

v);

a single detector for detecting the interference pattern in said interference arm; and

an evaluation circuit means, responsive to the detected interference pattern and an output signal from said means for modulating representing the modulation, for providing an output signal corresponding to the axial out of plane vibration of the test object.

2. A laser vibrometer according to claim 1, wherein the means for modulating modulates the frequency of the laser radiation through modulating the laser injection current.

3. A laser vibrometer according to claim 2, wherein the means for modulating modulates the laser injection current with a triangular-shaped or sawtooth-shaped modulation.

4. A laser vibrometer according to claim 3, wherein the triangular-shaped or sawtooth-shaped modulation takes place with an amplitude j of the current modulation and a frequency f of the current modulation.

5. A laser vibrometer according to claim 4, wherein the amplitude of the modulation is in the range of 2 to 15 mA and the frequency of the modulation is in the range from less than one up to several MHZ.

6. A laser vibrometer according to claim 1, wherein the means for splitting includes a polarization beam splitter provided with two quarter-wave plates, one in the reference beam path and one in the beam path of the object to be measured.

7. A laser vibrometer according to claim 1, wherein the measuring arm is the longer arm.

8. A method of measuring the vibration of devices or components thereof to detect points which cause great and/or unpleasant noise comprising the step of:

providing a laser vibrometer which operates according to the principle of the heterodyne interferometer, and which comprises means including a laser diode for producing a parallel beam of light radiation;

a Michelson interferometer including means for receiving the parallel beam and splitting same into first partial beam in a reference arm and a second partial beam in a measuring arm, with the first partial beam being reflected from a mirror disposed at a fixed position and the second partial beam being reflected from an object to be measured disposed at a known distance, and for combining the reflected partial beams to form an interference pattern in an interference arm, with one of the reference and measuring arms being considerably longer than the other by approximately 20 cm to approximately 100 cm;

means for modulating the frequency of the laser produced radiation to attain a necessary frequency shift (Δ

v);

a single detector for detecting the interference pattern in the interference arm; and

an evaluation circuit means, responsive to the detected interference pattern and an output signal from said means for modulating representing the modulation, for providing an output signal corresponding to the axial out of plane vibration of the test object;

positioning the vibrometer such that the device constitutes the object to be measured; and

,

measuring the axial out of plane vibration of the device with the vibrometer.

9. A method of measuring the vibration of devices or components thereof to detect points at which there exists risk of fracture and/or the risk of premature wear due to high and/or, with respect to frequency, particularly critical vibration amplitudes;

comprising the steps of;

providing a laser vibrometer which operates according to the principle of the heterodyne interferometer, and which comprises means including a laser diode for producing a parallel beam of light radiation;

a Michelson interferometer including means for receiving the parallel beam and splitting same into first partial beam in a reference arm and a second partial beam in a measuring arm, with the first partial beam being reflected from a mirror disposed at a fixed position and the second partial beam being reflected from an object to be measured disposed at a known distance, and for combining the reflected partial beams to form an interference pattern in an interference arm, with one of the reference and measuring arms being considerably longer than the other by approximately 20 cm to approximately 100 cm;

means for modulating the frequency of the laser produced radiation to attain a necessary frequency shift (Δ

v);

a single detector for detecting the interference pattern in the interference arm; and

an evaluation circuit means, responsive to the detected interference pattern and an output signal from said means for modulating representing the modulation, for providing an output signal corresponding to the axial out of plane vibration of the test object;

positioning the vibrometer such that the device constitutes the object to be measured; and

,

measuring the axial out of plane vibrations of the device with the vibrometer.

10. A a method of measuring the vibration of devices or components thereof to detect rhythmically recurring vibrations which, in terms of their evaluation, are convertible into rotational speeds of internal combustion engines comprising the steps of:

providing a laser vibrometer which operates according to the principle of the heterodyne interferometer, and which comprises means including a laser diode for producing a parallel beam of light radiation;

a Michelson interferometer including means for receiving the parallel beam and splitting same into first partial beam in a reference arm and a second partial beam in a measuring arm, with the first partial beam being reflected from a mirror disposed at a fixed position and the second partial beam being reflected from an object to be measured disposed at a known distance, and for combining the reflected partial beams to form an interference pattern in an interference arm, with one of the reference and measuring arms being considerably longer than the other by approximately 20 cm to approximately 100 cm;

means for modulating the frequency of the laser produced radiation to attain a necessary frequency shift (Δ

v);

a single detector for detecting the interference pattern in the interference arm; and

an evaluation circuit means, responsive to the detected interference pattern and an output signal from said means for modulating representing the modulation, for providing an output signal corresponding to the axial out of plane vibration of the test object;

positioning the vibrometer such that the device constitutes the object to be measured; and

,

measuring the axial out of plane vibrations of the device with the vibrometer.