PASSIVE TRANSLATIONAL VELOCITY MEASUREMENT FROM OPTICAL INFORMATION

• US 20110235028A1
• Filed: 12/16/2009
• Published: 09/29/2011
• Est. Priority Date: 12/18/2008
• Status: Active Grant

• Current Assignee: University of Zurich
• Original Assignee: University of Zurich
• Inventors: Rohrseitz, Nicola
• Granted Patent: US 8,711,337 B2
• Time in Patent Office: Days
• US Class Current: 356/229
• CPC Class Codes: G01C 11/16 in a common plane
• CPC Class Codes: G01P 3/36 Devices characterised by the use of optical means, e.g. using infrared, visible, or ultraviolet light G01P3/68 takes precedence; gyrometers using the Sagnac effect, i.e. rotation-induced shifts between counter-rotating electromagnetic beams G01C19/64
• CPC Class Codes: G01P 3/50 for measuring linear speed G01P3/56 takes precedence
• CPC Class Codes: G01S 11/12 using electromagnetic waves other than radio waves

Abstract

The invention is a passive method to measure the translational speed of a visual scene using the distribution of light intensities. Measuring the speed of translation is useful for control, safety, management of resources, fuel efficiency, and many more application fields. It is however technically challenging because a wide-field translating scene projects on an image plane as a heterogeneous field of apparent speeds. The invention solves this problem by combining two principles: perspective distortion matching over a broad field of view, and temporal filtering variation. In conventional systems, an acquired image is calibrated to obtain linear coordinates. Instead, the invention uses the perspective distortion of the image to sample the visual scene at different linear wavelengths over the visual field. The result is a spatial sensitivity map of the visual scene. The obtained signal is then temporally filtered with cutoff frequencies proportional to the spatial sensitivity. The final result is the wide-spectrum computation of the ratio between temporal and linear spatial frequencies, in other words linear speed. The technique is passive because it does not require the emission of a reference signal. This is an advantage over active speed sensors mainly because of reduced power consumption, but it is an enabling factor for other applications. Where it is difficult or impossible using standard device-centered techniques, like on aircrafts or in fluids, the invention enables measuring absolute linear speed. The advantage over non-device-centered techniques like—GPS—is the independence from external infrastructures. The small computational overhead makes it ideal for mobile applications.

Claims

1. A method for determining a linear speed (Δ

v) of an object, comprising the steps of;

a) sampling a series of light intensity courses I(t) over time t, each light intensity courses I(t) covering a different azimuthal direction, each different azimuthal direction defining a detection length (λ

_i;

i=1, . . . , k) of a series of detection lengths (λ

_i) as seen in a direction of the linear speed (Δ

v) to be determined;

b) transforming the series of detection lengths (λ

_i) for the azimuthal directions into a series of linear spatial frequencies (SF_i);

c) defining for each azimuthal direction a band pass or a high pass filter, relating for each detection length (λ

_i) characteristic filter frequencies to respective linear spatial frequencies (SF_i);

d) filtering sampled light intensity for each azimuthal direction with the respective band pass or high pass filter to obtain a set of filtered light intensities;

e) calculating the linear speed by dividing the characteristic filter frequencies through the linear spatial frequencies (SF_i) for at least one of azimuthal directions for which the filtered light intensity exceeds a predefined threshold; and

f) determining the linear speed by correlating the calculated linear speeds for those azimuthal directions for which the filtered light intensity exceeds a predefined threshold.

2. The method according to claim 1, wherein each azimuthal direction covers a range of detection lengths (λ

_i+Δ

λ

_i) yielding a range of linear spatial frequencies (SF_i).

3. The method according to claim 1, wherein each band pass or high pass filter assigned to an azimuthal direction covers a range of throughput frequencies.

4. A system for determining a linear speed (Δ

v) of an object, comprising;

a) a set of photoreceptors disposed on the object, composed of subsets of 3 photoreceptors, said subset of photoreceptors (35_i, o₁, o₂, o₃) measuring light intensities I(t) and being arranged to cover identical angular apertures pointing in different azimuthal directions, each different azimuthal direction defines detection length (λ

_i;

i=1, . . . , k) of a series of detection lengths (λ

) as seen in a direction of the linear speed (Δ

v) to be determined;

b) means (C) for calculation the linear speed (Δ

v) for evaluation subsets (36_j;

j=1, . . . , k−

1), each evaluation subset (36_j) comprising the intensity inputs of three adjacent photoreceptors (o₁, o₂, o₃), by performing for at least part of the subsets (36_j);

c) subtracting the light intensities inputs from two extreme photoreceptors (o₁, o₃) of each evaluation subset (36_j) to receive a first difference signal (s₁₃);

d) splitting the first difference signal (s₁₃) on two lines, one line delaying the difference signal (s₁₃) by a temporal delay (τ

);

e) subtracting the signals (s₁₃, delayed s₁₃) from the two lines to receive a second difference signal (ts₁₃); and

f) multiplying the second difference signal (ts₁₃) with the input from a central photoreceptor (o₂) to achieve an output signal (sso) being indicative for the linear speed (Δ

v).