06-11-2012, 04:53 PM
VISION-BASED NAVIGATION FOR RENDEZVOUS, DOCKING AND PROXIMITY OPERATIONS
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ABSTRACT
A novel approach for vision sensing and vision-based proximity navigation
of spacecraft is presented. We have recently invented a new sensor which
utilizes area Position Sensing Diode (PSD) photodetectors in the focal plane
of an omni-directional camera. These analog detectors inherently centroid
incident light, from which a line of sight vector can be determined. PSDs
are relatively fast compared to even high speed cameras, having rise times of
about ¯ve microseconds. This permits light sources to be structured in the
frequency domain and utilization of radar-like signal processing methods
to discriminate target energy in the presence of even highly cluttered ambient
optical scenes. We have developed the basic concepts, designed ¯rst
generation vision sensors based on this approach and carried out proofof-
concept experimental studies. Our results show that a beacon's line of
sight vector can be determined with an accuracy of one part in 5,000 (of the
sensor ¯eld of view angle) and at a distance of 30m with an update rate of
50Hz. In practice, measured directions toward four or more beacons would
typically be used. We have also developed an associated six degree of freedom
navigation algorithm that is readily applicable to rendezvous, docking,
and proximity operations; we have veri¯ed that this algorithm is robust and
compatible with real-time, on-board computational constraints. This paper
summarizes analytical, computational, and laboratory experimental results
supporting the e±ciacy and practicality of this approach.
Introduction
In rigid body dynamics, the determination of a body frame (position and orientation) in a given
reference frame is possible if at least three points of the body frame are known in the given frame.
The vision based navigation (VISNAV) system proposed in this paper implements this fundamental
truth. Target lights are ¯xed in a frame A (embedded in the target spacecraft), with a known
position in this frame, and an optical sensor is attached rigidly to a frame B (embedded in the
chase spacecraft). The sensor computer then orchestrates the lights, turning them on alternately,
and measures angles toward their lines of sight every time it detects them. Consequently, with a
modest amount of computation, making use of a Gaussian Least Squares Di®erential Correction
process, one is able to recover the relative position and orientation of frames A and B. Using an
extended Kalman Filter, we can also derive optimal estimates of rates and accelerations of relative
linear and angular motion. The system described in this paper comprises an optical sensor of a new
kind combined with speci¯c light sources (beacons) in order to achieve a selective or "intelligent"
vision. The sensor is made up of a Position Sensing Diode (PSD) placed in the focal plane of a wideangle
lens.
Hardware
The VISNAV hardware prototype consists of a set of beacons each radiating bursts of light modulated
at a carrier frequency of 38:4KHz, a small position sensing diode (PSD) `camera' that senses these
light packets, and controlling electronics for both the beacons and the PSD sensor. Figure 1 shows a
schematic of a VISNAV con¯guration using three beacons and ¯gure 2 is a schematic °ow diagram
that indicates the major electronic hardware subsystems. The sensor Digital Signal Processor (DSP,
see top LHS of ¯g. 2) decides which beacon is to be switched on and at what intensity, and this
information is then relayed in serial form to the beacon controller via an infrared or radio beam. The
beacon controller then commands the appropriate light via an analog switch and the latter radiates
light amplitude modulated at the carrier frequency. If this energy source is within the 90 degree ¯eld
of view of the PSD sensor, then four currents are generated, from the terminals of the PSD, each
also varying sinusoidally at 38:4KHz (see ¯g.1). The sensor electronics processes these currents,
removing the 38:4KHz carrier, and passes the ¯ltered results back to the DSP. From the imbalance
in these signals, the direction of the incident light (and therefore of the corresponding beacon) can
be determined. Once four or more sets of beacon data have been collected, the navigation algorithm
running on the DSP can compute the current position and attitude of the sensor with respect to the
object space frame of reference in the target spacecraft. The overall data update rate is 50Hz for
the current spacecraft VISNAV experiment. It is su±cient for controlling most proximity operations
anticipated. We now describe some features of the VISNAV hardware and software.
PSD Sensor
The PSD is a single silicon photodiode with an active area of 20mm£20mm. This diode is reverse
voltage biased in order to obtain the necessary signal bandwith (currently approximately 100KHz).
Four leads are attached, two to each side of the semiconductor diode. When photons meet the PSD
sensor active area electrical currents are generated that °ow through the four terminals. The closer
the incident light centroid is to a particular terminal, the larger the portion of current that °ows
through that lead. Comparison of the these four currents then determines the centroid location of
the incident light.
Wide angle lens
A wide angle lens is used to collect light from a cone of angle 90 degrees and focus the incident
energy onto the PSD. Currently a single piece 18mm diameter aspheric lens is used that has a focal
length of 12mm. A new lens with a wider diameter of 21mm (approx.) and a shorter focal length
of 11mm is presently being tested and this should capture more light, and keep more of this light
on the PSD active area (to improve signal to noise ratios outside a 35m range). Fresnel lenses may
also be used as only the PSD incident light centroid is of interest, and optical clarity (as required by
a conventional camera) is not needed. A narrow bandpass color ¯lter (centered on the color of the
beacon energy, 670nm in our case) is placed behind the lens and just in front of the PSD in order
to reject most ambient light and protect the silicon PSD from harmful light energy densities. This
also helps to reduce noise from ambient light sources.
Beacons
Figure 3 shows a schematic diagram of one of the VISNAV beacons. Here each beacon is an array
of light emitting diodes (LEDs) radiating energy over nearly a hemisphere, and driven with the
same current varying sinusoidally at the carrier frequency. The number of LEDs in this array will
depend upon cost, the type of LED, the required system signal to noise ratio, the max. operating
distance etc. The beacons in the following simulations are assumed to radiate at a power level of
approximately 1W and this requirement can be satis¯ed by 100 10mW LEDs mounted in an area
of 4in2 or less. Suitable LEDs are widely availible and inexpensive making this approach attractive.
The peak to peak value of the applied LED current is determined by the sensor DSP unit and set
by the beacon controller. Current control is used as LEDs provide a light intensity response that is
approximately linearly related to the LED current, whereas the voltage to light intensity relationship
is very nonlinear, and much energy would be wasted in harmonics of the carrier frequency. We have
found useful a further re¯nement; if we add a photodiode that feeds back a signal proportional
to the emitted light intensity then this allows better control of the actual light intensity, with
minimal harmonic generation. This technique is standard in laser diode drivers where accurate light
intensity control is very important for reliable operation. A typical beacon LED wavelength would
be 670nm (red) or 940nm (IR). A silicon PSD has a maximum response to a wavelength of 940nm
approximately, however in some cases visible LEDs might be preferred. The practical wavelength
(¸)range for our detector is about 450nm · ¸ · 950nm.
Beacon Orchestration
The sensor DSP unit controls the sequence of beacon lights by sending a two byte package of data to
the beacon controller via an infrared or radio data link (see ¯g.2) One byte determines which beacon
is to be turned on next (and therefore up to 255 di®erent beacons may be used with the current
system, however extra bytes of data can be easily added as required),
Modulation and Demodulation
Whether a VISNAV system is used in a laboratory or on-orbit, there is likely to be a large amount of
ambient light at short wavelengths and low carrier frequencies due to perhaps the sun, its re°ections,
incandescent or discharge tube lights, LCD and cathode ray tube displays etc. In many cases this
ambient energy would swamp a relatively small beacon signal and the PSD centroid data would
mostly correspond to this unwanted background light. In order for the beacon light to dominate
the PSD response, all energy except that centered on the color wavelength of the beacon is greatly
reduced by an optical color ¯lter, and furthermore a 38:4KHz sinusoidal carrier is applied to each
beacon control current. The resulting PSD signal currents then vary sinusoidally at approximately
the same frequency (depending on the movement of the PSD sensor with respect to the individual
beacons). These currents are converted to voltages by transimpedance ampli¯ers and then passed to
bandpass ¯lters also centered at 38:4KHz that reject virtually all of the lower frequency background
ambient light .