25-07-2012, 10:15 AM
VISION-BASED NAVIGATION
VISION-BASED NAVIGATION.docx (Size: 3.94 MB / Downloads: 32)
Aim of the project
Now days there are several navigation systems for positioning the objects. Several research efforts have been carried out in the field of Six Degrees of Freedom estimation for rendezvous and proximity operations. One such navigation system used in the field of Six Degrees Of Freedom position and attitude estimation is the VISion based NAVigation system. It is aimed at achieving better accuracies in six Degrees Of Freedom estimation using a simpler and robust approach. The system uses a Position Sensitive Diode (PSD) sensor for 6 DOF estimation. Output current from the PSD sensor determines the azimuth and elevation of the light source with respect to the sensor. By having four or more light source called beacons in the target frame at known positions the six degree of freedom data associated with the sensor is calculated. The beacon channel separation and demodulation are done on a fixed point digital signal processor (DSP) Texas Instruments TMS320C55x [2] using digital down conversion, synchronous detection and multirate signal processing techniques. The demodulated sensor currents due to each beacon are communicated to a floating point DSP Texas Instruments TMS320VC33 [2] for subsequent navigation solution by the use of co linearity equations.
Among other competitive systems [3] a differential global positioning system (GPS) is limited to midrange accuracies, lower bandwidth, and requires complex infrastructures. The sensor systems based on differential GPS are also limited by geometric dilution of precision, multipath errors, receiver errors, etc. These limitations can be overcome by using the DSP embedded system.
Sensor geometry
We have discussed that Position Sensitive Diodes are used for sensing purpose. The Position Sensitive Diode (PSD) is a single substrate photodiode capable of finding or locating a light beam within defined sensing area. When photons meet the PSD sensor active area electrical currents are generated that flow through its four terminals. The closer the incident light centroid is to a particular terminal, the larger the position of current that flows through that load comparison of these four currents determines the centroid location of the incident light With regards to the above figure the normalized voltage are as follows.
FACTORS AFFECTING MEASUREMENT
There is likely to be a large amount of ambient light at short wavelength and low carrier frequencies due to perhaps the sun, its reflections, incandescent or discharge tube lights, LCD and cathode ray tube displays etc. In many cases this ambient energy would swap a relatively small beacon signal and the PSD centroid data would mostly correspond to this unwanted background light.
In order to avoid this problem by modulating the beacon controller current by a sinusoidal carrier of high frequency. The resulting PSD signal currents then vary sinusoidal at approximately the same frequency and have to be demodulated to recover the actual current proportional to the beacon light centroid. This modulation or demodulation scheme leads high degree of insensitivity to variations in ambient light and it is a key to make the PSD sensing approach practical.
Another method for solving this ambient light problem is that all energy except that centered on the colour wavelength of the beacon is greatly reduced by an optical colour filter. Another problem that affects the measurement is that high power beacon signal may saturate output of the preamplifier which is used after the PSD. So incident light centroid can not measure accurately. In order to avoid this problem a feedback control is used to hold the beacon light intensity at a level that results in a maximum PSD current at approximately 70% of the Tran impedance amplifier input saturation level.
MODULATION
The PSDs are relatively fast compared to even high speed cameras, having rise time of about 5μs. 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 highly cluttered ambient optical scenes. If there is a single beacon excited by a sinusoidal oscillator operating at a frequency fc, the emitted light induces sinusoidal currents in the PSD with the frequency fc at the four terminals of the PSD sensor. Therefore, all the four currents can be processed in a similar fashion to estimate the amplitudes of the carrier waveforms. The amplitudes of these currents are related to the azimuth and elevation of the light source with respect to the image co-ordinate frame. If the PSD has a relative motion with respect to the beacon, the current envelopes are modulated by that relative motion and this modulation is analogous to amplitude modulation (AM).
CONCLUSION
A new method for operating beacons and demodulating the beacon currents for the sensor system is introduced here. It is shown that target differentiation based on FDM yields higher signal to noise ratios for the sensor measurements and the demodulation in the digital domain using multi rate signal processing techniques brings reliability and flexibility to the sensor system. The algorithm that is implemented on DSP is robust when there are four or more of line of sight measurements except near certain geometric conditions that are rarely encountered. It is shown that this algorithm is computationally efficient and achieves better results.