: An Artificial Neural Network-based error compensation method is proposed for improving the accuracy of resolver-based 16-bit encoders by compensating for their respective systematic error profiles. The error compensation procedure, for a particular encoder, involves obtaining its error profile by calibrating it on a precision rotary table, training the neural network by using a part of this data and then determining the corrected encoder angle by subtracting the ANNpredicted error from the measured value of the encoder angle. Since it is not guaranteed that all the resolvers will have exactly similar error profiles because of the inherent differences in their construction on a micro scale, the ANN has been trained on one error profile at a time and the corresponding weight file is then used only for compensating the systematic error of this particular encoder. The systematic nature of the error profile for each of the encoders has also been validated by repeated calibration of the encoders over a period of time and it was found that the error profiles of a particular encoder recorded at different epochs show near reproducible behaviour. The ANN-based error compensation procedure has been implemented for 4 encoders by training the ANN with their respective error profiles and the results indicate that the accuracy of encoders can be improved by nearly an order of magnitude from quoted values of ∼± 6 arc-min to ∼±0.65 arc-min when their corresponding ANN-generated weight files are used for determining the corrected encoder angle.
The accuracy of a low-cost, resolver-based 14-bit absolute encoder has been improved significantly from about 10 to about 2 arcmin (the resolution limit) by the use of a `look-up' table. The methodology adopted for generating this table and incorporating it into the drive-system software of our four-element -ray telescope array is described.
The TACTIC γ-ray telescope, equipped with a light collector of area ∼9.5m 2 and a medium resolution imaging camera of 349-pixels, has been in operation at Mt.Abu, India since 2001. This paper describes the main features of its various subsystems and its overall performance with regard to (a) tracking accuracy of its 2-axes drive system, (b) spot size of the light collector, (c) back-end signal processing electronics and topological trigger generation scheme, (d) data acquisition and control system and (e) relative and absolute gain calibration methodology. Using a trigger field of view of 11×11 pixels (∼ 3.4 • ×3.4 • ), the telescope records a cosmic ray event rate of ∼2.5 Hz at a typical zenith angle of 15 • . Monte Carlo simulation results are also presented in the paper for comparing the expected performance of the telescope with actual observational results. The consistent detection of a steady signal from the Crab Nebula above ∼1.2 TeV energy, at a sensitivity level of ∼5.0σ in ∼25 h, alongwith excellent matching of its energy spectrum with that obtained by other groups, reassures that the performance of the TACTIC telescope is quite stable and reliable. Furthermore, encouraged by the detection of strong γ-ray signals from Mrk 501 (during 1997(during and 2006(during observations) and Mrk 421 (during 2001(during and 2005(during -2006 observations), we believe that there is considerable scope for the TACTIC telescope to monitor similar TeV γ-ray emission activity from other active galactic nuclei on a long term basis.
A cost-effective methodology for operating the 349-pixel
Cerenkov light imaging camera of the imaging element of the TACTIC
γ-ray telescope array at stable single-counts rate and safe anode
current values has successfully been tried out despite variations in the light
of the night sky experienced by these pixels from time to time. Important
details of this unconventional scheme are discussed and reference is made to
the methods evolved for in situ absolute and relative gain
calibrations of the camera pixels, which are particularly required for this
mode of operation.
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