Carrier compensation for servomechanisms

Abstract

Abstract A carrier servo system is a servo system which employs carrier-frequency techniques for data transmission. The provision of compensation to improve the stability and the performance of such a system presents several unique problems. Theoretically, the compensation transfer characteristic should have an amplitude response which is symmetrical about the carrier frequency and a phase response which is skew-symmetrical about the carrier frequency, both considered relative to an arithmetical frequency scale. Transfer characteristics normally obtained from physically realizable structures possess the required symmetry if considered on a logarithmic frequency scale; therefore, they introduce distortion into the system. Accuracy of physically realizable transfer characteristics is limited by tolerances required on components of the compensation device and by the dependence of many of the devices on the absolute stability of the carrier frequency. Carrier-compensation transfer characteristics may be realized through the use of passive electrical networks, electrical or electromechanical demodulation, compensation, and remodulation systems, feedback devices, and other means. Two main types of passive electrical networks have been employed, namely, those involving resistances, inductances, and capacitances, and those involving only resistances and capacitances. The former type of networks is the most general but is limited by the non-linearities introduced by non-ideal inductances. Parallel-T and bridge-T networks of the latter type of networks have found wide usage in carrier lead compensation. Electrical demodulation, compensation, and remodulation systems consist of the application of readily available electronic and non-carrier compensation techniques; however, electromechanical systems for obtaining similar results have been developed for carrier servo systems. The electrical systems do make available for carrier compensation a wide variety of compensation transfer characteristics, whereas electromechanical systems are more limited. Feedback devices are useful for obtaining time derivatives of the output. At least one method for minimizing the effect of carrier-frequency instability has been developed. Passive electrical networks present perhaps the best method of obtaining carrier-compensation transfer characteristics; however, characteristics other than simple lead are normally difficult to obtain, and the use of non-carrier compensation techniques appears advisable. Carrier servo systems do have inherent advantages as compared with non-carrier systems and consequently require less compensation for acceptable response. Great advancement may be expected in the field of carrier compensation during the coming years.