Noise reduction techniques in pulse width modulated inductive switching systems of PSLV

The subject of the paper is the techniques employed in the Polar Satellite Launch Vehicle to reduce electromagnetic interference in control systems which have pulse width modulated inductive switching systems. There are 3 such control systems in PSLV. PWM servo amplifiers are generally preferred in control systems for better efficiency and to reduce dissipation losses. However, the PWM signal itself creates an EMI environment. Hence care must be taken in the design phase of control electronics to minimise the EMI effects of PWM. This includes radiation effects as well as interference due to switching spikes. In general, the main sources of interferences are the PWM switching signal, inductive switching of motors by PWM with heavy currents, and subsequent generation of large magnetic fields, ground loop formations between power and signal grounds and improper bunching and harnessing of cables. The paper attempts to categorise the various interference sources, and to share the experiences in PSLV programme as to how in the design phase these interferences are taken care of. The forms of interferences are studied and the methods of noise reduction techniques are illustrated in a case study of the 3rd stage control system