Modelling of quantization effects in current control for a synchronous servo drive

One major problem during development of a current control for a synchronous servo drive is to estimate the resulting torque ripple, which is generated by quantization and noise. Due to the non-linear character of quantizers, control engineers tend to minimize torque ripple caused by electronic components, a/d converters and sampling techniques experimentally by designing a prototype, thus generating avoidable costs. This paper proposes a technique based on stochastical linearization, which provides a simple linear model of a quantized current control loop. With aid of the linear model, the influence of quantization and noise on motor torque can be calculated. Thereby limits for current- and voltage resolutions are derived, which lead to non-linear system behaviour when neglected. Lastly, the results obtained are verified experimentally.