Cures for low-frequency mechanical resonance in industrial servo systems

Instability commonly results from mechanical resonance in servo systems. Resonance problems are usually caused by the compliance of power transmission components. Standard servo control laws are structured for rigidly-coupled loads. However, in practical machines some compliance is always present; this compliance often reduces stability margins, forces servo gains down and reduces machine performance. Mechanical resonance falls into two categories: low-frequency and high-frequency. High-frequency resonance causes instability at the natural frequency of the mechanical system, typically between 500 and 1200 Hz. Low-frequency resonance occurs at the first phase crossover, typically 200 to 400 Hz. Low-frequency resonance occurs more often in general industrial machines. This distinction, rarely made in the literature, is important in determining the most effective means of correction. This paper evaluates the effectiveness of several methods for dealing with low-frequency resonance, focusing on observer-based acceleration feedback. All methods are compared using an industrial motor and drive.