Control of Torque Pulsations and Mechanical Resonances: (93-0161R ERA Report)

To increase productivity and reduce costs, drive systems having faster dynamic performance are now being employed in industry in place of the previous slower systems. With the increased torque bandwidth associated with new motor and drive designs, the likelihood of mechanical vibration (such as torsional resonance) has increased. This report identifies some common sources of mechanical vibration, and includes an analysis of the torque ripple magnitude and frequency content for various machine types, to enable designers to select motors appropriate to the mechanical system. Methods of predicting mechanical resonances of some simple systems and the potential effects are reviewed. The impracticability of measuring high-frequency torque ripple and torsional resonance with existing strain gauge torque transducer has been shown, and alternative techniques using new designs of torque transducer and then calulating torque from angular accelleration are identified. An example drive system, suspected of having a mechanical resonance problem is analyzed and simulated. The simulation and analytical techniques developed will provide guidance for designers facing resonance problems. Methods to control and overcome mechanical resonance are investigated using both traditional mechanical approaches and advanced electronic control. The latter is simulated on modified versions of the example system. With the growth of high-performance motor controller systems and the advances in power electronics and digital controllers, these methods will become increasingly important in the future.

Table of Contents:
Introduction: the effects of torque ripple. Part 1 Sources of excitation of mechanical resonances: excitation torque time and frequency content; torque production in electric motors - category A, category B, category C, category D; practical deficiencies leading to torque ripple in electric motors - stator and rotor tooth and slot effects, winding distribution (space harmonic MMF distribution), current harmonics (time harmonic MMF distribution), miscellaneous parasitic effects; comparison of torque ripple effects in electric motors; gearboxes; load characteristics; misalignment and imbalance. Part 2 Predicting mechanical resonance: the natural resonances - two-mass system, multi-mass system, experimental techniques for identifying mechanical natural frequencies; the forced response - transient response, forced response, two-mass system, whirling of shafts. Part 3 The problems of measuring torque ripple and mechanical resonances: torque transducers and load cells; inference via angular acceleration; inference via vibration. Part 4 Overcoming torque ripple and mechanical resonances: traditional mechanical approaches - vibration isolation, dynamic absorption; advanced electronic control. Part 5 An example system: description of the system; the mechanical resonant frequencies and sources of excitation; extracting data for the model; model verification. Part 6 The simulated system performance: simulation environment; system blocks - demand voltage, demand gain, speed PI gain, current limit, noise filter, current gain, current PI gain, current feedback, motor, drum, tacho, motor-drum shaft, tacho-motor shaft, multi-body tacho-shaft-motor-shaft-drum model, lumped body approximation of tacho-shaft-motor-shaft-drum model, external load torque; the effect of mechanical resonance; the effect of motor torque ripple; the effect of tacho ripple; the combined effects of torque and tacho ripple; tuning of system loops; system sensitivity for the simplified system - shaft speed sensitivity to demand voltage, shaft speed sensitivity to external load torque and torque ripple, shaft speed sensitivity to tacho ripple; conclusions. Part 7 Methods to control the effects of mechanical resonance: performance with existing controller; performance with speed feedback; performance with shaft torque or position feedback; use of observers in place of sensors.