Jarir Mahfoudh

From transmission error measurements to angular sampling in rotating machines with discrete geometry

The benefits of angular sampling when measuring various signals in rotating machines are presented and discussed herein. The results are extracted from studies on transmission error measurements with optical encoders in the field of power transmissions and can be broadened to include phase difference measurements, such as torsional vibrations, and applied to control, monitoring and measurement in rotating machines with discrete geometry. The main conclusions are primarily that the use of angular sampling enables the exact location of harmonics and, consequently, the obtaining of spectral amplitude components with precision. This is always true even if the resolution of encoders is not directly related to the studied discrete geometry. It then becomes possible to compare these harmonics under different operating conditions, especially when speed varies, without changing any parameters in spectral analysis (window length, spectral resolution, etc.). Moreover, classical techniques of improving signal to noise ratio by averaging become fully efficient in the detection of defective elements. This study has been made possible thanks to the technique of transmission error measurement with optical encoders that allows the comparison of sampling procedures, based on the same raw data.

Modeling the climber fall arrest dynamics

Sports and mountaineering activities are becoming more and more popular. Equipment constructors seek to develop products and devices that are easy to use and that take into account all safety recommendations. PETZL and INSA have collaborated to develop a model for the simulation of displacements and efforts involved during the fall of a climber in the “safety chain”. The model is based on the classical equations of motion, in which climber and belayer are considered as rigid masses, while the rope is considered as a series of non-linear stiffness passing through several devices as brakes and runners. The main goal is to predict the forces in the rope and on the return anchor at the first rebound of the fall. Experiments were first performed in order to observe and determine the dynamic characteristics of the rope, and then to validate results stemming from simulations. Several fall configurations are simulated, and the model performs satisfactorily. It also provides a close approximation of the phenomena observed experimentally. The model enables the assessment of the existing equipments and the improved design of the future one.Copyright

Experimental Study of the Transmission Error for the Detection of Gear Faults

This study deals with the definition of preventive maintenance procedures for gearboxes. Faults can appear on one or several elements of the studied system. In this work, only gear faults are considered. Faults appear on spur gears, and the transmission error has been measured due to a progressive fault for several operating conditions. The transmission error is the system response that contains the main information about gear health. We intend to study the basic relationship between the presence of gear fault and the modification in the transmission error. We measured the transmission error using low pulse per revolution optical encoders. The operating test conditions were two speeds and three applied loads. Signals were processed in the time domain. Several indicators were examined with respect to the operating conditions. Trends of the system response with respect to fault evolution were established. Basic relationships between gear failure and the change of transmission error have been discussed.Copyright