Improved Approaches to the Measurement and Analysis of Torsional Vibration

A primary goal of NVH engineering is the identification and control of noise and vibration sources. In recent years the torsional vibration behaviour of engine and powertrain components has gained in significance. This paper discusses several aspects of measuring and analysing torsional vibration and related data. Several torsional vibration measurement techniques are presented, together with remarks on precautions against possible sources of error, and the order of accuracy to be expected of the test results. Two applications requiring multichannel measurement and analysis are outlined. Introduction Understanding and controlling torsional vibration is of profound importance in vehicle development, refinement and optimisation. The primary source of torsional vibration is the internal combustion engine. Periodic combustion impulses result in rotational speed fluctuations of the crankshaft. Ignition and combustion within a cylinder cause a rapid rise in gas pressure and an angular acceleration of the crankshaft. Gas compression in the next cylinder causes immediate deceleration. Torque pulsations result in crankshaft torsional vibrations which reach the camshaft(s) and auxiliaries via belt or chain drives. In addition, the torsional vibrations which enter the gearbox may be transmitted further via propeller shafts and differentials to the vehicle wheels [1, 2, 3]. Increasing demands to shorten development cycles mean that less time is available for both testing and mathematical modelling. Meaningful solutions may not be arrived at by treating vibration dampers, timing belts, gear stages, etc. as isolated components. Comprehensive studies of the complete system are required which take interactions between individual components into account. Rotec GmbH was formed in 1988 to develop portable, PC-based equipment for use in torsional vibration testing. The company places high emphasis on accurate acquisition of torsional vibration data and the primary analysis methods are based on the revolution domain as opposed Improved Approaches to the Measurement and Analysis of Torsional Vibration Seán Adamson, Rotec GmbH, Munich to the time domain. In recent years the company has exploited both the increased computational power of PCs and digital signal processing technology to help make multichannel measurement and analysis quicker and more effective. This paper begins by presenting the new generation of ROTEC-RAS equipment (RAS = Rotation Analysis System). Torsional vibration measurement methods are then reviewed. Sources of error are highlighted and the degree of accuracy of test results is discussed. Two applications in engine and powertrain testing are utilised to illustrate the capabilities of the equipment. The New Range of RAS Equipment Applications requiring multichannel torsional vibration measurement include optimising engine timing and auxiliary drive systems, tuning of vibration dampers, minimising clutch slip, reducing gearbox rattle and transmission error testing of gearsets. This type of testing may also require additional measurement of transverse vibrations synchronous to the rotational data [4, 5]. All RAS channels operate on a common time-base making accurate, phasematched, cross-channel analyses possible (e.g. accurate calculation of the angular displacement between two rotating shafts). The RAS rotational speed channels require square-wave TTL level signals as input. The time interval between rising (or falling) edges for each pulse period is measured using a high-speed counter/timer (10GHz/40-bit). Input of index pulse and rotational direction signals is also provided for. The RAS analogue channels sample at either 50kHz or 400kHz with 16-bit resolution. Digital downsampling, anti-aliasing protection, programmable gain, AC/DC coupling and differential or ICP inputs are provided. By making use of a commercially available interface board, Controller Area Network (CAN) signals may be directly input and analysed in a similar way to analogue signals. CAN information is increasing in relevance since this is the dominating serial bus system for in-vehicle networks of passenger cars, as well as trucks and buses. RAS systems are modular (both hardware and software), scalable to fit operational needs and used both in test cell and in-vehicle applications. Figure 1 summarises NVH signal types and sources. Angular Velocity Measuring Methods The digital measurement technique for torsional vibration is based on sampling at equidistant angular intervals around the rotating shaft. This is generally accomplished by one of three methods: (i) mounting an incremental rotary encoder onto the shaft, (ii) scanning a toothed wheel with a magnetic pickup, (iii) targeting reflective/non-reflective (black/white) bar patterns with an optical sensor. The sensor electronics generate an angular velocity signal in the form of a TTL pulse train. The frequency of the pulse train is directly proportional to the angular velocity of the shaft. Rotary encoders with high line counts, as used in single flank testing of gearsets, can provide thousands of pulses per revolution, whereas proximity sensors are limited to a maximum of several hundred pulses per revolution. Angular sampling provides a fixed number of samples per revolution and is independent of the rota24 Measurement and Analysis of Torsional Vibration Figure 1 Test Vehicle, Signals and RAS System