Measuring and analyzing rotational irregularities, caused by dynamic changes in torque, is important to understand dynamic design problems encountered in such products as engines, transmissions, ignition control systems, exhaust lines and ABS brakes. The phenomena that causes the rotational irregularity generates a forced vibration to coupled structures such as the transmission, the wheel, and the car, boat, motorcycle or compressor structure. By measuring and analyzing the dynamic change in rotation, one can determine the cause of a vibration or acoustic problem.

This paper presents an approach to measure this phenomena and easily analyze its influence on the noise behavior of an automobile and its rotating components. An accurate, economical measurement approach is presented. It provides inherent synchronization with other vibration and operational measurements so phase accuracy is maintained. The approach permits easy upgrade of existing test systems.

The RPM signal is provided by a tachometer or a sensor detecting the passing of a gear tooth. This pulse is provided to a high speed frequency-to-voltage conditioner that is unique by having an output update rate equal to the input rate. Thus the "rotational irregularity" analog signal is suitable for delivery to existing FFT or order measurement instruments and is proportional to the instantaneous RPM. This signal requires similar processing as any other vibration or acoustic "dynamic" signal. Transfer functions can be computed between the instantaneous RPM data and vibration or noise data. Magnitude and phase of intra-cyclic RPM change orders can be correlated with the magnitude and phase of noise and vibration orders to study their inter-dependence.

Design problems in the engine mechanics, in the electronic ignition system and in the exhaust line affect the RPM of a car engine and may lead to noise and vibration problems in coupled components. Besides mechanical health issues, rotational irregularities play an important role in the perceived comfort of cars. The dynamic torque forces can also be a source of unacceptable vibration at engine idle, even in larger, luxury cars.

A dynamic speed pattern for a 4-cylinder engine is shown in Figure 1. Note the asymmetry of the RPM speed changes, due to a faster acceleration in the explosion phase and a slower deceleration in the compression phase.

Direct measurement of rotational speed change is not easily, and especially not economically, achieved. Instead, the measurement of a passing gear is typically used, only to find that widely available measurement and analysis tools for vibration, acoustic and other dynamic phenomena have difficulty processing this signal. Yet without analyzing this signal, traditional order or FFT analysis of car vibration and acoustic phenomena only provides a picture of the consequences of all noise and vibration sources. We need to identify the cause of the design or integration problem. The dynamic change in rotation is a way to gain insight to achieve this.

MEASURING ROTATIONAL IRREGULARITIES

Directly measuring rotational irregularities involves measuring the evolution in time of the instantaneous rotating speed. Sensor candidates for this direct measurement could be angular accelerometers or angular velocity transducers, but they are bulky and too fragile to be mounted on the engine shaft.

Let us assume that the normal engine operation ranges between 600 and 6000 RPM. With 138 teeth/rev, a typical number for a flywheel, the frequency of this signal will vary from 1.38 kHz at 600 RPM to 13.8 kHz at 6000 RPM. A conventional integrator-type analog frequency-to-voltage converter, set for 15 kHz input frequency range (if programmable or selectable), will exhibit excellent "near" DC performance but poor dynamic performance. Even "high speed" FVC analog converters using a sample and hold technique will exhibit a settling time of several milli-seconds, limiting the bandwidth to a few hundred Hertz. 

Because of this, many test systems today will try to use more complicated computer architectures. An example is the use of counter boards for the dynamic rotational speed measurement and A/D inputs for physical vibration or sound data. On paper this is a sound choice, but in practice it is a hard choice to easily implement. Radical changes are required in computer software and computer bottlenecks must be avoided to insure accurate, synchronized measurements are achieved.
However, with a faster frequency or period to voltage conditioner, the measurement system remains the same and software developments are eliminated....

FVC02 : A DIGITAL-TECHNOLOGY FREQUENCY-TO-VOLTAGE CONVERTER

The FVC02 is a frequency/period/counter-to-voltage conditioner that combines analog technology for sensor signal conditioning (power supply, signal amplification, triggering) and digital technology for the angular position (count), pulse frequency or periodicity analysis, gating and averaging. By providing the output signal as a scaled, dynamic, analog signal all the existing measurement equipment in today's laboratories and production test facilities can be enhanced to perform instantaneous speed measurements.

The end-result, proportional to the frequency or to the periodicity of the input signal, is then converted to a ±10V analog signal by a 16-bit D/A converter, maintaining the high quality and accuracy of the measurement. The output is updated once for each incoming pulse, insuring maximized bandwidth, minimized delay and reduced phase shift.
The full scale range is user selectable by programming and can be adjusted to maximize the accuracy for the expected frequency span in the application. The achieved frequency resolution varies as a function of the full scale range and the measured frequency as shown in figure 4 :

This diagram shows that the resolution of the measured frequency stays close to ±0.6 Hz or 0.003% of full scale up to 3 kHz for a full scale of 20 kHz.

When applied to intra-cyclic RPM changes measurement, the accuracy and resolution will be close to 0.25 RPM in the above example, accurate enough to measure the slightest variations and provide quality data for subsequent analysis.

SIMPLIFYING THE PROCESSING OF DYNAMIC ROTATIONAL DATA

By converting the rotational speed data to a varying voltage signal, measurement system design is simplified and a wide variety of existing analysis tools are available to the engineer at economical cost. Most importantly, these measurement systems most likely already exist and will not require new learning curves or integration work from the test engineers. Strip chart recorders, FFT analyzers and vibration order analysis systems  can process this signal by the same techniques as used for the noise and vibration measurement channels. This allows direct and unbiased correlation between rotational speed orders and vibration or noise orders to be made. By analyzing both noise and rotational irregularities phenomena with the same measurement and analysis tool, understanding the mechanical, thermodynamic or aerodynamic origin of the problem is achieved.

It is then possible to access information on :

-  weak cylinders (order 0.5)

-  ignition point problems (variations of the phase of order 2 for a 4-cylinder engine, 3 for a 6-cylinder etc ...)

-  exhaust back pressure (local increase in a narrow RPM band of the magnitude of order 2 for a 4-cylinder engine, 3 for a 6-cylinder, etc...)

-  and many more engine internal phenomena.

CONCLUSION

The use of a high speed, period/frequency/counter-to-voltage conditioners to process angular pulse-coded signals provides detailed information about the instantaneous nature of RPM as well as angular position. It permits the test department to utilize standard, readily available measurement and analysis tools.  Little or no engineering integration work is required to achieve this new source location "troubleshooting" tool. 

The FVC02 is a unique frequency/period/counter-to-voltage conditioner that remains highly accurate even with an output update rate that matches the input rate. As the output of the FVC02 is a standard ±10V dynamic analog signal, it can now be used, just as any voltage signal from an accelerometer or microphone output is used, in any FFT analyzer or multi-channel data acquisition system. This signal becomes the much needed reference channel for all studies dealing with vibration and noise originating from an engine, transmission or braking wheel.

By correlating RPM variation orders with vibration or noise orders, a better understanding of the consequences of the engine rotational irregularities can be obtained. By applying order tracking methods, the same methods as used to understand noise and vibration problems, to the intra-cyclic RPM variation information, it is possible to relate these variations to the internal behavior of the power train.

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