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Until recent times, it has been not been considered possible to make accurate distance/thickness measurements using air coupled ultrasound. A primary reason for this was due to the nature of sound waves. Air coupled sound waves generated by conventional means, were deemed ambiguous whether produced by speaker, piezoelectric crystal, voice or any other means. Conventional sound waves produce an envelope of oscillations, that increase and decrease with the smallest of environmental changes. A conventional sound wave is shown to the right in Figure 1:
Measuring distance/thickness, using sound requires some algorithm to measure time of flight and convert time into distance. This measurement requires a starting point (when the sound is generated) and a stopping point (when the sound is returned from the target object or material. The starting point of the sound wave can be when a switch or gate is closed. The stopping point is much more difficult. As you will note in Figure 1, there can be 50 or more cycles or oscillations in the wave form envelope, deeming the sound wave to be ambiguous. Typical pulse echo systems trigger on the beginning of the return wave, the peak amplitude, etc. If the return time is even off by one cycle, the measurement will be in error by 0.300". In addition conventional sound systems operate at 40-50 kilohertz, making them vulnerable to naturally occurring sounds such as air lines, saws, motors, etc.
The Ultrasonic Arrays system uses an unambiguous sound wave, at a much higher frequency. The UAI unambiguous sound wave is shown to the right in Figure 2:
Because this sound wave is unambiguous, Ultrasonic Arrays can use a zero crossing technique to accurately measure time of flight (distance). This method removes any chance of error by electronically triggering on a set of events to ensure that only the correct part of the returned wave is used for measurement. To qualify as an accurate return from a target, the receiver comparator must see, a negative transition, a positive transition above a threshold voltage, then a negative transition through a zero crossing. Because of the steepness of the wave form (high frequency) and low noise receivers, the stopping point of the timer counter is essentially a vertical line crossing a horizontal line, giving pinpoint accuracy. Another reason why Ultrasonic Arrays gages operate at more than five times the frequency of conventional systems is noise immunity. There are no naturally occurring noises above 100 kilohertz. UAI systems operate at between 200-250 kilohertz.
Another benefit to producing an unambiguous sound wave is that the sound wave is also used to detect and measure angular alignment of the sensor to the target surface. When making a measurement of distance or thickness to a surface, it is necessary for the sensors to be normal or perpendicular to the surface. If this is not the case, the measurement will not be accurate. This is true for micrometers and other forms of contact measurement, lasers and UAI. The UAI system actually measures the angular alignment (or misalignment) by measuring it's own wave form. This provides an electronic means of mechanically adjusting alignment and an on line, real time method of knowing if the measurements being reported are accurate. This is especially critical if there is any case of the gaging fixture being knocked out of alignment by product transfer, or some other event.
