Technology

Technology Overview

Ultrasonic Arrays developed the first non-contact ultrasonic thickness gage in 1984. This gage, the TMS-1000 was developed to overcome the inherent problems in attempting to use contact and laser measuring devices in process applications.

This gage was revolutionary in that it was self calibrating for any factors that would affect measurement accuracy, even for fixture movement. An external reference bar target was used as a standard to eliminate variables that affected the speed of sound (such as temperature). More recently this technology has been improved by adding a second reference bar to each sensor.

Patented Reference-Bar Technology

Our commitment is to provide measurement systems that meet specification in the intended environment, day in and day out. A key component of this requirement is the UA patented double reference bar technology, used by the more recently developed TMS 5000 and DMS 5000. Components of any system can change over time, affecting accuracy and performance. The double reference bar provides a "gold standard", to remove any changes that occur in the UA gaging system. Shown to the right in Figure 1 and Figure 2, are different views of a UA double reference bar transducer.

Two external reference targets are precisely mounted at known distances, from the sound transmitting/receiving surface. Exact measurements of distance is measured to these fixed locations during the same period as thickness/distance measurements are taken to the target surface. If any factor (temperature, humidity, air pressure, electronic component drift, etc.) causes the sound waves to measure the target incorrectly, the reference bar compensation will correct the error. There is no lag or time delay and there is no aging or drift of the system. A warm up period is not required.

High frequency ultrasound is transmitted from the sound producing surface to the reference bar and back, 125 times per second. If the distance to the reference bar changes, this means that the speed of sound has changed. The microprocessor in the gage automatically corrects the measurement.

Most of the sound passes the transducer reference bar(s), and travels to the product surface and returns to the transducer receiver.

The thickness measurement is equal to the separation of the transducers S less the distance measured to the top surface d₁ and to the bottom surface d₂. The separation S is constantly measured and compared to what S was when the transducers were originally calibrated, correcting for any fixture movement.

Temperature, Vibration and Collision Compensation with Air-Flow Control

In a process line, the temperature swings caused by the process and ambient temperature variation between the hot part of the day and the cold part of the night can easily be 50 degrees F. This means that the structures of the line, including the fixtures supporting the sensors are moving. Fixtures can also move for other reasons, including collisions with product, personnel standing on the fixtures for maintenance, being bumped with a fork lift, internal stress relief in the fixtures, etc. UAI's patented AutoZcal feature, automatically corrects for movement, no matter what the cause.

Another very important reason why the TMS series of non-contact gages are so accurate is the supplied "air service". In a mill environment there are thermal air currents that affect the speed of sound. In addition products themselves may be at varying temperatures. These elements cause inaccuracy and instability in the measurement. The UA air service which is provided with every gage, acts as a wave guide to eliminate the effects of thermals. In the TMS series the ’air service” consists of an air filtration system, air supply and air lines to condition the sound path and protect the sensors.

Knowledge of Ultrasound

In 1979 UAI was started to develop a sensor technology that would measure materials and products, made in process industries, to better than micrometer accuracy. Development requirements were such that the technology had to work on any material in most process environments. Conventional methods, including laser, optical, capacitive, nuclear, eddy current and contact were rejected because they did not meet one or more constraints. Ultrasound was attractive but science at the time, said that ultrasound could not be transmitted through air accurately and reliably. Traditional ultrasonic measuring devices create a sound wave using a piezo crystal. This method generates sound by electrically vibrating a crystal within the sensor. Because of the mass of the piezo crystal, ambiguous sound waves are generated--it takes an extended period of time for the signal to increase and then decrease. Distance/thickness measurements taken with this method are unreliable and inconsistent.

UAI pioneered a more accurate measurement method by tensioning thin conductive film on a conductive plate. Charging the film and plate and then removing the charge results in a distinct pulse --an unambiguous sound wave--providing a consistent basis to measure the speed of sound and produce accurate measurements.

Ultrasonic Arrays research and development produced an ultrasound-based non-contact measurement system and method. Unlike other measurement methods, non-contact ultrasonic gauges do not require recalibration. Because UAI gauges do not actually touch the product, they do not sustain material buildup or cause damage to the product surface. The non-contact nature of this technology simplifies fixturing as the gages can be mounted in a fixed position and make accurate measurements over a wide range of stand off distances and thicknesses.

Proprietary Sound Wave

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.

Patented Reference-Bar Technology

Our commitment is to provide measurement systems that meet specification in the intended environment, day in and day out. A key component of this requirement is the UAI patented double reference bar technology. Components of any system can change over time, affecting accuracy and performance. The double reference bar provides a "gold standard", to remove any changes that occur in the UAI gaging system. Shown to the right in Figure 3 and Figure 4, are different views of a UAI double reference bar transducer.

Proprietary AutoZ-Cal Recalibration

UAI gages automatically and continuously measure and recalibrate themselves based on a patented principal of double reference technology. This method removes all environmental effects from the measurement. In an on line measurement application the sensors have to be permanently mounted to some form of fixturing, to automatically measure product as it is transferred through the gages. This fixturing needs to be robust to prevent any movement of the sensors so that accurate measurements can be made. UAI offers a variety of robust standard fixturing ("O", "C" and scanning frames) to suit specific industries and applications. Depending upon the material used to construct the fixturing, there will be fixture movement. The amount of movement of the fixture depends upon the coefficient of expansion/contraction of the material used and the temperature extremes in the plant. If the fixturing is made out of steel and the plant temperature varies from 50° F in the cold part of the night to 100° F during the hot part of the day, the fixture will expand by about 0.010 inches. Without correction the measurement of the gage would have a 0.010 inch error. UAI's patented AutoZ-Cal completely removes this error. Figure 5 illustrates the AutoZ-Cal feature:

When the thickness system is initially set up, the separation between the opposing sensors determines the target thickness. If the sensors move closer together because of fixture thermal expansion/contraction, the measurement will be in error. If the fixture movement were 0.010", the measurement would be off by 0.010" . If there were a product pileup or collision or some accident that caused the fixture to move by 0.050", the measurement would contain a 0.050" error. The AutoZ-Cal feature eliminates this error by continually measuring the separation between the two sensors and applying a correction for fixture movement.

TMS 1000 and DMS 1000 used single
reference bar (left). TMS 5000 and DMS
5000 use dual reference bar (right).

Figure 1: Conventional sound wave

Figure 2: Ultrasonic Arrays unambiguous sound wave

Figure 3

Figure 4

Figure 5: AutoZ-Cal Recalibration

Albion Devices, Inc.
538 Stevens Ave.
Solana Beach, CA., USA
Tel: 858-792-9585