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Performance Improvement of Rotary Ultrasonic Testers
J Venczel, Magnetic Analysis Corp., USA
Abstract



In Rotary Ultrasonic Testers the transducers rotate around the tube. The signal connection to and from the transducers represents a critical part of the tester. The task can be accomplished by one of three methods: slip rings, rotary capacitors and rotary transformers. Naturally, each has advantages and disadvantages. The paper analyzes these methods and offers an optimal solution. Detailed description of a working Rotary Tester is given with test results in tube and bar testing

Introduction

In order to insure a flawless product in the tube industry the final phase of production must be the nondestructive inspection. Eddy current inspection is capable of detecting surface flaws on the outside surface of the tubes but not on the inside surface or deep in the wall of the tube. Ultrasonic inspection can find flaws both on outside and inside surfaces, usually referred to as OD and ID, as well as internal flaws. Utilizing the circular symmetry of tubes, it is possible to achive100% inspection by rotating the tube in front of the transducer or rotating the transducer around the tube. Rotating a tube at 3000 to 4000 RPM is difficult to achieve, therefore rotating the transducer allows a higher inspection speed. Transducer rotation makes it more difficult to connect electrical signals to and from the transducer. There are three main methods for coupling electrical signals to rotating parts: slip rings, rotary capacitors, and rotary transformers. The main problem with slip rings is the electrical noise they generate; also, at high speed, it is difficult to maintain reliable electrical conductivity between the slip ring and the brush. Rotary capacitors are better with respect to noise but more difficult to make. Ultrasonic signals in case of nondestructive testing are in the frequency band of 0.5 MHz to about 20 MHz, or higher. To pass such a wideband signal, the coupling capacitors have to be in the range of 500 to 2000 pF for each channel. To realize this kind of capacitance the capacitor plates have to be placed close to each other and have a large surface. This, in turn, requires careful design and precision machining operation. The parasitic capacitance between plate and ground leads to loss of signal, while the mutual capacitance between plates of adjacent channels results in crosstalk between channels. Rotary transformers made of high frequency ferrite material are capable of handling the required high frequency signals but have problems transmitting the high voltage initial pulse, which may be 500 to 600 volt. Until recently, rotary capacitors were used predominantly in rotary testers. The application of rotary transformers is new but it has several advantages which will be discussed in the following.
 

Rotary capacitors vs. rotary transformers

A rotary tester requires several channels for a typical application. Four channels are required for a clockwise, counterclockwise, forward, and reverse shear wave testing and a fifth channel for wall thickness measurement. Higher inspection speed requirement leads to more channels. Testers often have seven or twelve channels for increased inspection speed. Failure of rotary testers is often caused by dirt or water in the rotary capacitors. Crosstalk between channels is a serious problem and it gets worse with increased number of channels. The coupling loss through capacitors is typically 7 to 10 dB and the signal path includes the ground connection between the rotor and the stator. Figure 1 shows the electrical connection between a pulser receiver and a rotary tester when using a rotary capacitor. The rotary capacitor has parasitic components on input to stator ground and on the output to rotor ground. These capacitances have a shunting effect, which decreases signal strength both in connecting the initial pulse to the transducer on the rotor and connecting the received echo signals to the pulser-receiver. The return path of electrical connection is accomplished through the ground connection between the stator and rotor. A conductive seal or a slip ring is used to make the electrical connection between the bodies of stator and rotor but it also acts as a noise source added to the input signal of the pulser-receiver. The application of a rotary transformer is shown on Figure 2. The coupling ratio is usually one-to-one but other ratios are also possible for better impedance matching. The effect of

Figure 1.

parasitic capacitances is minimal and so is the crosstalk between channels. The use of a conductive seal or slip ring type of ground connection is still required but it is no longer part of the signal return. The main problem, however, is that the ferrite transformers can not efficiently transmit the high voltage initial pulse. To overcome this problem a remote pulse preamplifier Was used. This, in turn, created a new problem by requiring electrical power Conducted to the rotor. This was accomplished by adding a power transformer to the set of signal transformers and a power supply board with rectifier and DC-to-DC converters. The end result was an ultrasonic rotary tester with rotor mounted electronic processing circuits.
 

 
Figure 2.

Rotary ultrasonic tester with on-board electronics

The example for this product is a seven-channel rotary tester. Three of the channels are set up to operate in compression mode for longitudinal wave testing and four to operate in shear wave mode. The pulser of the ultrasonic flaw detector is set up to operate with a reduced amplitude initial pulse, since it functions only as a synchronizing signal for the pulser circuit on the rotor. Each channel of the flaw detector is connected to the rotary by a single cable. This cable has to conduct signals in both directions. The approximately 10V amplitude initial pulse is generated in the flaw detector and connects to the rotor- mounted pulser via the
rotary transformer. The 500V amplitude output of the pulser is connected to the transducer. The echo signals received by the transducer are connected to the input of the preamplifier in the pulse-echo arrangement. You can set the gain of the preamplifier in four steps by slide switches on the PC board individually for each channel. For best signal-to-noise ratio the gain of the preamplifier has to be as high as possible without saturation. On the other hand, in some applications the signals are very strong and the amplifier gain has to be reduced to prevent saturation.

The slide switches allow a coarse setting of the gain in 6 dB steps. The output impedance of the preamplifier is 50 ohm to match the characteristic impedance of the interconnecting

 
Figure 3.

cable. To be able to use a single cable to conduct signals in both directions, we use a technique that is called “amplitude division multiplexing”. The amplitude range of +/-5 volts is assigned to the preamplified echo signals from the rotor to the flaw detector. The -5 to -10 volts range is assigned to the transmission of synchronizing initial pulse from the flaw detector to the pulser on the rotor. In evaluations the signal to noise ratio was found to be about 10dB better compared to the traditional rotary tester. Figure 3 shows the setup screen of the ultrasonic tester detecting a 0.3 mm deep and 0.3 mm wide surface notch. The bottom of the screen shows the strip chart recording of a calibration standard with signal amplitudes on and off the notch. On the A-scan presentation, the signal amplitude is 86% under the red gate while the noise level is 6% under the green gate. Ground loop noise is practically eliminated because of transformer coupling. It is hard to measure numerical improvement in resistance to electromagnetic interference, EMI, but it appeared to be less. Crosstalk between channels is so low that it is not a problem any longer. The most difficult problem we faced during the design was to mount the circuit boards in such a way that they can withstand the centrifugal forces during operation. Figures 4, 5, 6 and 7 show the mounting arrangement of the rotor-mounted printed circuit boards.
 

 
Figure 4
   
Figure 5
 
Figure 6
 
Figure 7

Conclusions

In the last few years, the rotary testers are facing a serious competition from phased array systems. Phased arrays offer many advantages. First of all there are no moving parts, the scanning is done electronically. Phased array systems also offer a number of advanced features, like variable focal length, electronic switching between compression and shear wave modes and possibly others, but many applications do not require those. The major disadvantage of a phased array system is the cost, which is much higher then that of a rotary tester. In multi channel applications the rotary testers offer a lower cost alternative to phased arrays and the addition of on board electronics improves the ability to find small defects reliably at high inspection speed. Plans for new developments include the addition of remote control for the rotary mounted pulser-preamplifier. Remote control of pulse amplitude, amplifier gain and damping can add flexibility to setup and operation. RF data links of 900 MHz or other frequency are quite suitable for application in rotary testers.


Contact Person:

John Venczel
Engineering Manager
Ultrasonic Development
Magnetic Analysis Corporation
Phone: (914)699-9450
Fax: (914)699-9837
Email:
 


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