WO1992019983A1

WO1992019983A1 - Ultrasonic measurement apparatus

Info

Publication number: WO1992019983A1
Application number: PCT/GB1992/000807
Authority: WO
Inventors: Ahmet Giral GURTUNA
Original assignee: MANCHESTER INSTITUTE OF SCIENCE AND, University of; MANCHESTER INSTITUTE OF SCIENCE AND TECHNOLOGY, University of
Current assignee: MANCHESTER INSTITUTE OF SCIENCE AND, University of; MANCHESTER INSTITUTE OF SCIENCE AND TECHNOLOGY, University of
Priority date: 1991-05-02
Filing date: 1992-05-01
Publication date: 1992-11-12
Anticipated expiration: 1993-11-02
Also published as: AU1667392A; GB9109552D0; GB9322859D0; GB2273983B; GB2273983A

Abstract

An apparatus for measuring the time taken for a signal to be transmitted through a medium such as a bolt. An ultrasonic signal is transmitted into the bolt and reflections of the transmitted signal are detected after transmission through the bolt. After a first transmission is initiated, each of a series of further transmissions is initiated in response to detection of a second reflection of the transmitted signal resulting from the preceding transmission. The total elapsed time between initiation of the first transmission and detection of the second reflection of the transmitted signal resulting from the last transmission is measured, and the sum of time delays elapsing between initiation of each signal and detection of a first reflection of the resulting transmitted signal is measured. The time taken for transmission of the signal through the bolt is calculated from the difference between the total elapsed time and said sum.

Description

ULTRASONIC MEASUREMENT APPARATUS

This invention relates to an apparatus for measuring the time taken for a signal to be transmitted through a medium to determine for example, the length of a structural member such as a bolt joining pieces of metal or other material. Such apparatus may be used to measure stress.

It is known to calculate the stress applied to a bolt joining, for example, two pieces of metal, by measuring the length of the bolt and calculating the stress from the difference between the bolt length when stressed and the bolt length when not stressed. The bolt length can be determined by transmitting a pulse of ultrasound radiation along the bolt and measuring the time taken for the pulse to be reflected back down the bolt.

Known techniques of stress measurement of this type suffer from a number of problems. The difference between the time taken for the pulse to travel down the bolt and back when the bolt is stressed, and the time taken when the bolt is unstressed is very short (of the order of nanoseconds), and consequently it is difficult to measure this time accurately. It is known to overcome this problem by detecting a number of reflections of a single pulse, as it travels up and down the bolt. Knowing the number of reflections detected, a more accurate time for one reflection can be calculated. However, the attenuation suffered by the signal is large, and often only four or five reflections can be reliably detected, depending on the bolt geometry. Thus the improvement in measurement accuracy is not very significant It should also be noted that there is also a delay between generation of an electronic signal indicating initiation of a transmission of ultrasound and the actual transmission of a pulse of ultrasound into the start of the medium. This delay is due to the electronic circuitry, the internal delay of the probe, and the coupling effect, and is difficult to accurately determine. This leads to more inaccuracies in the measurement, especially during long-term measurement.

It is an object of the present invention to obviate or mi tigate the aforementioned disadvantages.

According to the present invention there is provided an apparatus for measuring the time taken for a signal to be transmitted through a medium, comprising means for transmitting a signal into the medium and detecting a reflection of the transmitted signal after transmission through the medium, means for initiating a first transmission, means for initiating a series of further transmissions each in response to detection of a second or subsequent reflection of the transmitted signal resulting from the preceding transmission, means for determining the total elapsed time between initiation of the first transmission and detection of the said second or subsequent reflection of the transmitted signal resulting from the last transmission, means for determining the sum of time delays elapsing between initiation of each signal and detection of a reflection of the transmitted signal resulting therefrom, and means for calculating the time taken for transmission of the signal through the medium from the total elapsed time and said sum.

The time delays between each signal initiation and the first reflection may be summed and subtracted from the total elapsed time, each further transmission being initiated in response to the second reflection of the preceding signal. The result of this subtraction is the time taken for one reflection multiplied by the number of transmission initiations. Hundreds or thousands of initiations may be performed, enabling very accurate measurements to be made. Thus, by generating pulses of ultrasound in response to the detection of reflected signals a large number of pulses can be used, and the time elapsed can be measured very accurately. The problem of trying to detect reflections of small amplitude is also avoided. The delays inherent In the circuitry, the probe and the coupling effect, which are difficult to measure are simply subtracted out.

A specific embodiment of the present Invention will now be described by way of example, with reference to the accompanying drawings in which:

Figure 1 is a block diagram of the circuitry of the embodiment of the present invention; Figure 2 is a timing diagram illustrating the operation of the embodiment of Fig. I; and Figure i is a timing diagram illustrating the calculation oi the delav inherent in the time interval counter. Referring to Figure 1, the illustrated ultrasonic measurement apparatus comprises a central processor 1, a probe 2, a transmitter or pulser 3, a receiver 4 and associated devices.

The probe 2 is attached in a convenient manner to the end of a bolt indicated schematically by a broken line 5. The probe is also connected to a temperature sensor 6. The temperature sensor 6 is necessary because the length of the bolt and the acoustic velocity of the material alter with changes in temperature, and this needs to be taken into account in the final calculation.

The pulser 3 is responsive to a pulse controller 7 to apply a signal to the probe to cause a pulse of ultrasonic energy to be launched into the bolt 5. The pulse controller 7 is in turn responsive to a pulse initiation signal from the central processor 1 to initiate the transmission of ultrasound pulses.

The receiver 4 detects the output of the probe 2 indicating the arrival of reflections of transmitted ultrasonic pulses at the probe. An amplitude controller 8 controls the amplitude of the output of the receiver 4. A multi-target resolver 9 selects which reflected signal is to initiate pulses from the pulser 3 by setting a time window within which a reflected signal is to be detected. The multi-target resolver 9 is connected to the central processor via a target controller 10.

The multi-target resolver is also connected to positive and negative peak detectors 11. These ensure that the apparatus is not sensitive to the phase of the reflected pulse. It will be appreciated that a 180 degree phase change is caused on reflection. The peak detectors are connected to the central processor via a trigger level controller 12. The trigger level controller 12 sets the trigger level for a time interval counter 13 and the pulse controller 7. The peak detectors are also connected to the time interval counter 13, which is also connected to the central processor.

As illustrated in Figure 2, in use a measurement starting pulse is generated from the central processor. This is indicated by the leading edge of waveform M. This starts the time interval counter 13 to time the overall time of the measurement. The measurement starting pulse M also triggers a first pulse transmission initiation waveform S at time t₀. The waveform SI causes the pulser 3 to emit a pulse transmission control signal PI at time t. which causes the probe 2 to generate an ultrasonic pulse. The ultrasonic pulse travels back and forth along the bolt, each reflection being detected and represented by waveform Rl in Figure 2. The first reflection is received at time t₂, the second at time t₃. It will be noted that the amplitude of the reflected signal falls rapidly.

At time t₃, that is, the time at which the second reflection is received, a fresh pulse transmission sequence is initiated. This is represented by waveforms S2, P2 and R2. The first reflection of the second transmission is received at time t₄. A third transmission is then initiated at time t₅. The system discriminates between reflections from the first transmission and reflections from the second transmission by reference to the reflected signal amplitude, and the positions of the reflections relative to the positive edge of the waveform S2.

The retransmission process is repeated for a total of N times until the second reflection of the Nth transmission is received. As illustrated, the Nth transmission is initiated at time t, ~._N__{l )} , and the first and second reflections are detected at times t_{( 2N )} and t_{( 2N+1 )} . At time t,_{2N+ 1 )}, the measurement sequence is terminated.

During each transmission stage, the time between initiation of a transmission and reception of the first reflection is stored. These periods are indicated by waveforms Dl, D2 DN, representing delays

Tl, T2 .... TN. The central processor sums the delays Tl, T2 .... TN, and this sum is subtracted from the total duration of the measurement sequence. The result of this subtraction is NT_e, where T_e is the time taken for an ultrasonic pulse to travel the length of the bolt twice. In other words T_e is the measured elapsed time. Thus:

1 n = N

T = - [ (t_{( 2N+i )}-t₀) - ∑ T_n ]

N π= l

The change in the measured elapsed time T_e, relative to the original value when the bolt is unstressed, can be used to calculate the stress in the bolt in a conventional manner. During the calculation, the effect of the temperature on the acoustic wave velocity and the bolt length, and the effect of the applied stress on the acoustic wave velocity must be taken into account.

The description so far assumes that Tl, T2....TN can be measured with great accuracy. If circuit components such as the time interval counter 13 can be designed without a significant inherent delay the above equation will give accurate results. If the time interval counter 13 is subject to inherent delays, these should be compensated for in the calculation. If ΔD is the time interval counter delay, ΣT_n in the above equation will become Σ(T_n + &D). Thus, Δ must be determined in order to be able to calculate the actual elapsed time T_e', where T_e'=T_e if Δ D=0 (T_e is the measured elapsed time). It is reasonable to assume that Δ D is always the same for each pulse detection in any one series of measurements.

As illustrated in Figure 3, in the case that Δ D^O, the measured elapsed time T_e will be different from the actual elapsed time T_e' by D, the delay inherent in the time interval counter 13. To determine D, the sum of the total elapsed time K_n (where K_n=nT_e) is measured for different values of n (for example N_χ ,N₂....N_N) and then plotted as shown in figure 3. By extrapolation of the graph back to zero, Δ D can be calculated. That is, if D were zero, the curve would go through the origin. This calculation is performed by the central processor 1 at the end of a measurement sequence, and the actual elapsed time T_e' can be calculated by using the equation T_e'=T_e + Δ D. Delays can arise in circuit components other than the time interval counter 13, but it will be appreciated that the effects of these other delay sources will also be compensated for by the application of the above-described technique.

In the described embodiment of the invention, the second reflection of each transmitted pulse is used to trigger the next transmission. It will be appreciated that the third or subsequent reflections could be used if the reflected signal strength is adequate.

Claims

CLAIMS:

1. An apparatus for measuring the time taken for a signal to be transmitted through a medium, comprising means for transmitting a signal into the medium and detecting a reflection of the transmitted signal after transmission through the medium, means for initiating a first transmission, means for initiating a series of further transmissions each in response to detection of a second or subsequent reflection of the transmitted signal resulting from the preceding transmission, means for determining the total elapsed time between initiation of the first transmission and detection of the said second or subsequent reflection of the transmitted signal resulting from the last transmission, means for determining the sum of time delays elapsing between initiation of each signal and detection of a reflection of the transmitted signal resulting therefrom, and means for calculating the time taken for transmission of the signal through the medium from the total elapsed time and said sum.

2. An apparatus according to claim 1, comprising means for summing the time delays between each signal initiation and the first reflection, means for subtracting the summed time delays from the total elapsed time, and means for initiating each further transmission in response to the second reflection of the preceding signal.

3. An apparatus substantially as hereinbefore described with reference to the accompanying drawings.