Process and apparatus for activating a time gate in the ultrasonic testing of materials using the impulse-echo system

In the process whereby a time gate but especially the time gate for an expected error range 58 in a device by means of which materials are tested by ultra sound according to the impulse echo process involving a test head 20 which emits pulses that impinge diagonally on the surface 28 of a test piece 24, the test head 20 that emits diagonally is firmly connected to a test head 22 from which pulses are emitted vertically. Both test heads emit at given intervals 48, 49 ultra sound impulses and subsequently receive them. The echo signal 50 of the entry of sound of the test head 22 from which pulses emit vertically is utilized to activate the time gate of the test head from which sound is emitted diagonally.

BACKGROUND OF THE INVENTION 
The invention pertains to a process for activating a gate, more 
particularly a gate in a range where errors are expected in testing 
materials by the ultrasound method using the pulse echo process. The 
process incorporates a test head that emits diagonally onto the surface of 
the material to be tested. The invention pertains also to an apparatus 
that functions according to this process. 
In the procedure of this kind that is already familiar from the publication 
DE-23 21 699 the input echo is employed for activating the expected error 
gate of the same test head. This publication demonstrates that such an 
echo start device tier an expected error gate can only function when the 
signal of the sound entrance echo is at least of a certain amplitude. 
Different ways of attaching the device to the surface of the material to 
be tested cause changes in the echo signal, however, and in addition, 
variations occur in the height of the signal in the portion of the sound 
emitted from the surface of the test material. In consequence, activation 
of the evaluation gate can be delayed to the extent of several ultrasound 
wave lengths. 
Procedures of the above mentioned kind are also familiar from publications 
OS-41 50 577 and DE-30 17 900 C2. For general reference to this procedure 
see also the DE-publication "Ultrasonic Testing of Material" by J. and H. 
Krautkramer, 4th edition. 
For test heads from which the impulse is emitted diagonally the procedure 
that is already familiar and the device that functions according to this 
procedure have not always led to reproducible results. In test heads of 
this kind a relatively small and also broad sound input register is 
frequently present. In consequence, the echo start conditions can not be 
conclusively realized under practical conditions because on the one hand 
the strong dynamic can lead to loss of the start incident because the echo 
remains smaller than a given threshold, for which reason the test is not 
concluded. On the other hand, by reason of the dynamic and the broad echo 
signals, the start point for the expected error range changes, for which 
reason the gate/aperture position varies. 
The purpose of the invention is defined by these considerations, its aim 
being to improve the echo start steering for the aperture/gate position of 
an expected error range or of another time range in such a way that, for a 
test head from which the pulse is emitted diagonally, the best possible 
coordination between transmission of a sound impulse and activation of the 
time gate, e.g. an expected error range gate, is achieved. 
As was demonstrated in procedural tests, this task is solved by a device 
for activating a time gate but especially an aperture for an expected 
error range in testing materials according to the impulse echo process; 
said device incorporates a test head which emits pulses that impinge 
horizontally on the surface of a test body that is characterized by the 
test head from which the pulses are emitted vertically and which is firmly 
connected to a test head on which pulses impinge vertically; said test 
heads transmit at given intervals ultra sound impulses which are 
subsequently received; and the echo impulse of the entry of the sound into 
the test head from which the pulses are emitted vertically is utilized to 
trigger the time gate of the test head from which the pulses are emitted 
diagonally. 
With reference to the device, it was solved by a process characterized by a 
test head from which pulses emitted diagonally and a test head from which 
pulses transmitted in the main vertically are connected firmly together; 
and they are moveable relative to a test body; the receiver channel of the 
test head from which pulses are emitted vertically is fitted with a time 
gate for the input echo; this input echo, if necessary after traversing a 
time delay switch, is conducted as trigger signal of the time gate for an 
expected error range of the receiver channel of the test head from which 
pulses are emitted diagonally. 
In essence, the invention is based on evaluating the sound entry display 
for an additional test head that is connected to the test head from which 
the pulse is received diagonally. The pulse of this additional test head 
is essentially vertical. When the ultrasound waves are introduced 
vertically it is usual to receive a high, narrow signal for the portion of 
the sound that is reflected at the surface of the test material. For this 
reason it is possible to tune more finely the starting point for the time 
gate and, in particular, the gate for the expected error range. The sound 
input echo for the test head from which the sound is emitted vertically is 
now introduced in order to determine the instant for activating the 
expected error gate for the test head from which the pulse is emitted 
diagonally. In most applications the sound input echo for the test head 
from which the pulse is emitted vertically is delayed by a given period of 
time. For devices that function cyclically the delay time is added. 
The procedure that is in accord with the invention offers the advantage 
that the signal of the sound input echo is processed and can be prepared 
before it is required for activating the expected error gate. By reason of 
the present state of the technology, however, the entry signal continues 
to be processed in real-time, and under these conditions evaluation is to 
say the least difficult. As ultrasonic testing devices normally function 
cyclically, preparation and time delay for the pulse entry echo can be 
conducted simply because the processes proceed periodically. 
Even though the process is described in the main for activating an expected 
error gate no limitation to the patent sought is implied. The process can 
just as well be employed for activating any other time gate, for example, 
a gate for a variable interference/suppression point, and in this 
connection reference is made to publication DE 38 22 699 AI. It can also 
be employed for measuring run time or the like. In place of the concept 
pulse-echo-process the word pulse-reflection-method is frequently employed 
since it has the same meaning. 
It is normally not necessary to provide a separate test head from which the 
pulse is emitted vertically exclusively for the echo start of the time 
gate; instead, test heads which are present any way and from which the 
pulse is emitted vertically, such as for example test heads for 
determining the thickness of walls, or heads for ascertaining core faults, 
can be employed. In order to employ such test heads for the echo start of 
an expected error range of a test head from which the pulse is emitted 
diagonally they should impinge on the same surface point as the test head 
that impinges diagonally. Under these conditions, there occurs in a 
certain range of the lead advance a proportionality between forward run of 
the test head from which the pulse is emitted vertically and forward run 
of the test head from which the pulse is emitted diagonally. For this 
reason, changes in the forward run of the test head from which the pulse 
is emitted vertically can easily be transformed into changes in the 
forward run of the test head from which the pulse is emitted diagonally. 
It has proved of great advantage in determining the sound entry echo to 
take into account in each instance at least the last known echo of the 
test head from which the pulse is emitted vertically. In devices that 
function cyclically the time of occurrence of the last known input echo is 
then incorporated in the event that the input echo of the test head from 
which the pulse is emitted vertically should fail. In this way the test is 
not interrupted. In order to obtain in this way the median position for 
the input echo it is especially advantageous if a roving median range 
results from the last echo time instants. Determining the gate in the 
expected error range becomes more stable because of this, which is to say 
that it wanders back and forth less with every test.

DETAILED DESCRIPTION OF THE DRAWING 
As is shown in FIG. 1, two test heads 20, 22 are rigidly connected together 
and moved relative to a test body 24. An arrow 26 is included in the 
direction of movement; it is intended to indicate movement to the right of 
the test body 24. The test body in question 24 can be, for example, a pipe 
which, as is well known, is examined by a rotating test machine. In this 
case both test heads 20, 22 are arranged in a rotor, and the test body 24 
in the form of a pipe is moved axially in arrow direction. 
Pulses from the test head 20 impinge diagonally on the surface 28 of the 
test body 24, whereas the test head 22 is arranged on a norm to the 
surface 28 so that its pulses impinge vertically on the surface 28. Both 
test heads 20, 22 are so arranged that their central beams converge at a 
point 30 on the surface 28. This point 30 is the point of entry of the 
central beams of both test heads 20, 22 in the test body 24. Coupling both 
the test heads 20, 22 is effected according to the state of the art, that 
is, via a water forward run. In this way a considerable portion of the 
sound energy that is given off at certain instants by both test heads 20, 
22 enters the test body 24. By reason of the connecting surface between 
the coupling medium and the test body 24, a portion of the sound energy is 
nevertheless reflected. Even for test heads which are constructed 
identically, this portion is for the test head from which the pulse is 
emitted vertically 22 disproportionally larger than the portion for the 
test head from which the pulse is emitted diagonally 20. For this head 
(20) the reflected portion, proceeding from point 30, proceeds in the main 
upwards to the left, which is to say in a beam that is mirror symmetrical 
to the central beam 34. The norm 32 shows the symmetrical axis. 
Both test heads 20, 22 are operated in accord with the pulse echo 
principle. They emit ultra sound impulses and receive the echoes of these 
impulses. To this end a transmitter 36 or respectively 38 and a receiver 
40 or respectively 42 are fitted to each of the test heads 20, 22. This is 
shown in FIG. 1. By means of a line 44 to which reference will be made 
later, the receiver 42 of the test head 22 from which the pulses are 
emitted vertically is connected to the receiver 40 of the test head 20 
from which the pulses are emitted diagonally in order to activate the gate 
tier the expected error value in the receiver 40 of the test head from 
which the pulses are emitted diagonally. To this too reference will be 
made later. 
In the following the impulse processes that are in accord with FIG. 2 and 
FIG. 3 are described. A cycle provider 445, as is evident from FIG. 2, 
transmits at set intervals periodic impulses 48, 49 that are conducted 
alternately to the receiver of the test head from which pulses are emitted 
diagonally 36 and the receiver 38 of the test head from which pulses are 
emitted vertically where they at practically the same instant give rise to 
an ultra sound impulse (not shown). This impulse passes through the 
forward run (which is here not defined more exactly) and along the central 
beam to the surface 28 of the test piece 24. There a portion of the sound 
energy is reflected. The remainder enters the test body 24. The reflected 
amount produces the first activating cyclical signal 48 and subsequently 
the echo that follows the transmitted impulse. In the case of diagram FIG. 
2, which describes the relationships for a test head 22 from which the 
pulses are emitted vertically, the entry echo 50 follows at an instant dt 
subsequent to the instant that marks the triggering instant in the cycle 
48. If there is in the test body 24 no reflector of any kind, the coupled 
sound pulse proceeds further to the back wall 52, where a portion of it at 
least is reflected; and in its turn a portion of the echo from the rear 
wall proceeds to the test head 22, where it gives rise to a rear wall echo 
54. The time difference between the entry echo 50 and the rear wall echo 
54 can be utilized in the well known manner in order to determine the 
thickness of the material (and also the thickness of the wall) of the test 
body 24. In this connection attention is drawn once again to the 
publication DE 38 22 699 At. But for the process that is here described 
the entry echo 50 is important. 
The entry echo 50, which is to say the first echo that, following the 
transmitted pulse, is registered in the receiver 42, is processed in a 
circuit arrangement 56 (see FIG. 1 ) and delayed. This circuit arrangement 
56 is incorporated in the wire 44 and therefore conducted to the receiver 
40 of the test head from which the pulses are emitted diagonally 20. Here 
it is employed to trigger the expected error gate 58. 
In the model described, pulse 49, which follows the pulse 48 and which is 
transmitted at an instant that is a whole number multiplication of the 
cyclical time T.sub.s following the cyclical impulse 48 from the cyclical 
impulse trigger 46, triggers a transmitted pulse after the beat 48 from 
the cyclical trigger 46 of the test head 20 from which the pulses are 
emitted diagonally. In this way an ultra sound impulse traverses the 
central beam 34, and part of it enters through an area around the point 30 
the test body 24 while another portion gives rise to a reflection. A small 
portion of the reflected pulse appears as a weak input echo. In accordance 
with the available technology, this input echo 60 is utilized to activate 
the expected error gate 58. In the process described here, however, this 
input echo 60 is ignored. Instead, the expected error gate 58 is activated 
by a signal from the circuit arrangement 56 that is generated at an 
instant dt following the cyclical impulse 49. In this arrangement the time 
dt is, as described above, the elapsed time between the cyclical impulse 
48 and the input entry echo 50. In an improved version the time dt will be 
the varying mean value between the last n time differences between each 
cyclical pulse 48 that activates the test head 22 and the instant of the 
input entry echo 50, or it will constitute a different processing of 
several values. 
The entry echo 50 is evaluated and processed in the circuit arrangement 56 
in order to ascertain the time dt. Measuring the time for dt is for 
example stopped when the first leading edge of an input entry echo 50 
consisting of several half waves exceeds a given threshold value 62. In 
yet another version the instant of greatest amplitude of the input entry 
echo 50 can also be utilized for terminating the measurement of dt. 
Publication DE 35 19 797 AI describes a process whereby from an input echo 
that frequently consists of several half waves a signal for terminating 
the measuring of dt can be deduced. 
In the version described here the forward runs for both test heads 20, 22 
are of the same dimensions. The forward runs are also identical in terms 
of their physical properties. In consequence, following the triggering 
instant 48, 49, an ultra sound impulse requires for both test heads 20, 22 
the same amount of time to reach point 30 on the surface 28. In this case 
the time gate 58 (the expected error gate) in the receiver 40 of the test 
head from which pulses are emitted diagonally 20 is activated at an 
instant dt following the cyclical signal 49, and the actual expected error 
gate 58 then is activated following an additional time delay t.sub.a, 
which can be adjusted and which takes account of the decline in the input 
echo 60 so that this does not proceed into the expected error gate 58. 
This is shown in FIG. 3. The end of the expected error gate 58 is of no 
consequence for this description, as the expected error gate 58 in the 
version here described ends at sufficient distance from the rear wall echo 
64 that this too in its turn does not enter the expected error range. The 
echo 66 of an imperfection 68 in the test body 24 falls in the expected 
error range. 
In the event that the forward runs for each of the test heads 20, 22 are 
not identical an appropriate constant period of time t.sub.4 is 
introduced, and the expected error gate 58 is then activated at the 
instant dt+t.sub.v, following the triggering cyclical pulse 49. 
If the distance between the test head from which pulses are emitted 
vertically 22 and the surface 28 of the test body 24 changes the time 
period dt changes. Within the range of certain small changes it is not 
necessary to "take account of this influence. But if the change exceeds a 
certain threshold a correction must be made. Point of departure for this 
correction is the distance given when justifying the device. Should this 
distance exceed a certain amount the expected error gate can be activated 
at the wrong instant; it can be started too early or too late. To avoid 
this it is proposed in a subsequent development of the device to 
incorporate in the circuit arrangement 56 some compensation for the 
altered run. To this end the increment by which the time span dt at the 
time of justification was changed will be taken into account. The 
increment can be positive or negative. It is multiplied by a geometry 
factor which in essence is a trigonometric function of the input angle. In 
this way compensation is effected for the longer pre run for the test head 
20. It is longer because the pulses impinge vertically, with respect to 
changes in the distances between the test head 22 and the surface 28.