Method for measuring the wall thickness of a workpiece by ultra-sound
BACKGROUND OF THE INVENTION 
     The present invention relates to a method for measuring the wall thickness of a workpiece by ultra-sound, in which the surface echo of an ultrasonic wave generates a start pulse and the rear wall echo generates a stop pulse for a saw tooth generator and in which the amplitude of a saw tooth obtained when the stop pulse occurs provides a measure for the wall thickness. 
     For measuring the wall thickness of a workpiece by ultra-sound one records the surface echo and the rear wall echo obtained when irradiating ultrasonic waves perpendicularly upon the said surfaces, and measures the interval between the two echoes. This interval then provides a measure for the thickness of the workpiece. The recorded echoes cause corresponding triggering signals to be generated by the evaluation circuit. 
     The interval is measured by either of two methods. According to a first, namely an analog method of the type known from U.S. Pat. No. 3,427,866, the triggering signal produced by the surface echo starts a saw tooth generator which generates a signal rising linearly from zero. At the moment when the second triggering signal, the one produced by the rear wall echo, occurs one measures the instantaneous amplitude of the signal of the saw tooth generator. This analog method provides the disadvantage that the rising flank of the saw tooth must be very steep if high resolution is desired for the interval measurements. This however, considerably restricts the total measuring range. 
     According to the second, namely the digital evaluation method, one measure high-frequency counting pulses occurring between the two triggering signals. If in this case high resolution is desired for the interval measurements, the counting pulses must have a very high frequency. For example, if a resolution of 1 micron is desired in the case of steel, the counting pulses must have a frequency of approximately 3 GHz. Such high frequencies can be controlled only with a very considerable input in circuitry. 
     German Disclosure Document No. 31 26 138 describes a digital evaluation method of this type. However, in order to improve the resolution while reducing the counting frequency to every second triggering signal, the counting pulses are shifted in this case by a pre-determined phase angle. One then counts the number of counting pulses occurring between the two triggering signals over a given number of measuring cycles and determines thereafter the wall thickness by dividing the number of counting pulses by the number of measuring cycles. 
     According to the method known from German Disclosure Document No. 34 29 409, the residual time remaining after appearance of the second triggering pulse is recorded as a fraction of a counting pulse. The residual time is than also counted according to the Nenius principle by detuning an oscillator. 
     SUMMARY OF THE INVENTION 
     Now, it is the object of the present invention to improve this method so that the wall thickness can be measured with very high resolution over a very large range of wall thicknesses. 
     This object is achieved according to the invention by the fact that the saw tooth generator generates a sequence of equal triangular saw teeth, that a counting pulse is generated at each reversal point of the signal curve of the saw teeth and that the number of the counting pulses produced before the stop pulse appears and the amplitude of a saw tooth existing when the stop pulse appears are recorded by an adder, each counting pulse defining a constant unit of the wall thickness of the amplitude defining a fraction of this unit of the wall thickness. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other details, advantages and features of the invention will become apparent not only from the claims and the features to be derived from the latter--individually and/or in combinations thereof--but also from the following description of certain preferred embodiments of the invention which are represented in the drawing in which 
     FIG. 1 shows one embodiment of a circuit arrangement for carrying out the method according to the invention; and 
     FIG. 2 shows curves of the type encountered when carrying out the measurements. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     When an ultrasonic signal (20) is generated by a measuring head, then this signal is reflected by the surface of the workpiece whereby a surface echo (2) is produced. The ultrasonic waves penetrating into the workpiece are reflected by the opposite rear wall of the workpiece and, accordingly, received by themeasuring head as rear wall echoes (22). 
     These conditions are illustrated in FIG. 2a. The surface echo (21) and the rear wall echo (22) are supplied to a logical measuring-gate circuit (1) where the surface echo (21) produces a triggering pulse (23) according to FIG. 2b, while the rear wall echo (22) produces a second triggering pulse (24) according to FIG. 2c. The measured wall thickness is determined by the time interval between the rising flanks of the two rectangular triggering pulses (23 and 24). These two rising flanks define the beginning and the end of the output signal (2) of the logical measuring-gate circuit. The output signal (2) constitutes the so-called measuring gate. 
     It is supplied to a saw tooth generator (3) which generates a sequence of saw teeth (25) of stable frequency and exhibiting symmetrical rising and dropping flanks, as can be seen in FIG. 2d. The saw tooth generator (3) is started when the rising flank of the signal (2) appears. A trigger circuit 4 provided in the saw tooth generator (3) generates a counting pulse (26) every time a change in direction occurs in the saw tooth curve (25). The counting pulses (26) are supplied from the saw tooth generator to a counter (5). 
     When the dropping flank of the rectangular output signal (2) appears, corresponding to the appearance of the rising flank of the triggering signal (24), the instantaneous analog value of the saw tooth signal is recorded. If the dropping flank of the output signal (2) coincides with a rising flank of the saw tooth signal (25), then this amplitude value is supplied directly to an analog-to-digital converter (7). However, when the dropping flank of the signal (2) coincides with a dropping flank of the saw tooth signal (25), then the converter (7) is supplied with a value equal to the maximum amplitude of the saw teeth (25), reduced by the instantaneous amplitude value. This means that the converter (7) is in this case supplied with the complementary value. As soon as the dropping flank of the signal (2) appears, the saw tooth generator (3) is stopped. The contents of the counter (5) is input into an adder (10). In addition, the before-mentioned analog value (9), which had been supplied from the saw tooth generator (3) to the converter 7, is converted to a digital value and supplied to the adder (10). 
     The digital signals supplied to the adder (10) are weighted by the latter as follows: 
     Every counting pulse (26) corresponds to one unit of wall thickness (E) while the analog value (9) corresponds to part of this unit of wall thickness (E). The greater the analog signal (9), the greater is the said part. In the embodiment illustrated in FIG. 2, three counting pulses (26) are encountered which means that the wall thickness corresponds to three units of wall thickness (E). The analog signal (9) corresponds, for example, to a value 0.8 E, so that the wall thickness measured is equal to 3.8 E. If the ultrasonic waves pass a length of 2 mm of the workpiece between the occurrence of two counting pulses (26), then the unit of wall thickness E of equal to 2 mm. This means that the wall thickness of 7.6 mm has been measured in the example just described. 
     The sum formed by the adder (10) is multiplied by the factor (11) determined by the sound velocity, i.e. by the unit of wall thickness, and the value of the wall thickness so obtained is output by an output device (12), denominated in millimeters. 
     Although no particular demands are placed either upon the analog or upon the digital part of the circuit arrangement, one still obtains very high resolution of the measured value. 
     The saw tooth generator (3) may also produce a sequence of saw teeth exhibiting an inclined rising flank and a vertical dropping flank. The counting pulses (26) are generated every time the vertical dropping flank appears. In this case, it is not necessary for the saw tooth generator (3) to supply a complementary value to the converter (7). Rather, the converter (7) is always supplied with the instantaneous amplitude value of a saw tooth given at the moment when the dropping flank of the signal (2) appears. 
     The method can be applied similarly when the wall thickness is recorded as the time interval between two rear wall echoes, for example.