Ultrasonic sensor for controlling casting processes

Abstract

This patent application describes a device for performing ultrasonic measurements. According to an example, the device comprises at least one ultrasonic transducer and an electronic circuit including a transmitter and a receiver. The transmitter is configured to generate a control signal with a sequence of control pulses for the ultrasonic transducer. The receiver is configured to receive a sensor signal supplied by the ultrasonic transducer with a sequence of echo pulses and to generate an envelope signal representing the envelope of the echo pulses. The receiver further comprises a first filter circuit and a second filter circuit, both configured to smooth the envelope signal. The second filter circuit has a larger time constant than the first filter circuit, and therefore reacts more slowly to changes in the envelope signal.The receiver section further includes a comparator designed to detect that an output signal of the first filter circuit falls below an output signal of the second filter circuit by a defined amount.

Description

TECHNICAL AREA The present invention relates to a method and a device for the non-contact detection of liquid media in cavities within metallic structures. BACKGROUND The detection of liquid media in cavities within metallic structures is relevant, for example, in monitoring casting processes. In an injection molding process, the liquid medium is a plastic melt (located in the injection mold), while in a die casting process it is a metallic melt. Pressure or temperature sensors can also be used to detect the flow front in casting processes. However, such concepts have the disadvantage that the sensors must be in direct contact with the melt. This requires drilling a hole through the cavity wall inside the mold. Due to the high pressures that can occur, for example, in plastic injection molding processes (often exceeding 1000 bar), these sensor bores must be manufactured with extreme precision to prevent leaks during the filling process. Also due to the high pressures (and temperatures of the melt), the sensors must be extremely robust. This means that the low pressures exerted by the melt's flow front (typically 1-10 bar) are too low when the melt flows over the sensor to detect the flow fronts consistently and accurately.The necessary pressure resistance also leads to very slow reaction times for temperature sensors, which is problematic for the extremely short filling processes. Furthermore, methods are known for determining the flow front of a melt during injection into a mold using ultrasonic measurements. Ultrasonic systems can relatively easily detect through the mold wall whether air or liquid (melt) is present inside the cavity at the sensor position. For this purpose, an ultrasonic pulse is sent at regular intervals towards the cavity wall of the mold. As long as air is present in the mold (at the sensor position), almost all of the sound energy is reflected at the mold / air interface due to the high density differences. When the liquid / melt reaches the sensor position, a small portion is transmitted into it, and only the remaining portion is reflected. The melt can be detected by measuring the amplitude of the reflected pulse. Such measurements are typically performed using a reflection setup. The ultrasonic transducer is excited by a short high-voltage pulse; the resulting ultrasonic pulse travels through the metal and is reflected at the interface, travels back in the opposite direction to the transducer and is converted there into an electrical signal, subsequently amplified, digitized by means of an analog-to-digital converter and digitally post-processed. The inventors have set themselves the task of improving existing circuit designs so that it becomes possible to avoid the digitization of the signals and the subsequent signal processing. SUMMARY The aforementioned problem is solved by the device according to claim 1. Different embodiments and further developments are the subject of the dependent claims. The following describes a device for performing ultrasonic measurements. According to an example, the device comprises at least one ultrasonic transducer and an electronic circuit including a transmitter and a receiver. The transmitter is configured to generate a control signal with a sequence of control pulses for the ultrasonic transducer. The receiver is configured to receive a sensor signal supplied by the ultrasonic transducer with a sequence of echo pulses and to generate an envelope signal representing the envelope of the echo pulses. The receiver further comprises a first filter circuit and a second filter circuit, both configured to smooth the envelope signal. The second filter circuit has a larger time constant than the first filter circuit, and therefore reacts more slowly to changes in the envelope signal.The receiver section further includes a comparator designed to detect that an output signal of the first filter circuit falls below an output signal of the second filter circuit by a defined amount. Further embodiments relate to applications of the device described above. BRIEF DESCRIPTION OF THE DRAWINGS The invention is explained in more detail below with reference to the examples shown in the figures. The illustrations are not necessarily to scale, and the invention is not limited to the aspects shown. Rather, emphasis is placed on illustrating the principles underlying the invention. In the figures: Fig. 1 schematically illustrates the reflection of an ultrasonic pulse at an interface. Fig. 2 schematically illustrates the detection of a flow front in a casting tool using ultrasonic measurements. Fig. 3 illustrates an example of a sensor circuit. Fig. 4 is an exemplary oscilloscope image of a high-voltage pulse for exciting the ultrasonic transducer. Fig. 5 is an exemplary oscilloscope image of the sensor signal containing the ultrasonic echoes. Fig. 6 contains various timing diagrams to illustrate the function of the circuit from Fig. 5.Figure 7 is an exemplary oscilloscope image to illustrate the detection of a flow front in an injection molding process. DETAILED DESCRIPTION The method presented here introduces a circuit design for a simple and cost-effective circuit that enables the highly dynamic, reliable, and real-time detection of flow fronts during rapid filling processes (often in less than one second). Flow front detection during filling processes can be used for a variety of applications. The following section first explains the fundamental relationships for the reflection of ultrasound pulses at interfaces. Such an interface 20 can be, for example, the cavity wall in a mold 21. Figure 1 schematically illustrates these relationships. The ultrasound pulse 50 is generated by exciting the piezoelectric ultrasound transducer 10 with a short electrical voltage pulse (burst) lasting a few microseconds. The coupled ultrasound pulse 50 propagates in the wall of the metallic mold 21 until it encounters the interface 20 between the mold 21 and the cavity 23, which contains, for example, a polymer melt 22. When an ultrasonic pulse strikes an interface between two different materials with different acoustic impedances Z (where Z is the product of the speed of sound c and the density ρ of each material), part of the pulse is reflected and returns to the ultrasonic transducer as an echo, where it is converted into an electrical signal. The other part is transmitted into the other material. In the example shown, the ultrasonic pulse 50 coupled into the mold has a (maximum) amplitude AE, and the echo (the reflected pulse 52) has an amplitude AR. The ultrasonic pulse 51 transmitted into the melt 22 has an amplitude AT. The amplitude AR of the reflected pulse 52 indicates the difference between the acoustic impedances Z1 and Z2 (and thus, approximately, the densities ρ1 and ρ2) of the two materials (metal of the mold and melt / air).The reflection coefficient R is the ratio AR / AE = (Z2 - Z1) / (Z1 + Z2), where the following applies to the acoustic impedances Z1 (impedance of the material of the mold 21) and Z2 (impedance of the material in the cavity 23): Z1 = c1ρ1 and Z2 = c2ρ2. The parameters c1 and c2 denote the respective sound velocities (in m / s) and ρ1 and ρ2 denote the respective densities (in kg / m3). In the case of a steel / air material combination, almost 100% of the coupled pulse is reflected at the interface due to the high density differences (R ≈ 1). For the steel / plastic material combination, this value is, for example, between 95 and 98% (R ≈ 0.95 ... 0.98). Figure 2 illustrates the principle of flow front detection. When the tool closes, the cavity 23 inside the mold 21 is empty, and nearly 100% of the coupled ultrasonic pulse is reflected (see diagram (a) of Figure 2). To ensure continuous monitoring, ultrasonic pulses 50 are emitted and received at a sufficient repetition frequency, and the reflected amplitude AR is evaluated (see also Figure 1). When the flow front reaches the sensor position during the injection process (see diagram (b) of Figure 2), a small portion of the ultrasonic pulse 50 is coupled into the melt, correspondingly reducing the amplitude of the reflected pulse 52. The measuring system detects this amplitude reduction due to the flow front (as a voltage change) and sends, for example, a digital signal to the injection molding machine or other external control devices for further processing. The examples described here concern electronic circuits for detecting such flow fronts or fill levels in casting molds or similar containers. These circuits should largely meet the following criteria: (i) no external or internal adjustment of the required gain, (ii) simple separation of the transmitting and receiving sections, (iii) no complex analog-to-digital conversion and digital signal processing, (iv) robustness against external influences (such as temperature changes). Depending on the application, it is possible that not all of the above criteria are met. An example of a suitable circuit is shown in Fig. 3. In the illustrated example, the sensor circuit 30 receives a trigger signal STRIG, for example, from an external control unit (controller). The trigger signal STRIG indicates the start of the casting process and is generated, for example, when the mold is closed. The trigger signal Son activates a signal generator PWF whose output signal has a frequency that represents the desired pulse repetition frequency of the ultrasonic pulse 50. For example, the signal generator PWF delivers a square wave signal with a frequency corresponding to the desired pulse repetition frequency. This pulse repetition frequency can be adapted to the specific application. The output signal of the signal generator PWF is fed to the pulse generator PG, which generates high-voltage pulses for the ultrasonic transducer 10 at the desired pulse repetition frequency. An oscilloscope image of such a high-voltage pulse is shown in Fig. 4. The high-voltage pulses (excitation signal) are fed to the ultrasonic transducer 10, which couples corresponding ultrasonic pulses 50 into the mold 21 (see Fig. 2). It should be noted that Fig. 4 is merely an illustrative example. The specific signal waveform can vary depending on the application and depends, among other things, on the ultrasonic transducer used. The signal generator PWF and the pulse generator PG together form the transmitter section of the circuit 30. The remaining circuit components form the receiver section of the circuit 30. The reflected ultrasonic pulses (echoes 52, see Fig. 2) are converted back into electrical signals (sensor signals) by the ultrasonic transducer 10, and the transmit / receive switch 31 (S / E switch, see Fig. 3) separates the excitation signal (high-voltage pulses) from the sensor signal (receive signal), so that (ideally) only the latter is processed by the receiver section of the circuit 30 (which does not mean that crosstalk from the transmit section to the receiver section cannot still occur). The S / E switch 31 thus has the task of keeping the high voltages of the excitation pulse away from the rest of the receiving electronics and only allowing the comparatively low voltages of the sensor signal (which is based on the echo pulses) to pass through. Suitable implementations of S / E switches are known per se and are therefore not explained in detail here. A typical transmit / receive switch contains a diode bridge and a control circuit.To eliminate the need for the control circuitry of the transmit / receive switch, a structure consisting of an inductor L1 with two anti-parallel diodes DL1 and DL2 (limiter, amplitude limiting circuit) is proposed. Anti-parallel diodes (without inductance, but often used with a series resistor) are frequently used for a simple implementation of a transmit / receive switch, but in this simple form often have a negative impact on signal quality. In the example shown, the inductance L1 has a very low value and can be easily implemented using a meandering stripline on a printed circuit board (PCB). Fig. 5 shows a cycle of the resulting sensor signal (after the S / E switch 31 and amplified) with the echo pulses during an ultrasonic reflection measurement on a steel reference block. The initial burst upon excitation of the ultrasonic sensor, the primary echo (first reflection), and the secondary echoes (second and third reflection) are clearly visible. With the S / E switch described above (with inductance), an amplitude approximately twice as high is achieved in this example compared to an S / E switch that only has a diode limiter with a series resistor. The cycle shown in Fig. 5 repeats according to the pulse repetition frequency. The receiver section of circuit 30 is configured to receive a sensor signal (containing a sequence of echo pulses) supplied by the ultrasonic transducer 10 and to generate an envelope signal Venv, which (approximately) represents the envelope of the echo pulses. For this purpose, the receiver section of circuit 30, according to the example shown in Fig. 3, comprises an amplifier AMP, a rectifier GR, and a smoothing capacitor CGR. The amplifier AMP amplifies the output signal of the S / E switch 31 (sensor signal with the echo pulses), and the rectifier GR rectifies the amplified sensor signal. The smoothing capacitor CGR is connected to the output of the rectifier GR. It thus performs integration or envelope detection. The resulting envelope signal at the output of the rectifier GR is denoted Venv.It goes without saying that the circuit described here is merely one of many known possibilities for determining the envelope signal Venv. According to the examples described here, the receiver section of circuit 30 further comprises a first filter circuit and a second filter circuit. The filter circuits are designed to smooth the envelope signal Venv, with the second filter circuit having a larger time constant than the first filter circuit, which is why the second filter circuit reacts (significantly) more slowly to a change in the envelope signal Venv than the first filter circuit. In the example shown in Fig. 3, an impedance converter IW1 and IW2 (buffer amplifier) ​​are located between the smoothing capacitor CGRam at the output of the rectifier GR and the aforementioned filter circuits. The buffer amplifiers IW1 and IW2 essentially serve to prevent the output of the envelope detection circuit (i.e., the capacitor CGR) from being loaded by the filter circuits. Depending on the implementation of the envelope detection, the buffer amplifiers IW1 and IW2 may also be omitted. The first filter circuit essentially comprises a smoothing capacitor C1, and the second filter circuit a smoothing capacitor C2 whose capacitance is significantly larger (e.g., by a factor of 100) than that of smoothing capacitor C1. A resistor R2 is connected in parallel with capacitor C2, thus forming an RC filter. Capacitor C1 can also have a parallel resistor, in which case the existing parasitic resistances (represented by R1 in Fig. 3) are sufficient, and an explicit resistor is not strictly necessary. However, in some embodiments, a dedicated resistor R1 may be present. The rectified signal Venv is buffered, as mentioned, by means of an impedance converter IW1 and smoothed by means of capacitor C1 (over a large number of cycles). The smoothing to a DC voltage signal yields a voltage level proportional to the amplitudes of the echo pulses (after amplification, rectification, and envelope detection). The smoothed (voltage) signal at capacitor C1 is denoted V1. The rectified signal Venv, as mentioned, is fed via another impedance converter IW2, a diode D, and a transistor T1, which is switched on when the trigger signal STRIG is active, to another capacitor C2. Both capacitors C1 and C2 are charged, with the voltage V2 across capacitor C2 being equal to the voltage V1 minus the voltage drops across diode D and transistor T1. Importantly, the capacitance of capacitor C2 is higher than that of capacitor C1 (e.g., C2 = 100 × C1), which is why the capacitor voltage V2 reacts more slowly to changes in the envelope signal. The relatively large difference between the capacitances of capacitors C1 and C2 means that in the event of an abrupt voltage change (e.g., due to a flow front moving past the sensor position in a casting mold), the voltage V1 (across capacitor C1) should drop abruptly below the voltage V2 (across capacitor C2) of the echo signals. Such an event is detected by comparator K, whose output value is stored by an RS flip-flop (latch). That is, a high level at the output of comparator K sets the RS flip-flop to logic 1 (SOUT=1 corresponds to the event "flow front detected"). One advantage of the concept described here is that the circuit is robust against a reduction in the signal amplitudes AR of the echo signals, which can be caused, for example, by an increase in temperature in the casting tool. That is, heating the tool can lead to a reduction in the amplitudes AR. To prevent the circuit from triggering a false detection, resistor R2 is connected in parallel with capacitor C2. Resistor R2 is dimensioned such that the voltage V2 across capacitor C2 can follow relatively slow changes in the voltage Ve. The time constant τ2 is known to be R2·C2. A resistor R1 can also be connected in parallel with capacitor C1. However, the existing (relatively high) parasitic resistances are often sufficient, since C1 is significantly smaller than C2. The corresponding time constant τ1 is R1·C1. After successful detection (indicated by SOUT = 1), the circuit must be reset before the next process to be monitored. In the example shown, this is done by resetting (deactivating) the level of the trigger signal STRIG from high to low (logic 1 to logic 0). This deactivates the signal generator PWF, and no more ultrasonic pulses are emitted. Simultaneously, transistor T1 is switched off, and capacitor C2 is disconnected from the impedance converter IW2. Inverter I switches on transistor T2, which shorts out capacitor C2 and consequently discharges it. At the same time, the RS flip-flop is also reset (SOUT = 0). After the next activation of the trigger signal (setting the level of STRIG to high), the next detection can occur. Transistors T1 and T2, together with inverter I, can be considered a reset circuit designed to reset according to the state set by STRIG (i.e.,(Depending on its logic level) the second capacitor C2 is discharged, resetting flip-flop 32. Discharging the second capacitor C2 resets the second filter circuit (R2, C2) to a defined initial state. The first filter circuit (R1, C1) can also be reset to a defined initial state, but this is not strictly necessary (depending on the implementation) due to the small time constant of the first filter circuit. Figure 6 summarizes the output signals of the various circuit components of the circuit shown in Figure 3. The first diagram (from the top) shows an example output signal VPWF of the signal generator PWF. In this example, a square wave signal with period Tcyc is used to drive the pulse generator PG with the required pulse repetition frequency Tcyc-1. The pulse generator P produces the required excitation pulse VPULS for the ultrasonic sensor (see Figure 6, second diagram). The reflected signals are routed through the S / E switch 31 and amplified. The signal Videal shows the amplified sensor signal at a preset gain without considering saturation effects due to the finite supply voltages (see Figure 6, third diagram). In reality, some reflections may saturate. The actual output signal Vreal of the amplifier AMP is shown in the fourth diagram of Figure 6.However, individual echo pulses always fall within the saturation voltage limits VB+ and VB-, thus allowing changes in amplitude to be detected. A separate gain adjustment (neither via an internal circuit nor an external setting) is therefore unnecessary. The rectified signal VGR (without smoothing by capacitor CGR) is shown in the fifth diagram of Fig. 6. The sixth diagram shows the voltage VV across capacitor CGR (envelope of the echo pulses). The last diagram in Fig. 6 (below) shows the resulting capacitor voltages V1 and V2 across capacitors C1 and C2, respectively. When the trigger signal STRIG (i.e., STRIG = 1) is activated, the RS flip-flop is reset, and it remains in the reset state as long as there is air in the cavity and consequently the amplitudes AR of the echo pulses are constant. In this situation, the voltages V1 and V2 differ only by the amount of the voltage drops across diode D and transistor T1. The detection of a flow front can be observed using the oscilloscope image in Fig. 7. When a flow front arrives at the sensor position (time t1), the amplitudes of the echo pulses in the sensor signal decrease (because some of the sound energy is transmitted into the melt). The voltage V1 across capacitor C1 drops abruptly, whereas the voltage V2, due to the higher capacitance of capacitor C2 (C2 >> C1), cannot follow the voltage V1. Consequently, the difference V2-V1 increases, which is detected by comparator K. Comparator K activates (sets) the flip-flop as soon as a voltage difference V2-V1 > VTH is detected (VTH is a predefined threshold value of the comparator). The set state of the flip-flop is indicated by the output signal (SOUT = 1). After time t1, the voltage V2 across capacitor C2 also slowly drops (through discharge via resistor R) below the voltage V1. In the example shown, the trigger signal STRIG is reset at time t2 (STRIG=0), which results in the flip-flop being reset (SOUT=0) and capacitor C2 being discharged (via transistor T2, see Fig. 3). At time t3, the trigger signal is set (e.g., by closing the mold again), and the voltages V1 and V2 return to their initial levels, allowing the next casting process to begin.

Claims

A device comprising: at least one ultrasonic transducer (10); and electronic circuit comprising a transmitter (PWF, PG) and a receiver, wherein the transmitter (PWF, PG) is configured to generate a control signal with a sequence of control pulses for the ultrasonic transducer (10), and wherein the receiver is configured to receive a sensor signal supplied by the ultrasonic transducer (10) with a sequence of echo pulses and to generate an envelope signal (Venv) representing an envelope of the echo pulses, wherein the receiver further comprises a first filter circuit (C1) and a second filter circuit (C2), both configured to smooth the envelope signal (Venv), wherein the second filter circuit (C2) has a larger time constant than the first filter circuit (C1), and wherein the receiver further comprises a comparator (K) configured to detect,that an output signal (V1) of the first filter circuit (C1) falls below an output signal (V2) of the second filter circuit (C2) by a defined amount (VTH). The device according to claim 1, wherein the receiving part for generating the envelope signal (Venv) comprises an envelope detector circuit. The device according to claim 2, wherein the envelope detector circuit comprises a rectifier (GR) and a capacitor (CGR) connected downstream thereof. The device according to claim 2 or 3, wherein an amplifier (AMP) is connected upstream of the envelope detector circuit, which is configured to amplify the sensor signal. The device according to any one of claims 1 to 4, wherein the first filter circuit comprises a first capacitor (C1) and the second filter circuit comprises a second capacitor (C2), wherein the second capacitor (C2) has a larger capacitance than the first capacitor (C1). The device according to claim 5, wherein a second resistor (R2) is connected in parallel to the second capacitor (C2) and a first resistor (R1) is connected in parallel to the first capacitor (C1). The device according to claim 6, wherein the first resistance (R1) is formed by parasitic resistances. The device according to one of claims 1 to 7, wherein the receiving part further comprises: a flip-flop (32) which is connected to an output of the comparator (K) such that it is set when the comparator (K) indicates that the output signal (V1) of the first filter circuit (C1) is less than the output signal (V2) of the second filter circuit (C2) by a defined amount (VTH). The device according to claim 8, wherein the receiving part further comprises: a reset circuit configured to reset the flip-flop (32) according to a trigger signal (STRIG). The device according to claim 5, wherein the receiving part further comprises: a reset circuit configured to discharge the second capacitor (C2) according to a trigger signal (STRIG) and the flip-flop (32). The device according to one of claims 1 to 10, wherein the transmitter part is configured to output the sequence of control pulses to the ultrasonic transducer (10) depending on the level of a trigger signal (STRIG). The device according to one of claims 1 to 11, further comprising: a transmit / receive switch (31) which forwards the sensor signal with the received sequence of echo pulses to the receiver, wherein the transmit / receive switch (31) comprises an inductor (L1) and an amplitude limiting circuit (DL1, DL2). A method comprising: generating a sequence of ultrasonic pulses using an ultrasonic transducer (10), wherein the ultrasonic pulses are coupled into a solid; receiving a sensor signal with a sequence of echo pulses using the ultrasonic transducer (10); determining an envelope signal (Venv) which represents the envelope of the sensor signal with the sequence of echo pulses; filtering the envelope signal (Venv) with a first filter circuit, wherein the second filter circuit (C2) has a larger time constant than the first filter circuit (C1), which is why the second filter circuit (C2) reacts more slowly to a change in the envelope signal (Venv); detecting that an output signal (V1) of the first filter circuit (C1) falls below an output signal (V2) of the second filter circuit (C2) by a defined amount (VTH). The method according to claim 13, wherein the generation of the sequence of ultrasound pulses is started depending on the level of a trigger signal (STRIG). The method according to claim 13 or 14, wherein at least the second filter circuit (C2) is reset to a defined initial state before the generation of the sequence of ultrasonic pulses begins. The method according to any one of claims 1 to 15, wherein the first filter circuit comprises a first capacitor (C1) and the second filter circuit comprises a second capacitor (C2), wherein the second capacitor (C2) has a capacitance that is 10 times, in particular 100 times, greater than that of the first capacitor (C1).

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