CONDENSATE FREE ULTRASONIC TRANSMITTER

Abstract: A distance or level measuring system in accordance with the present invention is employed to measure the distance between a predetermined location and a material having a surface. The inventive system comprises a transducer (10), a heater (18) for heating the transducer so as to prevent the formation of condensation in an area adjacent the transducer and between the transducer and the material surface, and an electrical control circuit (80, 80') coupled to the heater for controllably energizing the heater.

Description: CONDENSATE FREE ULTRASONIC TRANSMITTER Field of the Invention The present invention relates generally to ultrasonic systems for measuring distance. Background of the Invention In the most common ultrasonic distance measuring system, a single piezoelectric crystal is used both as a transmitter of ultrasonic pulses and as a receiver of the return echo from the surface whose distance is to be measured. A short pulse, typically 100 micro-seconds duration, of ac voltage of an appropriate frequency is applied to the piezoelectric crystal. The crystal vibrates, and transmits an ultrasonic pulse into the medium separating the ultrasonic transducer from the surface whose distance is to be measured. The echo reflected from the surface causes the crystal to vibrate and produce an electrical voltage between its faces of the same frequency and of an amplitude proportional to the strength of the echo. The time between the start of the transmitter pulse and the received pulse is measured (corrected for temperature of the medium) to determine the distance. The crystal is supplied with dampening such that its vibration will be negligible by the time the return echo is received. As shown in Figure 1 , an ultrasonic level measuring system may comprise an ultrasonic transducer 10 coupled to a housing 20 containing control circuitry and a head temperature sensor 30. In addition, in this example, the transducer 10 is attached to a head 40 (at a temperature T H ) by a threaded neck region 12, and is disposed within a cylindrical neck 50 extending from a wall of the container for the substance 60 (granular) or 70 (liquid) whose surface level is to be measured. As shown, the transducer 10 has a front surface 14 at a temperature designated T s . The process material temperature (T M ) is typically higher than the head temperature (T H ). The transducer surface temperature (T s ) will consequently be very close to the head temperature and may be cooler than the dew point of the air around it. This condition may produce condensation on the transducer surface, and the condensation will interfere with the measuring function. Condensation will cause absorption, reflection and scattering of the transmitted energy, reducing the energy transmitted to the material. In addition, the surface condensation will cause reflection of the energy reflected (or echo) from the material surface. In many cases, these effects will be sufficient to prevent sensing of the material level. If the surface temperature T s is below 32°F, the condensate will freeze as it forms and successive layers of ice will still further limit the transducer function. Accordingly, there is a need in the ultrasonic distance/level measuring art for a system and method for preventing condensation from interfering with the correct operation of the measuring system. Summary of the Invention The present invention combines a heater under the transducer surface and a control circuit that causes heat to be added to the surface as required to maintain the surface temperature at a desired value (selected value) sufficient to prevent condensation independent of the head temperature (T H ). A distance or level measuring system in accordance with the present invention is employed to measure the distance between a predetermined location and a material having a surface (as discussed above). The inventive system comprises a transducer (10), heater means (18) for heating the transducer so as to prevent the formation of condensation in an area adjacent the transducer and between the transducer and the material surface, and an electrical control circuit (80, 80') coupled to the heater means for controllably energizing the heater means. In presently preferred embodiments of the invention, the power supplied to the heater by the control circuit supplies is proportional to the degree to which the head temperature is less than the target surface temperature. In presently preferred embodiments of the invention, the transducer is an ultrasonic transducer for controllably transmitting acoustic pulses toward the material surface and receiving return acoustic pulses reflected from the material surface. The distance between the transducer and the material surface may be determined in accordance with well known techniques (which are not described herein). In addition, in the presently preferred embodiments, the heater means comprises an electrical resistor disposed over a supporting substrate. Further, the system also includes an impedance matching layer situated as disclosed below between the transducer and the material surface, and the heater means is disposed adjacent to the impedance matching layer. The impedance matching layer is preferably characterized by a thickness, measured in a direction of acoustic energy propagation from the transducer, of approximately one-quarter wavelength at a predetermined operating frequency. The system may also include a safety barrier situated between the transducer and the material surface, and the heater means may be disposed adjacent to the safety barrier. The safety barrier may have a thickness, again measured in a direction of acoustic energy propagation from the transducer, of approximately one-quarter wavelength at the operating frequency. In addition, the presently preferred embodiments of the invention include a layer of acoustic couplant disposed between the transducer and the material surface. For example, the acoustic couplant may be situated between the impedance matching layer and the material surface, or between the safety barrier and the transducer. The heater means is preferably embedded in the safety barrier but may alternatively be situated on a surface of the safety barrier. The safety barrier thus performs the function of the matching layer. Another aspect of the present invention is an electrical control circuit for use in supplying power to a heater. The inventive control circuit includes means for receiving as a first input a signal indicative of a target temperature and as a second input a signal indicative of a measured temperature, and means for suppling power to the heater proportional to the degree to which the measured temperature is less than the target temperature. Other features of presently preferred embodiments of the invention are disclosed below. Brief Description of the Drawings Figure 1 schematically depicts an ultrasonic distance or level measuring system. Figure 2 schematically depicts, in partial section, an ultrasonic transducer in accordance with the present invention. Figure 3 schematically depicts front cross-sectional and side views of a heater for an ultrasonic transducer in accordance with the present invention. Figure 4 schematically depicts a first embodiment of an electrical heater circuit in accordance with the present invention. Figure 5 schematically depicts a second embodiment of an electrical heater circuit in accordance with the present invention. Figure 6 schematically depicts in more detail the embodiment of the electrical heater circuit of Figure 4. Figure 6' schematically depicts in more detail the embodiment of the electrical heater circuit shown in Figure 5. Figure 6A schematically depicts a power supply circuit for use in an ultrasonic distance or level measuring system in accordance with the present invention. Figure 6B is a schematic diagram used to explain the operation of the heater control circuit. Figure 6C is a waveform diagram used to explain the operation of the heater control circuit of Figure 6. Figure 6D is a graph of the temperature coefficient of nickel 270, a material useful for making a temperature sensing heater of the kind used in the embodiments of Figures 5 and 6'. Figure 6E is a waveform diagram used to explain the operation of the heater control circuit of Figure 6'. Figure 7 is an assembly diagram, in cross section, of a "non-intrusive" ultrasonic distance or level measuring system in accordance with the present invention. In this configuration, a piezo transmitter/receiver is not cemented to a safety barrier but is held in tight contact with it and the joint is filled with acoustic "gel." Figure 7 A is a partial cross-sectional view of the ultrasonic transducer in Figure 7. Figure 8 is a view of the portion of the transducer including a one-half wavelength barrier and a one-quarter wavelength impedance matching layer. In this embodiment, the impedance matching layer is cemented to the crystal. Figure 8 A depicts a configuration of the transducer in which the transducer is pressed against a half- wavelength barrier and the joint is sealed with acoustic couplant. Figure 9 depicts an embodiment of the half- wavelength barrier with an embedded heater. Figure 10 depicts an embodiment of the one-half wavelength barrier with a separate heater. Detailed Description of Preferred Embodiments The heater location is shown in Figure 2. It is a thin layer 18 cemented between the acoustic impedance matching layer 17 and the transducer face 14. The ultrasonic crystal 16 is also shown, as are the wire leads 16a, 18a, 18b for energizing the crystal 16, and heater 18, respectively. Construction of the heater 18 is shown in Figure 3. As show, the heater comprises a very thin metallic resistor having a serpentine shape and being supported on either side with suitable plastic films 18c and 18d. The resistor can be made of a flat metal ribbon. Both the metal and the plastic layers are thin enough so as not to interfere with the acoustic energy transmission. A first embodiment of a heater circuit is shown in Figure 4. This embodiment comprises a head temperature (T H ) sensor 30, a control circuit 80, and a switching transistor 90 operatively coupled as shown to the heater element 18. Thus, the heater 18 and transistor 90 are connected in series across a 12V power supply and ground. The control circuit 80 receives as inputs the measured head temperature T H and the target surface temperature, and adjusts the heater current by appropriately setting the conduction time of the transistor 90. In other words, the control circuit is supplied a voltage proportional to the desired surface temperature (target surface temperature). Also supplied to the control circuit is a voltage from the head temperature sensor. The control circuit supplies power to the heater proportional to the degree to which the head temperature is less than the target surface temperature. Table 1 shows exemplary results for a situation where the target temperature is 100°F. When the head temperature is equal to or greater than the target temperature, no added heat is required to maintain the target temperature. As the head temperature drops below the target temperature value, the heat is supplied proportional to the difference in temperature. TABLE 1 The rate for the transducer in Table 1 was determined to be 0.018 wattΛF. In another example, the material temperature may be up to 180°F and the head temperature may vary from 70°F to 100°F. When the target temperature is selected to be 180°, the results are as shown in Table 2. Again, the surface will be maintained at a value which will prevent condensation. TABLE 2 An alternative method to control the heater is to use the heater itself as a temperature sensitive resistor as shown in Figure 5. In this approach, the heater is included as one leg of a resistance bridge, which includes a control circuit 80' and a differential amplifier 92 in addition to the switching transistor 90 and 12V power supply. Current supplied to the bridge causes an increase in the heater temperature. The current continues to increase until it reaches the target value at which the bridge is balanced and no further increase in current takes place. Thus, the heater is maintained at the target temperature, and as the surface is closely coupled to it, the surface temperature is maintained very close to the target temperature. The control circuit 80, 80' is depicted in greater detail in Figures 6 and 6 A, and in Figure 6'. In Figure 6, the control unit comprises four basic functional sections: 1. Temperature Sensor 2. Ramp Generator 3. Comparator U, connections 5, 6 and 7 4. Field Effect Switching Transistor (with low "on" resistance) The temperature sensor is mounted to respond to the head temperature (T H ) and produces a voltage proportional to the head temperature. The Ramp Generator produces a triangular voltage which has a maximum value equal to the output of the temperature sensor when the temperature is equal to the target temperature for the surface temperature T s . The minimum voltage from the Ramp Generator is made equal to the head temperature (T H ) at which maximum power is required. The voltage from the temperature sensor is connected to one input (#6) of the comparator, while the triangular voltage from the Ramp Generator is connected to the other input (#5) of the comparator. When the voltage from the Ramp Generator is greater than the temperature sensor voltage, the control switch Ql is closed, applying power to the heater. Figure 6C shows this action. Potentiometer R^ adjusts the Ramp Generator output corresponding to the target temperature. Resistor R 4 is selected to set the value of the Ramp Generator voltage corresponding to the temperature for full power. In some cases, as described, it is sometimes necessary to heat the transducer face by a considerable degree, and maximum efficiency is required to reduce the energy required. By limiting the heating to just the exposed face, a maximum temperature rise for minimum energy is achieved. In a preferred embodiment of this invention, the heating element is located immediately behind the transducer surface and is uniformly distributed across the entire surface. By locating the heater and connection entirely within the transducer body, they are not exposed to the atmosphere around the transducer and thus will not constitute a hazard when the transducer is used in a flammable medium. In many cases, such as uses involving pharmaceuticals, it is essential to be able to check and/or replace the transducer without opening the vessel, which could contaminate the contents. This is accomplished as shown in Figures 7 and 7A. A safety barrier is sealed to a nozzle on the vessel while the transducer is outside of the vessel and measuring through the safety barrier. The safety barrier thickness is selected to constitute, along with the transducer face, 1/4-wavelength at the operating frequency. Thus, the barrier and transducer face provide an impedance match between the piezoelectric crystal and the air or gas within the vessel. In the embodiment shown in Figure 6', the heater is made of a material having a known, nonzero temperature coefficient of resistance (such as nickel 270). A switching transistor Q3 is selected to have an "on resistance" which is small compared to the resistance of the heater. For example, a heater having a resistance of 70 Ohms at 75 °F rising to 98.36 Ohms at 200 °F can be used with a switching transistor such as an MTP3055E, having a typical "on resistance" of 0.05 Ohm. The resistance of the heater (R H in Fig. 5) is included in a Wheatstone bridge with resistors (Rl, R2, and R s in Fig. 5) respectively including (R16, R20, and Rprogl in parallel), (R19 and Rprog2 in parallel), and R21. R16 and R20 are chosen such that their parallel resistance will be equal to 187 times the resistance of the heater about 8 °F below the target temperature. (This is approximately the temperature at which full power is applied to the heater.) The resistor selected for R16 is chosen from a chart based on measurements of the actual resistance of the heater and R20, thus compensating for variations in their resistances due to manufacturing tolerance. R21 is chosen to be small compared to the resistance of the heater, to avoid wasting power in it, but is large enough to produce sufficient voltage to enable accurate control of the temperature of the heater. A substantially triangular wave varying between 1.1 volts and 6.1 volts and having a period of approximately 12 seconds is generated on capacitor Cl by comparator U1A. At the positive peak of the wave, the (substantially rectangular) output of U1A switches from approximately 11 Volts to a voltage near ground. The negative transition of this voltage is differentiated by C3, R8 and R9, and applied to the base of Ql, which turns off for approximately 120 milliseconds. R10, acting through D2, develops approximately 10 volts across R13 and the gate of Q3, forcing Q3 to turn on for 120 milliseconds during each control cycle, regardless of the heater temperature. Simultaneously, Rl 1 begins charging C5, raising the voltage at the gate of Q2. After approximately 2 milliseconds, Q2 turns on, connecting the "error voltage" of the Wheatstone bridge to U2A, which amplifies it by approximately 2000 (the ratio of R7 to R17). The timing of the control waves is illustrated in Figure 6E. Still referring to Figure 6', the output of U2A is stored on C4 during the time Q2 is turned off (there is a small decay due to the bias current of U2A discharging C4 but this has a negligible effect on the control). After 120 milliseconds, Ql turns back on, discharging C5 through Dl, and thus turning off Q2, and allowing Q3 to be controlled by UIB (if the temperature of the heater were substantially higher than the target temperature, Q3 would turn off at this time). The voltage at the output of U2A is compared to the triangle wave by comparator UIB. If the output of U2A is lower than the triangle wave, R6 pulls up the output of UIB. This is coupled through diode D3 to R13 and the gate of transistor Q3, raising it to approximately 10 Volts and turning on the transistor. The conduction time of Q3 varies substantially linearly with the temperature of the heater, from a minimum of about 120 milliseconds to the full cycle time. U2B, D4, R12, Rl 4 and Rl 5 prevent thermal runaway if the connection to R16 and R20 is lost. R14 and R15 develop a voltage of approximately 72 millivolts at the positive input to U28. The voltage at the junction of R16, R19 and R20 (the reference side of the Wheatstone bridge) is applied to the negative input. If the connection to R16 and R20 is lost, the output of U2B goes high and forces the voltage at the negative input of UIB high through diode D4. This limits the conduction of Q3 to the 120 millisecond minimum established by Ql. As shown in Figure 7, the piezo transmitter/receiver is not cemented to the barrier but is held in tight contact with it and the joint is filled with an acoustic couplant compound (such as, e.g., acoustic grease, silicone oil, or other material that conducts acoustic energy well). An alternate configuration is shown in Figure 8. In this case an impedance matching layer with 1/4-wavelength thickness is cemented to the crystal. The matching layer is held against the barrier and the joint is sealed with acoustic couplant. In this case, the barrier is one-half wavelength in thickness. The system shown in Figure 8A has also been demonstrated. In this configuration, a standard transducer was pressed against the half wavelength barrier and the joint was sealed with acoustic couplant. In the standard transducer, the crystal was cemented to the impedance matching layer, which in turn was cemented to a 0.030 inch thick face. In this case, the face plate was CPVC. The face of the barrier toward the process can, if cooler than the dew point in the vessel, collect condensation interfering with the operation. In this case, a heater described earlier is in the preferred form embedded in the barrier near the process face, as shown in Figure 9. In this form, the layer supporting the thin metallic heater should be of the same material as the barrier. In an alternatives approach, which will function but much less effectively, the heater can be cemented to the outside of the barrier as shown in Figure 10. When the barrier is required to tolerate greater pressure, the barrier thickness is selected to produce the 1 and 1/4 wavelengths along with the transducer face. Any air space between the transducer face and the safety barrier will substantially reduce the energy transmitted in the vessel and further reduce the echo reaching the transducer. To prevent this, the space between the transducer face and the safety barrier is filled with acoustic couplant, and an "O" Ring prevents the couplant from bleeding out or drying. Experience has shown that some difficulty may be experienced from acoustic energy being transmitted into the vessel nozzle 50 with resulting resonance. The resonance can be eliminated by clamping rubber or other acoustic absorbing material to the outside of the nozzle. The scope of protection of the following claims is not intended to be limited to the specific presently preferred embodiments of the invention disclosed above. Those skilled in the art will recognize that variations and modifications may be made to the above-described embodiments without departing from the true spirit of the invention.

Claims: We claim: 1. A distance or level measuring system for measuring the distance between a predetermined location and a material having a surface, comprising: (a) a transducer (10); (b) heater means (18) for heating said transducer so as to prevent the formation of condensation in an area adjacent said transducer and between said transducer and the material surface; and (c) an electrical control circuit (80, 80') coupled to said heater means for controllably energizing said heater means. 2. A system as recited in claim 1 , wherein said transducer is an ultrasonic transducer for controllably transmitting acoustic pulses toward the material surface and receiving return acoustic pulses reflected from the material surface, whereby the distance between said transducer and the material surface may be determined. 3. A system as recited in claim 2, wherein said heater means comprises an electrical resistor disposed over a supporting substrate. 4. A system as recited in claim 3, wherein said heater means is characterized by an electrical resistance whose value varies as predetermined function of temperature, whereby said electrical resistance is indicative of the temperature of said heater means. 5. A system as recited in claim 4, wherein said system further comprises an impedance matching layer situated between said transducer and the material surface, and said heater means is disposed adjacent to said impedance matching layer. 6. A system as recited in claim 5, wherein said impedance matching layer is characterized by a thickness, measured in a direction of acoustic energy propagation from said transducer, of approximately one-quarter wavelength at a predetermined operating frequency. 7. A system as recited in claim 4, wherein said system further comprises a safety barrier situated between said transducer and the material surface, and said heater means is disposed adjacent to said safety barrier. 8. A system as recited in claim 7, wherein said safety barrier is characterized by a thickness, measured in a direction of acoustic energy propagation from said transducer, of approximately one-quarter wavelength at a predetermined operating frequency. 9. A system as recited in claim 4, further comprising a layer of acoustic couplant disposed between said transducer and said material surface. 10. A system as recited in claim 5, further comprising a layer of acoustic couplant disposed between said impedance matching layer and said material surface. 11. A system as recited in claim 7, further comprising a layer of an acoustic couplant disposed between said safety barrier and said transducer. 12. A system as recited in claim 7, wherein said heater means is embedded in said safety barrier. 13. A system as recited in claim 7, wherein said heater means is disposed on a surface of said safety barrier. 14. A system as recited in claim 7, wherein said electrical resistor comprises nickel 270. 15. A distance or level measuring system for measuring the distance between a predetermined location and a material having a surface, comprising: (a) at least one ultrasonic transducer for controllably transmitting acoustic pulses toward the material surface and receiving return acoustic pulses reflected from the material surface, whereby the distance between said transducer and the material surface may be determined; (b) heater means for heating said transducer so as to prevent the formation of condensation in an area adjacent said transducer and between said transducer and the material surface, said heater means being characterized by an electrical resistance whose value varies as predetermined function of temperature, whereby said electrical resistance is indicative of the temperature of said heater means; (c) an electrical control circuit coupled to said heater means for controllably energizing said heater means, wherein said control circuit receives as a first input a signal indicative of a target temperature and as a second input a signal indicative of a measured temperature, and supplies power to the heater means proportional to the degree to which the measured temperature is less than the target temperature; and (d) an impedance matching layer situated between said transducer and the material surface, said impedance matching layer being characterized by a thickness, measured in a direction of acoustic energy propagation from said transducer, of approximately one-quarter wavelength at a predetermined operating frequency. 16. A system as recited in claim 15, wherein said system further comprises a safety barrier situated between said transducer and the material surface. 17. A system as recited in claim 16, wherein said safety barrier is characterized by a thickness, measured in a direction of acoustic energy propagation from said transducer, of approximately one-half wavelength at a predetermined operating frequency. 18. A system as recited in claim 17, further comprising a layer of acoustic couplant disposed between said transducer and said material surface. 19. A system as recited in claim 18, wherein said heater means is embedded in said safety barrier. 20. A system as recited in claim 18, wherein said heater means is disposed on a surface of said safety barrier. 21. A system as recited in claim 18, wherein said electrical resistor comprises nickel 270. 22. A system as recited in claim 15, wherein said electrical resistor comprises nickel 270. 23. An electrical control circuit for use in supplying power to a heater, comprising means for receiving as a first input a signal indicative of a target temperature and as a second input a signal indicative of a measured temperature, and means for suppling power to the heater proportional to the degree to which the measured temperature is less than the target temperature.

Inventors: FELDMAN ALEX, KRAMER L JONATHAN, LADYANSKY STEPHEN J, MALTBY FREDERICK L, FELDMAN, ALEX, KRAMER, L., JONATHAN, LADYANSKY, STEPHEN, J., MALTBY, FREDERICK, L.
Applicants: DREXELBROOK CONTROLS, FELDMAN ALEX, KRAMER L JONATHAN, LADYANSKY STEPHEN J, MALTBY FREDERICK L, DREXELBROOK CONTROLS, INC., FELDMAN, ALEX, KRAMER, L., JONATHAN, LADYANSKY, STEPHEN, J., MALTBY, FREDERICK, L.
Application year: 1998
Main classification: G01S15
IPC codes: B06B1, G01F23, G01S7