Vibrating Fork Liquid Level Switch with Verification

Abstract

The vibrating fork liquid level switch (10) includes a vibrating fork assembly (11) arranged to vibrate at a first frequency when in contact with a process fluid (18) and at a second frequency when in contact with air. A drive circuit (54) connected to the vibrating fork assembly (11) is configured to drive the vibrating fork assembly (11) into oscillation. A detection circuit (52) detects the frequency of oscillation of the vibrating fork assembly (11). An output circuit (70 / 72) provides a first output when the detected oscillation is at the first frequency and a second output when the detected oscillation is at the second frequency. A control circuit (80) controls the power applied to the vibrating fork assembly (11) by the drive circuit (54) between a first power level and a second power level. A verification circuit (68) verifies the frequency of oscillation of the vibrating fork assembly (11) when the power applied to the vibrating fork assembly (11) by the drive circuit (54) is varied.

Description

[Technical field]

[0001] The present invention relates to a vibrating fork assembly of the type used for liquid level sensing of process fluids, and more particularly to verifying operation of a vibrating fork liquid level switch. [Background technology]

[0002] Vibrating fork level switches are commonly used to detect when the surface of a fluid in a tank is at a particular level, which is the height at which the fork is placed in the tank. Typically, vibrating fork level switches are configured to self-oscillate in an electronic circuit via a positive feedback loop, and the phase delay between the transmitting and receiving elements is carefully adjusted so that the positive feedback signals are in phase.

[0003] During operation, in a "normally dry" application, the vibration frequency is at one level when the forks are in the air ("dry") and when the liquid in the tank rises, it drops and touches the forks ("wet"). In a "normally wet" application, the opposite applies: the frequency increases when the liquid drops below the level of the forks.

[0004] It is desirable to verify the operation of a vibrating fork level switch to ensure it can accurately detect the level of a process fluid. One type of verification process requires an operator to change process conditions. In such a verification test (or "calibration test"), the operator monitors the output from the liquid level switch while the liquid level is adjusted so that the forks of the switch change from a wet to a dry condition, or from a dry to a wet condition. The operator monitors the output from the vibrating fork level switch to ensure that the output changes within an expected period of time. However, such testing is problematic for a number of reasons. For example, the test could result in the tank being overfilled if the switch is faulty or if a safety event occurs while the calibration test is being performed. Additionally, the test requires the operator to manually control the liquid level. Summary of the Invention [Means for solving the problem]

[0005] The vibrating fork liquid level switch assembly includes a vibrating fork assembly arranged to vibrate at a first frequency when in contact with a process fluid and at a second frequency when in contact with air. A drive circuit connected to the vibrating fork assembly is configured to drive the vibrating fork assembly into oscillation. A detection circuit detects the oscillation frequency of the vibrating fork assembly. An output circuit provides a first output when the detected oscillation frequency is at the first frequency and a second output when the detected oscillation frequency is at the second frequency. The first and second outputs each indicate one of a wet and a dry condition. A control circuit controls power applied to the vibrating fork assembly by the drive circuit between a first power level and a second power level. A verification circuit verifies the oscillation frequency of the vibrating fork assembly when the power applied to the vibrating fork assembly by the drive circuit is varied. [Brief description of the drawings]

[0006] [Figure 1] Figure 1 shows a vibrating fork level switch in air. [Diagram 2] FIG. 2 shows the fork or sensor of the switch of FIG. 1 fully immersed in the medium used (the process fluid). [Diagram 3] FIG. 3 is a simplified block diagram showing an electrical circuit that vibrates the fork assembly level switch of FIGS. [Figure 4] FIG. 4 is another exemplary electrical circuit embodiment for vibrating the fork assembly level switch of FIGS. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0007] Referring initially to Figures 1 and 2, the present invention provides a vibrating fork level switch 10. In the configuration shown, the switch 10 includes a level sensor in the form of a fork assembly 11 mounted at one end of a cylindrical tube 12. A housing 13 is mounted at the opposite end of the cylindrical tube 12. The fork assembly 11 includes a pair of tines 14 extending from a diaphragm 15. The cylindrical tube 12 typically includes a mounting 16 to allow the device to be mounted to the wall of a process vessel and housed within the housing 13 the operating electronics including a microcontroller (not shown in Figures 1 and 2). The switch 10 may be battery powered and may include an antenna to enable the switch to communicate wirelessly with a remote control facility.

[0008] In one configuration, the output from the switch 10 is provided to a two-wire process control loop 40. In a specific configuration, the two-wire process control loop 40 is a 4-20 mA process fluid control loop, where different values ​​of current through the loop are indicative of different states of the switch 10. For example, a level off or low level condition can be indicated by setting the current through the process fluid control loop 40 to a low value of 8 mA. Similarly, a level on or high level condition can be indicated by setting the current through the process fluid control loop 40 to 16 mA. High and low error alarms can be provided by regulating the process fluid control loop current to the extreme conditions, i.e., 4 mA and / or 20 mA. The two-wire process control loop can also be used to power the switch 10. Information can be transmitted to a remote location, such as a process control room 42, illustrated as a power source 44 and an electrical resistor 46. In some configurations, the process fluid control loop can also be used to carry digital commands and information, for example, according to the HART® communication protocol. Generally, process fluid control loop, as used herein, refers to both wired configurations, such as those illustrated in Figures 1 and 2, as well as wireless configurations where information is transmitted using radio frequency. Examples of wireless process fluid control loops include WirelessHART® (IEC 62591) or ISA 100.11a (IEC 62734), or another wireless communication protocol such as WiFi®, LoRa, Sigfox, BLE, or any other suitable protocol, including a custom communication protocol. Another exemplary output is simply a switch that changes between an open and closed position based on the detected liquid level.

[0009] Operation of this type of device requires knowledge of the Dry Fork Frequency (DFF), the natural or resonant frequency at which the fork assembly 11 vibrates in air. This is usually established at the time of manufacture, but may also be established or adjusted when the switch is installed in the environment in which it will be operated. In FIG. 1, the switch 10 is shown in air above a medium (process fluid) 18. When a surface 19 of the medium 18 rises and contacts the fork assembly 11, the vibration frequency of the fork drops from the DFF by a predetermined frequency change, and the switch 10 operates. Typically, the switch 10 is configured such that the switching point corresponds to a defined level of immersion of the fork assembly 11 in the medium 18. However, because the change in frequency of the fork 11 changes with changes in the density of the medium 18, the switching point may also change.

[0010] It is important to be able to verify the correct operation of the circuit during installation and testing of the switch 10 as well as during operation. One verification test is known as a partial calibration test, in which the switch 10 is caused to go into a test mode. This can be done through receipt of a command or through some type of local user input. When the switch goes into the test mode, the output on the two-wire process control loop 30 is set to a low alarm value, for example a 4 mA value, for a predetermined period of time. After this period of time has elapsed, the output is raised to an off or low indicator value, for example 8 mA, for a second period of time. After the second period of time has elapsed, the output of the two-wire process control loop 40 is adjusted to an on or high output level, for example 16 mA, for a third period of time. After this third period of time has elapsed, a high alarm value, for example 20 mA, is set on the process fluid control loop 30 until the test is terminated.

[0011] The measurement circuit and switch 10 monitor the current levels present on the two-wire process control loop during calibration testing and provide a verification error output if the various measured current levels are outside predetermined ranges from their correct values. Such calibration testing is useful in testing the output circuitry of the switch. However, as discussed herein, it may be desirable to test further aspects of the switch 10.

[0012] FIG. 3 is a simplified schematic diagram illustrating an electrical circuit 50 of the vibrating fork level switch 10. The circuit 50 includes a sensor circuit 52 coupled to a tuning fork assembly 11. The circuit 52 includes an oscillator 54 connected to a piezoelectric stack 56 via a switch 58. The stack 56 is physically coupled to the tuning fork assembly 11, for example, via a diaphragm 15 as shown in FIGS. 1 and 2. The stack 56 is also coupled to an operational amplifier 60 having a feedback resistor 62 and an output connected to a lock-in amplifier 64. An output from the lock-in amplifier 64 is provided to a frequency comparator 66 and a validation circuit 68. The validation circuit 68 provides a validation output as discussed herein. The frequency comparator 66 provides a first output f1 and a second output f2 to output circuits 70 and 72, respectively. Outputs f1 and f2 indicate that the vibrating fork assembly 14 is vibrating at a first frequency f1, which indicates that the fork is not in contact with the process fluid 18 and is in contact with, for example, air, and that the vibrating fork assembly 11 is vibrating at a second frequency, which indicates that the vibrating fork assembly 11 is immersed in the process fluid 18. A control circuit 80 is used to control the operation of the oscillator 54 and the switch 58, and is optionally configured to receive commands from an input / output circuit 82.

[0013] In operation, the switch 10 is used to monitor the level of the process fluid 18 based on the frequency of the vibrating fork assembly 11, as described above. The system is configured to enter a calibration test mode to verify the operation of the vibrating fork. This calibration test mode can be initiated based on any criteria. For example, a command can be received from the control room 42 using the input / output 82 via the process fluid control loop 40, received wirelessly, received from the local user interface or control circuitry 80 via the I / O 82, or the calibration test can be initiated based on unique criteria, such as periodically.

[0014] The verification circuit monitors the frequency of the vibrating fork assembly based on the output from the lock-in amplifier 64. When a calibration test is initiated, the verification circuit measures a first frequency measurement f initial This information may be stored, for example, in memory 69. Control circuitry 80 then opens switch 58, disconnecting the oscillator from piezoelectric stack 56. After a user selectable or fixed period of time, control circuitry closes switch 58, causing vibrating fork assembly 11 to once again begin vibrating in response to the vibration input from piezoelectric stack 56. At this time, the verification circuitry obtains a second frequency measurement f final This second frequency measurement is compared to the first frequency measurement stored in memory 69. This comparison can be made within validation circuit 68 or by control circuit 80. If the first and second frequencies differ by more than a predetermined or user selectable amount, it is determined that there is a fault or impending fault in sensor circuit 52. This may be due to changes in the vibrating fork assembly, fluctuations in piezoelectric stack 56, or other changes in the electronics associated with circuit 52. Based on this comparison, a validation output is provided that may indicate that switch 10 is operating properly or that switch 10 is operating out of specification. The validation output may be, for example, an alarm signal provided on process fluid control loop 40, a digital communication signal provided on process fluid control loop 40, a wireless signal communicated over a wireless process fluid control loop, an output provided through a local operator interface, or an output provided locally to a wireless local operator interface via a wireless communication link, etc. In this configuration, the power applied to the vibrating fork assembly is regulated between an operating level and a no power (or 0) level. The operating level may be a power level used during normal operation, or some other power level.

[0015] 4 is a simplified schematic diagram of another exemplary configuration of the vibrating fork assembly 10. In the configuration of FIG. 4, an analog-to-digital converter 92 is used to digitize the output from the lock-in amplifier 64 and provide a digital signal to the controller 90. In another exemplary configuration, the circuit 92 includes a frequency-to-digital converter, which provides a digital frequency value that is the output from the lock-in amplifier 64 to the controller 90. In either case, the controller 90 determines the frequency of the output from the lock-in amplifier 64 and operates as the verification circuit 68 shown in FIG. 3. Thus, the controller 90 compares the frequency at which the vibrating fork assembly 70 is vibrating with a previous frequency measurement stored in the memory 69 to verify the operation of the circuit 52. In a related configuration, the operation of the frequency comparator 66 and the analog-to-digital converter 92 is performed in a single functional block, either alone or in combination with the controller 90.

[0016] The controller 90 controls the operation of the control circuit 80. In this configuration, rather than using the switch 58, the control circuit 80 can be used to directly disable and de-energize the oscillator 54. In another exemplary configuration, the control circuit 80 is used to adjust the power provided by the oscillator 50 to the piezoelectric stack 56. For example, the power provided to the piezoelectric stack 56 can be decreased or increased and a second frequency measurement can be obtained by the analog-to-digital converter 92 and the controller 90. In such a configuration, a change in the frequency, or frequency drift, of the vibrating fork assembly 11 can be monitored as a function of the power input provided to the piezoelectric stack 56. A change in this measured frequency can indicate a failure or impending failure of the circuit 52.

[0017] For example, before, during, and / or after adjusting the power supplied to the vibrating fork assembly 56, the operation of the circuitry 52 is verified based on a comparison of the measured frequencies. Based on this comparison, the controller 90 can provide a verification output to the process fluid control loop 40 via the input / output circuitry 82. During normal operation, the controller 90 is also used to provide an indication of the liquid level based on the frequencies f1 and f2, as described above. As previously described, the input / output circuitry 82 can provide an output locally, remotely, using a wired connection, such as a two-wire process control loop 40, or via a wireless connection, such as compliant with the WirelessHART® protocol.

[0018] While the present invention has been described with preferred embodiments, those skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the present invention. The various outputs and inputs discussed herein may be provided locally, received remotely, or a combination of local and remote communication. The outputs may be used to directly drive a control element, e.g., a valve, and / or may be communicated locally or remotely. Calibration testing of the output circuits discussed herein may be performed as an optional component of the validation testing. The various functions discussed in connection with the functional blocks of Figures 3 and 4 may be performed by digital implementations including the use of individual circuits, combinations of circuits, microprocessors or microcontrollers, and the like. Other types of circuits may be used to oscillate the fork assembly 11, and the circuits shown herein are merely exemplary. During validation, the power applied to the piezoelectric stack is applied at a first power level and the frequency is measured. The power applied to the piezoelectric stack is then adjusted to a second power level and the frequency is additionally measured. Finally, the power applied to the piezoelectric stack is adjusted back to the first power level and the frequency is measured again. The operation of the vibrating fork liquid level switch is verified by observing one or more of the frequency measurements, for example, by comparing the frequency measured at the first power level.

Claims

1. a vibrating fork assembly arranged to vibrate at a first frequency when in contact with the process fluid and to vibrate at a second frequency when in contact with air; a drive circuit connected to the vibrating fork assembly and configured to oscillate the vibrating fork assembly; a detection circuit for detecting an oscillation frequency of the vibrating fork assembly; an output circuit configured to provide a first output when the detected oscillation is at the first frequency and a second output when the detected oscillation is at the second frequency; a control circuit configured to control power applied to the vibrating fork assembly by the drive circuit between a first power level and a second power level; and a verification circuit configured to verify the oscillation frequency of the vibrating fork assembly when power applied to the vibrating fork assembly by the drive circuit is varied.

2. 2. The vibrating fork liquid level switch of claim 1 including a switch operated by said control circuit to selectively disconnect said drive circuit from said vibrating fork assembly, thereby controlling the power applied to said vibrating fork assembly.

3. The vibrating fork liquid level switch of claim 2 , wherein said verification circuit measures a first oscillation frequency of said vibrating fork assembly when said switch is in a closed state.

4. 4. The vibrating fork liquid level switch of claim 3, wherein said control circuit opens said switch for a period of time, thereafter closes said switch, and measures a second oscillation frequency of said vibrating fork assembly when said switch is then in a closed state.

5. 5. The vibrating fork liquid level switch of claim 4 including a frequency comparator for verifying an oscillation frequency of said vibrating fork assembly by comparing said first oscillation frequency with said second oscillation frequency.

6. The vibrating fork liquid level switch of claim 1 , wherein said control circuit adjusts the power level that said drive circuit applies to said vibrating fork assembly.

7. 2. The vibrating fork liquid level switch of claim 1, wherein said control circuit controls the power applied by said drive circuit to said vibrating fork assembly to said first power level, then to said second power level, then to said first power level.

8. 8. The vibrating fork liquid level switch of claim 7, wherein said verification circuit measures said oscillation frequency when said drive circuit is at said first power level prior to said second power level.

9. 9. The vibrating fork liquid level switch of claim 8, wherein said verification circuit measures a second oscillation frequency of said vibrating fork assembly when said drive circuit operates at said first power level subsequent to operating at said second power level.

10. 10. The vibrating fork liquid level switch of claim 9, wherein said verification circuit compares said first oscillatory frequency to said second oscillatory frequency and provides a verification output in response to the comparison.

11. The vibrating fork liquid level switch of claim 1 , wherein the verification circuit is implemented in a microcontroller.

12. 2. The vibrating fork liquid level switch of claim 1, wherein the verification test by the verification circuit is performed in response to a locally received command.

13. 2. The vibrating fork liquid level switch of claim 1, wherein the verification test by the verification circuit is performed in response to a command received from a remote location.

14. The vibrating fork liquid level switch of claim 1 , wherein said first output and said second output are provided on a two-wire process control loop.

15. causing the vibrating fork assembly to vibrate at a first frequency when in contact with the process fluid and at a second frequency when in contact with air; oscillating the vibrating fork assembly using a drive circuit; detecting an oscillation frequency of the vibrating fork assembly; providing a first output when the detected oscillation is at a first frequency and providing a second output when the detected oscillation is at a second frequency; controlling power applied to the vibrating fork assembly between a first power level and a second power level; and verifying operation of the vibrating fork liquid level switch based on a frequency of the vibrating fork assembly when power applied to the vibrating fork assembly is changed.

16. The method of claim 15, comprising controlling a switch that selectively disconnects a drive circuit from the vibrating fork assembly.

17. The method of claim 15 , further comprising adjusting a power level applied by the drive circuit to the vibrating fork assembly.

18. 16. The method of claim 15, comprising controlling the power applied to the vibrating fork assembly to the first power level, then to the second power level, then to the first power level.

19. 20. The method of claim 19, wherein verifying comprises measuring the oscillation frequency when a vibrating fork assembly is at the first power level prior to the second power level.

20. 20. The method of claim 19, wherein verifying includes measuring a second oscillation frequency of the vibrating fork assembly when the drive circuit operates at the first power level subsequent to operating at the second power level.

21. 21. The method of claim 20, wherein verifying comprises comparing the first oscillator frequency to the second oscillator frequency and providing a verification output in response to the comparison.

22. The method of claim 15 , wherein the verifying occurs in response to receiving a command.

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