IMPROVEMENTS IN OR RELATING TO VIBRATING FORK LEVEL SWITCHES

Abstract

The invention provides a method and apparatus for checking the condition of a self-oscillating vibrating fork level switch. The switch includes a test facility that operates when the switch is taken from a closed loop feedback operating mode into an open loop test mode. Amplitudes of the received test signals are subjected to comparison with predetermined thresholds to establish the health of the switch.

Claims

1. A method of testing the function of a vibrating fork level switch configured to self-oscillate in a normal working mode by means of a closed feedback loop, said method comprising interrupting said normal working mode; driving said switch in an open-loop pulsed working mode with a test signal generated by a test facility integrated into said switch; and comparing amplitudes of received test signals at a set of times with predetermined amplitude thresholds at corresponding times, wherein said test signals are generated at a frequency related to a frequency observed in normal mode working prior to interruption thereof.

2. A method as claimed in claim 1, wherein the received test signals are subjected to envelope detection.

3. (canceled)

4. A method as claimed in claim 1, wherein said switch includes an integral microcontroller, said method comprising using said microcontroller to generate said test signals.

5. A method as claimed in claim 4 comprising generating a test driving signal of a discrete number of cycles

6. A method as claimed in claim 4, further comprising programming said microcontroller to interrupt said normal working mode prior to generating said test signals.

7. A method as claimed in claim 1, wherein the act of interrupting said normal working mode is effected by manual intervention.

8. A method as claimed in claim 7, wherein said manual intervention is effected from a remote location.

9. A method as claimed in claim 1, further comprising providing a visual indication of a condition of said switch.

10. A method as claimed in claim 9, comprising providing an indication of an existence of fault as well as a nature thereof.

11. A vibrating fork level switch configured to self-oscillate in a normal working mode by means of a closed feedback loop, said switch further including a test facility operable, on interruption of said normal working mode, in an open loop pulsed test mode, said test facility being configured and operable to drive said switch with a test signal and to compare amplitudes of received test signals at a set of times with predetermined amplitude thresholds at corresponding times, wherein said test facility is configured to generate test signals at a frequency related to a frequency observed in normal mode working 20 prior to interruption thereof.

12. A vibrating fork level switch as claimed in claim 11, further including an envelope detection facility configured for application to received test signals

13. (canceled)

14. A vibrating fork level switch as claimed in claim 11, including an integral microcontroller, said test facility being integrated at least in part, in said microcontroller.

15. A vibrating fork level switch as claimed in claim 14, wherein said microcontroller is configured or programmed to generate a test signal of a discrete number of cycles.

16. A vibrating fork level switch as claimed in claim 14 or wherein said microcontroller is programmed to interrupt said normal working mode prior to operation of said test facility.

17. A vibrating fork level switch as claimed in claim 11, further including a control to effect interruption of said normal working mode and to initiate operation of said test facility.

18. A vibrating fork level switch as claimed in claim 17 wherein said control is operable manually or remotely by manual or process control intervention.

19. A vibrating fork level switch as claimed in claim 11, wherein said test facility is further operable to provide a visual indication of a condition of said switch.

20. A vibrating fork level switch as claimed in claim 18, including an LED lamp programmed to indicate an existence of a fault as well as a nature thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] One preferred form of the invention will now be described with reference to the accompanying drawings in which:

[0032] FIG. 1: shows a schematic hardware diagram for performing fault detection according to the invention;

[0033] FIG. 2: shows a frequency response of a vibrating fork level switch operating in air where the frequency of the drive signal is at the resonant frequency of the fork in air;

[0034] FIG. 3: shows a frequency response of a vibrating fork level switch operating in water where the frequency of the drive signal is at the resonant frequency of the fork in water;

[0035] FIG. 4: shows a frequency response of a vibrating fork level switch operating in silicon oil at the resonant frequency of the fork; and

[0036] FIG. 5: shows a frequency response of a vibrating fork level switch operating as in FIG. 4 but not in resonance and indicating the presence of a fault, the amplitude scale being less than that of FIG. 4.

DETAILED DESCRIPTION OF WORKING EMBODIMENT

[0037] Referring to the drawings the invention provides a method of checking the operation and/or condition of a vibrating fork level switch 10 without removing or disturbing the switch from its normal operating position. The invention further provides a level switch configured with an internal fault checking or testing facility such that the functionality of the switch can be checked or tested while the switch is in situ. As will be apparent from the description that follows, the test procedure and apparatus can also encompass fault finding and diagnosis.

[0038] Referring to FIG. 1, the level switch 10 conventionally works in self-oscillating mode, fork 12 being driven by piezo drive element Tx 13 and the resulting fork vibration being sensed by piezo sensing element Rx 14. In the conventional manner a positive feedback loop in the electronic circuit, which contains suitable amplifiers and filters, establishes and maintains the fork 12 vibrating in resonance. The Rx signal is also fed to a waveform shaper 15 and thence to microcontroller 16 which compares the Rx frequency with one or more thresholds to determine if there has been a change of medium in contact with the fork 12.

[0039] In its broadest form, the invention involves integrating a test facility into the switch 10. This is conveniently implemented by adding additional hardware such as further drive and receive amplifiers to the microcontroller 16 which enable the normal operating cycle of the switch to be interrupted and an open loop test mode initiated in which test signals are generated to drive the existing transmit element 13 to vibrate the fork. The responses to the test signals are then picked up by the existing Rx sensor 14 and subjected to analysis preferably by passing the received test signals to an envelope detector 17 in which the amplitudes of the received test signals are recorded and compared by microcontroller 16 with predetermined thresholds at a given set of times. In essence this allows the rate of decay of the test signal to be observed. Based on a comparison with known data, the patterns of receive signal decay can be translated into an indication of the health of the vibrating fork sensor. This indication can then be outputted by way of a visual indicator such as LED lamp 18.

[0040] To enable the normal self-oscillating working mode of the switch 10 to be interrupted, and the fork 12 to be driven in open-loop test mode, a gate switch 20 is conveniently included in the feedback loop such that the feedback can be interrupted and the test signal from microcontroller 16 applied to the transmit element 13. At the end of a test cycle the gate switch is re-positioned to re-establish the working closed feedback loop. The gate switch may be operated under the control of microcontroller 16 or may include an additional control that allows external intervention to effect switching. This additional control may permit manual operation of the gate switch 20 and/or may allow remote operation of the gate switch via a wired or wireless communication facility.

[0041] The test signals are conveniently 50 cycle square waveforms, the frequencies of which may be set in a number of different ways.

[0042] Firstly, the test frequency may be set to the normal ‘dry’ or normal ‘wet’ frequency depending on what the switch is indicating immediately prior to the test cycle being initiated. Secondly the frequency observed prior to interruption of the working mode and initiation of the test cycle, is used for the test signal. A third option is to apply test cycles over a range of frequencies looking for a point at which the fork will resonate. Obviously, if no resonance is observed during this sweep of different frequencies, it can be concluded that the switch is not functioning.

[0043] Those skilled in the art will appreciate that the microcontroller 16 can be programmed to effect any one or more of the above options.

[0044] Following transmission of the test signal the resultant response or receive signal is passed to an envelope detector 17. Thereafter, the microcontroller 16, by way of analogue to digital convertor 21, samples the amplitudes within the envelope at, say, times 0.2 ms, 20 ms, 40 ms, 60 ms, & 80 ms. The amplitudes at each time point are then compared to pre-set threshold values to determine if the switch is operating in an acceptable manner. As stated above, a visual indication of the health of the switch may be provided by way of LED lamp 18. This, in turn, can be programmed to indicate different forms of fault.

[0045] Referring now to FIG. 2, the test transmit pulse is shown in the bottom part of the figure while the response of the fork, which is in air, is shown above. It will be noted that the signal decay upon termination of the test pulse is relatively gradual indicating a switch in good condition.

[0046] FIG. 3 shows a similar test to FIG. 2 but with the fork in water. While the amplitudes of the receive signal or response are lower overall, due to the damping effect of water, once again the decay after termination of the transmitted test pulse is relatively gradual, indicating a switch in good condition.

[0047] FIGS. 4 and 5 are simulations to provide a comparison between a switch in good operating condition, and a faulty switch. FIG. 4 shows the response of a switch, operating in silicon oil which is fairly viscous (1086 cp). As expected in fluids of such viscosity, the response decays quite rapidly upon termination of the test signal, even though the switch may otherwise be in good condition. FIG. 5 is a simulation of a defective switch operating in the same conditions. In FIG. 5, the amplitude (vertical) scale is nearly twice that of FIG. 4 so the amplitudes of the receive signal depicted are about half those of FIG. 4 even when the test signal is still transmitting. Given that the transmit or test signal is the same, this is an indication that a fault is present in the switch.

[0048] The invention as described above enables the identification and indication of faults such as:

Correct/wrong fork condition when dry
Correct/wrong fork condition when wet
A broken wire within the switch
Dis-bonding or decay of bonding in the sensor drive/receive assembly;
Damage to a fork tine.

[0049] These faults may be characterized by suitable programming of the microcontroller 16 and indicated individually by programming the operation of the LED lamp 18, for example by allocating different flashing rates or different colours to different faults.

[0050] Thus the present invention not only enables a vibrating fork level switch to tested in situ but also, by using a communications facility in such a switch, may be tested remotely which is of considerable advantage in this art.