GB2160656A

GB2160656A - Monitoring fluctuating physical variables

Info

Publication number: GB2160656A
Application number: GB08416012A
Authority: GB
Inventors: Charles John Timothy Clarke
Original assignee: SEASIM CONTROLS Ltd
Current assignee: SEASIM CONTROLS Ltd
Priority date: 1984-06-22
Filing date: 1984-06-22
Publication date: 1985-12-24
Also published as: GB8416012D0

Abstract

A monitoring system for monitoring a fluctuating variable in particular the level of a liquid surface has a monitoring circuit which relies upon the current flow between two probes 1, 2 extending into the liquid by an amount depending upon the level of the liquid to produce an output signal representative of the level. A compensating circuit is provided and has a second pair of probes 3, 4 which are totally immersed in the liquid. The monitoring circuit and the compensating circuit are energised 12, 5 by electric carrier waves. The voltage amplitude of the carrier wave of the compensating circuit is regulated 10, 6 by a feedback signal to produce a wave form of constant current amplitude and the voltage amplitude of the carrier wave of the monitoring circuit is regulated from the compensating circuit by a related amount. <IMAGE>

Description

SPECIFICATION Monitoring fluctuating physical variables The invention relates to a system for monitor ing a fluctuating physical variable by monitor ing the environment in which the variable exists to provide an electrical output signal which is representative of the fluctuations in the variable. The electrical output signal may be fed into a data recorder or it may be used as a feed back signal to exert an appropriate control on the physical variable.

The invention has particular but not exclusive application in measuring the level of a liquid surface such as may be required for monitoring the quantity of liquid in a container or for monitoring real and model sea wave and tidal characteristics.

The changing liquid level can be monitored by a probe arrangement which relies upon current flow through the water between two probe members extending into the liquid.

Thus the amount of current flowing is dependent upon the height to which the water reaches up the probes (hereafter called level probes); the higher the water level the greater the current.

In order to provide a meaningful output signal representative of the physical variable, it is necessary to calibrate the monitoring system. However, the environment in which the physical variable exists may change causing a change in the datum on which the calibration of the monitoring system was based. For example, in liquid level monitoring systems the conductivity of the water may change.

To avoid having to continuously re-calibrate the system for such changes in the environment, it is necessary to compensate automatically for such changes and it is the object of the present invention to provide such an automatic compensation.

Such automatic compensation may be achieved by using a separate pair of probe members (hereafter called calibration probes) to determine the datum and amplitude transfer function of the fluctuating physical variable measuring system.

According to the invention, the calibration probes control the calibration by utilising an electrical carrier wave whose voltage amplitude is regulated to produce a waveform of constant current amplitude. The regulated voltage amplitude is applied through a known factor (which may be unity) to another electrical carrier wave which is present on the level probes (or the same as may be more convenient in the case of a multiplexed probe arrangement). Thus the voltage amplitude of the carrier waveform on the level probes is related (by the known factor) to that on the calibration probes which is known to satisfy the constant current requrements.

The invention will now be further described with reference to the accompanying drawing which is a block circuit diagram of one embodiment of monitoring system in accordance with the invention which in particular illustrates the system for use with separate carrier wave frequencies on the two probe pairs.

The block circuit diagram shows a system for measuring changes in water height in a tank. In this description the level probes comprise a pair of parallel wires 1 and 2 which dip into the water and the change in level of the water surface is detected by the change in current flowing between the two wires 1 and 2 through the water. The calibration probes in this description are similarly constructed of wires 3 and 4 which by being totally immersed are able to detect the liquid conductivity through the voltage required to pass a fixed current.

The electronic system has in this embodiment an oscillator 5 for generating the calibration carrier wave and a separate oscillator 1 2 for generating the level carrier wave. In both cases, the carrier waves are fed through controllable attenuators 6 and 12, whose function will be described, to bridge circuits 7 and 1 5 incorporating the probes 1 and 2 and 3 and 4. A carrier wave whose amplitude is proportional to the current flowing between two wires on each probe arrangement is provided as the output from each bridge such that the wave amplitude from bridge 7 is related to the liquid conductivity while that from bridge 1 5 is related to the immersed length of the level probes 1 and 2.These waveforms are fed to rectifiers 8 and 1 6 and thence to offset devices 9 and 1 7. The output from the offset device 1 7 is passed through a filter 1 8 to remove the carrier and the resulting output voltage is fed to a gain control circuit 1 9. The output from the gain control circuit 1 9 provides the output signal which is representative of the variable liquid level.

The output from the offset device 9 is fed to an integrator 10 whose output is fed back as a control input to the attenuator 6 to control the amlitude of the carrier voltage that energises the bridge 7.

The mean of the offset rectified waveform from 9 is zero. If this were not the case, the integrator output would continue to change in such a way as to control the attenuator 6 to change the amplitude of the carrier waveform fed to the bridge 7 causing the mean output from the offset device 9 to be zero. Thus the circuit establishes the amplitude of the bridge energisation carrier to satisfy a zero mean waveform into the integrator 1 0.

The bridge energisation voltage is also fed to a gain stage 20 which determines the known factor between the amplitude of the carrier waves into bridge 7 and bridge 1 5.

Stages 21 and 22 rectify and smooth the carrier to provide a d.c. voltage which is fed to the differential amplifier 14. Similarly, the carrier into bridge 1 5 is also fed into this differential amplifier 14 via rectification and filtering stages 23 and 24 to provide a second d.c. input to the amplifier. If these two d.c.

levels are not the same, the differential amplifier controls the attenuator 13 to cause the carrier amplitude into the bridge 1 5 to equal that from the gain stage 20.

The circuit that establishes the amplitude of the bridge energisation carriers to satisfy a zero mean waveform into the integrator 10 is responsible for the automatic compensation for different liquid conductivities. The zero mean signal level into the integrator 10 determines the calibration because the amplitude of the carrier signal into the bridges 7 and 1 5 establishes their sensitivity to changing resistance between the probe pairs. This is because the lower the probe resistance, the lower is the carrier amplitude and the less sensitive the bridge. Notwithstanding, with highly conductive liquids (low probe resistance), a small change in level causes a large change in resistance between the probe wires 1 and 2 but this is the bridge in a state of low sensitivity. Correspondingly, the output from the bridge 1 5 changes the same amount for a given change in liquid level irrespective of the liquid conductivity.

Claims

1. A monitoring system for monitoring a fluctuating physical variable by monitoring the environment in which the variable exists comprising a monitoring circuit having a probe arrangement which in use relies upon current flow through the environment between two probe members to provide an output signal which is representative of changes in said variable, characterised in that a compensating circuit is provided having a second probe arrangement which has a second pair of probe members the current flow between which is not responsive to changes in said variable and which provides a feed back signal to regulate said compensating circuit in order to maintain the current flow between said second pair of probes substantially constant, said compensating circuit providing a compensating control on said monitoring circuit related to its own regulation.

2. A monitoring system for monitoring the changes in level of a liquid surface, comprising a monitoring circuit having a probe arrangement which in use relies upon current flow through the liquid between two probe members immersed in the liquid to a varying amount as said level varies in order to provide an output signal which is representative of changes in said level, characterised in that a compensating circuit is provided having a second probe arrangement which has a second pair of probe members which in use are immersed in the liquid to a constant amount irrespective of changes in said liquid level and which relies upon the current flow between said second pair of probes to provide a feed back signal to regulate said compensating circuit in order to maintain the current flow between said second pair of probes substantially constant, said compensating circuit providing a compensating control on said monitoring circuit related to its own regulation.

3. A monitoring system according to claim 2, wherein said second pair of probes are adapted to be totally immersed in said liquid.

4. A monitoring system according to any preceding claim, wherein said monitoring circuit and said compensating circuit are energised by electric carrier waves, wherein the voltage amplitude of said carrier wave of the compensating circuit is regulated by said feedback signal to produce a wave form of constant current amplitude, and wherein the voltage amplitude of the carrier wave of the monitoring circuit is regulated from said compensating circuit by a related amount.

5. A monitoring system substantially as hereinbefore described with reference to the accompanying drawings.