GB2306660A

GB2306660A - Fluid electrical measurement apparatus and method

Info

Publication number: GB2306660A
Application number: GB9615712A
Authority: GB
Inventors: Christopher John Collister
Original assignee: Individual
Current assignee: Individual
Priority date: 1995-07-28
Filing date: 1996-07-26
Publication date: 1997-05-07
Also published as: GB9615712D0; GB9515506D0

Abstract

Measurement apparatus for electrical measurement of fluid such as oil, or brake fluid for use in an engine, vehicle or hydraulic machine is provided in which a circuit includes a sensor sensing variables dependent on complex relative permittivity of the fluid in the sensor. The circuit provides a first value dependent upon the imaginary part, and a second value dependent on the real part of the complex relative permittivity. A number of capacitive sensing arrangements for use in the apparatus are disclosed. The apparatus may be used for continuous or periodic monitoring of the fluid, and may provide an audible signal, the frequency or volume of which is indicative of the permittivity of the fluid.

Description

FLUID ELECTRICAL MEASUREMENT APPARATUS AND METHOD The present invention relates to apparatus and a method for electrical measurement of fluids, for example, during use of an engine.

It is known to monitor the quality of machine oils such as lubricating and hydraulic oils by detecting the presence of contaminants and impurities. Known methods for the detection of contaminants such as particle counters and spectral analyzers tend to be cumbersome and expensive. It is also known, for example from GB2230098A and GB21491 17A, to measure electrical parameters of oils under test. These have involved measurement of a single electrical parameter only such as capacitance or conductance.

The present invention in a first aspect provides electrical measurement apparatus for use in an engine, vehicle or hydraulic machine, the apparatus comprising a circuit including a sensor for detecting values dependent on complex relative permittivity of a fluid in the sensor, the circuit providing a first value dependent upon the imaginary part of the complex relative permittivity and a second value dependent upon the real part of the complex relative permittivity. The first value can be representative of tan8 and the second value representative of E/. Preferably the fluid is oil, such as lubricating oil, hydraulic fluid or brake fluid.

Preferably the fluid is passed continuously through the sensor in use. The sensor can be a capacitive sensor and the circuit can be a resonant circuit, the resonant frequency being dependent upon the real part of the permittivity and the decay constant being dependent upon imaginary part of the permittivity. To a second order, the resonant frequency is also dependent on the imaginary part of the permittivity.

Measurements can be made continuously or intermittently. Results of measurement can be stored over time for processing. In particular, changes in permittivity (real and imaginary components) over time can be detected. Processing can be undertaken under microprocessor control and data can be presented in numeric form or graphically for inspection by a technician. Measurements can be made at least several times per second and averaged over predetermined periods for accuracy.

The measurement apparatus can be used with an engine, such as a vehicle engine, to monitor lubricating oil quality, or with an hydraulic machine to monitor hydraulic fluid quality.

Preferably the oil or hydraulic fluid passes through the sensor for return to the engine or machine, possibly via a filter. The sensor can include a threaded inlet port and outlet port so as to fit to and between an engine main body and an oil filter, for example of a vehicle engine, or the sensor can be mounted in the sump of the engine, or oil can be drawn from a pressurised outlet.

The present invention advantageously allows measurements to detect the presence of water or other contaminants in a fluid such as oil or hydraulic fluid. In its preferred embodiments the apparatus can detect the presence of wear debris such as metallic and carbon particles, for example soot, in oils, as well as water and acid combustion products to provide an indication of the condition of the oil or hydraulic fluid. It will be seen that the apparatus is useful for engine health monitoring in motor cars, heavy goods vehicles, stationary engines and hydraulic machinery.

The monitoring of parameters relating to complex relative permittivity of a fluid can provide a user or operator of an engine or machine using that fluid with an early warning of potential failure. The user may then change the fluid and/or rectify the fault before the engine or machine is damaged.

Measurement apparatus according to the present invention preferably includes indicator means operative dependant on the detected complex relative permittivity or related parameters to indicate the condition of the fluid under test. The indicator means can be at least one indicator light, or a sound generator. In particular, a sound generator can provide an audible change in tone upon a permittivity value within a predetermined range being detected.

Brake fluid is a vital but often neglected oil used in a vehicle. As time passes, the brake fluid, which is generally hygroscopic, absorbs water or other contaminants from the atmosphere, leading to a significant reduction in its boiling point and the danger of catastrophic loss of braking effectiveness. Water can also cause internal corrosion of brake components such as pistons and cylinders, leading to seal failure and reduced performance.

Measurement apparatus according to the present invention can be used to monitor brake fluid continuously or periodically. The apparatus can be permanently fitted to the vehicle, or can incorporated in a hand held instrument for measurement during routine servicing.

The operation of refilling a brake circuit after draining via a bleed nipple is conventionally difficult to carry out without two people, one to pump the fluid through the system using a foot pedal, and one to watch for the cessation of bubbles in the emerging fluid. In a preferred embodiment of the present invention, small variations in both the real and imaginary parts of permittivity (or equivalently in E1 and Tan6) due to the presence of air bubbles may be detected. Since the fluid and bubbles are in motion through the sensor, the permittivity of this two phase fluid will be changing rapidly with time, and may be used to modulate the sound from an audible tone generator.When bubbles cease, the tone from the device will be continuous, rather than modulated, providing the operator with the knowledge that the bleed nipple can be closed.

Both the real part and the imaginary part of brake fluid permittivity are typically much higher than engine oils. The measurement circuitry can be adjusted to make allowance for this.

The present invention in its first aspect also relates to an engine or machine including measurement apparatus for measurement of fluid, and to corresponding methods.

The present invention in a second aspect relates to electrical measurement apparatus for use in monitoring brake fluid comprising a circuit including a sensor for detecting a value dependent on the relative permittivity of the brake fluid so as to determine whether contaminants, such as water, have been absorbed. Preferably, the circuit includes a signal generator which provides audible signal indicating contamination. In particular, the value is dependent on the modulus of the relative permittivity or the tan8 value. The monitoring can be continuous or periodic. The present invention in its second aspect also relates to a corresponding method.

There are important differences in the dielectric properties of mineral lubricating oils and brake fluids which are usually based on some form of ester. The complex permittivity of brake fluids shows a strong dependence on frequency, such that Tan8 is often a minimum between 1MHz and 10MHz, and the design of any circuitry involving the use of brake fluid as a dielectric has to take this into account. It remains true, however, that for brake fluid both ' and Tan8 increase significantly with quite small concentrations of water, and measurement of either one of these parameters will provide an indication of water contamination.

The operation of refilling a brake circuit after draining is conventionally difficult to carry out without two people, one to pump the fluid through the system by means of the foot pedal, and one to watch for the cessation of bubbles in the emerging fluid. In a preferred embodiment of the present invention, small variations in either the real and imaginary parts of permittivity (or equivalently in lE or TanG) due to the presence of air bubbles may be detected.

Since the fluid and bubbles are in motion through the sensor, the permittivity of this two phase fluid will be changing rapidly with time, and may be used to modulate the sound from an audible tone generator. When bubbles cease, the tone from the device will be continuous, rather than modulated, providing the operator with the knowledge that a bleed nipple can be secured.

Preferred embodiments of the present invention will now be described by way of example and with reference to the drawings in which: Figure 1 is a block schematic illustrating real permittivity measurement apparatus, Figure 2 is a block schematic illustrating loss tangent measurement apparatus, Figure 3 is a top view of a sensor of stacked disc type, Figure 4 is a cross-sectional side view of the sensor shown in Figure 3, Figure 5 is a top view of a sensor of linear comb type, Figure 6 is an oblique view of the sensor shown in Figure 5 with part of its casing and some of the "comb teeth" removed, Figure 7 is a top view of a further sensor of linear comb type, Figure 8 is a cross-sectional side view of the sensor shown in Figure 7, Figure 9 is a top view of a sensor of flat spiral type, Figure 10 is a cross-sectional side view of the sensor shown in Figure 9, Figure 11 is a top view of a sensor of "bed of nails" type, Figure 12 is a side view of the sensor shown in Figure 11, Figure 13 is an oblique cross-sectional view of a sensor of concentric cylinders type, Figure 14 is a more detailed cross sectional view of the sensor shown in Figure 13, Figure 15 is an expanded view showing a sensor according to the invention fitted between an engine block and oil filter, Figure 16 is a cross-sectional side view of a further sensor suitable for brake fluid monitoring, Figure 17 is a cross-sectional end view of the sensor shown in Figure 16, Figure 18 is a schematic diagram illustrating the sensor shown in Figures 16 and 17 in use, and Figure 19 is a graph illustrating one example of the voltage across the fluid filled sensor after a short pulsed excitation.

Phvsical Basis The physical parameters sensed by the apparatus of the material under test, are the real part e' of the complex relative permittivity and the ratio El//E/ (denoted TanG) where E11 is the imaginary part of the complex relative permittivity.In the case of engine oil, the accumulation of electrically conductive particles of metal and carbon causes the oil to become more lossy, ie the imaginary part E/l of the permittivity increases as does the Tan8 value (which is 11/E/). There is also a slight increase in the real part of the permittivity e' and an increase in the modulus, which is denoted il and equals E' (1 + tan28). Permittivity changes also occur should water or acid combustion products accumulate in the lubricating system, or should water accumulate in a hydraulic system.

From measurement of tans, e' is determined as explained below.

Basic Circuitrv To sense the permittivity of the test material, the test material is used as part of the dielectric of a capacitive sensor, the complex admittance of which varies in proportion to the complex permittivity. The capacitive sensor forms part of an oscillator circuit which may be LC, RC, or crystal controlled. Optionally, the oscillator is built into the sensor using components which function at up to 120"c.

Tan5 Determination The tan6 term is sensed in any of a variety of ways, of which one is the measurement of the potential difference across the tuned circuit for example as shown in Figure 2. It is well known that the impedance of a parallel tuned RLC circuit increases as the exciting frequency approaches the resonant frequency, and that the voltage across the circuit also increases in proportion. This apparent amplification is often referred to as the "Q" of the circuit, which is generally the ratio of reactance to resistance in a parallel tuned circuit.The permittivity of the capacitative element of the circuit is complex, and capacitance therefore follows: C=C0(l -jtan#) (Equation 1) where Co = KE/, K being a constant dependent on the geometry of the capacitor and E' being the real part of the permittivity.

Assuming, for simplicity, that the inductance possesses no resistance, the impedances of the inductor and capacitor are respectively: ZL = jcoL (Equation 2) Zc = (j#C0(1 - jtan#))-l (Equation 3).

It can be shown that resonance occurs at Ct)o2 = (LC0)-, at which frequency the magnitude of the impedance is given by I |Z| = (L/C0)"2.1/tano (Equation 4) With the resonant circuit driven by a sinusoidal current of constant amplitude, it follows that the measured potential difference across the circuit is inversely proportional to tans, all other losses being ignored. It can be shown that for values of tan# < 0.01, capacitative losses tend to dominate resistance losses in practice for the preferred inductors which are used.A small relative change in potential difference V is given by A V = - (tan#) V tan8 (Equation 5) From this relation it will be seen that where tan8 is small, small changes in tan6 result in relatively large changes in the potential difference V.

As shown in Figure 2, LC oscillator 3 is used in determining a signal dependent on Tan.

LC oscillator 3' is connected to a capacitive sensor (Csense'). The oscillator 3' output is capacitively coupled (to remove d.c. components) to a high impedance JFET buffer amplifier 25, which determines the voltage across the inductor in oscillator 3. The a.c. output from amplifier 25 is converted to d.c. by detector circuit 27, and applied, together with a reference voltage 29, to differential amplifier 31. The output from the amplifier 31 is low-pass filtered by low-pass filter 33 which provides an analog d.c. voltage. Thus a signal is provided of the amplitude of the a.c. voltage across the inductor in the oscillator 3/, the signal being inversely proportional to tan 6.

The overall gain of the amplifier is controlled by one or more temperature sensitive elements such as thermistors (not shown) in a feedback loop to provide temperature stability.

Another method to sense Tan6 is the measurement of the amplitude of adjacent or successive peaks in the exponential decay of the excited resonance, as shown in Figure 19. These values giving the decay constant directly, provided the resonant frequency coo, which is obtained by a separate measurement, is known. Excitation is achieved by applying a short pulse to the resonant LC circuit. The voltage across the circuit is given by V = exp(-(ssOt (K + Tand/2))Cos(o)nt) (Equation 6). where on = 0 (1 -1/4 tan26)2 (Equation 7) where CO is the natural frequency and K is a term which includes other losses in the circuit due to resistance and radiation.If the amplitude of the pulse is V0, then the amplitude of the next peak is V = V0 Exp(-2 (K + Tan#/2)), (Equation 8) and (Tan8)/2 + K = (1I27t) loge(VJV,), (Equation 9) or, more generally, (Tan8)/2 + K = (1/2n#) logc(V0/Vn), (Equation 10) where n is the n'th successive peak. Thus it is necessary only to measure the amplitude of successive peaks in the decaying waveform to obtain the total losses in the circuit.Since K is a constant, any change in Tan8/2 + K is due entirely to changes in Tan#. This method has the advantage that only one capacitive sensor is required to extract both 0 and Tan. The calculations are most conveniently performed in software.

In the example shown in Figure 19, V0 = 1, K = 0.001, tan#. = 0.1 and o = 1.

E/ Determination Turning now to determination of a value representative of real part of the permittivity, first the resonant frequency f of the oscillator circuit which depends on the capacitance of the sensor is determined. Obtaining the resonant frequency f is straightforward, and requires comparison with a crystal oscillator of accurately known frequency and the use of a frequency to voltage converter as described below.

The preferred apparatus used to sense the permittivity real part is shown in Figure 1 and consists of the capacitive sensor (C sense) connected to an LC oscillator 3 which is sensitive to changes in real permittivity between electrodes of the sensor so as to slightly adjust its resonant frequency fo. The oscillator 3 is supplied with a steady voltage by a precision regulator 5. The output signal from the LC oscillator 3 is passed to a divider 7 to reduce the frequency and to a mixer 9 where the signal is further reduced in frequency by mixing with the signal from a stable oscillator 11. The output signal is low pass filtered by filter 13 and further down-converted by divider network 15 before frequency to voltage conversion by convertor 17. The convertor 17 output is applied, together with a voltage reference signal 19, to a differential amplifier 21.The output signal is low-pass filtered by filter 23 to provide a voltage signal representative of resonant frequency fo.

A signal proportional to the real part of the permittivity e' is then determined once tan8 has been measured, as follows: The natural frequency fo is that expected if there were no loss, fo = 1/2Z(LCo)l'1 and E/ is proportional to Co. Thus is inversely proportional to the square of the natural frequency fo.

The natural frequency fo of the circuit is higher than the measured resonant frequency fm and is assumed to be given by the approximate relation: fo = fm (1 + 1/8 (tan?6, + tan' 42)) (Equation 11) where 6, is a small constant phase shift associated with the components of the circuit, and a2 is a small variable phase shift associated with the losses in the test material such as oil.

Since the geometry of the sensor is constant as is the value of the inductance L and the loss term 6,, the real part El of the permittivity is given by E1 = A (B - Tan262) (Equation 12) fm m where A and B are constants.

In another embodiment of the invention, sensors Csense and Csense' are the same sensor, and there is a single LC oscillator.

Temperature Compensation Complex relative permittivity is, in general, both frequency and temperature dependent, with ElI varying roughly as exp (-C/T), where T is temperature and C is a constant. Since the frequency of the tuned circuit will vary by, typically less than one percent, and a few percent at most, the frequency dependence of the permittivity is ignored as negligible.

The sensing circuit is preferably for use with an engine or hydraulic machine, to monitor oil.

Although the circuit is designed to be operated only when an engine or machine has reached its normal operating temperature, cooling system thermostats in engines or machines possess considerable hysteresis, and oil temperature fluctuations of 10"C or so can often be expected, especially as oil temperature is not usually in phase with coolant temperature. The effect of fluctuating temperature is significant, and unless compensated for is quite likely to mask the small changes due to the presence of contaminants in the fluid.

Temperature compensation or control is achieved in one of four ways, or with a combination of all four: first, the sensing element is placed in very close proximity to an identical sensor which, instead of being continuously flushed with circulating oil as described in more detail below contains a fixed quantity of fresh oil. Since both sensors are at the same temperature, any differences in the output, such as would be sensed by a bridge circuit, will be due to other factors, such as the presence of contaminants. Secondly, if it is known a priori what the temperature dependence of the fluid is, then an appropriate correction can be made by means of a stored calibration curve or mathematical function.

Thirdly, the geometry of the sensor, together with the different rates of expansion of its components can be selected so as to minimise the effects of temperature variation. Fourthly, the temperature of the sensor can be maintained at some fixed level above the maximum expected temperature by means of a thermostatically controlled electric heating element. In any event. the importance of thermal effects requires that the capacitive sensor be fitted with a probe which measures the temperature at the sensor itself.

Sensors A preferred sensor, when used for oil measurement, has a capacitance of between 5pF and 500pF, is mechanically and electrically stable, and is immune to the damaging effects of oil and temperature. The value of the capacitance is selected to be sufficiently large that small changes in the capacitance of the connecting leads are insignificant. A preferred sensor has a set of flat parallel plates or discs, stacked like a conventional air spaced radio tuning capacitor, and with the oil dielectric between the plates (as shown in Figures 3, 4, 5 and 6).

Another preferred sensor is in the form of concentric or eccentric cylinders within which there can be more than one gap (as shown in Figures 13 and 14) and can be perforated to allow free flow of oil. This sensor can be incorporated in an oil drain plug. Other preferred sensors include: 1. a flat sheet of copper clad board, etched with interdigitating elements, and with a small gap between each element (as shown in Figure 7 and 8), 2. similar to the above, but manufactured on flexible plastic film which can be rolled up into a compact cylinder, 3. a flat sheet similar to the above, but with the electrodes arranged in the form of a double spiral (as shown in Figures 9 and 10), 4. two identical metal plates covered with square or round pins like a bed of nails, brought together so that each pin is close to, but not touching, its neighbour (as shown in Figures 11 and 12), and Referring to the Figures 3 and 4, the stacked-disc type sensor 2 has a cylindrical earthed outer casing 4 with an oil inlet port 6 and an oil outlet port 8. The sensor includes a set of alternate positive and negative charge plates 10 of which the negative plates are earthed and the positive plates are connected together and to a signal output port 12. The plates 10 are separated by insulating spacers 3 (of material B) of a material having a different thermal expansion coefficient to that of the metal plates (material A).

Figures 5 and 6 show a linear comb type sensor 14 including a metal outer case (material A) and supporting pillars 16 (material B). Materials A and B have different thermal expansion coefficients. Differential expansion causes separation X shown in Figure 5 to change in such a way as to provide temperature compensation (ie. minimise the effect of temperature variation).

The linear comb type sensor 14 includes an oil inlet port 18, an oil outlet port 20 and electrical conducting wires 22 from the charge carrying surfaces 24, 26 of the sensor 14 to the outside via a seal 28. The charge carrying surfaces 24, 26 are formed into teeth-like parts of the comb type sensor 14. The teeth-like parts may optionally be tapered to provide a reduced overall variation of capacitance with temperature.

A further sensor 201 is shown in Figures 7 and 8 consisting of two conductors 56, 58 having thin electrodes 60, 62 enmeshing but not touching in a manner referred to as "interdigitating", that is, like the fingers of a pair of clasped hands. There is a connection lead 64 to the positive conductor 58, an oil inlet port 66, an oil outlet port 68, and the sensor 20' includes a casing 70.

The electrodes 60, 62 of the sensor 20' shown in Figures 7 and 8 are separated by a gap 72 and act as a capacitor due to electric flux between the electrodes 60, 63. The thickness, and real and imaginary parts of permittivity of the substrate 74 on which the electrodes are mounted should be as small as possible in practice to maximise sensitivity to oil quality. One suitable material for the substrate is PTFE (E' = 2.1).

The capacitance depends on the gap: width ratio of the fingers and also by the number of fingers. Given that the minimum practicable gap 72 is about 0.2mm, there is an optimum electrode width apart (shown by 72' on Figure 7) which maximises the capacitance of a sensor of given size, and is typically around twice the gap width.

A further sensor 30 is shown in Figures 9 and 10 consisting a pair of electrodes 32, 34 spiralling non-contactedly on a sheet 36. The sheet includes perforations 38 for oil passage.

The sensor 30 has oil inlet and outlet ports 40, 42 and a signal output port 44. The oil may alternatively pass across the spiral electrodes between an alternative inlet 45 and an alternative oil outlet 46.

A further sensor 46 is shown in Figures 11 and 12 of the "bed of nails" type. This offers reduced resistance to oil flow. It includes two sets of round or square shaped protrusions ("nails")* 48, 50 which are brought close but do not contact. This sensor similarly includes an oil inlet port 52 and oil outlet port 54.

A further sensor is shown in Figure 13 and 14 having electrodes 51 of concentric cylinders separated by spacers 53 which are narrow spacers 53a or wide spacers 53b. The cylinders can be perforated to facilitate the exchange of fluid within the gaps.

It is important that there should always be good flow through a sensor, with no regions of stagnant flow where a build up of particulates could occur. In any event, the sensor is designed to be easily removed for cleaning in a suitable solvent should this be necessary. It is also important that the sensor be insensitive to changes in fluid pressure or flow rate; this is achieved through regard to the direction and magnitude of the pressure gradient within the sensor and by ensuring that the design is mechanically robust.

To avoid a need for special oil outlet pipes in an engine block requiring changes to an engine manufacturer's production schedule, as shown in Figure 15, a sensor 76 is configured to fit in the place of the engine oil filter 78 by being screwed onto the engine block 80. The filter 78 is then screwed into the sensor 76 which acts as an adaptor remaining permanently in place on the engine block 80 and not being disturbed during oil changes. Oil flow in use is shown in Figure 15. Ring seals 82 are provided between the sensor and engine block, and sensor and filter.

To minimise the requirement for temperature compensation, the preferred sensors and associated electronics are designed to be inherently insensitive to temperature. For sensors, this is done by selecting parallel or coaxial arrangements where the relative rates of thermal expansion of plates and spacers respectively are appropriately chosen as shown, for example, in the sensors shown in Figures 4, 7 and 14. In the Figure 14 embodiment, the wide spacers 53b have the effect of with increasing temperature, pushing the cylindrical electrodes towards the central axes of the sensor thereby causing a reduction in capacitance with increasing temperature. Away from the sensor itself, the electronics is optionally stabilised by the signal from the temperature probe, or by housing the circuitry in a temperature controlled environment.

Calibration In the range of frequencies of interest (2MHz to 100MHz) it turns out that the differences in permittivity between samples of unused mineral oils are small, El being around 2.2, and Tan8 being around 0.01. No calibration for oil type is therefore necessary for an embodiment which is intended to be as cheap as possible, although it is a simple matter to provide a reset button to zero the output at each oil change in other embodiments.

In other embodiments of the invention, relative permittivity values, in particular complex relative permittivity components are determined; in particular by calibration using known permittivity reference samples.

As for temperature variations, the preferred instrument includes a processor, and can be selfcalibrating for temperature. After turning on the engine there will be a period of 10 to 15 minutes during which the temperature will rise, and the output will change. The process can also be carried out while the sensor cools after the engine is turned off. Either way, a calibration curve of output vs. temperature is obtained which will allow all measurements to be referred to, say, 850C. During this brief calibration period it is reasonable to assume that the permittivity of the oil remains constant.

Use in Periodic Measurements E1 and tanS are continuously monitored with measurements taken every few minutes over the projected life of the oil (say 100 to 300 hours) to build up a curve of operating history which may be stored in solid state memory. The shape of these two curves representing E/ and tan8 versus time (or engine revolutions) contains information about the state of the oil or other fluid under test. Information is contained not only in the values, but also in the first and higher order derivatives of the curves. Such rates of change may be important when, for example, seal or gasket failure leads to a sudden increase in water content.

If water (the real part of the permittivity of which is approximately 80) leaks into the oil under test (approximate real part of the permittivity of 2.2), the real part of the permittivity increases. If water leaks into a concentration of, say, 0. 1 % the real part of the permittivity will increase by, a measurable, 0.7%, and Tan8 will increase by considerably more depending on the number of ions in the test sample. Both these changes are detectable. On the other hand, if the test sample becomes contaminated with soot or wear particles, this would cause a measurable increase in tano but not E/. In consequence, measuring both E' and tano means that these two types of failures can be identified and distinguished between.

A sudden change in the slope of either one or both of curves representing E' or tan8 versus time curves will tend to indicate a problem in the machinery, which is preferably indicated immediately on a suitable display panel. In addition, having been stored in solid state memory the whole history of the fluid from new is available for inspection via a suitable interface between the electronics module and an external computer. The downloading of this data can form part of the normal service routine, and enables a technician to spot any subtle deviations from the expected shapes which might have been missed by the software.

Data storage over time can be made with the clock of the processor being turned on and off with the engine. Alternatively, data can be stored as a function of engine revolution, a parameter which is often measured in modern vehicles.

Since preferably many measurements are made per second, it follows that a voltage recorded at the end of, say, a 100 second period can be the average of many hundreds of such voltages, for which confidence in the accuracy of the mean will be high. This will help to reduce the effects of noise in the data due, for example, due to transient bubbles in the fluid or electrical interference. Typically, 300 hours worth of data is, for example, stored as 8 bit words, so one data point recorded every 270 seconds will provide 4000 data points, requiring a 32Kbits (ie 4Kbytes) per data stream.

Sensors for use in Brake Bleedin Preferred sensors intended for use in the process of brake bleeding are physically small so that the presence of small air bubbles in the oil has a relatively large effect on the capacitance of the sensor. A typical capacitance might be 10pF for a sensor filled with brake fluid. Such a sensor 100 is shown in Figures 15 to 18.

It is envisaged that the preferred sensor 100 is part of an LC, RC, or crystal oscillator circuit running at a few MHz such that changes in capacitance of a few pF will cause a considerable change in frequency. Changes in capacitance are due to changes in the modulus of the relative permittivity of the brake fluid. This change in frequency is converted to an audible tone change as follows.

Alongside the first circuit is another identical oscillator running at a frequency differing by a few kHz. Mixing these two signals results in an audible difference frequency after demodulation by rectification and filtering. When the sensor 100 is empty, the device emits a low frequency whistle, at, for example, 500 Hz. When pure brake fluid passes through it, it will emit a continuous whistle at a somewhat higher frequency, SkHz, say, while the passage of bubbles will cause it to emit a warbling sound, giving a clear indication that the bleeding process is not yet complete. The human ear is extraordinarily sensitive to changes in pitch and frequency modulation of just a few Hz is easily detectable. Furthermore, the presence of water will cause the continuous frequency to be somewhat different from that expected from pristine fluid, thereby providing an effective indication of the general condition of the fluid.

Since the fluid and bubbles are in motion through the sensor 100, the permittivity of this two phase fluid will be changing rapidly with time, and may be used to modulate the sound from an audible tone generator 102. When bubbles cease, the tone from the generator 102 will be continuous, rather than modulated, providing the operator with the knowledge that the bleed nipple can be secured.

Since the bubbles are generally small, the sensor 100 itself is also small so that the bubbles occupy a significant fraction of the volume. The sensor 100 is typically a pair of concentric cylinders 104, 106 with a small gap 108 between them such that the capacitance when filled with fluid is between 10 pF and 100 pF (Figures 16 and 17). The sensors 100 is of robust construction such that insignificant dimensional changes occur due to the internal pressure of the fluid. To prevent fluid from flowing back into the system when pressure is released from the brake pedal, a one-way valve 110 (shown in Figures 16 and 17) is fitted between the sensor 110 and a bleed nipple 112 as shown in Figure 18. A tube 114 connecting the nipple to the sensor is made from a good quality flexible rubber such as silicone, which is resistant to the chemical or solvent effects of brake fluid.To avoid the use of an excessive length of the tube 114 between the nipple 112 and the permittivity measurement apparatus (since the nipple may be quite high off the ground), the sensor 100 itself, which is small and lightweight is fitted a few centimetres downstream of the nipple, and contains the high frequency electronics 116 which may be installed on a very small printed circuit board 118 using miniature surface mount components. This circuitry obtains its power from a flexible connecting lead 120 which connects with a separate box 122 containing the tone generator 102, loudspeaker 124, and battery 120. The tone generator 102 is controlled by a separate lead (not shown) within the same flexible connector 120'.

Sensor Circuit Output Signals The output signal produced from a sensor and its associated circuitry will depend upon the precise application to which the sensor is put. In the case of the monitor for engine or hydraulic oils, indications on the driver's dashboard might be "OK" (green light) "Change oil soon" (amber light), "Change oil immediately" (red light), or "Bearing/seal failure" (flashing red light). Additionally, the data contained within a circuit memory may be accessible via a suitable interface for examination by service personnel.

Processing of signals from sensors will preferably involve a dedicated microprocessor to perform the necessary statistical smoothing functions, detection of rates of change and calculation of El, see Equation 12.

Claims

1. Apparatus for electrical measurement of a fluid for use in an engine, vehicle or hydraulic machine, the apparatus comprising a circuit including a sensor responsive to complex relative permittivity of the fluid in the sensor, the circuit providing a first value dependent upon the imaginary part of the complex relative permittivity and a second value dependent upon the real part of the complex relative permittivity.

2. Apparatus for electrical measurement according to claim 1, in which the first value is representative of tang, and the second value is representative of El, where tan# = #" #', E/ is the real part of the complex relative permittivity, and ElI is the imaginary part of the complex relative permittivity.

3. Apparatus for electrical measurement according to claim 2, in which the first value is inversely proportional to tans.

4. Apparatus for electrical measurement according to any preceding claim, in which the sensor is a capacitive sensor and the circuit is a resonant circuit.

5. Apparatus for electrical according to claim 4, in which the first value is the amplitude of potential difference across the sensor at resonance.

6. Apparatus for electrical measurement according to claim 4, in which a rectangular excitation pulse is applied to the resonant circuit, tan8 being determined from the ratio between the amplitudes of a first and next resonance peaks in response.

7. Apparatus for electrical measurement according to any of claims 4 to 6, in which the second value is determined from measurement of the resonant frequency of the circuit and the first value.

8. Apparatus for electrical measurement according to any preceding claim, in which the fluid is lubricating oil, hydraulic fluid or brake fluid.

9. Apparatus for electrical measurement according to any preceding claim, in which the fluid is passed continuously through the sensor in use.

10. Apparatus for electrical measurement according to any preceding claim, including controlling means for controlling continuous or intermittent repetition of measurements.

11. Apparatus for electrical measurement according to any preceding claim, including storage means operative to store results of measurement over time for processing.

12. Apparatus for electrical measurement according to claim 11, including processing means operative to determine changes in and/or the rate of change of measured values.

13. Apparatus for electrical measurement according to claim 12, in which measurements are made at least several times per second and averaged over repeated periods for accuracy.

14. Apparatus for electrical measurement according to any preceding claim, including indicator means operative dependent on measured values to indicate the condition of the fluid under test.

15. Apparatus for electrical measurement according to claim 14, in which the indicator means comprises at least one indicator light.

16. Apparatus for electrical measurement according to claim 14 or claim 15, in which the indicator means comprises a sound generator operative to provide an audible change in tone upon a value within a predetermined range being detected.

17. Apparatus for electrical measurement according to claim 16, in which variations in permittivity due to gas bubbles in a liquid test fluid are detected and modulate the tone provided by the sound generator.

1 8. Apparatus for electrical measurement according to any preceding claim incorporated in a hand-held instrument for measurement during servicing.

19. Apparatus for electrical measurement according to any preceding claim, for use in detecting the presence of water or other contaminants in the fluid.

20. Apparatus for electrical measurement according to any preceding claim, in which temperature is monitored and temperature variations are compensated for.

21. An engine, a brake circuit or an hydraulic machine including apparatus for electrical measurement according to any preceding claim, in which the sensor is connected to the engine, brake circuit or machine so that fluid passes from the engine brake circuit or machine through the sensor for return to the engine brake circuit or machine.

22. An engine, brake circuit or hydraulic machine according to claim 21, in which the sensor is adapted to be fitted between an engine main body and an oil filter so that oil passes continuously through the sensor and the oil filter.

23. A method of electrical measurement of a fluid for use in an engine, brake circuit or hydraulic machine, the method comprising (i) providing a circuit including a sensor for fluid, (ii) applying an electrical signal to the fluid in the sensor, (iii) sensing variables dependent on complex permittivity of the fluid in the sensor, and (iv) providing a first value dependent upon the imaginary part of the complex relative permittivity and a second value dependent upon the real part of the complex relative permittivity.

24. A method of electrical measurement according to claim 23, in which the first value is representative of tan6, and the second value is representative of e', where tan6 = E/ is the real part of the complex relative permittivity and all is the imaginary part of the complex relative permittivity.

25. A method of filling or refilling a brake circuit of a vehicle with brake fluid, the brake circuit including an outlet port to which a sensor is connected and through which brake fluid is drained during filling or refilling, including a measurement method including the steps of (i) providing a circuit including the sensor, (ii) applying an electrical signal to the sensor, (iii) sensing variables dependent on complex relative permittivity of the fluid in the sensor, (iv) providing a first value dependent upon the imaginary part of the complex relative permittivity and a second value dependent upon the real part of the complex relative permittivity.

26. A method according to claim 25, in which variations in permittivity due to gas bubbles and/or other contaminants in the fluid are detected.

27. A method according to claim 26, in which the variations in permittivity values are used to modulate an audible signal.

28. Apparatus for use in electrical monitoring of fluid comprising a circuit including a sensor for determing a value dependent on the relative permittivity of the fluid so as to determine whether gas bubbles or other contaminants are present, the circuit including a signal generator which provides audible signal tone the frequency or volume of which indicates the detected value dependent upon relative permittivity, and hence level of contamination.

29. Apparatus according to claim 28, in which the value is dependent on the modulus of the relative permittivity of the fluid.

30. Apparatus according to any of claims 28 or claim 29, adapted to be connected to the outlet of a brake fluid circuit of a vehicle, the value being dependent upon the presence of bubbles or other contaminants in the brake fluid in the sensor.

31. Apparatus according to any of claims 28 to 30, in which the sensor detects values continuously or periodically.

32. A method of electrical measurement of a fluid in an engine, brake circuit or machine including providing a measurement circuit including a sensor sensing a variable dependent on relative permittivity of the fluid in the sensor at a time to provide a value so as to determine whether gas bubbles or other contaminants are present, providing an audible signal, the frequency of volume of which is dependent on the value.

33. A method of electrical measurement according to claim 32, in which values are measured over time and a change in value produces an audible change in frequency or volume.

34. Apparatus for electrical measurement of a fluid as hereinbefore described with reference to the Figures.

35. A method of electrical measurement of a fluid as hereinbefore described with reference to the Figures.

36. A method of filling or refilling a brake circuit of a vehicle with brake fluid as hereinbefore described with reference to the Figures.