GB2454045A

GB2454045A - Inductive linear displacement sensor with temperature compensation

Info

Publication number: GB2454045A
Application number: GB0813089A
Authority: GB
Inventors: Peter Kenneth Smith
Original assignee: ACTIVE SENSORS Ltd
Current assignee: ACTIVE SENSORS Ltd
Priority date: 2007-07-20
Filing date: 2008-07-17
Publication date: 2009-04-29
Anticipated expiration: 2028-07-17
Also published as: GB0714208D0; GB0813089D0; GB2454045B

Abstract

The sensor comprising: an inductive winding 1 wound in a single direction thereby having both inductance and resistance; a length of conductive wire 2 having resistance but negligible inductance; a high permeability core 13 situated within the winding: and measurement circuitry for providing an output representative of the position of the core while compensating for temperature using the resistance of the negligible-inductance wire. The winding may have a higher concentration of turns at one end 1 to compensate for non-linearity. The negligible-inductance wire may be co-extensive with the winding and may comprise two oppositely-wound coils, or may be wound in a figure-of-eight arrangement.

Description

INDUCTIVE LINEAR DISPLACEMENT SENSOR

FIELD OF THE INVENTION

The invention relates to linear inductive sensors used to indicate the position of a moveable member and is particularly concerned with compensating for the effects of temperature on the said linear inductive sensor.

BACKGROUND OF THE INVENTION

Generally, the mechanical design of linear inductive sensors consists of one or more copper windings wound on a cylindrical tube in which a high permeability core is inserted. The windings are impregnated and a magnetic screen encases the windings to prevent any interference from the outer environment. The core is mechanically connected to the object to be monitored, and any axial movement of the object will alter the position of the core within the windings and will therefore change the electrical characteristics of the windings. This change is detected by signal conditioning electronics and an output signal is generated relative to core position.

Linear inductive sensors have become common in industrial, automotive and aerospace applications as they can be designed to withstand severe vibration, temperature and humid environmental conditions.

There are mainly 3 types of winding configurations, these are: * I) Transformer. (3 coils) Transformer types of linear inductive sensor contain 3 windings and they are commonly referred to as LVDT' s. (Linear Variable Differential Transformer). The * construction takes the form of a primary winding and two symmetrical secondary windings wound on a cylindrical tube. The primary is supplied with an alternating current and voltages are induced in each secondary by each coils mutual inductance with the primary. Axial movement of the core within the tube changes the mutual * . inductance between the windings and therefore the amplitude of induced voltage. The secondary coils can be connected in differential or ratio-metric measurement modes.

Examples of LVDT type sensors are disclosed in the following US patents, US 2,459,210 (1949), US 3,456,132 (1969) and US 5,087,866 (1992).

2) Bridge. (2 Coils) Bridge type contain two windings and can be connected in half or full bridge measurement configuration and are commonly referred to as LVIT' s (Linear Variable Inductive Transducer). The construction takes the form of two symmetrical windings, where axial core movement increases the inductance/impedance of one winding whilst the inductance/impedance of the other decreases, and vice versa, which imbalances the bridge centre tap voltage. Examples of LVIT type sensors are disclosed in the following patents US 3,961,243 (1976), CH 575115(1976), US io 4,623,840 (1985) and US 5,521,496 (1996).

In both transformer and bridge configurations the coils are normally energised with a 1 to 6 Vrms sine wave at a frequency of between 400Hz and 20 KHz, the electronics required to extract the displacement signal would contain at least some of the following signal conditioning blocks, voltage regulators and references, sine wave voltage oscillators, rms to dc converters, synchronous demodulators, S/H amplifiers, low pass filters, voltage dividers and offsetigain amplifiers.

The windings of variable inductance linear sensors (LVIT) can also be connected so that each winding forms part of a variable LIR timing circuit. The difference between the times that the voltage at the tap between each L and R takes, from a start to a reference level, is measured and taken as the displacement signal.

The signal conditioning blocks required to obtain the displacement signal would be, * voltage regulators and references, electronic switches, voltage comparators, flip flops, * 25 PWM circuits, digital counters and microprocessors. Examples of this type of sensor S...

are disclosed in the following UK patents, GB 2,005,844 (1979), GB 2,053,487 (1981), GB 2,059,604 (1981) and GB 2,272,295 (1994).

The advantages of transformer and bridge linear sensors are that they can posses very low thermal drift errors as the windings are operated in differential or ratio-metric measurement modes and the sensors can be designed to achieve high output sensitivities, which increases the sensors signal to noise ratio, which means the sensor will exhibit a low drift and low noise displacement signal.

The disadvantages are that they are relatively expensive as they require many turns of copper wire per winding to achieve a high output sensitivity and the sensors also exhibit a high winding to stroke length ratio, that is, the sensors body length can be 1.5 to 2.0 longer than the sensors operating range, which can cause difficulties in the mechanical installation of the sensor. Another disadvantage of the symmetrical winding linear sensor is that as the measurement range of the sensor increases, the body diameter will also need to increase, as more turns of copper wire are required to achieve an acceptable electrical performance, which also increases the weight of the sensor.

3) Single Coil.

There are mainly 3 types of single coil measurement techniques, these are: a) Bridge.

As the high permeability core enters the displacement measurement winding, the inductance and impedance of the coil increases, it is this change that is detected and taken as the displacement signal. This type of linear sensor can also be operated as a variable reluctance linear sensor by winding the displacement coil on a high permeability rod, which produces a high impedance coil. A low resistance material tube is moved over the top of the winding, which will decrease the coil impedance due the effect of induced eddy currents and again it is this change that is taken as the displacement signal. The winding is connected in one half of a bridge with the other half containing resistive or reactive components. The bridge would be normally * , energised with a I to 6 Vrms sine wave at a frequency of between 1 KHz and 20 KHz.

The electronics required to extract the displacement signal would contain the * same signal conditioning blocks as those required for transformer linear sensors.

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Examples of this type of sensor are disclosed in patents US 3,688,187 (1972), US 4,667,158 (1987) and EP 0,875,734 (1998).

b) Constant AC or AC&DC.

By supplying the displacement coil with a constant rms current, sine wave in form, at a frequency of between 5KHz and 20KHz, and the core is inserted into the winding, the impedance of the coil increases. As the current is constant, the mis voltage seen across the coil will increase. It is this voltage that is measured and converted into a displacement signal.

The displacement coil would have a very high thermal drift error, unless techniques are employed to compensate for changes in the displacement signal due to changes in temperature.

This can be achieved by incorporating a separate temperature compensating winding or a temperature sensitive component and can also be achieved by generating a temperature signal, which changes in proportion to the change in the dc resistance of the displacement measurement coil. The temperature signal is used to adjust the displacement signal and keep the value the same, independent of temperature changes. Examples of this type of sensor are disclosed in patents US 4,954,776 (1989), US 5,1 15, 193 (1992) and WO 0,113,070 (2001).

c) L/R Timing.

The displacement winding forms part of a variable L/R timing circuit. The time that the voltage at the tap between the L and R takes, from a start to a reference level, is measured and taken as the displacement signal. The sensor will have a very high thermal drift error unless temperature compensation techniques are employed.

This can be achieved by introducing a temperature sensitive component, such as a NTC Thermistor, or by generating a signal which is proportional to the dc resistance * of the coil, this signal becomes a pointer in a look up table where temperature correction values are obtained. **** * . S...

The signal conditioning blocks required to obtain the displacement signal *:. would be, voltage regulators and references, electronic switches, voltage comparators, flip flops, digital counters and microprocessors. Examples of this type of sensor are disclosed in the following patents US 3,973,191 (1976), EP 0368128 (1990) and DE *..: 10,022,821 (2001).

The advantages of a single coil linear inductive sensor are that it posses a low winding to stroke length ratio together with a small body diameter for long operating ranges.

The disadvantages are that the signal conditioning electronics can be complicated and in many cases precludes it from being contained in the sensor casing, which can cause sensor installation problems. Another drawback is that sensor temperature compensation adjustment can be difficult and cumbersome and if the coil posses a low number of turns, a low displacement signal is obtained, which means that the sensors signal to noise ratio would be quite low, which is an undesirable feature The signal would need a large amount of amplification to obtain a useful output level.

Another disadvantage is, if the sensor posses a temperature compensation winding or temperature sensitive component, that does not extend the length of the displacement measurement coil, and if they are not at the same temperature, that is to say that a temperature gradient appears along the length of the sensor, a large error will occur in the displacement signal. A large error will also occur if the sensor is subjected to any rapid temperature fluctuations. Further disadvantages of a single coil sensor with a dc current flowing through the displacement measurement coil which generates a dc magnetic field, is that a velocity sensor has been created, and when the conducting core moves within the coil, voltages are induced in the core, which produce eddy currents, and a velocity voltage is induced back into the coil. This * * velocity signal becomes superimposed on the temperature compensation signal, which generates a velocity error in the displacement signal and another drawback when generating low strength dc magnetic fields is the interference that can occur from the

Earth's magnetic field.

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I

OBJECT OF THE INVENTION

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The object of the invention is to provide a linear inductive sensor that consists of a simple and accurate measurement technique, that is compensated for the effects of temperature and will contain a compact and low component count signal conditioning electronics, that can be enclosed within the sensor casing without increasing the sensors body diameter.

THE INVENTION

According to the invention there is provided an inductive linear displacement sensor comprising: * an inductive winding of conductive wire about a central void, the winding being continuous in a single direction, whereby it has inductance, which is measurable and changes both with displacement being measured and with temperature independently of displacement, and resistance, which changes with temperature independently of displacement; * a length of conductive wire arranged in heat conductive contact with the said winding, this arrangement of conductive wire having resistance, which changes with temperature as does the change in resistance of the said inductive winding, but negligible inductance; * a high permeability core movable within the central void to cause a change in the inductance of the winding; and * measurement circuitry adapted to provide an output in proportion to the change in inductance with core movement, the circuitry including: * means compensating via a function of the change in resistance of the said arrangement of conductive wire with temperature for the effect of the changes in inductance and resistance of the winding with temperature. * S. * S *

It should be noted that materials having a relative permeability of greater than * *** 1 OOO.tr are considered to have high permeability. S. * S * *S.

* Preferably, the inductive winding has a local concentration of its turns at its end corresponding to small extension of the core into the central void, to compensate S. S5** :. 30 for non-linearity of the sensor at the end of its stroke corresponding to this small extension.

Normally the winding and the said arrangement will be of the same wire, conveniently copper; however the arrangement can be envisaged to be of a material having higher resistivity, reducing its length, and substantially the same coefficient of resistance with temperature.

Preferably, the said arrangement of the conductive wire is substantially co-extensive with the said inductive winding for uniformity of temperature of the wire and the winding.

Whilst other arrangements of the conductive wire can be envisaged to be without inductance, such as figure of eight lay out laid on a substrate wrapped around the core; in the preferred embodiment, the arrangement comprises two oppositely wound windings, the opposition of the windings causing the inductance of the one to be cancelled by the inductance of the other. These windings and the inductive measurement winding all being wound on a common tube about the central void. The windings are impregnated with a varnish for insulation and heat conduction to a common temperature. A metallic case, providing a Faraday cage, surrounds the coils to prevent any electro-magnetic interference. The high permeability core is inserted into the tube and mechanically connected to the object to be monitored.

The preferred measurement circuitry comprises an operational amplifier LR relaxation oscillator circuit. The displacement measurement coil, the non-inductive coil, and a fixed resistor (RC) are connected in series, in the timing arm of the circuit.

* * The reference arm of the oscillator contains fixed resistors that set the oscillator's threshold voltage level (Vth). If the object moves axially relative to the inductive sensor, the high permeability core will move within the windings and the inductance (LA) of the measurement coil changes, which changes the circuit's time constant and the output pulse period from the oscillator. The pulse period is measured and an output signal is generated which is proportional to the core displacement.

S. 5,15 * *30 *..: When there is a rise in sensor temperature, the inductance (LA) and dc resistance (RA) of the displacement measurement coil increase and so will the dc resistance (RB) of the non-inductive oppositely wound coils, this reduces the oscillator's time constant as this equates to LA/(RA+RB+RC), which reduces the output pulse period. There will also be a reduction in the steady state potential divided final voltage (Vt), which increases the output pulse period and compensates for the reduction in the pulse period caused by a reduction in the oscillator time constant.

This maintains a consistent pulse period and output signal independent of sensor temperature changes at any given core displacement.

THE DRAWINGS

To help understanding of the invention, a specific embodiment thereof will now be described by way of example and with reference to the accompanying drawings, in which: FIG. I shows an axial cross-section view of the linear inductive sensor embodiment according to the invention.

FIG. 2a shows a circuit diagram illustrating the general form of the LR relaxation oscillator.

FIG. 2b displays voltage waveform diagrams with timing calculations for the LR relaxation oscillator.

FIG. 3 illustrates graphically the uncompensated and linear displacement coil inductance with relation to core displacement.

FIG. 4 illustrates graphically the effect of temperature on the displacement coil inductance in relation to core displacement. * S.

FIG. 5 illustrates graphically the effect of temperature on the oscillators LIR time constant at different core displacements. * S * S..

:. FIG. 6 illustrates graphically the effect of temperature on the oscillator circuits Vf voltage at various RB values.

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THE PREFERRED EMBODIMENT

Fig. I displays a vertical section view, passing through the axis of the cylindrical linear inductive sensor, that is comprises two longitudinal coils 1, 2 (comprised of two halves 2a,2b) wound on a cylindrical guide tube 14, and mechanically bound by a front cap 16 and an inter-connection bobbin 15. This bobbin also supports the signal conditioning electronics P.C.B 18, to which the windings are electrically connected. A metallic case 17 surrounds the coils, to prevent any electro-magnetic interference and an electrical cable 19 is provided, enabling power to be supplied to the electronics and a displacement signal to be returned from the sensor.

The displacement measurement coil I consists of several layers of copper wire wound, in the same direction, on a guide tube 14. The coil 1 will exhibit mainly electrical resistance (RA) and inductance (LA) attributes. If the coil I possesses a uniform number of turns per unit length and a martensitic stainless steel core 13, having a high relative permeability of 4000.tr, is inserted into the guide tube 14, the inductance of the displacement measurement coil I will increase, as shown in the uncompensated inductance trend of Fig.3. If the coil is uniform, the inductance behaves in a non-linear manner when the core is first inserted into the coil. This non-linear effect will increase the inductive sensor's winding to stroke length ratio, which is an un-desirable feature. The solution is to increase the turns per unit length and therefore the winding inductance over the affected region, as shown in the displacement measurement coils 1 profile displayed in Fig. 1. The result is an inductance (LA) which is now linear and proportional to core displacement as shown in Fig.3. The inductance, for a medium measurement range coil can change from 1 OmH when the core is retracted to I OOmH when the core is fully inserted and the linear inductive sensor now possess a low winding to stroke length ratio of around 1.1:1.

By altering the profile and inductance (LA) trend of coil I, in this manner, *:. another benefit has been achieved, and that is the thermal offset error has been reduced to zero. Explained another way, with reference to Fig.4, it can be seen that ** SSSS * 30 the error obtained in the value of the coil 1 inductance, expressed as a percentage of *..: reading at 20°C, remains constant at 10%, when the core 13 is inserted into the coil I. In other words, the coil's thermal offset error has been reduced and there is only a thermal gain error present, which means that compensating the displacement measurement coil I from the effects of temperature change would be an easier task to achieve as only one parameter needs to be controlled. It should be noted that the percentage of reading error of 10% shown in Fig.4 is a typical value and is dependent on the mechanical dimensions of the inductive sensor and the core 13 and on the type of materials that they are manufactured from.

As shown in the enlarged view in Fig.1, the first section of coil 2, identified as 2a, consists of several turns of copper wire wound over the displacement measurement coil 1, extending longitudinally over the full operating range of the inductive sensor and wound in the same direction as coil 1. The second section of coil 2, identified as coil section 2b, consists of the same number of turns as that wound in coil section 2a and also wound over the same longitudinal length, but these turns are wound in the opposite direction to section 2a. The magnetic fields generated by the current flowing through each turn from the first section 2a cancels with the magnetic fields generated by the same current flowing through the corresponding turn in the other section 2b, the net effect is zero coil inductance, independent of core 13 position, which creates a single non-inductive resistive coil 2 (RB) as there is no break in the copper wire between each section.

With reference to the signal conditioning electronics schematic Fig 2a, a voltage regulator 8 conditions the input and a lower single stable voltage supply rail is generated together with a half rail buffer 7, which powers the remaining circuit. The displacement measurement coil 1, the non-inductive resistive coil 2 and a fixed * ** resistor 3 are connected in series between the output terminal of an operational amplifier 4 and the half rail buffer 7. This forms a series LR timing circuit with the tap between the coil 1 and coil 2 connected to the inverting input to the amplifier 4.

* Resistor 5 and resistor 6 are connected in series between the output of amplifier 4 and the half rail 7. This potential divider creates a threshold voltage level, which is applied to the non-inverting input of amplifier 4. This component configuration S. 5.55 * 30 creates a LR operational amplifier relaxation oscillator circuit, which produces repetitive output pulses 11 and is similar to the classic RC operational amplifier relaxation oscillator, which is well known in the art. The period of the repetitive pulse 11 is a function of the inductance (LA) of the displacement measurement coil I, the dc resistance of the timing circuit of the oscillator (RtRA+RB+RC), the

II

threshold voltage (Vth) 9 and the steady state final voltage (\f) 10 values as shown in the oscillator waveform diagrams and equation I of Fig.2b.

If the object moves axially relative to the inductive sensor, the high permeability core 13 will move within the windings then the inductance (LA) of the displacement measurement coil 1 will change, which changes the circuit's time constant (LA/(Rt) and the output pulse period 11. This pulse period (tH) is measured with a micro-controller timer and converted to a DAC, PWM, LIN or CAN bus output 12, which is proportional to core 13 displacement.

When there is a rise in sensor temperature, say to 100°C for example, the inductance (LA) of the displacement measurement coil 1 increases as shown in Fig.4, and so will the coils dc resistance (RA) by approx 31.5% as the temperature coefficient of resistance of copper is 0.394%/°C and the temperature rise is 80°C. This will reduce the oscillator's time constant (LAIRt) as shown in Fig.5, which reduces the oscillator's pulse period (t(H). The rise in sensor temperature will also increase the dc resistance (RB) of the non-inductive resistive coil 2. This will decrease the potential divided steady state final voltage (Vf) 10 seen at the inverting input of the oscillator operational amplifier 4, as shown in Fig.6. It can be seen from equation I of Fig.2b that this process increases the oscillator's pulse period (t(H)).

Compensating the linear inductive sensor from the effects of temperature change is achieved by varying the dc resistance values of the non-inductive coil 2 and the fixed resistor 3 until the reduction in steady state final voltage (Vf) 10 compensates for reduction in the pulse width 11 caused by the reduction in the oscillator's time constant. * * * *..

This ensures that a consistent pulse period 11 and output 12 is maintained independent of sensor temperature changes at any given core 13 displacement. *

* * 30 It will be appreciated that the above described inductive linear displacement *.. sensor has the following advantages: It possesses a low winding to stroke length ratio together with a small body diameter.

It possesses good immunity to errors in the displacement signal that occurs when a thermal gradient appearing along the length of the sensor and also exhibit low errors when the sensor is exposed to rapid temperature fluctuations.

It provides high signal levels and therefore a high signal to noise ratio.

It exhibits no velocity errors in the displacement measurement signal and a good immunity to interference from the Earth's magnetic field as the sensor's coils are energised with an ac current and there are no low dc magnetic fields generated in the measurement technique of the invention.

It exhibits low power consumption and is powered by a single unregulated positive supply voltage, as those obtained in battery supply systems in automotive applications. * S. * * . * .. * . * . * S.. * S..

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Claims

CLAIMS: 1. An inductive linear displacement sensor comprising: * an inductive winding of conductive wire about a central void, the winding being continuous in a single direction, whereby it has inductance, which is measurable and changes both with displacement being measured and with temperature independently of displacement, and resistance, which changes with temperature independently of displacement; * a length of conductive wire arranged in heat conductive contact with the said winding, this arrangement of conductive wire having resistance, which changes with temperature as does the change in resistance of the said inductive winding, but negligible inductance; * a high permeability core movable within the central void to cause a change in the inductance of the winding; and * measurement circuitry adapted to provide an output in proportion to the change in inductance with core movement, the circuitry including: * means compensating via a function of the change in resistance of the said arrangement of conductive wire with temperature for the effect of the changes in inductance and resistance of the winding with temperature.
2. An inductive linear displacement sensor as claimed in claim 1, wherein the said inductive winding has a local concentration of its turns at its end corresponding to a small extension of the core into the central void, to compensate for non-linearity of the sensor at the end of its stroke corresponding to this small extension.
3. An inductive linear displacement sensor as claimed in claim I or claim 2, wherein * ,.. the said inductive winding and the said arrangement of conductive wire are of the * ** * .s. 25 same wire. * *
4. An inductive linear displacement sensor as claimed in claim 3, wherein the said same wire if of copper.
5. An inductive linear displacement sensor as claimed in claim 1 or claim 2, wherein the said arrangement of conductive wire is of a material having higher resistivity, ** S...

:. 30 reducing its length, and substantially the same coefficient of resistance with * temperature, in comparison with the material of the winding.
6. An inductive linear displacement sensor as claimed in any preceding claim, wherein the said arrangement of the conductive wire is substantially co-extensive with the said inductive winding for uniformity of temperature of the wire and the winding.
7. An inductive linear displacement sensor as claimed in any preceding claim, wherein the said arrangement of the conductive wire is wound to be without inductance.
8. An inductive linear displacement sensor as claimed in claim 7, wherein the said arrangement of the conductive wire is wound with a figure of eight arrangement.
9. An inductive linear displacement sensor as claimed in any one of claims I to 7, wherein the said arrangement of the conductive wire comprises two oppositely wound windings, the opposition of the windings causing the inductance of the one to be cancelled by the inductance of the other.
10. An inductive linear displacement sensor as claimed in claim 8, wherein the two oppositely wound windings and the said inductive measurement winding are all wound on a common tube about the central void.
11. An inductive linear displacement sensor as claimed in claim 9, wherein the two oppositely wound windings and the said inductive measurement winding are impregnated with a varnish for insulation and heat conduction to a common temperature.
12. An inductive linear displacement sensor as claimed in claim 10, including a metallic case, providing a Faraday cage, surrounding the coils.
13. An inductive linear displacement sensor as claimed in any preceding claim, wherein: * ** . the measurement circuitry comprises an operational amplifier LR relaxation oscillator circuiV S... , * * the said inductive winding of conductive wire and the said arrangement of conductive wire are connected in series in the timing arm of the circuit; and * a fixed resistor which sets the oscillator circuit's threshold voltage level is connected in the reference arm of the oscillator, S. . *** * 30 whereby, if an object to be monitored causes the high permeability core to move *..: axially within the said inductive winding and its inductance to change, the circuit's time constant and the output pulse period from the oscillator change.