METHOD AND DEVICE FOR DRIVING AND TEMPERATURE COMPENSATING AN INDUCTIVE POSITION SENSOR

Abstract: The invention relates to a method for the control and regulation of a driving unit for an inductive position sensor intended for the measurement of linear motion and compensation for temperature variations which arise in the sensor, in which a constant alternating current generator AC is used together with a constant direct current generator DC for supply of a circuit including a winding (1) which is part of a position sensor and a current measurement sensor (6) connected in series with the winding, whereby the voltage over the winding is measured both to partly obtain a signal which corresponds to the position of a sensor element (2) which moves relative to the winding and which influences the impedance of the winding, and to partly obtain a signal which corresponds to the temperature of the winding. In order to achieve a temperature compensation which is continuous, an alternating current AC which is superposed onto a direct current DC is used as drive signal for the inductive sensor and this signal is supplied in a current branch of the drive unit (5) which is common for both the winding (1) and the current measurement sensor (6), whereby the differential voltage of the component of the alternating voltage which appears across the winding is measured, and used to form the signal which corresponds to the position of the sensor element (2) relative to the winding (1), and the direct voltage component which appears across the winding is measured and used to form the signal which corresponds to the temperature appearing in the winding.

Description: METHOD AND DEVICE ?OR DRIVING AND TEMPERATURE COMPENSATING AN INDUCTIVE POSITION SENSOR The present invention relates to a method for driving an inductive position sensor for the measurement of linear movements according to the introduction to claim 1 , and a device for carrying out the method according to claim 6. Because of their reliability and robustness, inductive position sensors are used in a range of industrial applications to measure the relative distance between objects. Examples of their areas of application include measuring the position of a piston in a hydraulic cylinder and its speed of motion, measuring the relative motions of machines, measuring motion in shock absorbers, measuring the motion of sliders in hydraulic valves, measuring clutch position in automatic gear boxes and measuring motion of so-called "joysticks". Inductive position sensors generally include an inductor, generally in the form of what is known as an empty spool or a cylinder of non-magnetic, so-called paramagnetic material, an electrical conductor wound around the said empty spool into the form of a coil, and a sensor element of ferromagnetic material, normally rod-shaped, which moves relative to the empty spool, for example, along an axis which extends through the empty spool. This element is influenced by the object whose relative motion or position is to be measured. When the sensor element moves inside the spool, an electrical voltage is induced in the conductor the magnitude of which is proportional to the number of wire loops of the coil. The induced voltage gives rise to a current through the conductor in a direction such that the original flow of the magnetic field is resisted and the induction in the spool thus varies. Thus the change in induction in the spool can be measured by a suitably designed electrical circuit, and in this way a measured value obtained for the position of an object or its relative motion. The output from inductive position sensors of the type referred to here has a resistance to alternating current, known as an impedance, which is affected by temperature to a relatively large extent, at the same time as the position sensor is usually used in a range of different environments with the existence of temperature variations. For this reason the electronic circuits of the sensor must compensate for the influence on the sensor signal which is caused by the temperature. From US 4 954 776, an inductive position sensor for the measurement of linear motion is known which includes a sensor element and a spool that have a first and a second winding which each are connected in series to their respective resistance and wound around a common spool. A constant RMS alternating current AC is supplied across the first winding and a constant direct voltage DC is supplied across the second winding. Since they are wound around a common empty spool, they will be affected by essentially the same temperature. The change in inductance which occurs in the first winding of the spool due to temperature changes is compensated for by using the change of resistance with temperature of the second winding, that is, the winding which is fed by the direct current DC is used to measure the change in resistance which depends on variations in temperature. An alternative embodiment is further known from the said document, in which a single winding is used for the measurement both of the position of the sensor element relative to the winding and of the temperature of the inductive sensor. To make this possible, the winding is arranged via a time- controlled multiplexer switch to be connected in an alternating manner in series with a first or second resistance and in an alternating manner be connected with an alternating current source and a direct current source respectively. By using sample and hold circuits arranged for this, the change in inductance across the winding and one of the said resistances or the direct voltage across the winding and the other resistance can be measured in an alternating manner, whereby the data arising from the impedance of the sensor (position) and its resistance (temperature) respectively are compared and processed in a number of subsequent steps for temperature compensation of the measured signal giving the position of the sensor. A disadvantage of known inductive position sensors is that the calculations of the actual temperature in the winding of the sensor are relatively complicated since the measurement occurs both across the winding and the resistor, while in practice, the temperatures of these are different, since normally the winding forms part of the sensor whereas the resistor forms part of the electronic measuring circuit which is normally located at a distance from the actual sensor. A further disadvantage of known inductive position sensors is that the temperature compensation does not occur continuously, which in turn influences the precision which it is possible to achieve for the output signal of the sensor, and makes the possibilities of measuring rapid motion changes with high precision more difficult. There has long existed a desire for a simple and, not least, continuous method of achieving compensation for the inductance change that occurs in a winding of an inductor and that depends on the existence of temperature variations, and the aim of this invention is to produce a method which achieves this need. Another aim of this invention is to produce a device for performing the method. To be more precise, what is aimed at is to achieve an inductive position sensor with one winding, but whose position-determining output signal, despite this, can be continuously compensated for the existence of temperature variations. These aims are achieved by the method and device described in the invention having the characteristics specified in the claims. The invention is described more closely below with reference to the attached drawings, which schematically exemplify an embodiment of the invention and in which: Fig. 1 shows a simplified block diagram of the electrical components which according to the invention are used to drive and to compensate for the temperature of an inductive position sensor, Fig. 2 shows in more detail parts of a unit which comprises the driver of the electrical function blocks which are shown in Fig. 1, and Fig. 3 shows a diagram of the DC and AC reference signals which are applied across the windings of the inductive spool, and of the AC and DC measurement signals which are measured across it. Fig. 1 shows the principle in the form of a functional block diagram for a device for an inductive position sensor which in principle comprises a winding 1 which surrounds a spool of paramagnetic material, not shown in the figure. A sensor element 2 in the form of a rod made from ferromagnetic material is placed in the normal way into the cavity of the spool in a sliding axially displaceable manner, and suitably arranged to affect the movement of the element whose position or relative motion is to be measured. As shown by the functional blocks in Fig. 1, there is a power supply such as an voltage generator 3 which is fed with a voltage preferably between 8 V and 35 V, and which in turn provides a direct voltage for the subsequent steps. Block 4 contains an amplitude- stable sinusoidal oscillator designed for the particular driving frequency of the winding 1. This driving frequency normally lies between 10 - 20 kHz. The amplitude-stable sinusoidal signal is superposed onto a stable direct voltage level (the reference voltage). In other words, an amplitude-stable sinusoidal signal AC superposed onto a stable direct voltage DC is supplied from the oscillator. The said amplitude-stable sinusoidal signal, superposed onto a stable direct voltage level, is used as the signal for a driving step. The appearances of the signals mentioned above are more evident if Fig. 3 is studied more closely. In the figure, these signals are denoted (Vp 0S.Ref ) and (Vτ em p . Re f ), respectively, and are shown with continuous lines. Driving step 5 is designed to control the current supply to winding 1 in what is known as a feedback manner, that is, the current through the winding can be held constant independently of the impedance and resistance changes which occur in the winding 1 during the motion of the sensor element 2 and during variations in temperature. More specifically, the alternating current through the winding 1 is held constant by regulation of the amplitude of the output signal from driving step 5 based on the voltage drop which is measured over the current measuring sensor 6 which is arranged in series with the winding 1. In this embodiment, the current measuring sensor consists of a resistance, or a so-called resistor 6, but it could just as well include other current measuring principles, for example, a "Sense- FET", transistors with current measurement outputs, or a so-called Hall element. The value of the named resistance 6 is chosen such that the amplitude of driving step 5 can be held within the supply voltage region of the driving step, and in relationship to the basic inductance of the winding in general. As is evident from Fig. 1, two measurement signals are taken from driving step 5, one of which, forming a position signal for the sensor element, passes through an AC/DC demodulator 7 and a linearisation step 8, before it is finally addressed to a temperature compensation step 9, while the second output signal from the drive step, forming a temperature signal 10 for the winding, is addressed directly to the said temperature compensation step 9. After the temperature compensation step, a step for the adaptation of the signal 1 1 follows, from which the final temperature-corrected output signal is derived in the form of a signal which gives the position of the sensor element 2 relative to the winding 1 , that is, in the form of a temperature-compensated position-specifying signal. The driving step 5 which is shown in Fig. 1, and the winding 1, include, as is evident in Fig. 2, an OP-amplifier 12, to whose positive input is connected as an input voltage the signal from the amplitude-stable sinusoidal signal superposed onto a stable direct-current voltage shown in block 4 of Fig. 1. The winding 1 is connected between the output of the OP- amplifier 12 and its negative input so that the winding 1 is driven by what is a per se known feedback and regulating method, whereby the voltage V R specified in the figure over the resistor 6 is maintained at a constant level both regarding the AC and the DC components, while the equivalent voltage V L over the winding 1 will vary. By choosing a suitable value of the resistor 6 which is arranged in series with the winding 1 , the current through the winding 1 can be balanced to a suitable level, and to a level which is also suitable for the rest of the circuit in general. When the position of the sensor element 2 relative to the winding 1 is changed, the impedance of the winding is influenced by an equivalent amount. As has been described above, the voltage across winding 1 varies due to the maintained constant current through the winding 1 , which gives a measurable signal (V Pos ) which is used to form what is known as a "position voltage" (ΔV Pos ), for details, see also Fig. 3. Thus, by measuring the voltage directly across the winding 1, a measure of this change can be obtained in the form of a so-called raw sensor signal, which can be addressed to the AC/DC demodulator 7, and which, as is shown in Fig. 3, corresponds to the difference in amplitude between the position voltage (Vp os ) and the reference voltage (Vp os R e f), or, in other words, in the form of the differential voltage of the alternating voltage component across the winding 1. A closer study of Fig. 2 shows that this so-called raw sensor signal is obtained by means of a differential voltage sensor 13 which is connected across the winding 1, and which in the embodiment of the invention described here consists of a differential amplifier. In addition to the raw sensor signal or position signal AC, the temperature signal DC can be obtained from the driver step 5 in a similar manner, which occurs by the AC component of the raw sensor signal being decoupled to earth. As is shown in Fig. 3, the temperature signal (ΔV Temp ) is defined as the difference in potential between the DC component of the reference voltage (Vτ e m P Ref ) and the DC component of the raw sensor signal (Vj e mp). To be precise, this decoupling to earth occurs by the output signal from the OP amplifier 12 being connected to earth across a condensor 14 which is connected to the winding 1 and to the resistor 6, and a current limiter such as a resistor which is connected in series with this. In order to be able to later put it under load, the outputted temperature signal, which, it should be realised, is linear, is connected to a voltage follower 16 before it is finally led to the temperature compensation step 9. The direct voltage which is received from the AC/DC demodulator, on the other hand, is not linear, and this is led through a linearisation step 8 before it finally is connected onwards to the temperature compensation step 9. Fig. 1 shows schematically the functional blocks which are used to achieve the linearisation, the temperature compensation and the signal adaptation of the signals which can be received from the inductive position sensor. The said circuits and functions are already well known and do not in themselves comprise any part of the invention, and this is why no closer descriptions of these will be given. Because the circuit electronics which are described above use only a single winding, and because both the position signal AC and the temperature signal DC can be taken from a single signal which passes through the winding, the essential advantage achieved is that temperature compensation of the output signal from the inductive sensor can take place in real-time and in a completely comparable manner, since both signals are conducted and measured as output from the same winding. Furthermore, the advantage achieved is that temperature correction of the position signal of the sensor is very simple in practice because the measurement can be made directly across the winding. An interesting feature of this device for driving an inductive position sensor is that it is the temperature of the winding of the inductor which provides the signal to the temperature compensation step, and since the inductor at the same time also forms the actual position sensor, very short response times can be achieved for the temperature compensation. The current invention, however, is not limited to that described above and shown in the figures, but can be changed and modified in a number of ways within the scope of the concept of the invention as stated in the following claims.

Claims: Claims 1. Method for the control and regulation of a driving unit for an inductive position sensor intended for the measurement of linear motion and compensation for temperature variations which arise in the sensor, in which a constant alternating current generator AC is used together with a constant direct current generator DC for supply of a circuit including a winding (1) which is part of a position sensor and a current measurement sensor (6) connected in series with the winding, whereby the voltage over the winding is measured both to partly obtain a signal which corresponds to the position of a sensor element (2) which moves relative to the winding and which influences the impedance of the winding, and to partly obtain a signal which corresponds to the temperature of the winding characterised in that an alternating current AC which is superposed onto a direct current DC is used as drive signal for the inductive sensor, and that this signal is supplied in a current branch of the driving unit (5) which is common for both the winding (1) and the current measurement sensor (6), by which the differential voltage of the component of the alternating voltage which appears across the winding is measured, and used to form the signal which corresponds to the position of the sensor element (2) relative to the winding (1), and that the direct voltage component which appears across the winding is measured and used to form the signal which corresponds to the temperature appearing in the winding. 2. Method according to claim lcharacterised in that the winding (1) and the current measurement sensor (6) are connected to a circuit which includes a differential voltmeter sensor (13) with which the differential voltage of the alternating current component of the drive signal is measured. 3. Method according to claim 1 or 2 characterised in that a circuit is connected to the winding (1) and the current measurement sensor (6) by which the alternating voltage component AC of the drive signal is decoupled to earth and that the remaining direct voltage component of the drive signal is measured. 4. Method according to claim 2characterised in that a differential amplifier connected across the winding (1) is used as a differential voltmeter sensor (13). 5. Method according to claim 3 characterised in that a condensor (14) connected across the winding (1) and the current measurement sensor (6) together with a current-limiting component (15) connected in series with the condensor is used as a circuit for the decoupling of the alternating current component AC of the drive signal to earth. 6. Device for the control and regulation of a driving unit for an inductive position sensor of the type which is described in the introduction to claim l characterised in that the drive signal for the inductive position sensor specified by the drive unit (5) includes an alternating current AC supeφosed on a direct current DC, supplied in a current branch of the drive-unit which is common to the winding (1) and the current measuring sensor (6), that it includes a step for measurement of the differential voltage of the component of the alternating voltage which appears across the winding, and on the basis of this forms the signal which corresponds to the position of the sensor element (2) relative to the winding (1), and that it includes a step for the measurement of the direct voltage component which appears across the winding and on the basis of this forms the signal which corresponds to the temperature appearing in the winding. 7. Device according to Claim 6 characterised in that the step for measurement of the differential voltage of the alternating voltage component of the drive signal across the winding includes a differential voltmeter sensor (13) arranged on the winding (1) and the current measurement sensor (6). 8. Device according to Claim 6 or 7 characterised in that the step for measurement of the differential voltage of the component of the direct voltage which appears across the winding includes a circuit which is designed for the decoupling of the alternating voltage component AC of the drive signal to earth. 9. Device according to Claim 7 characterised in that the differential voltmeter sensor (13) includes a differential amplifier connected across the winding (1). 10. Device according to Claim 8 characterised in that the circuit for decoupling the alternating current component AC of the drive signal to earth includes a condensor connected across the winding (1) and the current measurement sensor (6), and a current-limiting component (15) connected in series with the condensor.

Inventors: DERNEBO LARS, DERNEBO, LARS
Applicants: DERNEBO LARS, LD DESIGN ELECTRONICS AB, DERNEBO, LARS, LD-DESIGN ELECTRONICS AB
Application year: 1999
Main classification: G01D5
IPC codes: G01B7, G01D3