GB2415046A

GB2415046A - A variable differential transformer in which the coil voltage is measured at the zero current point

Info

Publication number: GB2415046A
Application number: GB0412770A
Authority: GB
Inventors: Andrew Paul Bridges; William Peter Stuart-Bruges
Original assignee: Sondex Ltd
Current assignee: Sondex Ltd
Priority date: 2004-06-08
Filing date: 2004-06-08
Publication date: 2005-12-14
Also published as: WO2005121708A1; GB0412770D0

Abstract

A Linear Variable Differential Transformer (LVDT) is disclosed which addresses measurement errors arising out of the resistance in the VDT coil windings. The LVDT comprises current sensing means for sensing when the current flowing in the LVDT coil is zero. At the zero-current point, the voltage on the mid-point of the coil is measured to give an indication of the position of the moveable core in the coil. At the time when zero-current is flowing, the voltage induced across the resistive part of the winding is zero, and the voltage measurement taken is therefore free of resistive errors. The apparatus can also be implemented as a Rotary Variable Differential Transformer (RDVT).

Description

24 1 5046 A Variable Differential Transformer This invention relates to a

variable differential transformer (VDT), and in particular to a linear variable differential transformer (LVDT) in which measurement errors arise as a result of the resistance in the windings of the coil LVDTs are frequently used to electrically measure linear displacement in a mechanical system. A known LVDT 2, shown in Figure 1, consists of a primary winding 4, an electrically isolated centre tapped secondary winding 6, and a magnetic slug 8 mounted on a linear actuator 10. The secondary winding is grounded at its mid-point. The magnetic slug varies the magnetic coupling between the primary winding and each half of the secondary winding according to its position. In operation, the primary winding is driven with an excitation waveform, usually a sinusoidal voltage, by an alternating current voltage source 12. The ratio of the voltages developed in the two halves of the secondary winding is then measured by ratio detecting circuit 14 to give an accurate indication of the linear position of the actuator.

The secondary winding is terminated into a high impedance to minimise the current in the winding. If significant current is allowed to flow, an additional voltage is developed in each half of the secondary winding due to the effect of the current flowing through the winding's own resistance, and the reading of the measurement is degraded as a result. The current in the primary winding has a similar effect on the total voltage developed across the secondary winding, but this has no effect on the reading if the ratio of the voltages developed across the two halves of the secondary is used to establish the position of the actuator.

In some applications, however, usually due to the lack of space, it is not possible to provide a separate isolated primary winding. The centre tapped secondary winding must then be used to both drive the LVDT and measure the position of the actuator, as shown in Figure 2. The mid-point of the secondary or sensor winding is now connected to the ratio detecting circuit.

In this configuration, the LVDT drive current will therefore have an effect on the voltages developed across the two halves of the winding, thus introducing a measurement error.

The magnitude of this error can be reduced, but not completely eliminated, by reducing the resistance of the winding. One way to achieve this is to wind the coil of the LVDT using a larger diameter and hence lower resistance wire In some applications however, again due to space constraints, the windings must be made with small diameter wire and the resulting winding resistance will be high.

We have therefore appreciated that there is a need for an improved LVDT in which the measurement errors of the type described above are reduced or eliminated.

Summary of the Invention

The invention is defined by the independent claims to which reference should now be made. Advantageous features are defined in the dependent claims.

A Variable Differential Transformer is provided which addresses measurement errors arising out of the resistance in the transformer coil windings. The variable differential transformer comprises current sensing means for sensing when the current flowing in the coil is zero. At the zero-current point, the voltage on the mid-point of the coil is measured to give an indication of the position of the moveable core in the coil. At the time when zero-current is flowing, the voltage induced across the resistive part of the winding is zero, and the voltage measurement taken is therefore free of resistive errors.

Brief Description of the Drawings

The invention will now be described in more detail, by way of example, and with reference to the drawings in which: Figure 1 illustrates a known LVDT comprising a primary and a secondary coil; Figure 2 illustrates a known LVDT comprising only a single coil; Figure 3 illustrates a preferred embodiment of the invention; and Figure 4 illustrates the voltages and currents across the LVDT coil of the preferred embodiment over time.

Detailed Description of the Preferred Embodiment

The preferred embodiment 20 of the LVDT apparatus, shown in Figure 3, comprises a centre tapped sensor coil 22 connected at its mid-point M to an Analogue to Digital Converter (ADC) circuit 24. The coil is driven by four transistors Q1 to Q4 arranged in a full bridge configuration. The gates of Q1 and Q4, are connected to a first drive signal generator 26, and the gates of Q2 and Q3 are connected to a second drive signal generator 28. The coil 22 is connected between the transistors Q1 to Q4 such that one end of the coil A is connected to the source of Q1 and the drain of Q3, while the other end of the coil B is connected to the source of Q2 and the drain of Q4.By appropriate timing of the drive signals therefore, the coil 22 can therefore be alternately electrically connected between transistors pairs Q1 and Q4, and Q2 and Q3 respectively.

Although, not shown in Figure 3, it Will be appreciated that the coil is magnetically coupled to a moveable core and actuator, in the same way as illustrated in Figures 1 and 2.

The bridge is driven from a stable reference voltage provided by a power supply unit 30 connected to the drains of Q1 and Q2, as well as to the reference voltage terminal of the ADC 24.

The sources of Q3 and Q4 on the other hand are connected to ground. The source of Q3is connected via a resistor 32. A comparator circuit 34 is connected with its inputs terminals arranged in parallel across the resistor, and its output terminal connected to the ADC. ADC 24 has an output 36 for providing an indication of the measured voltage at the midpont of the coil.

The measurement error introduced by the effects of drive current in known LVDT windings is caused by the voltage induced across the resistive part of the winding impedance This error voltage is proportional to the current flowing In the winding.

If the driving voltage is an AC voltage, the current in the windings of the coil will periodically fall to zero, regardless of the actual excitation waveform used. At these zero-current points the voltage induced across the resistive part of the winding impedance is zero, and the ratio of the voltages across the two halves of the winding gives a true indication of the position of the actuator free of the measurement errors resulting from the resistance of the coil windings.

Rather than using the usual continuous wave voltage measuring techniques, the preferred embodiment of the LVDT sensor therefore measures the current flowing in the coil and samples the voltage across the two halves of the coil only at the instant that the current In the winding passes through zero. A number of samples taken on consecutive cycles of the excitation is preferably then averaged to offer improved noise immunity.

The operation of the preferred embodiment will now be described in detail. The first and second drive signal generators produce gate drive signals that are anti- phase so that the current path through the bridge is alternately through Q1 and Q4, and Q2 and Q3. The drive signals are illustrated in the top two graphs a) and b) of Figure 4.

Assuming that Q1 and Q4 have been turned on for some time, a current will have built up in the LVDT winding and will still be increasing at a rate determined by the bridge voltage, the LVDT inductance and the total resistance in the current path. This current is now flowing from left to right in the coil, as shown in figure 3.

If Q1 and Q4 are now turned off, and Q2 and Q3 are turned on, the driving voltage across the LVDT is reversed. However, the current in the LVDT cannot change instantaneously, and Will still be flowing from left to right, drawing current from ground through the resistor 32 and Q3, and passing it through Q2 into the reference voltage. With the driving voltage reversed the current will start to reduce, pass through zero, and start to build up in the opposite direction.

The corresponding voltages at the coil terminal points A and B are illustrated in graphs c) and d). Referring to graph d), it can be seen that the voltage at terminal point B alternates between the reference voltage and zero volts potential respectively, as it is alternately connected directly between the reference voltage line and ground. Referring to graph c), the voltage at terminal A of the coil can be seen to be the reference voltage, when the current is flowing through Q1 and Q4, as the terminal is connected directly to the reference voltage power supply line.

However, when the coil is connected between Q2 and Q3, the transitional current flow in the opposite direction to the voltage appears as a negative voltage drop across the resistor 32. As one side of the resistor 32 is grounded, this voltage drop initially appears as a potential negative to the reference voltage at terminal A. However, as the current flowing in the coil and resistor approaches zero, the voltage drop across the resistor 32, and the potential at A will also approach zero (becoming gradually more positive), until at the zero current point, the potential at A is zero.

The current flow across the coil is illustrated in graph e). As can be seen the zero current point through the coil lags the transition of the gate drive signal and corresponds to the zero voltage position at terminal A. The comparator circuit 34 measures the voltage across the resistor 32 to determine the zero current position. Once this position is sensed by the comparator, it provides an output to the ADC, causing the ADC to sample the voltage on the centre tap M of the LVDT. At this instant there is no current flowing anywhere in the system, so the left hand end of the LVDT will be at ground potential and the right hand end will be at the bridge reference voltage. The voltage on the centre tap is thus measured with respect to ground, and ratometric to the bridge reference voltage.

This technique thereby avoids errors associated with the resistance of the coil windings. Although the resistor 32 adds to the error voltage when a current is flowing, its contribution at the zero-current point is zero.

The voltage on at the mid-point or centre tap of the coil is illustrated in graph c, and the comparator output in graph 9).

Although the sampling of the voltage occurs at the zero-current point, while ever Q2 and Q3 remain switched on, an increasing current will flow in the coil and through the resistor 32 As the current flow across the resistor increases from the potential at point A also increases.

Once the current has built up in the reverse direction, from right to left in figure 3, Q2 and Q3 turn off and Q1 and Q4 turn on. Once again the driving voltage across the LVDT reverses, and the current starts to reduce, passes through zero, and builds up again in the original direction. This completes one cycle of operation.

Preferably, the ADO stores the values of the voltage on the mid-point of the coil measured across several cycles, and takes the average of the these values to output a final voltage reading to output 36.

Thus, the preferred embodiment of the invention uses current sensing to ensure that the sample point of the voltage on the coil occurs when zero current is flowing, regardless of the drive technique and the way in which the LVDT is sampled.

In the preferred embodiment described above, the voltage measurement is only sampled once every cycle. However, it will be appreciated that the circuit could be modified so that Q4 is also connected to ground through a resistor, and an additional comparator is provided to measure the voltage across this resistor.

This would allow a measurement to be taken on each half cycle of the excitation waveform, instead of once per complete cycle.

The excitation waveform may be a conventional sinusoidal voltage, but any waveform that causes the current in the winding to periodically reverse may be used. A square wave drive is preferred as it is simpler to generate and is less lossy.

The zero-current measurement technique described above In connection with the preferred embodiment may be implemented in a variety of alternative ways. The full bridge implementation described above is preferred however as it has the added advantage that the energy stored in the inductance of the LVDT winding at the end of one half cycle is returned to the power supply in the first portion of the next half cycle. This charge recycling can significantly reduce the LVDT drive power requirement.

Although, the preferred embodiment has been described as comprising MOSFETs, it will be appreciated that other types of transistors could alternatively be used.

Although the preferred embodiment has been described entirely in terms of hardware, it will be appreciated that an alternative embodiment of the invention various functions may be implemented in software as would be apparent to the man skilled in the art.

Although the preferred embodiment of the invention has been described with reference to a linear variable differential transformer, it will be appreciated that it need not be limited to such. Alternative embodiments of the invention may for example comprise a rotary variable linear transformer (RDVT).

Claims

Claims 1. A variable differential transformer comprising: a sensor coil;

current sensing means, for detecting zero current flowing in the sensor coil, and; voltage sensing means arranged to detect the voltage difference across at least a part of the coil in dependence on the current detecting means detecting zero current flowing in the coil.
2. A variable differential transformer according to claim 1, having only a single coil coupled to a drive current source and to the voltage sensing means.
3. A variable differential transformer according to any preceding claim, wherein the current sensing means comprise a resistor connected in series with the coil.
4. A variable differential transformer according to claim 3, wherein the current sensing means comprise a comparator connected in parallel with the resistor, for detecting the current flowing through the resistor.
5. A variable differential transformer according to claim 4, comprising a first and a second pair of transistors, a first drive signal generator for supplying a drive signal to the first pair of transistors, and a second drive signal generator for supplying a second drive signal to the second pair of transistors, wherein the transistors are arranged in a full bridge configuration and driven such that the sensor coil is alternately electrically connected between the source and drain of the two transistors in one pair of transistors, and the source and drain of the two transistors in the other pair of transistors.
6. A variable differential transformer according to claim 5, wherein the resistor Is connected in series with the coil and the first pair of transistors.
7. A variable differential transformer according to claim 6, wherein the current sensing means comprise a second resistor, connected in series with the coil and the second pair of transistors.
8. A variable differential transformer according to claim 7, comprising a second comparator connected in parallel with the second resistor.
9. A variable differential transformer according to any preceding claim wherein the voltage sensing means comprises an Analogue to Digital Converter 1 0 (ADC).
10. A variable differential transformer according to any preceding claim wherein the ADC comprises a memory for storing a plurality of measurements performed by the voltage sensing means, and an averaging circuit for outputting an average of the stored measurements.
11. A variable differential transformer comprising: a coil; an alternating current source for causing an alternating current to flow in the coil; a moveable core magnetically coupled to the coil; current detecting means for detecting current through the coil; and voltage detecting means for detecting the voltage difference across at least a part of the coil to detect the position of the moveable core, wherein the voltage detecting means is operable in dependence on the current detecting means detecting zero current flowing in the coil
12. A method of detecting the displacement of the actuator in a variable differential transformer, wherein the actuator is connected to a moveable core magnetically coupled to a coil, the method comprising: a) measuring the current is flowing in the coil; b) measuring the voltage difference across at least a part of the coil only when the current flowing in the coil is determined to be zero; and c) determining the position of the actuator from the measured voltage.
13. A computer program product comprising a computer readable medium on which code is recorded, the code when executed on a computer causing the computer to perform the steps of method claim 10.
14. A variable differential transformer substantially as described herein and with reference to Figures 3 and 4 of the drawings
15. A method substantially as described herein and with reference to Figures 3 and 4 of the drawings.
16. A computer program product substantially as described herein and with reference to Figures 3 and 4 of the drawings.