EP1360466A1

EP1360466A1 - Force sensor device

Info

Publication number: EP1360466A1
Application number: EP02710844A
Authority: EP
Inventors: Lutz Axel May
Original assignee: Fast Technology AG
Current assignee: Fast Technology AG
Priority date: 2001-02-07
Filing date: 2002-02-06
Publication date: 2003-11-12
Also published as: GB0103037D0; WO2002071018A1

Abstract

A force sensor device establishes a balance of forces on an axially-displaceable sensor member (70) which is at least partly of magnetic material. For a horizontal sensor member (70) the balance is between a constant magnetic bias force (F4) and an opposed counteracting magnetic force (F1) generated by an energised coil (L1) whose energisation is controlled in a feedback loop (88) dependent on the sensed axial positions of the sensor member (70). For a vertical sensor member the balance is between gravity (weight) and the counteracting magnetic force (Fig. 2). The feedback loop (88) is preferably of the second-order kind (fig. 7). The generation of magnetic force is by having an end (75) of a magnetic portion (72) of the sensor member terminate within a coil (L1) to generate a solenoid-like attractive force thereon. A magnetic-position sensing device detects the position-sensitive coupling of two coils (L2:L3) by a magnetic portion (74) of the sensor member (70) which extends entirely through one coil (L2) and terminates within the other. A force (T) to be measured is applied axially to the sensor member (70) acting to disturb the force balance. The force sensor device has application to measuring tension in a running thread (Fig. 4).

Description

Title: Force Sensor Device

FIELD OF THE INVENTION

This invention relates to a force sensor device and to a position- sensing device.

The invention has arisen in relation to the measurement of tension in a longitudinally-moving flexible body such as a thread, filament, cord, string, and so forth. The invention finds application in the measurement of the tension in a thread that is moving lengthwise in textile machinery, for example in weaving machinery, where the thread may be drawn under tension from one part of the machinery to another. However, the force and position techniques proposed in accord with the invention are of wider utility. The invention will be discussed and exemplified in the context of measuring tension in a lengthwise moving thread.

BACKGROUND TO THE INVENTION

The conventional device for measuring the tension in a lengthwise moving (running) thread is diagrammatically illustrated in Fig. 1. The path of the thread 10 includes two fixed rollers 12 and 14 between which the thread engages a freely-rotatable roller or wheel 16 supported by a spring 18. The weight of the wheel 16 acts downwardly to extend the spring. The path of the thread follows a V-shaped path that is apex down with the thread engaging the underside of wheel 16. The interior angle θ (= θi + Θ2) determines the magnitude of a vertically upward component T of the tension in the thread. This component produces a force upwardly counteracting the weight of the wheel so that the extension of the spring is a measure of the tension as is diagrammatically indicated by pointer/scale 19. The spring is in continuous small movement following the changes in tension in the thread. It has been found that the spring wears or fatigues so that replacement is necessary at relatively frequent intervals.

SUMMARY OF THE INVENTION

The present invention enables us to provide a solution to the above problem which does not involve the use of a mechanical spring and in which the point of contact of the measuring device with the thread is essentially at a fixed or constant position. The device provided in accord with the present invention utilises magnetic forces. In one embodiment a tension-dependent magnetic force is employed to balance a combination of the force of gravity, i.e. a weight, and a component of thread tension. In a second embodiment a tension-dependent magnetic force is employed to balance a combination of a constant magnetic force and a component of thread tension.

More generally the invention is based on the concept of measuring the effect of a force to be measured on a balance of a magnetic force and gravity or on a balance of two opposed magnetic forces, with or without gravity being a factor in the balance.

Another aspect of the invention, which may be employed in a force sensor device, but is itself of wider utility is a position sensing device which relies on a position-dependent coupling of two electrical coils.

Aspects and features of the invention are set forth in the claims following this description.

The invention and its practice will be further described with reference to Figs. 2 to 8 of the accompanying drawings in which: BRIEF DESCRIPTION FO THE DRAWINGS

Fig. 1 diagrammatically illustrates a known device for measuring tension in a running thread;

Fig. 2 is a diagrammatic illustration of a first embodiment of a force sensor device the invention employing a vertically-oriented sensor device;

Fig. 3 illustrates a magnetic position sensor device usable with the embodiment of Fig. 2;

Fig. 4 illustrates an embodiment combining the teachings of the force sensor device of Fig. 2 with the position sensor device of Fig. 3;

Fig. 5 is another modification of the embodiment of Fig. 2 to incorporate the magnetic-position measurement arrangement of Fig. 3.

Fig. 6 is a diagrammatic illustration of a second embodiment of a force sensor device of the invention employing a horizontally-oriented sensor;

Fig. 7 is a block diagram of a feedback loop controlling a vertically- oriented sensor but applicable to a horizontally-oriented sensor; and

Fig. 8 is a diagrammatic illustration of a means of supporting and actuating the horizontally-oriented sensor of Fig. 5.

The invention is of general applicability to the measurement of forces but in the following description the force to be measured will be exemplified as a component of the tension in a running thread.

Description of Preferred Embodiments

The first embodiment to be described is one in which the tension in a running thread is used as a counterweight to gravitational force (weight). An upward component of tension is added to a controllable magnetically- generated force, the control being exercised to maintain a fixed position of a magnetisable member, that is a tension affected balance between magnetic force and gravity. In the embodiment to be described the magnetic force is generated by an electromagnet and the control is exercised through the current supplied to the electromagnet. The current is measured as a measure of the tension in the thread.

Fig. 2 shows a sensor member in the form of an elongate core 20 of ferromagnetic material axially movable within a coil 22 of an electromagnet diagrammatically represented as inductor L1. The coil is energised in a circuit 24 from a D.C. source 26 through a current-control unit 28 shown for simplicity of explanation as a variable resistor R. Associated with unit 28 is a current-measuring unit 29 - which may be part of unit 28 - for providing a signal Vi representing the current I. The coil 22 is wound on a former (not shown) which is mounted to have its axis vertical (aligned with the local direction of gravity). The core member 20 is dimensioned and mounted to be freely movable along the axis of the coil so that in the absence of energisation the member 20 will tend to drop vertically out of the coil 22 under its own weight W. Energisation of the coil 22 produces a magnetic field acting on the core to generate a magnetic force M to draw the core 20 upwardly into the coil assuming the core is in a position within a part of the coil, that is does not extend out of the upper end of the coil. This is a solenoid-like action to retract an upper portion 20a of the core into the coil counter to the force of gravity. The current required to maintain the core suspended in a fixed or constant vertical position is a measure of the weight of the core.

|f the core is also subjected to an applied upward force component T this will counteract the core weight W (combine additively with the magnetic force M) so that the current to maintain the same vertical position of the core can be reduced in proportion to the magnitude of force component T. The current supplied to the coil can thus be measured as a measure of force T. The measurement can also be effected by way of another parameter of the coil energisation circuit related to the current I. For example, if the source 26 is a constant voltage source, the resistance R can be measured: if the resistance R is made constant and a variable voltage source 26 employed, the same voltage can be measured. In general:

W- T = f(l).

The next stage shown in phantom in Fig. 2 is to convert the balancing principle explained above into a measurement device for automatically measuring the force T, particularly when that force is subject to variation. To this end, the vertical position of the core member 20 is measured. This is shown purely diagrammatically in Fig. 1 as a pointer 30 attached to the member 20 against a scale 32. In theory small variations in T will be reflected in the deviation of the pointer from a norm or reference index position 33 so that T can be measured. However, such a system involves axial movement of the core 20. What is required is a position sensing system which will act in conjunction with the circuit 24 to control the current to maintain member 20 in a fixed vertical position or at least reduce the amount of vertical deflection . To this end in a case where R is the variable element, a position sensor 34 provides a position-dependent output signal V_p at 36 to the current control unit 28 forming a negative feedback loop 38 to maintain the core 20 in a fixed position. Preferably, the sensor 34 is of a non-contacting type with respect to member 20 so as to avoid any additional contributions to the forces acting on the core member.

The position sensor 34 can use any one of a number of known non- contacting sensor technologies, usually referred to as proximity measurement techniques. These include optical and optical-electronic devices, ultrasonic, radar, and magnetic field sensors. There will be described below a magnetic- based position-sensor technique which has been devised for the practice of the present invention.

In measuring the tension in a running thread by way of a force component T of it, the lower end of core 20 will be provided with means to engage the thread as will be discussed below.

In constructing a practical implementation of the device of Fig. 2 it is preferred that the core 20 is of a shape, e.g. circular cross-section, having a smooth surface with a view to ensuring least friction between the core and the coil former. Preferably the core 20 is suspended out of contact with the former. Coatings may be used to obtain surfaces having a low coefficient of friction. In general the core should be suspended and positioned within the former with the least friction possible.

The core 20 can be of any material attractable by magnetic force. To prevent the weight of the core changing over time in an uncontrolled environment, e.g. due to humidity changes, the core material is preferably one which does not absorb water or water vapour or any other liquids or vapours, or gases to which the core may be exposed in use. To meet these objectives, the core may be given an impervious coating to protect and seal it.

The current sensing unit 29 produces an output signal Vi representing the force measured. This signal may be processed (digitally or in analog form) by hardware or software to provide a reading of the force T sensed by the core. Referring back to Fig. 1, it will be appreciated that by holding the core at a fixed vertical position, the angles θι, Θ2 (normally equal) of the thread to the vertical remain constant despite variations in thread tension so that the vertical component T is a constant proportion of the actual tension in the thread. Also the maintaining of the fixed core position should avoid any irregularities or non-linearities that would occur in plotting the vertical position of the core as a function of current I over a greater excursion of position (all other forces remaining constant), i.e. the feedback loop open. As will be further described below with reference to Fig. 6, the feedback loop 38 is preferably a second order integrating loop acting to maintain the vertical position of the core fixed for all values of applied force. A first order loop would allow some variation of the core position as a function of force T, but reduced by the gain of the feedback loop.

It is thus considered that the above-proposed embodiment of the invention should provide both a sensitive and accurate measurement of the force T.

The measurement of the vertical position of the core 20 can be done by a magnetic technique as will now be described. The principle is illustrated in the position sensor device of Fig. 3 which can be combined into the force sensor device or may be more generally applied. It will be described in terms of measuring displacement of the core 20 in response to the force T in a magnetic force-gravity balance as described for Fig. 1.

In Fig. 3, the core 20 is shown to extend through a coil L2 upwardly to partially enter a coil L3 but preferably without contact with the coil L2 or L3. For the present the magnetic force M acting upwardly on the core 20 will be taken to be generated by means of a separate coil L1 as already described and not shown in Fig. 3. In this position sensor device the coil L2 is energised by alternating current from an A.C. source 40 at a constant amplitude which does not generate a significant force on the core or else any force is a constant. The core 20 extends out of each end of coil L2 so that there is no change in the magnetic coupling of the coil field to the core within the expected range of movement of the core which may be made small in any event. By contrast the upper portion 20a of core 20 partially couples to the field of coil 43 so that a position-sensitive coupling is established. The core 20 provides a position-sensitive coupling between windings L2 and L3 so that, in the absence of any connection to the terminals of L3, the signal Vp from coil L3 is a function of the vertical position of the core. In practice, this signal is processed at 42 and supplied for use as a negative feedback signal 44 to control the current in L1 to act to maintain core in fixed the position, the controlled current being a function of measured force T. Because the core is maintained at a fixed position any irregularities or non-linearities in the coupling between coils L2 and L3 over a greater excursion of the core position are reduced to a minimum.

Reverting to the force sensor device of Fig. 2, the application of the position sensor device of Fig. 3 to the design of Fig. 2 requires the core to extend partially through L1 and L3, that is to have two end portions terminating in L1 and L3 respectively. This can be achieved in various ways. The position sensor device of Fig. 3 can be inverted to have the lower end of a longer core terminate in L3, the core extending upwardly through L2 and the upper end terminating in L1. Other solutions are possible.

L1 could be located to act on the lower portion of the core. That is L1 located below L2. To this end, it may be appropriate to shape the core 20 to ensure that the energisation of coil L1 provides the necessary upward attractive force on it. For example, the lower portion of the core entering into coil L1 could be of greater cross-section than the upper portion continuing through L1 and into coil L2 so that the energised coil L1 is acting to attract the lower portion of the core upwardly against gravity. An alternative is to introduce a non-magnetic portion into the sensor member 20 as is done in the modified embodiment of Fig. 5.

It will be recognized that, the force sensor device so far described establishes a balance between a magnetically-generated force and a gravitational force. As shown the force T to be measured acts against gravity and adds to the magnetic force, acting to disturb the balance. The force T could be arranged to act in the opposite sense, i.e. downwardly in Fig. 1 , provided the circuit is operating in a sense to restore the balance.

Fig. 4 illustrates a variation of providing a position sensor acting on the principle of Fig. 2 with the force sensor of Fig. 1. In this variation the coil L2 acts also as the coil L1 to provide the magnetic force balancing the sensor body as well as being an element of the position sensor. In this case the ferromagnetic core 20 is a magnetisable portion of an extended member 50 having a non-magnetic lower portion 52. Thus it is the weight of member 50 which is to be counterbalanced by the force exerted by coil L1/L2 on core portion 20. The coil L1/L2 acts both to generate magnetic force and as the source coil for the position sensor. The upper end portion 20a of the core 20 is positioned within a part of coil L1/L2 so as to have an upward attractive force exerted on it. The lower portion of core 20 extends through the coil L3 to terminate below the coil so as to provide a position-sensitive coupling between the coils. The lower end of the core 20 may terminate within coil L3 provided that a position-sensitive coupling is achieved to obtain a position output signal V_P.

The non-magnetic portion 52 projects beneath the coil L3 and its lower end is shaped at 54 to provide a low friction contact with the running thread 1 o. This could be provided by a low friction groove or a freely rotatable wheel could be provided at the lower end of portion 52 to be engaged by the thread. If the thread is to engage the lower end of portion 52 directly, it can be formed to have a hard, smooth, low friction surface, e.g. a ceramic material.

It will be understood that the means by which the sensor member is engaged by the thread for a component T of thread tension to be communicated to it, as described in the preceding paragraph, is applicable to the embodiments of Fig. 2 and 3. The A.C. source 40 for energising coil L1/L2 is made a controllable current source. It is controlled in a negative feedback loop by the output V_P of coil L3 via the unit 42 to derive a suitable signal for application to control input 46 of controllable source 40. If the feedback loop is of a first order kind a small change in position of core 20 will accompany a change in sensed force T, the position change depending on loop gain. If an integrating loop is employed, the position will settle to a fixed reference position following a change in component T.

Fig. 5 shows a modification of the embodiment of Fig. 2 which can be considered as a hybrid of the ideas of Figs. 2 and 3. Fig. 5 shows a measurement device employing the magnetic coupling, position measurement concept of Fig. 3 but separated from the sensor weight-counterbalancing magnetic coil. The latter may be provided as described in Fig. 2.

In Fig. 5 the vertically-suspended sensor member 60 has an upward force applied to it by a D.C. energisation circuit 24 including coil L1 whose energisation current I is controllable by a circuit 24 as described with reference to Fig. 2. The circuit 24 is connected in a negative feedback circuit and the current in it is controlled by the position-dependent signal V_P from coil

L3. In this case the signal coupled to L3 is obtained from source inductor L2 energised by a stable A.C. source 40. The position control circuit 24 could also be A.C. energised and coil L1 can be physically separated from coupling to coil L3 as will now be described or the frequencies of source 40 and circuit

24 can be sufficiently different to enable the wanted signal to be separated and detected at L3. Even if L1 and L3 are not coupled physically, the use of different frequencies would help ensure avoidance of the detection at L3 of any L1 signal due to stray coupling. It may be considered that the use of separate D.C. for weight balancing and A.C. for position sensing is in fact an example at one extreme of using different frequencies, one being zero. In any event L1 has to generate the upward magnetic force on the sensor member 60 and the physical separation concept enables this to be done.

Thus, in the modification of Fig. 5, the member 60 has to be attracted upwardly by the magnetic force generated in it by coil L1 and at the same time the coupling to coil L3 should be position-dependent. Also in the device of Fig. 5, the position sensor energising coil L2 should not influence the position of member 60, though even if it does because it is a constant signal, its influence will be small and it will slightly modify the gravitational force acting on member 60. The maintenance of the member at a fixed vertical position assists in minimising the effects on the measurement of any such contribution from L2.

To satisfy these requirements the structure of the member 60 is in three portions: a lower portion 66 of ferromagnetic material whose upper end terminates within coil L1 as in Fig. 2 and whose lower end is adapted (by means not shown) to engage the running thread as already described; an upper portion 68 of ferromagnetic material whose upper end terminates within coil L3 as in Fig. 3 and which extends downwardly through coil L2 so as to provide a position-sensitive coupling between L2 and L 3 as in Fig. 3; and an intermediate portion 69 of non-magnetic material which separates the weight- balancing part of the sensor device from the position-measuring part. Each ferromagnetic portion 66 and 68 may be a simple circular rod of ferromagnetic material.

An alternative structure for sensor member 60 is that previously mentioned with regard to the embodiment of Fig. 3. The member 60 can be made ferromagnetic throughout. The establishment of an upward attractive force on the member could be realised by differential dimensioning of the upper and lower portions of the core within coil L1 , e.g. of lesser and greater cross-section respectively. Measures to avoid unwanted signal coupling between coils L1 and L2 could then be applied as already described. This technique could also be applied to the embodiment of Fig. 6 described below.

So far all the embodiments discussed and the variations of them have used a vertical sensor member whose vertical position is maintained at a fixed or set reference level by balancing gravitational force on the member (weight) with an upward force due to magnetic field. What will now be described is a force sensing device which does not rely on gravity and may thus be used in a horizontal plane. In this case, it is proposed to substitute the gravitational force by a magnetically-generated force which is counterbalanced by a current controlled field as already described, that is a balance between two magnetic forces.

Fig. 6 shows an embodiment of the invention in accord with the principle stated in the preceding paragraph. The structure of the force measurement device of Fig. 6 is like that of the device of Fig. 5 but oriented horizontally rather than vertically. The elongate sensor member 70 has a force-balancing portion 72 and a position-sensor portion 74 separated by a non-magnetic portion 76. The elongate sensor member 70 is supported for free horizontal movement along its axis. Further discussion on the supporting of the member 70 in this horizontal orientation is given below. The member 70 is subject to a constant bias force - arrow F4 - acting on portion 72 to move it to the right in the figure. This force is counteracted by the controllable force due to the current in coil L1 also acting on portion 72 and indicated by arrow F1. The bias force is magnetically generated enabling non-contacting means to be used. Specifically this is a coil L4 energised in a constant current circuit 78 comprising D.C. source 80; and current-setting unit 82 shown as resistor R2 to establish a constant current 12 in the circuit. It will be seen that the right-hand end 73 of portion 72 (as seen in Fig. 6) terminates within coil L4 so that the field generated thereby acts to attract the member 70 into the coil (i.e. to the right) with a constant magnetic force. The left-hand end 75 of portion 72 terminates within coil 22 (L1) controllably energised in a circuit 24 as in Fig. 2. The energisation current is here denoted 11 Portion 72 is thus subject to a controllable force F1 counteracting force F4.

The non-magnetic portion 76 is inserted in sensor member 70 so as to separate the ferromagnetic portions 72 and 74 with portion 74 extending through position sensor energisation coil L2 fed from a constant amplitude source 40 and terminates within a position-sensing coil l_3 so as to provide a position-sensitive coupling between L2 and L3. The A.C. signal Vp from coil L3 is amplitude detected at 86 and the detected signal utilised in a negative - feedback loop 88 to control the energisation current 11. If the loop is an integrating loop the control signal developed to control circuit 24 will be a measure of the force component T and can be used as a measurement output signal Vout.

The magnetic bias force acting on portion 72 could be realised by an

A.C. energised coil or by a permanent magnet arrangement. The right-hand end of member 70 is provided with a non-magnetic extension 84 intended to engage a running thread similar to the end 54 in Fig. 4. In this case the thread will run over a V-shaped path in a substantially horizontal plane engaging the extension 84 at the interior angle of the apex to provide a force component T of the tension in the member 70. However, the part 84 could be provided at the other end of the member 70 as indicated in phantom at 84'. It will be understood that the member 70 and particularly the portion 72 of it, is held as regards horizontal linear movement in equilibrium by the counterbalancing magnetic forces provided by circuits 78 and 24 (L4 and L1). The feedback loop will act to counter any force on the member 70 which is in the axis of linear movement and which tends to disturb the horizontal equilibrium. This reasoning also applies to the vertically-oriented, gravity-biased embodiments described previously. In these embodiments the sensor member is movable along a linear vertical axis and the sensor member is effectively suspended at a given vertical position by the counterbalancing of a magnetic force and of a gravitational force (weight). Any force applied to along the vertical axis and acting to disturb that equilibrium will be counteracted by the negative feedback loop. The vertically-oriented embodiments show the sensor member having a force component T applied to it at its lower end. However, the running thread could be taken in an upwardly-directed V-shaped path (apex up) to act on the upper end of the sensor member.

Reverting to the use of a single coil (L1/L2) to provide magnetic force on the sensor member and to couple through the sensor member to the position sensing coil L3, Fig. 7 shows a more complete block diagram of such a control circuit. In Fig. 7 the sensor member is generally shown at 90. The sensor device may be vertically or horizontally oriented . Details of its structure are in accord with the teachings given above to provide counterbalancing forces on the sensor member. They are not detailed in Fig. 7. The force T to be measured acts along the axis of member 90 to displace it. A single coil 92 acts as both the position-sensor energising coil L2 and the magnetic force generating coil L1. The coil 92 is driven from a constant amplitude A.C. source 94 through an amplifier 96 feeding a driver 98 to energise coil 92 at the frequency of the source 94. This frequency is preferably distinct from mains or supply frequency (e.g. 50 or 60 Hz) to reduce potential interference problems. Between the source 94 and the amplifier 96 a controllable gain-control unit 100 is inserted to control the level of signal applied to coil 92. It will be appreciated that the level control could be integral with source 94 to provide a controllable amplitude source or could be exercised in a gain-controllable amplifier 96. A negative feedback path 102 is closed from position-sensing coil L3 to a gain-control input 104 on the unit 100. The feedback loop 102 comprises a filter 106 for the position- dependent signal from L3 at the frequency of source 94. The filtered A.C. signal passes to a peak-to-peak detector 108 and the rectified (D.C.) peak-to- peak value is integrated by integrator 110. The integrated signal is passed to D.C. amplifier 112 from which, with such offset adjustments as may be required, there is obtained an analogue signal Vout at point 114 which is a measure of the force component T on member 90. Signal Vout may be further processed, in analogue form or digitally, as required. To close the feedback loop the signal Vout is applied to gain control unit 100. The unit 100 also receives a pre-set bias value for setting the equilibrium position of the member 90 as indicated at 116. By virtue of the integrator in the feedback loop, the loop is a second-order feedback loop acting to maintain the member 90 at a fixed vertical position.

In the circuit loop of Fig. 7 it is also possible that the sensor loop frequency - the L2 function of coil 92 could be separated from the frequency of the magnetic force function - the L1 function. Separate frequency generators could be used, with equivalent gain control applied to each as regards the respective signals in coil 92. It will also be understood that the integrating loop feedback of Fig. 7 can also be applied to the control loop 88 of Fig. 6.

The provision of a magnetically-generated bias force as by circuit 78 is not limited to the horizontally-oriented embodiment of Fig. 6. It could be added to the vertically-oriented embodiments to add to or subtract from the gravitational force (weight).

The support of the sensor member 90 in the horizontally-oriented embodiment of Fig. 7 requires to be as friction-free as possible. Fig. 8 shows schematically one proposal for this which also frees the sensor member of direct contact with the running thread.

Fig. 8 shows a horizontal sensor member 120 which is to be supported within coils L3, L1/L2 and L4 which are as previously described. A single coil performs the L1/L2 function in this case. The member 120 should be non- contacting with the interior of the coils or the formers on which they are wound. It is proposed to support the member 120 at a point 122 along its length by means of a transverse horizontal member 124 e.g. perpendicular to the sensor member. The support member 124 is carried on a low friction bearing arrangement 126, for example using ceramic bearings 127. The deflections of the member about the vertical pivot provided by bearing arrangement 126 will be very small, especially where a second-order loop control is used to maintain a fixed position of the sensor member. As part of a balancing of moments about the pivot in the vertical plane the support member 124 can be linearly extended beyond the pivot 126 as shown at 128. The end of part 128 can be provided with thread-engaging means acting to generate a force component - as indicated by the arrows T that in turn acts to deflect the part 128 and member 124 to communicate a force along the axis of sensor member 120 that is a function of thread tension. This force acts to move the sensor member linearly along its axis and is counteracted by a negative feedback loop as described above. The use of the pivoted level 124 enables the selection of the ratio of the force to be measured to that acting on member 122 in dependence on the lever ratio.

What has been described above is a force sensor device in which a sensor member is supported for movement along an axis. At least one portion of the sensor member is magnetic. It may be of ferromagnetic material and may itself be a permanent magnet. This portion is acted upon by a controlled magnetic field generated by an energised coil to generate a controlled force acting to move the member along the axis. The controlled force acts in opposition or counter to a second force acting on the member. The second force may be a constant bias force as provided by gravity (that is the weight of the member including any other parts carried therewith, particularly where the axis of movement is vertical) or by a magnetic field source such as another coil energised to provide a constant field acting on the aforementioned portion or another magnetic portion of the member.

A position-sensing system senses the position of the member along the axis and provides a position-dependent signal. This signal is used in a negative feedback loop which controls the energisation of the first-mentioned coil to provide a controlled magnetic field acting to maintain the member in a fixed or equilibrium position. An external force applied to the member disturbs or acts to disturb the equilibrium so that within the negative feedback loop a signal is developed which is a measure of the applied force. This signal may be obtained within a feedback path by which the coil energisation is controlled or within the energisation circuit for the coil.

The equilibrium position at which the negative feedback loop acts to maintain the sensor member may be proportional to the force (in a first order loop) so that deviation (error) of the position arise in dependence on the applied force. The negative feedback loop may include an integrator (in a second order loop) so that the loop acts to very closely maintain the member at a fixed position.

The position-sensing system described is itself magnetically-based. The member includes a magnetic portion coupling a pair of coils in a position- sensitive manner so that energisation of one coil with an A.C signal produces a position-dependent signal at the other coil. One of the pair of coils may also be the coil energised to provide the above-mentioned controlled magnetic field. The equilibrium in which the sensor member is supported, whatever the orientation of its axis of movement may be regarded as a balance between a controlled magnetic force and a bias force, such as gravity (weight) or/and a constant magnetic force that is modulated by an external force that is to be measured.

It is a feature of the embodiments of the invention that have been described that the counter-balancing or opposed forces that act on the sensor member are generated without the use of mechanical parts such as springs.

Furthermore, these force-generating means act on the sensor member in a contactless manner.

The force sensor device of the invention finds one particular application in textile machinery in which a thread runs lengthwise in a path which includes a V-shaped portion at which a thread engaging means coupled to the sensor member is located to receive a force component that is dependent on the tension in the thread.

Claims

1. A force sensor device comprising

a coil having a hollow interior and an energising circuit for controllably energising the coil to generate a controlled magnetic field in the hollow interior thereof,

a sensor member having at least a portion thereof of magnetic material, said member being supported for movement along an axis that extends into the hollow interior of the coil such that the member is subject to a first, magnetic, force on said portion dependent on said controlled magnetic field,

bias means for subjecting said member to a second force counter to said first force,

position-sensing means for providing a position signal dependent on the position of said member along said axis,

a feedback circuit connected between said position-sensing means and said energising circuit and acting to provide therewith a negative feedback loop for maintaining said member at a given position,

means associated with said member for applying an external force to be measured to said member, and

output means in said negative feedback loop providing an output signal dependent on the applied external force.

2. A force sensor device comprising a coil having a hollow interior and an energising circuit for controllably energising the coil to generate a controlled magnetic field in the hollow interior thereof,

means for subjecting said member to a second force counter to said first force,

said means for subjecting said member to said second force being arranged to be responsive to an external force to be measured which modulates said second force, and

output means in said negative feedback loop providing an output signal dependent on the external force.

3. A force sensor device as claimed in Claim 1 or 2 in which said feedback circuit includes an integrator such that the negative feedback loop acts to maintain said member at a fixed position.

4. A force sensor device as claimed in Claim 3 in which said output signal is obtained at a point in the feedback circuit following the integrator.

5. A force sensor device as claimed in Claim 1 , 2 or 3 in which the output signal is derived from a controlled parameter, preferably current, of the energising circuit.

6. A force sensor device as claimed in Claim 1 or any claim dependent thereon in which said bias means at least comprises a gravitational force.

7. A force sensor device as claimed in Claim 6 in which said bias means comprises the weight of at least said member.

8. A force sensor device as claimed in Claim 6 or 7 in which said bias means is constituted by weight.

g. A force sensor device as claimed in Claim 6, 7 or 8 in which said axis of movement is vertical.

10. A force sensor device as claimed in Claim 1 or any one of Claims 3-5 as dependent thereon, wherein said bias means comprises a further coil having a hollow interior into which said axis of movement extends, and an energising circuit for energising the further coil to generate a predetermined, preferably constant, magnetic field, a magnetic portion of said sensor member being influenced by said predetermined magnetic field to have said second force exerted on the member.

11. A force sensor device as claimed in Claim 10 or in Claim 2 wherein said axis of movement is horizontally-oriented.

12. A force sensor device as claimed in any preceding claim in which said position-sensing means comprises a pair of coils each having a hollow-interior into which said axis of movement extends, a magnetic portion of the sensor member acting to couple said pair of coils, the coupling between the pair of coils being dependent on the axial position of said member, and means for energising one of the coils with an A.C. signal, said position signal being obtained at the other of said coils.

13. A force sensor device as claimed in Claim 12 in which said pair of coils are disposed in line along said axis.

14. A force sensor device as claimed in Claim 12 or 13 in which one of said pair of coils is the coil energised by said energising circuit.

15. A force sensor device as claimed in Claim 1 or any claim dependent thereon, in which said means for applying an external force to be measured comprises means for engaging a running flexible body such that the external force is a component of the tension in the flexible body.

16. A force sensor device as claimed in Claim 2 or any claim dependent thereon, in which said means for subjecting said member to said second force comprises means providing a force component of fixed magnitude and means for providing a force component that is variable.

17. A force sensor device as claimed in Claim 16 in which said means for providing a variable force component is adapted to engage a running flexible body such that the component is a component of the tension in the flexible body.

18. In textile machinery, a force sensor device as claimed in Claim 15 or 17 in which the flexible body comprises a running thread moving along a path including a V-shaped portion at the apex of which said means that engages the thread is located to be subject to a force component dependent on tension in the thread.

19. A position-sensing device comprising a member supported for movement along an axis, said member comprising at least a magnetic portion,

a pair of coils each having a hollow interior located in-line along said axis, said magnetic portion being movable along said axis to couple said coils and terminating in a first of the coils to provide a position-sensitive coupling between the coils,

means for energising one coil of the pair with an A.C. signal to produce a signal at the other coil that is a function of the position of said member.

20. A position-sensing device as claimed in Claim 19 in which said magnetic portion extends through the second of the coils.

21. A position-sensing device as claimed in Claim 19 in which said magnetic portion terminates within the second of the coils so as to be subject to a force due to a magnetic field established by energisation of the second coil.