GB2289947A

GB2289947A - Position and force measuring apparatus

Info

Publication number: GB2289947A
Application number: GB9411105A
Authority: GB
Inventors: Nigel Roland Evans
Original assignee: Bohlin Instruments Ltd
Current assignee: Bohlin Instruments Ltd
Priority date: 1994-06-03
Filing date: 1994-06-03
Publication date: 1995-12-06
Anticipated expiration: 2014-06-03
Also published as: GB2289947B; GB9411105D0

Abstract

Apparatus, (e.g. a rheometer) including means for determining the rotational and axial positions and/or axial forces on a shaft 42, comprises a gas bearing 39 for supporting the shaft, the gas bearing having a housing 41 in which a disc 43 carried by the shaft is supported, in use, by gas pressure for rotational movement about the axis 40 of the shaft and linear movement parallel to that axis, means 1, 2, 3 for generating an electrical current within a circuit 4, 5, 6, 37 mounted on the shaft or disc, detector means 7, 8, 9, 36 mounted to the housing and inductively coupled to the circuit, so that current flowing in the circuit produces first and second electrical signals therein which vary according to the axial and angular displacements respectively of the shaft, and measuring means 13, 14, 15 for measuring said first and second signals and deriving therefrom an indication of the axial and rotational movement and/or axial force on the shaft. <IMAGE>

Description

MEASURING APPARATUS This invention relates to apparatus for measuring the rotational and axial movement of a shaft and thereby the axial force on the shaft. The invention relates to such a shaft supported in a gas bearing and is particularly concerned with a rheometer where the shaft supports a probe and may be part of a drag cup motor located in the rheometer. Rheometers are well known instruments for measuring the rheological properties of samples, normally placed between the facing surfaces of upper and lower probes or plates, one of which is driven relative to the other. Conventionally the rheometer includes at least two separate measuring means for measuring axial and rotational positions such as position gauges and separate measuring means for measuring forces such as strain gauges for measuring the movement strains induced between the probes in accordance with the properties of the sample.

Conventionally the strain and position gauges are located at different positions in the rheometer and not associated with the air bearing.

An object of the present invention is to allow the simultaneous determination of the angular deflection about the axis of rotation of the spIndle/shaft of a gas bearing, and the linear deflection along the axis of rotation. By discovering the mechanical characteristics of a gas bearing through measurement, and by subsequently measuring the instantaneous axial deflection of the bearing spindle in response to an externally applied unknown axial force, the instantaneous axial force acting on the bearing spindle can be deduced.

Another object of the invention, in its preferred form, is to use such measurements in a rheometer to achieve the desired rheological property measurements.

Accordingly, in one aspect the invention provides apparatus for determining the rotational and axial positions and/or axial force on a shaft, comprising a gas bearing supporting the shaft, the gas bearing having a housing in which a disc carried by the shaft is supported, in use, by gas pressure for rotational movement about the axis of the shaft and linear movement parallel to that axis, means for generating an electrical current within a circuit mounted on the shaft/disc within the bearing, and detector means mounted within the housing and inductively coupled to the circuit, so that current flowing in the circuit produces first and second electrical signals therein which vary according to the axial and angular displacements respectively of the shaft1 and measuring means for measuring said first and second signals and deriving therefrom an indication of the axial and rotational movement and/or axial force on the shaft.

The means for deriving the axial force may include a microprocessor which has been calibrated by using the apparatus with known forces. The rotational force is known or measured by other means.

The apparatus is preferably part of a rheometer.

Preferably the generating means is mounted on the housing and inductively coupled to said circuit, which is a closed circuit including a plurality, preferably three, coils. The generating means will preferably generate an alternating current, which is with advantage a sine wave, and includes a coil in close proximity to a first of the coils in the circuit. The second of the coils in the circuit is in close proximity with two coils (forming part of the detectIon means) arranged in such a way that the currents induced in the two coils are 90 degrees out of phase with each other, such that when the magnitude of current flowing in one of the two coils is at maximum, the magnitude of the current flowing in the other coil is a minimum. A third of the coils in the circuit is in close proximity with a further coil (forming part of the detection means) in such a way that the current induced in the further coil will give an indIcation of axial displacement of the shaft within the housing.

One embodiment of measuring apparatus, in accordance with the invention, will now be described, by way of example only with reference to the accompanying drawings of which: Figure 1 is a diagrammatic view of one form of a gas bearing incorporating a shaft and measuring apparatus in accordance with the invention, Figure 2 is a similar view of an alternative form of gas bearing incorporating a shaft, Figure 3 is a schematic circuit diagram of the measuring system, Figure 4 is a schematic diagram of the demodulator unit of Figure 3, and Figure 5 illustrates one relationship between axial position and axial force.

Referring first to Figures 1 and 2, a shaft 42 is supported for rotation about its axis 4C in a gas bearing IndIcated generally at 39. The shaft 42 carries a coaxially arranged disc 43 extending in a plane perpendicular to the axis 40. The disc is positioned between fixed surfaces 38 protruding from a fixed housing 41 around the spindle, forming a sandwich construction.

The bearing has one or more gas inlet ports 44 and one or more gas exhaust ports 45.

When a positive gas pressure difference is applied between the inlet and exhaust ports1 gas flows through the bearing and is forced around the disc fixed to the bearing shaft. The flow of gas through the bearing produces forces which act on the surfaces of the bearing. These forces cause the shaft to move to a position where the forces are brought into equilibrium with the other forces acting on the bearing1 such as the force of gravity acting on the mass of the bearing shaft.

The gas bearing may be constructed so that it operates with no mechanical contact between the rotating shaft and the fixed housing since they are separated by a cushion of flowing gas. This enables the bearing to be constructed with very low friction. Further, if the mass of the shaft is kept small, the bearing may also have a low inertia.

Such a bearing, In this invention, is incorporated in a known form of rheometer to be used as the bearing or bearings for a drag cup motor. The shaft 42 will carry one of the probes of the rheometer. The gas bearing of Figure 2 is similar to but simpler than that of Figure 1; like parts have been given the same reference numerals but with the suffix a and will not be redescribed.

Referring now to Figure 3, the measuring system comprises broadly four parts: a transmitter formed by a sine wave oscillator generator 1, an amplifier 2, and a coil 3; a closed circuit formed by coils 4, 5 and 6 connected in series; a set of receivers formed by coils 7, 8 and 9 with respectively amplifiers 10, 11 and 12; and a set of decoders formed by a resolver to digital converter 13, a demodulator 14 and a displacement to force calculation means 15. The set of coils 4, 5 and 6 are fixed to the bearing shaft 42 or disc 43 and rotate with it. The remainder of the parts are stationary relative to the bearing housing. The device is constructed so as to achieve a high mutual inductance between coils 3 and 4, coils 6 and 9, and coils 5, 7 and 8, while producing a low mutual inductance between these different sets of coils.

The generator 1 produces a reference sine wave (typIcally at lOkHz) which is also supplied to the resolver to digital converter 13 as a reference. The sine wave is amplified by a power amplifier 2 which drives current into the transmitter coil 3, The transmitter coil is coupled by mutual inductance to the coil 4 mounted on the bearing shaft. This coupling induces a current in the coil 4 which in turn drives current through the pair of series coils 5 and 6. One of these coils 5 is used to derive the rotational position of the shaft, while the other coil 6 is used to derive the axial position of the shaft. The coil pair 5 and 6 may be constructed from wire tracks printed on to a printed circuit board fixed to the bearing shaft in a plane perpendicular to the axis of rotation of the shaft.

Such a circuit board is indicated diagrammatically in Figures 1 and 2 at 37 and 37a. The coils 7, 8 and 9 may be constructed from wire tracks printed onto a printed circuit board fixed to a part of the bearing housing whose surface is parallel to and in close proximity with coils 5 and 6.

Such a circuit board is indicated diagrammatically in Figures 1 and 2 at 36 and 36a. In practice these two sets of coils must be positioned within a few hundred pm of each other in order to achieve a high enough mutual inductance.

Coil 5 is therefore coupled by mutual inductance to coils 7 and 8. The current flowing in coil 5 induces current to flow in coils 7 and 8. The device is arranged in such a way that the currents induced in coils 7 and 8 are 90 degrees out of phase with each other. When the magnitude of current flowing in coil 7 is a maximum, the magnitude of the current flowing in the other coil 8 will be a minimum. The currents flowing in coils 7 and 8 produces voltages across them which are amplified by amplifiers 10 and 11 respectively. The outputs of these amplifiers are 9G degrees out of phase with each other and may be referred to as the sine, on line lOa, and the cosine, on line 11a, of the current flowing in coil 5, whose phase is fixed relative to the reference signal received from the oscillator 1, all the signals being passed to the converter 13.

Some adjustments may be made electronIcally to the phases of the outputs of the amplifiers 10 and 11 to ensure that they are a good representation of the sine and co-sine of the phase of the signal appearing at the reference input to the resolver to digital converter 13. This convertor 13 is of a known form already used in our instruments and based on a commercially available Integrated circuit manufactured by Analog Devices Inc. The output of the resolver to digital converter 13 is a signal on line 1 3a giving the rotational position of the bearing shaft.

The device is constructed so that the coil 6 is coupled to the coil 9 and the current flowing in coil 6 induces a current in coil 9. The position of coil 6 is fixed relative to the bearing shaft5 while coil 9 is fixed relative to the bearing housing. However, the device is constructed so that both coil 6 and coil 9 are rotationally symmetrical about the axis of rotation of the bearing so that, as far as is practical, the measurements will be independent of the rotational displacement of the shaft.

Any movement of the shaft along the axfs of the bearing modulates the mutual inductance between the coils 6 and 9 which in turn modulates the amplitude of the current flowing through the coil 9. The current flowing through coil 9 induces a voltage across coil 9 which is amplified by the amplifier 12 and supplied to the demodulator 14.

The demodulator 14 detects changes in the amplitude envelope of the signal and derives a signal which is an indication of the axial position, this signal being supplied on line 14a and a line 14b to a displacement to force calculating means 15. The parts 13, 14 and 15 all may all form part of a microprocessor.

An example demodulator circuit is shown in Figure 4.

An RMS to CC convertor 21 is used to detect the amplitude envelope of the output signal of the amplifier 12. An offset voltage 22 is then subtracted from the output of the RMS to =C convertor 21. The amplitude of the offset voltage 22 should be equal to the amplitude of the envelope when the bearing shaft is at its equilibrium position. The output of the amplifier 23 will then be zero when the bearing is in equilibrium. The output of amplifier 23 will tend to be positive when axial forces act on the bearing shaft which tend to push the coils 9 and 12 closer together, and negative when axial forces act on the bearing shaft which tend to pull the coils 9 and 12 further apart.

In practice, when measurIng small displacements of the order of few m, the gain required to detect small axial displacements is very large and this offset voltage must be set very accurately in order not to saturate the amplifier 23. This may be achieved using a closed loop feedback circuit under the control of the micro-computer.

The output from the demodulator 14 is a signal which depends on the axial displacement of the bearing shaft. By measuring the deflection of a given gas bearing In response to a range of known applied forces, the gas bearing may be calibrated for the measurement of unknown forces. The output signals from parts 14 and 15 may not be non linear with respect to axial displacement and force however, once calibrated a measure deflection in response to an unknown externally applied force may be used to determine the magnitude of the external force.

Thus the deflection of the spindle relative to the housing is some function, S(), of the applied force, as per equation (El) Da = S(Fa) ....(E1) Where Fa is the force applied along the axis of rotation of the spindle, and Da is the resultant displacement along the axis.

Calibration allows the determination of the function S() which gives the stiffness of the bearing at a given deflection. From S(), the inverse function Sol() may be calculated. By knowing S-l() and measuring the displacement, Das the force acting on the spindle, Fas may be derived from equation (E2).

Fa = S(Da) ....(E2) One example of relationship between position and force is shown in Figure 5. The axial displacement signal and the output of the demodulator 14 is squared in unit 31 to produce a signal representing the axial force acting on the bearing shaft.

In practice, the output signals from units 14 and 15 may be digitized, and the micro-computer may be used to implement any arbitrary calibration. The system may be calibrated by initially applying a series of known axial forces and observing the output signals measuring the deflection and then calculating the equations (eel) and tE2) to be used for further calibrations.

Claims

1. Apparatus for determining the rotational and axial positions and/or axial forces on a shaft, comprising a gas bearing for supporting the shaft, the gas bearing having a housing in which a disc carried by the shaft is supported, in use, by gas pressure for rotational movement about the axis of the shaft and linear movement parallel to that axis, means for generating an electrical current within a circuit mounted on the shaft/disc, detector means mounted to the housing and inductively coupled to the circuit, so that current flowing in the circuit produces first and second electrical signals therein which vary according to the axial and angular displacements respectively of the shaft, and measuring means for measuring said first and second signals and deriving therefrom an indication of the axial and rotational movement and/or axial force on the shaft.

2. Apparatus according to claim 1 in which the circuit mounted on the shaft/disc comprises a printed circuit board extending in a plane perpendicular to the axis of the shaft.

3. Apparatus according to claim 2 in which the detector means comprises a printed circuit board fixed to the bearing housing in a plane perpendicular to the axis of the shaft and closely adjacent to the printed circuit board carried by the shaft.

4. Apparatus according to any of claims 1 to 3 including means for deriving the axial force on the shaft, which means includes a microprocessor which has been calibrated by using the apparatus with known forces.

5. Apparatus according to any of claims 1 to 4 in which the generating means is mounted on the housing and inductively coupled to the circuit carried by the shaft.

6. Apparatus according to any of claims 1 to 5 in which the generating means is arranged to generate an alternating current, preferably a sinewave.

7. Apparatus according to any of claims 1 to 6 in which the circuit carried by the shaft is a closed circuit including a plurality of coils arranged in series.

8. Apparatus according to claim 7 in which the closed circuit includes three coils.

9. Apparatus according to claim 7 or claim 8 in which a first of the coils in the closed circuit is arranged in close proximity to a coil forming part of the generating means.

10. Apparatus according to any of claims 7 to 9 in which a second of the coils in the closed circuit is located in close proximity to two coils, forming part of the detection means, said two coils being arranged in such a way that the currents induced in the two coils are substantially 90" out of phase with each other.

11. Apparatus according to any of claims 7 to 10 in which a third of the coils in the closed circuit is in close proximity with a further coil, forming part of the detection means, in such a way that the current induced in the further coil will give an indication of axial displacement of the shaft within the housing.

12. Apparatus according to claim 10 in which signals from the two coils forming part of the detection means are supplied to a resolver to digital converter to give an indication of the rotational position of the shaft.

13. Apparatus for determining the rotational and axial positions and/or axial force on a shaft substantially as described herein with reference to or as illustrated in the accompanying drawings.

14. A rheometer incorporating apparatus according to any of claims 1 to 13.