GB2204701A - Viscometer - Google Patents

Abstract

A viscometer 1 comprises a synchronous motor having an output connected to a bob through a torque transducer 6 and bearing 7. The transducer 6 includes a spring 12, two arcuate capacitive drive plates 15, 16, an annular capacitive pick-up plate 17 surrounded by ground plates and a screen plate 14 intermediate the drive plates 15, 16 and pick-up plate 17. The pick-up plate 17 is connected to pick-up electronic circuits 20 and the drive plates 15, 16 are connected to drive electronic circuits 21. Rotation of the screen plate 14 provides a signal to the pick-up plate 17 which provides a measure of viscosity. <IMAGE>

Description

A Viscometer.

The present invention relates to viscometers.

Viscometers available generally comprise a motor driving a bob immersed in a fluid whose viscosity is to be measured, drive being provided through a resilient spring element.

When the motor drives the bob, the rotational lag of the bob vis-a-vis the motor drive is measured as this lag is proportional to the viscosity of the fluid. Many transducers have been employed for measuring this rotational lag, however, none of the known transducers are completely satisfactory. For example, one such transducer includes one arm fixed to the drive shaft and another to the driven shaft and the time elapsing between the passage of these arms past optical detectors is measured. This time interval is proportional to both the speed of rotation and the relative angular displacement of the drive and driven shafts. The main problems with this transducer are that it outputs viscosity data once per revolution only, and that windage of the rotating arms can cause errors at high speeds.

Capacitive transducers are also available for measuring rotational lag to obtain viscosity data. Such capacitive transducers generally use either a conventional AC bridge or a Blumlein AC bridge. However, a conventional AC bridge is prone to stray capacitance effects and a Blumlein AC bridge requires the use of a specially wound transformer. Further, such AC bridges must be excited by true sinusoidal signals, usually at a relatively high frequency, which signals are difficult to produce within the required tolerances.

The motors used for conventional viscometers are generally DC servo motors or mains-energised synchronous motors with a mechanical gearbox. A D.C. servomotor has not proved to be satisfactory because a relatively long time usually elapses before the drive speed stabilises. On the other hand a mains-energised synchronous motor requires a constant and known mains supply frequency, is cumbersome, and may not easily be adapted to remote control.

The present invention is directed towards providing a viscometer to solve at least some of these problems with known viscometers.

According to the invention there is provided a viscometer of the type comprising a drive motor driving a bob through a resilient torque element and a torque transducer for measuring the angular displacement of the bob relative to the drive when the motor drives the bob, in which, the torque transducer comprises: at least one electrode means fixed to the motor drive; at least one electrode means fixed to the bob; means for supplying electrical signals to one electrode means; and means for detecting electrical signals in the other electrode means.

Ideally, the torque transducer comprises: at least one capacitive drive electrode means fixed to the motor drive; one pick-up electrode means fixed to the motor drive; an intermediate screen means fixed to the bob; means for transmitting electrical signals to the or each drive electrode means; means for detecting current flow in the pick-up electrode means; and processing means for outputting viscosity data in response to the detected current flow in the pick-up electrode means.

In a preferred embodiment of the invention each electrode means comprises a capacitive plate.

Preferably, there are two capacitive drive plates.

Preferably each of the drive plates is of arc sector, shape in plan view.

Preferably the screen means comprises a screen plate interposed between the drive and pick-up electrode means.

Ideally, the screen plate is a semi-circular earthed metal plate.

In one embodiment of the invention the motor is a synchronous motor having electronic commutation circuits for exciting the motor with electrical signals, the frequency of which vary as a function of the desired motor speed.

Preferably one electrical signal is transmitted to each drive plate, and each signal is a voltage ramp signal of substantially constant slope magnitude, the two signals being substantially 1800 out of phase.

In this latter embodiment, the means for detecting current flow in the pick-up plate detects the amplitude of the current flow and converts it's value to digital form.

In another embodiment of the invention the processing means is a microprocessor having inputs indicating of the bob-type, pick-up plate current amplitude signals, and motor speed signals.

The invention will be more clearly understood from the following description of an embodiment thereof given by way of example only with reference to the accompanying drawings in which: Fig. 1 is a diagrammatic front view of a viscometer, according to the invention, in use; Fig. 2 is a diagrammatic front view of portion of the viscometer of Fig 1; Figs. 3 to 5 are plan views of portions of the viscometer; Fig. 6 is a partial block and circuit diagram of portion of the viscometer; Figs. 7(a) to 7(d) are graphs representing electrical signals for the circuit of Fig. 6; and Fig. 8 is a block diagram of the operation of another portion of the viscometer.

Referring to the drawings and initially to Fig. 1 there is illustrated a viscometer, indicated generally by the reference numeral 1, for measuring the viscosity of a fluid 2, in a vessel 3. The viscometer 1 comprises a synchronous motor 4 having an output connected to a bob 5, through a torque transducer 6 and a jewel bearing 7. The torque transducer 6 is also connected to processor electrical circuits 8, which are in turn connected to digital and/or analog output devices, indicated generally by the reference numeral 9.

Referring now to Figs. 2 to 5 the torque transducer 6 and the jewel bearing 7 are illustrated in more detail. The synchronous motor 4 is connected by a motor shaft 10, to a transducer housing 11 which is in turn, connected to a resilient torque element, namely, a spiral spring 12 adjacent its lower end. The spiral spring 12 is in turn connected adjacent its upper end to a driven shaft 13 which is connected to the bob 5 via the jewel bearing 7.

The jewel bearing 7 is of known construction and requires no further explanation.

Mounted within, and fixed to the transducer housing 11 there are two capacitive drive plates 15 and 16 respectively, each of which is of arc sector shape, in plan as will be apparent from Fig. 3. Also mounted within and fixed to the transducer housing 11 are an annular capacitive pick-up plate 17, surrounded both internally and externally by annular ground plates 18 and 19 respectively, each of which is connected to earth. The pick-up plate 17 is connected to pick-up electronic circuits 20, and the drive plates 15 and 16 are connected to drive electronic circuits 21. A screen plate 14 forming a semi-circle in plan, is fixed to the drive shaft 13, and is positioned to be substantially parallel to and intermediate the drive plates 15, 16 and the pick-up plate 17.Slip-rings are provided for transmitting supply electrical signals to and from the drive electronic circuits 21 and the pick-up electronic circuits 20.

The operation of the torque transducer 6 will be more clearly understood from the following description of the operation of the drive and pick-up electronic circuits 21 and 20 respectively.

Referring to Figs. 6 and 7 of the operation of the torque transducer 6 is diagrammatically illustrated in more detail, and parts similar to those described with reference to the previous drawings are identified by the same reference numerals. The drive electronic circuits 21 comprise an oscillator 30 which in this case has a frequency of approximately 10KHz connected to a divider circuit 31 for dividing by two, and thence to two coupled op-amps 32 and 33. The op-amp 32 is connected to the drive plate 15 to supply a voltage ramp wave-form similar to that illustrated in Fig. 7(a) and the op-amp 33 is connected to the drive plate 16 to supply a voltage ramp wave-form similar to that illustrated in Fig. 7(b). The screen plate 14 is connected to earth.The pick-up plate 17 is connected to the virtual earth input of an inverting op-amp 34, the output of which is connected to an amplifier op-amp 35. The output of the amplifier op-amp 35 is in turn connected to a Phase Sensitive Detector (PSD) 36, which is also connected at it's drive input to the divider circuit 31. The output of the PSD 36 is connected to an op-amp 37, which is connected to the output line.

In operation, the synchronous motor 4 drives the bob 5 to rotate in the fluid 2. The viscous drag of the fluid 2 generates a torque acting on the bob 5, which torque is proportional to the fluid viscosity. The action of this torque on the spiral spring 12 causes the rotating driven shaft 13 to lag behind the rotation of the transducer housing 11. As a result, the screen rotates-by an angle determined by this lag in the plane of the drive plates 15, 16 and the pick-up plate 17. Meanwhile, the drive plate 15 receives a voltage ramp signal similar to that in Fig. 7(a) and the drive plate 16 receives a voltage ramp signal similar to that shown in Fig. 7(b). These ramp signals have a phase difference of 1800 and are capacitively coupled to the pick-up plate 17. However, the screen plate 14 varies the capacitance between each of the drive plates 15, 16 and the pick-up plate 17.When the screen plate 14 is so orientated that capacitances between each drive plate 15 and 16 and the pick-up plate 17 are equal, then no net signal is coupled to the pick-up plate. Rotation of the screen plate 14 from this position causes a net signal to be coupled to the pick-up plate 17, and the magnitude of this current signal is directly proportional to the angle of rotation from the zero signal position, and the phase relation of the net signal to one of the drive plate signals is related to the direction of rotation.

It will be appreciated that because the pick-up plate 17 is surrounded by the annular ground plates 18 and 19 no currents, other than the capacitively-coupled displacement currents from the drive plates 15 and 16, flow to the pick-up plate 17. The ground plates 18 and 19 also improve the field distribution between the drive plates 15 and 16 and the pick-up plate 17.

The net current flowing into the pick-up plate 17 is as given by i = C1 dvl - C2 dv2 dt dt where:- C1 is the capacitance between the drive plate 15 and the pick-up plate 17; C2 is the capacitance between the drive plate 16 and the pick-up plate 17; V1 is the input ramp voltage to the drive plate 15; and V2 is the input ramp voltage to the drive plate 16.

If the angular rotation of the screen plate 14 from the position at which the pick-up plate current i = O is denoted by e, and the change in C1 or C2 is cF per degree, then the current i may be expressed as: i = cG dvl + dv2 dt dt Because the slopes dv1/dt, dnd, dv2/dt are constant in magnitude, but change sign once per cycle, the current will be a square wave, and therefore the voltage output of the inverting op-amp 34 shown in Fig. 7(c) will be a square wave with amplitude proportional to 6. It will therefore be appreciated that the signals V1 and V2 from the op-amps 32 and 33 need only have constant slope and variations in frequency or magnitude will not effect the output of the torque transducer 6.

The square-wave voltage signal outputted from the inverting op-amp 34 is amplified and synchronously rectified and filtered by the amplifier op-amp 35, the PSD 36 and the op-amp 37 to give a DC voltage signal output proportional to 6.

It will be appreciated that the torque transducer 6 has very little stray capacitance, does not require a transformer and uses voltage ramp signals which are relatively easy to generate to the desired tolerance. It will further be appreciated that if the speed of rotation of the bob 5 is selected, so that the screen plate 14 does not rotate at more than +450 with respect to the transducer drive plates 15 and 16 and the pick-up plate 17, then field-fringing effects will be avoided and the transducer output will be linear.

Referring now to Fig. 8, the inter-connection of the processor circuits 8, the displays 9 and the synchronous motor 4 are illustrated in block diagram form and parts similar to those described with reference to the previous drawings are identified by the same reference numerals.

The processor circuits 8 comprise an Analog to Digital Converter (ADC) 40 connected to the transducer output.

The ADC 40 is in turn connected to an interface circuit 41 for a microprocessor 42, two Digital to Analog Converters (DAC)'s and amplifiers 43 and 44 and a display 45. The DAC's and amplifiers 43 and 44 are connected at their outputs to the synchronous motor 4. The transducer output is also connected through an amplifier 46 to an analog output device, not shown. The interface circuit 41 is also connected to receive the following inputs: User speed selection; Bob selection indication; User display selection; Motor on/off control; and Zero calibration mode.

Display Hold In use, the ADC 40 converts the transducer output signals to digital form and then transmits them through the interface circuit 41, to the microprocessor 42. The microprocessor 42, is programmed in ROM to convert the transducer output signals to viscosity data signals, which signals are transmitted back through the interface circuitry 41 to the display 45. The interface circuitry 41 uses user speed-select switch setting inputs to output two motor drive signals to the DAC and amplifiers 43 and 44, which in turn, excite the synchronous motor 4 with sinusoidal signals of a similar frequency, but 900 out of phase.

To operate the viscometer 1, the transducer 6 is rotated in air and the microprocessor 42 stores the corresponding viscosity data as zero offset viscosity data. The bob 5 is then placed in the fluid whose viscosity is to be measured, the user selects a motor speed using a switch on the processor circuits 8, and a bob to be used is selected with a similar switch. The microprocessor uses these two switch inputs and the transducer output to produce viscosity data signals from which it continuously subtracts the zero offset viscosity data. The resulting net viscosity data signals are continuously transmitted to the display 45, which may display the data in many different forms, such as centipoise or kilopoise.

It will be appreciated that because a synchronous motor with electronic commutation circuits is used, the desired speed may be set very quickly and the motor speed is very accurate in comparison with that of present viscometers.

The invention is not limited to the embodiment hereinbefore described but may be varied in construction and detail.

Claims (12)

1. A viscometer of the type comprising a drive motor driving a bob through a resilient torque element and a torque transducer for measuring the angular displacement of the bob relative to the drive when the motor drives the bob, in which, the torque transducer comprises: at least one electrode means fixed to the motor drive; at least one electrode means fixed to the bob; 10 means for supplying electrical signals to one electrode means; and means for detecting electrical signals in the other electrode means.

2. A viscometer as claimed in claim 1 wherein the torque transducer comprises: at least one drive electrode means fixed to the motor drive; one pick-up electrode means fixed to the motor drive; an intermediate screen means fixed to the bob; means for transmitting electrical signals to the or each drive electrode means; means for detecting current flow in the pick-up electrode means; and processing means for outputting viscosity data in response to detected current flow in the pick-up electrode means.

3. A viscometer as claimed in claim 1 or 2 wherein each electrode means comprises a capacitive plate.

4. A viscometer as claimed in claim 3 wherein the viscometer includes two capacitive drive plates.

5. A viscometer as claimed in claim 4 wherein each of the drive plates is of arc sector, shape in plan view.

6. A viscometer as claimed in any of claims 2 to 5 wherein the screen means comprises a screen plate interposed between the drive and pick-up electrode means.

7. A viscometer as claimed in claim 6 wherein the screen plate is a semi-circular earthed metal plate.

8. A viscometer as claimed in any of claims 1 to 7 wherein the motor is a synchronous motor having electronic commutation circuits for exciting the motor with electrical signals, the frequency of which vary as a function of the desired motor speed.

9. A viscometer as claimed in claim 8 wherein a single electrical signal is transmitted to each drive plate, and each signal is a voltage ram signal of substantially constant slope magnitude, the two signals being substantially 1800 out of phase.

10. A viscometer as claimed in claim 9 wherein the means for detecting current flow in the pick-up plate detects the amplitude of the current flow and converts the detected signal to a digital form.

11. A viscometer as claimed in any preceding claim wherein the processing means is a microprocessor inputs indicating the bob-type, pick-up plate current amplitude signals, and motor speed signals.

12. A viscometer substantially as hereinbefore described with reference to the accompanying drawings.