GB2192463A - Non-destructive rheological testing - Google Patents

Abstract

A closed carton 30 (Figure 1) is secured to a turntable 10 driven by an electric motor 12 from an initial state of rest with an appropriate drive torque. The change of angular velocity of the turntable occurring in a determined time interval is measured so that the rheology of the carton's contents can be inferred. Yoghurt, semi-yoghurt and milk may be distinguished by the different acceleration curves produced. The angular velocity of the motor measured by tachogenerator 14 is temporarily recorded in transient recorder 18 and reproduced in chart form by recorder 22. The apparatus is calibrated in terms of known moments of inertia by replacing the carton by aluminium rings of identical diameter but varying heights. In a second method, the cartons are accelerated in one direction and then decelerated quickly to rest whereupon the motor is de-energised, the tachogenerator then measuring the angular velocity from the time the turntable recommences motion due to the still spinning liquid contents of the container. <IMAGE>

Description

SPECIFICATION Non-destructive testing The present invention relates to the non-destructive testing ofthe fluid or semi-fluid contents of containers, especially, but not exclusively, closed containers.

All previous attempts to observe non-destructively the rheology of the fluid or semi-flu id contents of a closed container have used an oscillating, torsional pendulum approach. In these methods, the container and its contents may be saidto form part of the 'bob' ofatorsional pendulum and measurements ofthe dampedtorsional sinusoidal oscillations ofthe pendulum bob are used to infer the viscoelastic properties ofthe container's contents.

Depending on the method used, the torsional oscillations may comprise a damped transient responseto an initial pertubation, e.g. as described by Moisio, T. and Kreula M. (1973) (Milchwissenschaft28 477-478)1973, Hsu, E. H.Y.

Whitney, L.F. and Peleg, M (1980) in J. Fd. Sci. 45, 204-208 and by Whitney, L.F and Prasad, A (1973) in Trans ASAE 1194-1199, or they may comprise sustained steady state oscillations driven continuouslyfrom outside, e.g. as suggested by Smith,N.D.P.andWright,T.A. (1981) in British PatentSpecification 1591892.

According to the present invention, however, a method of non-destructively testing the fluid or semi-fluid contents of a container comprises the steps of applying a torque to the container and monitoring the change of angularvelocity of the container occurring in a determined time interval.

The effectiveness of the invention relies upon the fact that the mechanical coupling between the rotating container and its contents depends upon the viscosity and/orsolidity of the contents. The greater these are, the better is the coupling between the two and the greater the effective moment of inertia ofthe contents. The present invention provides a method of determining the extent of the container/contents coupling by investigating the apparent moment of inertia of the container and its contents.

Conveniently, in the method of the present invention, the container is rotated from rest by applying a well-defined drive e.g. substantially constant torque, to the container. Conveniently, in this case, the change of angularvelocity is monitored to give an indication ofthe acceleration of the container e.g. by measuring the angular velocity at different times. As will be explained hereinafter, for rapid, accurate estimates, continuous measurements of angular velocity and motor current during the acceleration may be used to derive a single value which, is proportional to the moment of inertia of the container and its contents.

Alternatively, the container may be rotated from rest by the spinning contents of the container in which casethevelocity is monitored to determine the overall time taken by the container to accelerate from rest to a predetermined greatervalue orto come back to rest.

The invention also includes an apparatus comprising a support for a load, drive means for rotating the support and its load, and monitoring meansfor monitoring the changeofangularvelocity of the support and its load occurring in a determined time interval e.g. by measuring eitherthe acceleration imparted to the support and its load or the period required for the support and its load to acquire a preselected speed, which may be zero.

Conveniently, the apparatus includes switching means operative to reverse the drive to bring the support to rest and thereafterto remove the drive to allow the supportto be driven by the inertial effect of its load.

Conveniently, the apparatus includes first integrating means for integrating a firstsignal which is proportional to the motor current, second integrating means for integrating a second signal which is proportional to the angular velocity of the motor armature, and a divider operative to provide a value representative ofthe value ofthe first signal divided by the value of the second signal.

Embodiments of the invention will now be described, by way of example only, with referenceto the accompanying drawings in which Figure lisa diagrammatic illustration of an apparatus according to the present invention; Figures2 and 3 are schematic representations of the results achieved using the apparatus of Figurel in two different modes of operation; Figure4shows results illustrating the apparent inertia of the container and its contents; and Figure5shows a second apparatus in accordance with the present invention.

Thus referring first to Figure 1 ofthe drawings, an apparatus according to the present invention comprises a circular perspexturntable 10(101 mm diameter, mass 38 g) driven by a precision DC motor 12 (Maxon Ltd., winding resistance 8 Ohms) to give a well-defined drive torque.

The motor 12 which is driven under switch or relay control from a regulated DC powersupply 13 isalso equipped with a tachogenerator 14(500 mv = 1000 r.p.m.) mounted on the same axis, the inertia of motor rotor (and its associated tachogenerator) being only 30 g cm2. The torque constant ofthe motor is 63 g cm/A.

If the actual position of the turntable needs to be known, and perhaps adjusted,then the tachogenerator 14may be accompanied by an optical encoder (not shown) of the sector disc variety (e.g. producing 112 pulses per revolution) with one unique "home" pulse per revolution produced on another separate optical channel to indicate accumulated revolutions and the absolute attitude of the armature.

The output of the tachogenerator 14 is fed to a - transient recorder 18. The transient recorder 18feeds information to a chart recorder 22.

The apparatus is completed by a switch 26 operated by a pulse generator 28 to drive the motor with a steady rotation (switch 26 set as in Figure 1 ),to decelerate the motor (switch 26 set at opposite extreme to that shown), orto allowthe motor and turntable etc. to freewheel (switch 26 set at mid-position).

The switch 26 can conveniently be within a relay or an arrangement of solid-state switches.

The container whose contents are to be tested in this case comprises a closed yoghourt carton 30 suitablysupported on and/orsecured tothe turntable 10! e.g. by tape, so as to have an axis of symmetry along the rotation axis oftheturntable.

It isto be understood that lowviscosity contents will result in low coupling between the contents and the container, thereby leading to a low value for the apparent inertia ofthe system and a correspondingly high acceleration rate for a given drive to the turntable.

As the viscosity of the contents increases, so too does the coupling between the contents and the carton. This results in the apparent inertia ofthe system also increasing with a corresponding drop in theturntable's acceleration rate.

In both the above cases, the effective inertia ofthe system will inevitably be less than that anticipated for a rigid but otherwise identical body, such as will be the case,for example, when the contents are frozen.

~Becausethe acceleration rate is ultimately determined by the viscosity ofthe carton's contents, a measurnmentofthe acceleration can be used to distinguish substances of different viscosity within the carton.

Thus, in accordance with a first approach proposed underthe present invention, the switch 26 is moved from its mid-position to that shown in Figure 1 so as to accelerate the carton 30 from rest and apply a weli-defined or constant drive torque to the turntable 10. The angularvelocity ofthe motor, as measured bythetachgenerator 14; is temporarily recorded in the transient recorder 18 and reproduced in chartform bythe chart recorder22 so as to give an indication oftheacceleration oftheturntable and its load.A simplified version of the plots that will be produced by recorder 22 is reproduced in Figure2 where curve (a) represents the plot obtained for UHT milk in carton 30; curve (b) represents the plot obtained for semi-yoghourt; and curve (c) represents the plot obtained for a true yoghourt. The temperature in each case was 20"C and the carton was a 500 ml "Tetra Brik" with a total weight (i.e.

carton plus contents) of 530 g.

Curve (d) represents, for reference purposes only, the plotthatwould be obtained if the carton 30 were a rigid body of the same mass as the container and its contents investigated as in the other three plots.

It is readily apparent that these plots confirm that the three different contents produce markedly different rates of acceleration. When the contents is very fluid (a), the early effective moment of inertia of the system is small and acceleration rapid. The acceleration reduces as angularvelocity is gradually coupled inwardstowardsthe axis of rotation. When the contents is viscous or semi-solid (b) (c), the early effective moment of inertia of the system is higher due to better coupling ofangularvelocitywithin tie contents and there is little decrease in acceleration rate with time.When the contents are solid (d), then, as expected, the acceleration is slower still,the inertia of the system is time-independent and rotation rate increases linearly with time.

It is evident that the apparent moment of inertia of yoghourt is somewhat less than that characteristic of a package containing frozen (and hence rigid) UHT milk. Semi-yoghourt, and even more markedly UHT milk itself, exhibit lower moments of inertia at any particulartime and the values gradually ascend (as the rotation is gradually imposed upon the contents) towards a lower steady state. A rigidly frozen contents has a higher moment of inertia than any of these three.

Similar experiments with the carton replaced by aluminium ringsofvarying heightbutidentical diameter enable the system to be calibrated in terms of known moments of inertia and this calibration may be used to construct plots of apparent moment of inertia against acceleration time for frozen milk, yoghourt, semi-yoghourt and UHT milk in like experiments.

Records of acceleration are from the stationary state (+ 1 0V applied). Provided the rotation rate is kept sufficiently smell (as will be hereinafter described), the back e.m.f. ofthe motor does not develop sufficiently to seriously affect the motor current during this brief period of acceleration and so the motorcurrent is similar and remains practicallyconstantthrough the imposed acceleration in all these records.

In operation ofthe apparatus as proposed by a second method in accordance with the present invention, the carton 30 and its contents are spun as in the first method but then the switch 26 is switched to the opposite extreme to that illustrated so as to deceleratethe-turntable and its load quickly to rest.

On the package coming to rest, the switch 26 is set to its mid-position so as to apply no external driveto the turntable 10. However, this latter immediately begins two rotate again, due to the action ofthe still-spinning contents of the carton 30. The tachogenerator 14 is used, as before, to plot the motion of the motor armature from the instant at which it starts to rotate again.

The morefluid is the contents of the carton,the greater is the residual rotation rate and the greater the re-acceleration, permitting yoghourt, semi-yoghourt and milk to be readily distinguished, one from the other, asbefore.

The corresponding plotsthatwill be produced by the chart recorder 22 are illustrated, in simplified- form, in Figure 3 where curves (a), (b), (c), (d) have been identified as before andthetime course of rotation speed is that produced by rapid deceleration (-1 0V) from a steady rotation rate (+2.7V) to zero.

Subsequent recovery of rotation is greaterwith a UHT milk contents (a) than with semi-yoghourt (b) and no recovery is seen with a yoghourt contents (c).

These tests were carried out at a contents temperatu re of 20"C, the carton being a 500 mlTetra Brikwith atotal weightof530g.

As already suggested above, Figure 3 indicates that after rapid deceleration from a steady rotation speed, the extent of recovery of rotation is greater with milk as contents than with semi-yoghourtand that with yoghourt, no recovery was observed (the subsequent steady deceleration reflects the constant decelerating torque applied through the motor bearings). Such a result, however, does not permit thievery rapid measu rement (typically 50 milliseconds or less) achieved with the first method and hence the first method is normally to be preferred as a measure ofthe rheology ofthe contents of the carton 30.

Figure 4, shows the apparent moment of inertia of the carton and its contents as a function of time and is derived from the information represented in Figures 2 and 3, the apparent moment of inertia being derived from the-total angular velocity achieved after a particular acceleration time. The contents are as follows; frozen UHT milk (A), yoghourt(B),semi-yoghourt(C),UHTmilk(D).The rests for (B), (C) and (D) were conducted at a contents temperature of 20"C.

As compared with the prior art systems referred to above, the method ofthe present-invention has the following advantages. Firstly, it simplifies analysis of the system under study, since one no longer has to considerthe elastic restoring forces which act on an oscillating system. Secondly, even if the container (a cardboard carton, for example) is not ideally rigid, this is not important as the mechanical characteristics ofthe container no longer have much influence on the-observedresults. Thirdly, measurements can now be made considerably faster than hitherto since there is no longer any requirementfora minimum measuring time large enough to embrace the oscillation period as in previous systems.

In practice, the rotation rate signal will be contaminated with noise (particuiarly commutation noise), and due to commutation effects, the drive currentdeliveredtothe motor from the source of constant voltage will fluctuate and so is not accurately constant overtime. Accordingly, if the sensitivity of the method ofthe present invention is to be exploited to the full, a smoothing operation is required on the rotation rate signal and compensation forthe drive currentfluctuation is necessary.

One means offulfilling these requirements is illustrated by the apparatus of Figure where smoothing is effected byintegration and where, at the end of an acceleration run, a single parameter "0/8" (see below) may be derived which is independent of the drive current. This parameter is produced from the continuous measurements of motor angularvelocity and drive current, and is proportional to the momentof inertia ofthe container and its contents (for a constant acceleration time) as will be hereinafter explained.

Thus referring now to Figure 5, the current flowing through the motor 12 is controlled by a power MOSFETtransistorswitch TRi,thecurrent being monitored by measurement of the potential difference generated across the resistance R1. Thus a signal Iwhich is proportionaltothe motorcurrentis fed via a resistance R2 and solid-state switch S3 to a current-integrating unit 34 operative to provide a value Qwhich is proportional to the electric charge supplied to the motor 12 during the integration period.

A second solid-state switch S1 is connected across the integrating unit 34, as shown.

The output signal 11) from the tachogenerator 14 is fed via a third resistance R3 and, via another solid-state switch S4, to a second current-integrating unit 36 operative to provide a value 0 which is proportional to the angular displacement ofthe motor armature during the integration period.

A last solid-state switch S2 is connected across the integrating unit 36, as shown.

A resetting monostable 38 is connected with switch Si and with a second monostable 40-acting as a timer. This latter is also connected with the transistor switch TR1 and with switches S3 and S4.

Reference numeral 42 indicates a divider unit operative to divide the output 0 of integrator 34 by the output û of integrator 36, the integrated and derived values 0, Q/B and û being supplied to output terminals 44,45,46 respectively.

The system is actuated by an external trigger applied to terminal 48 ofthe monostable 38. In the inoperative condition illustrated, all five switches TRi, Si, S2, S3 and S4 are open as shown.

To commence operation, a trigger pulse is applied to the monostable 38 via terminal 48 to close switches S1 and S2. This discharges the integrators 34and36.

At the end ofthetimed pulse provided by monostable 38, the monostable reverts to its original state to reopen switches S1 and S2 and close switches S3 and S4. This brings the integrators 34,36 back into circuit and activates the second monostable 40 which operates to close the transistor switch TR1 and put the motor into motion.

At the end of the timed pulse provided by monostable 40, the monostable reverts to its original state to open switches S3, S4 and TR1 thereby to return the system to the situation illustrated in Figure 5.

As already explained,the values Oand 0 supplied at terminals 44 and 46 respectively, are proportional to the electric charged delivered to the motor 12 over the integration period and to the angular displacement of the motor armature during this time. During the early phase of acceleration where the rotation rate of the motor armature is small by comparison with the final (steady state) rotation which will ultimately be achieved (e.g. up to 1/4, say, ofthisfinalvalue),thevalueO/Osuppliedatterminal 45 is proportional to the moment of inertia ofthe container plus the contents.

The parameter 0/8 is important because it is independent ofthe drive currentforthe motor and this avoids difficulties introduced by the irreproducibility ofthe motor current. This irreproducibility arises because the armature current delivered by a constant voltage supply is not accurately constant, slight variation occurring due both to the back e.m.f. ofthe rotating motor and due to commutation effects which depend upon armature position.

Apparatus constructed in accordance with the present invention, may also be used as a viscometer with the particular benefit that the measurement of viscosity does not require contact between the apparatus andthefluid under test. If the resolution of the apparatus is to be improved in this context' the motor characteristics-become important since in order to measurelowviscosities (near that of water), low rates of acceleration are necessary and the motor bearing resistance and stiction" must be as low as possible.

Apparatus according to the present invention also has the advantage that measu rementscan be made very rapidly (e.g. in 50 milliseconds or less with apparatus ofthe sort illustrated in Figure 1) permitting a throughputof, perhaps, Sto 10 second1. In addition, the invention would appearto provide a suitable method for a wide range offood products and types of package where a non-invasive rheological measurement is required.

Claims (11)

1. Method of non-destructively testing the fluid or semi-fluid contents of a container comprising the steps of applying a torque to the container and monitoring the change of-angularvelocity ofthe container occurring in a determined time interval.

2. A method as claimed in Claim 1 in which the container is rotated from rest by applying a well-defined drive to the container.

3. Amethodasclaimed in Claim 2 in which the drive is a constant torque drive.

4. A method as claimed in Claim 2 or Claim 3 in which change ofangularvelocity is monitored to give an indication ofthe-acceleration ofthe container.

5. A method as claimed in Claim 1 in which the container is rotated from rest by the-spinning contents of the container and the velocity-is monitored to determine the overall time taken by the containerto accelerate from rest to a predeterm ined greater value orto-come backto rest.

6. An apparatus comprising a supportfor a load, drive means for rotating the support and its load, and monitoring means for monitoring the change of angular velocity ofthe support and its load occurring in a determined time interval.

7. An apparatus as claimed in Claim 6 in which the monitoring means operates by measuring the acceleration impartedto the support and its load.

8. An apparatus as claimed in Claim 6 in whichthe monitoring means operates by measuring the period required forthe support and its load to acquire a preselected speed,which may be zero.

9. An apparatus as claimed in any of Claims 6 to 8 for use in the method of Claim 5 including switching means operative to reverse the driveto bring the supportto rest and thereafter to remove the drive to allowthesupportto be driven by the inertial effectof the load.

10. Apparatus as claimed in anyofClaims6to9 including first integration means for integrating a first signal which is proportional to the motor current, second integrating means-for integ rating a second signal which is proportional to the angular velocity of the motor armature, and a divider operative to provide a value representative ofthe value ofthefirst signal divided bythe value ofthe second signal.

11. An apparatus or method substantially as hereinbefore described with reference to, and/or as illustrated in, the accompanying drawings.