A vibration-type rheometer comprises a vibration-type viscometer having a pair of vibrator subassemblies which resonate as in a tuning fork. The pair of vibrator subassemblies constituting a tuning fork vibrator each has at its free end a sensor plate formed from a thin metal plate places into a sample to be measured, and the vibrator subassemblies are driven at the same frequency but in opposite phase relationship to each other by an electromagnetic driving unit together with the sensor plates. A control unit supplies a time-varying driving current whose magnitude varies with time to the electromagnetic driving unit in order to change the vibration-applying force applied to the pair of vibrator subassemblies. The amplitude of vibration of the vibrator subassemblies changes as a function of the viscous resistance encountered by the sensor plates from the sample and the amplitude is electrically detected, and the detected value is sent to a recording unit together with the value of the driving current to thereby indicate the behavior of the sample.

FIELD OF THE INVENTION 
The present invention relates to a rheometer for measuring a phenomenon 
such as the deformation and flow of a fluid, and more particularly, to a 
vibration-type rheometer including a pair of tuning fork-like members 
capable of being vibrated in a fluid sample. 
BACKGROUND OF THE INVENTION 
A flowability of simple liquids, for example, such as water, alcohol, 
glycerine or the like, is different in viscosity, but these liquids 
exhibit a Newtonian viscosity, that is, a straining rate proportional to a 
stress during the flow. On the other hand, it has been known that thick 
liquids having a relatively complicated construction, for example, such as 
paint, toothpaste, mayonnaise and cold cream, exhibit a non-Newtonian 
viscosity which will not start the flow unless an external force exceeds a 
predermined value. On the other hand, the property of gels which change 
into sol when subjected to shaking or vibration, and then return again to 
gels when left standing is called thixotropy. 
A measurement of Newtonian viscosity or non-Newtonian viscosity, 
particularly, the degree of thixotropy, can be determined by evaluating an 
area of a hysteresis loop based on the number of revolutions of a rotary 
viscometer. In the rotary viscometer, the viscosity is obtained by 
rotating a cylindrical body in a viscous fluid and measuring a torque due 
to the viscosity exerted on the cylindrical body. In a coaxial double 
cylindrical meter, a fluid is put between an inner tube and an outer tube, 
and a torque exerted on the inner tube when the outer tube is rotated is 
measured. This measurement is carried out by hanging the inner tube by 
means of a torsion wire and obtaining a torsional angle of the torsion 
wire. If a design is made so that an angular velocity of a rotary body is 
made variable so as to vary a shear rate corresponding thereto, it can be 
applied to the measurement of the flow characteristics of non-Newtonian 
viscous fluids. 
However, it is necessary to vary the shape of a rotary body depending on a 
sample to be measured. Furthermore, since it is cumbersome to clean and 
wash the rotary body after being used, a problem arises in handling and 
using the apparatus. In addition, being affected by the inertia of the 
rotary body or by the flow of the sample, the range of a controllable 
angular velosity of the rotary body is narrow, and various measuring 
patterns cannot be selected. 
SUMMARY OF THE INVENTION 
It is a general object of the present invention to provide a new rheometer 
apparatus which can eliminate the disadvantages as noted above with 
respect to a rheometer using a conventional rotary viscometer. 
Particularly, it is an object of the present invention to provide a 
rheometer which can widely change the manner of applying an external force 
to a sample to be measured and a strain rate so that various measuring 
patterns may be selected. 
It is a further object of the present invention to provide a rheometer 
which is simple in handling and easy in operation. 
According to the present invention, a vibration-type rheometer apparatus 
includes a viscometer having a pair of tuning fork-like members capable of 
being vibrated in a sample. A vibration-type viscometer having a pair of 
tuning fork-like members is known from U.S. Pat. No. 4,602,505 
(corresponding to Europe Patent Laid-Open No. 112,156) entitled "APATUS 
FOR MEASURING VISCOSITY" issued to the present inventors on July 29, 1986. 
Also, an improved vibration-type viscometer of this type has been proposed 
in U.S. Pat. No. 4,729,237 (corresponding to Europe Patent Laid-Open No. 
233,408) entitled "TUNING FORK VIBRATION-TYPE VISCOSITY MEASURING 
APATUS" issued to the present inventors on Mar. 8, 1988. These 
vibration-type viscometers each comprise a tuning fork vibrator including 
a pair of vibrator subassemblies, each vibrator subassembIy having at its 
free end a sensor plate to be inserted into a sample to be measured, a 
driving unit for applying vibrations to the pair of vibrator 
subassemblies, and a detector for detecting the vibration amplitude of the 
pair of vibrator subassemblies which changes due to a viscosity resistance 
applied to the sensor plates when placed in the sample and for converting 
the vibration amplitude into an electric signal. The driving unit 
comprises a combination of an electromagnetic coil and a permanent magnet, 
in which the pair of vibrator subassemblies are vibrated in a phase 
opposite to each other, that is, a phase difference of 180 degrees under 
the same frequency. In the vibation-type viscometers so far proposed, the 
driving frequency is 30 Hz, and one-side amplitude at the time of no-load 
is 20 microns, which is constant. 
A characteristic of the present invention resides in the further provision 
of a control unit for changing the driving current applied to the 
aforesaid driving unit according to a predetermined pattern, and a 
recording means for plotting a change in the amplitude value of the pair 
of vibrator subassemblies in response to a change in the driving current 
in an output of the detector, in addition to the constituent elements of 
the known vibration-type viscometers as described above. Since the 
continuous change in the magnitude of the driving current with respect to 
the driving unit appears in the continuous change in the 
vibration-applying force to the pair of vibrator subassemblies, if the 
change in the amplitude of vibrator due to the change in the 
vibration-applying force is continuously detected, it is possible to 
measure the rate of change with time in the motion of a fluid. In this 
case, in contrast with the rheometer using a conventional rotary 
viscometer for generating a concentric circular flow in the fluid, slight 
vibratIons are merely generated by the pair of vibrator subassemblies 
according to the present invention, and therefore the measuring pattern 
resulting from the control of the applying the vibration-applying force 
and the magnitude may have a considerable freedom. The control of the 
manner of applying the vibration-applying force and the magnitude may have 
a considerable freedom. The control of the driving current in typical 
examples of the measuring pattern is as follows: 
Pattern 1: From time t0 to t1, steplessly and continuously increased, and 
then from t1 to t2, steplessly and continuously decreased. 
Pattern 2: Similar to the pattern 1, continuously increased till the time 
t1, and held constant after the time t1. 
Pattern 3: Similarly to the pattern 1, continuously increased till the time 
t1, and thereafter held constant till the next time t2, and then cut off 
to 0. 
Pattern 4: Similar to the pattern 1, continuously increased till the time 
t1, and conversely steplessly and continuously lowered from the time t1 to 
the next time t2. These up and down cycles are repeated. 
These measuring patterns can be obtained easily by program-controlling the 
driving current. 
According to the present invention, the driving current applied to the 
driving unit may, for example, be steplessly an continuously changed and 
the change in the amplitude value due to the change in the 
vibrationapplying force in the pair of vibrator-subassemblies is 
continuously detected whereby the rheology of the fluid can be measured. 
Therefore, the measurement can be simply conducted without giving rise to 
a problem in handling encountered in the use of a conventional rotary 
viscometer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
First, referring to FIG. 3, a vibration-type rheometer apparatus according 
to the present invention includes a vibration-type viscometer generally 
indicated at 50. This vibration-type viscometer comprises an 
electromagnetic driving unit 51, an amplitude detector 52 and a 
thermometer 53, which will be described in detail later. The 
electromagnetic driving unit 51 has a permanent magnet 13 (FIG.1) and an 
electromagnetic coil 12 (FIG. 1) cooperating therewith, the 
electromagnetic coil 12 receiving a driving current, whose magnitude 
steplessly and continuously changes, from a control unit 56 including an 
oscillator 54 and a variable amplifier 55. This control unit 56 is 
provided with an ammeter 57 for measuring the magnitude of the driving 
current to be supplied to the electromagnetic driving unit 51. On the 
other hand, the amplitude detector 52 comprises, for example, a 
non-contact system eddy current loss detection type displacement detector 
14 (FIG. 1), and an output signal of this detector 14 is sent to an 
amplitude display unit 58. This amplitude display unit 58 includes an 
amplifier 59 connected to an output side of the detector 14 and a 
voltmeter 60 for measuring an output of the amplifier 59 as a voltage 
value. The thermometer 53 has a temperature probe 21 (FIG. 1), an output 
signal of which is supplied to a temperature indicator 61 A value measured 
by the ammeter 57 and a value measured by the voltmeter 60 are sent to a 
recording or processing unit 62 such as an X-Y recorder, and the recording 
unit 62 plots a detection voltage representative of a change in an 
amplitude value from the amplitude detector 52 corresponding to a driving 
current having a magnitude which continuously and steplessly changes sent 
to the electromagnetic driving unit 51. 
The vibration-type viscometer per se used in the present invention is 
disclosed in U.S. Pat. Nos. 4,602,505 and 4,729,237. Referring to FIG. 1, 
the vibration-type viscometer is provided with a hollow support block 2 
formed of a rigid material firmly secured to a frame shaft 1 extending 
from a base (not shown), the support block 2 having a downwardly extending 
support column 3. A pair of vibrator subassemblies 4 constituting tuning 
fork-like vibrators are secured to the lower end of the support block Z, 
and these vibrator subassemblies 4 downwardly extend from the support 
block 2 and occupy sides opposite each other of the support column 3. The 
vibrator subassemblies 4 each include a leaf spring 5 with one end secured 
to the support block 2 by means of a screw 6 through a stop 7, a long 
intermediate plate 8 firmly mounted on the other end of the leaf spring 5, 
and a sensor plate 9 secured to the end of the intermediate plate 8 by 
means of a screw 10. The leaf spring 5 is preferably made of constant 
elastic spring steel, and the intermediate plate 8 is preferably made of a 
relatively light material having a rigidity, for example, such as 
aluminum. The sensor plate 9 is preferably made of stainless steel which 
is as thin as 0.2 mm or so, is flat and has a chemical resistance, the 
sensor plate having a free end formed into a disk 11 having a diameter of 
20 mm or so, for example. 
One vibrator subassembly 4 is arranged symmetrically with the other 
vibrator assembly, and a permanent magnet 13 which cooperators with a pair 
of electromagnetic coils 12 mounted on the support column 3 is provided on 
the intermediate plate 8. The combination of the electromagnetic coils 12 
and the permanent magnet 13 functions as a driving device 51 for vibrating 
the corresponding vibrator subassemblies 4, the driving device 51 being 
supplied with a driving current having a magnitude which steplessly and 
continuously changes from the control unit 56 (FIG. 3) as described above 
to thereby vibrate the pair of vibrator assemblies 4 by a 
vibration-applying force having a magnitude which steplessly and 
continuously changes in phases opposite to each other, that is, a phase 
difference of 180 degrees under the same frequency. According to a 
desireable example, the driving frequency is 30 Hz, and the driving 
current linearly changes from 0 to 1000 mA. The pair of sensor plates 9 
are distributed within the same imaginary vertical plane, and as a result, 
a torsional reaction in the support block 2 generated in the case where 
the sensor plates, are distributed in the different imaginary vertical 
planes can be avoided. While the relative arrangement of the 
electromagnetic coils 12 and the permanent magnet 13 may be reversed, the 
provision of the electromagnetic coils 12 on the side of the support 
column 3 as in the example shown in the drawing is suitable in that a lead 
wire 15 of the coil 12 can be guided to a terminal metal 16 (upward) 
passing through the support column 3. 
The support column 3 between the support block 2 and the electromagnetic 
coil 12 is provided with a displacement detector 14 opposed to the leaf 
spring 5 of the vibrator subassembly 4, the displacement detector 14 
converting the amplitude of one vibrator subassembly 4 into an electric 
signal. In this case, a further displacement detector may be provided for 
the other vibrator subassembly, but since both the vibrator subassemblies 
4 exhibit substantially the same amplitude, one will suffice. When the 
pair of sensor plates 9 are placed into a sample as will be described 
later, the amplitude of the vibrator subassemblies 14 is affected by the 
change in the viscous resistance, and therefore the displacement detector 
14 electrically detects the amplitude, and the viscosity of the sample is 
arithmetically calculated from that detected value in a well known manner. 
The displacement detector 14 can be, for example, of a well known 
non-contact system eddy current loss detector but in the case where this 
well known displacement detector is used, the leaf spring 5 opposed 
thereto is formed of a magnetic spring steel. A well known optical 
displacement sensor can be also used in place of an eddy current loss 
detection type displacement sensor. A lead wire 17 of the displacement 
detector 14 is also guided to a common terminal element 16 passing through 
the support column 3. 
Turning to FIG. 2, a thermometer generally indicated at 20 is mounted on 
the lower end of the support column 3, and a sheathed probe 21 of the 
thermometer 20 extends downward. This temperature probe 21 occupies an 
intermediate position between the pair of sensor plates 9 and is 
distributed in the same imaginary vertical plane, the probe 21 having its 
lower end distributed in substantially the same imaginary horizontal plane 
as the pair of the sensor plates 9. Since the temperature probe 21 is 
aligned in the same imaginary vertical plane as the pair of sensor plates 
9, an occurrence of sample turbulence due to the presence of the 
temperature probe 12 between these sensor plates is prevented. The 
thermometer 20 can be of a well known type, for example, in which a 
platinum temperature measuring resistance is provided within a sheath, 
which well known thermometer has a circuit unit 22 including an amplifier 
at the base end of the sheath. A lead wire 23 of the circuit unit 22 
reaches a common terminal element 16 passing through the support column 3. 
An external thread 30 is formed at the lower end of the support column 3, 
and a carrier device 32 having an adjusting nut member 31 threadedly 
engaged with the external thread 30 is mounted on the support column 3. 
The carrier device 32 detachably carries a sample container 33, and 
functions as a lid for closing an open end of the sample container 33. The 
sample container 33 is conveniently made from a transparent glass like a 
beaker. This container has a flange 34 around the open edge thereof, and 
has an index comprising two parallel lines 36 representative of an 
allowable amount of a sample 35 to be placed therein. The carrier device 
32 includes a lid member 37 formed of a synthetic resin excellent, for 
example, in heat insulating property having a plane size just fitted into 
the sample container 33, the lid member 37 having a flange 38. The lid 
member 37 is provided with a pair of well known clamp elements 39, and 
when these clamp elements 39 are brought into engagement with the flange 
34 of the sample container 33, the sample container 33 can be mounted on 
the carrier device 32. The adjusting nut member 31 threadedly engaged with 
the external thread 30 of the support column 3 has a stopper 40 at the 
lower part thereof, and the axial movement is restricted by the stopper 
40. The lid member 37 is formed with a hole 41 through which the lower end 
of the support column 3 may pass and a pair of head-diffusion preventive 
narrow slits 42 through which the pair of sensor plates 9 may pass. 
Normally, the carrier device 32 is mounted on the lower end of the support 
column 3, and the sample container 33 is detachably mounted on the carrier 
device 32. Two pins 44 are downwardly secured to the lower end of the 
support column 3, the pins 44 being located on opposite sides of the 
temperature probe 21, occupying a position between the probe and the 
sensor plate 9 and being aligned in the imaginary vertical plane in which 
the sensor plates 9 and the temperature probe 2 ar distributed. The pens 
44 each have an end tip which functions as an indicator representative of 
a desirable surface level of the sample 35 within the container 33. More 
specifically, if there is non-coincidence between the end tips of the pin 
44 and the surface level of the sample 35, the adjusting nut member 31 of 
the carrier device 32 is rotated to axially move the sample container 33 
along with the carrier device 32 toward the support column 3, until the 
end tips are coincident with the sample surface. As a result, even if 
samples in different amounts within an allowable range between the two 
index lines 36 are provided in the sample container 33, the sensor plates 
9 and the temperature probe 21 can be always inserted by a predetermined 
length into the sample without difficulty and it is not necessary that the 
sample be provided in the sample container 33 in a strictly determined 
amount. 
FIGS. 4, 5 and 6 are respectively graphs showing the results obtained by 
measuring the flow characteristics of three kinds of samples during a 
measuring cycle using a vibration-type rheometer manufactured in 
accordance with the preferred example of the present invention. In these 
graphs, the axis of ordinate indicates the magnitude of a driving current 
signal I corresponding to a vibration-applying force applied to the pair 
of vibrator subassemblies 4 while the axis of abscissa indicates the 
magnitude of the detected voltage signal E corresponding to the amplitude 
of the pair of vibrator subassemblies 4. In this case, FIG. 4 of the case 
where the sample is mayonnaise, FIG. 5 is of the case where the sample is 
cold cream, and FIG. 6 is of the case where the sample is milky liquid. 
According to FIGS. 4 and 5, a curve obtained by gradually increasing the 
vibration-applying force and a curve, obtained by gradually decreasing the 
vibration-applying force depict a hysteresis loop, from which can be 
learned that these samples exhibit a thixotropyic non-Newtonian viscosity. 
According to FIG. 6, it can be seen that the change in the amplitude with 
respect to the change in the vibration-applying force is linear, and this 
sample is a material exhibiting a Newtonian viscosity. 
FIGS. 7a-7d are graphs showing the state of driving current in various 
measuring patterns, and in each graph, the axis of ordinate indicates the 
current value I and the axis of abscissa indicates the time T. The states 
of the measuring patterns represented by these graphs are as follows: 
FIG. 7a--pattern 1: From time tO to t1, driving current increased 
steplessly and continuously, and then from t1 to t2, decreased steplessly 
and continuously. 
FIG. 7b--pattern 2: Similar to the pattern 1 in that the driving current is 
increased continuously till time t1, but then held constant after the time 
t1. 
FIG. 7c--pattern 3: Similar to the pattern 12 in that the driving current 
is increased continuously till time t1, but thereafter held constant till 
time t2, and then abruptly cut off to 0. 
FIG. 7d--pattern 4: The driving current is increased steplessly and 
continuously from time to time t1, and then lowered steplessly and 
continuously from time t1 to time t2. 
These up and down cycles are repeated. 
The aforementioned measuring patterns merely comprise exemplary 
embodiments, and according to the present invention various modifications 
can be made and the invention is not limited to these measuring patterns.