Patent Application
20170102309
The invention relates to a method for actuating an electric motor for an oscillatory rotation of a driveshaft, in particular for a rheometer. Furthermore, the invention relates to a configuration for exerting an oscillatory rotation of a driveshaft, in particular for a rheometer for measuring the viscosity of a sample.
The prior art has disclosed various closed-loop actuation controls for electric motors, which excite an electric motor to carry out an oscillatory rotation of the driveshaft. In particular, such methods are used to measure the nonlinear, rheological properties of media, wherein the driveshaft of the motor is brought into the region of a medium to be examined and, by moving the driveshaft in the relevant medium, the nonlinear, rheological properties of the latter are established. In that case, a rotating oscillation with large deflection amplitudes is particularly preferred since the used media or samples exhibit a nonlinear behavior when certain thresholds are exceeded by the employed deflection amplitudes. The prior art has disclosed, in particular, the practice of examining the deformation behavior under cyclical loads, in particular expansion and compression between two measuring parts, wherein at least one of the measuring parts is connected to the driveshaft of the motor. A so-called rotational rheometer which is thus embodied has shearing plates, between which the sample to be examined is disposed, wherein one of the shearing plates is connected to the driveshaft of the electric motor.
The prior art has disclosed rotational and oscillatory rheometers as instruments for determining the flow behavior of viscoelastic samples by using different trial positions, such as e.g. rotation, relaxation and oscillation trials. In the process, it is possible to examine both the flow behavior of liquids and the deformation behavior of solids. In general, real samples exhibit a combination of elastic and plastic behavior. The sample material to be examined is introduced into a measurement space between two measuring parts and the distance between the two measuring parts is determined by using a height adjustment and suitable sensors. The upper measuring part and lower measuring part are moved counter to one another in a relative manner about a common axis of rotation. The sample is exposed to a shearing load due to the rotation of the measuring parts against one another. Both rotating and rotating oscillatory movements are possible in such a measurement setup. In principle, different geometries can be used for such a trial setup, in particular measurement systems in which the medium is clamped between two plates, or measurement systems in which the medium is clamped between a cone and a plate, or measurement systems in which the medium is disposed between two concentrically disposed cylinders which rotate counter to one another.
The prior art disclosed various rheometers, in which the determination of the torque is effected by using a motor constructed for driving and determining torque. However, the torque can alternatively also be determined by way of two mutually separated units for driving and rotation, which are each assigned to one of the measurement parts. Moreover, devices with two measurement motors are also known, as emerge, for example, from Austrian Patent AT 508.706 B1, corresponding to U.S. Pat. No. 8,453,496 and U.S. Patent Application US 2007/0292004.
Independently of the type of motor, it is possible to use synchronous motors with permanent magnets, or else asynchronous motors, within the scope of the invention. The amplitude of the oscillatory motion, the oscillation frequency, the rotational speed of the motor or the torque acting on the sample may be predetermined within the scope of the invention.
In general, the torque can be measured by way of the power consumption of the respective electric motor, wherein there is a functional relationship with the power uptake of the motor for the torque, depending on the type of the motor or device being used: N=c1×I, or N=c2×I2, where the two constants c1 and c2 are device specific.
The deflection of the oscillating motor can be established in different ways, in particular optically.
The goal of the measurement of a sample lies in obtaining different measurement values for different amplitudes, deflections and frequencies, which may be modified independently of one another. The measurement values thus established are referred to as a rheological fingerprint of the material to be examined.
However, the substantial problem existing is that the respective excitation is also modified by the nonlinear behavior of the medium or the sample.
It is accordingly an object of the invention to provide a method for actuating an electric motor and a configuration for exerting oscillatory rotation of a driveshaft, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and configurations of this general type and which develop an actuation of an electric motor for an oscillatory rotation, in which it is possible to set either the time profile of the torque or the time profile of the deflection freely in advance. In particular, it is an object of the invention for the time profile of the torque or of the deflection to assume the form of a sine oscillation or cosine oscillation with great accuracy. To this end, the invention proposes a specific actuation of the electric motor.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for actuating an electric motor for an oscillatory rotation of a driveshaft, in particular for a rheometer. Provision is therefore made for:
With the objects of the invention in view, there is also provided a configuration for exerting oscillatory rotation of a driveshaft of a motor in particular for a rheometer for measuring viscosity of a sample.
In this case, significant improvements arise when using large signal amplitudes, in which the medium to be examined or the sample to be examined is operated in the nonlinear force or tension range. In particular, the invention renders it possible to predetermine a very exact sine profile and cosine profile of the torque or of the deflection of the electric motor.
In order to be able to take better account of the frequency dependence of the individual nonlinear effects of the sample, provision can be made for sine torques and cosine torques to be used as a base function.
In order to be able to generate a spectrum of different frequencies in a simple manner, provision can be made for a first base function to have a predetermined basic shape and for the further base functions each to be compressed in relation to the first base function by an integer value, in such a way that fn(t)=f1(n*t).
For the purposes of reducing the required computational time, provision may be made for the number of the selected base functions to be less than 5.
A preferred embodiment of the invention, which enables fast signal adaptation in real time, provides for the base functions to be predetermined as periodic functions and for the sampling to be selected in such a way that more than one hundred samples are taken during the period duration of the base function with the longest period.
For the same purpose, provision can be made for the base functions to be predetermined periodically and for the time window, within which the samples are undertaken, to have a duration of between 25% and 50% of the period duration of the base function with the longest period.
The adaptation, as described in steps h) to k) is preferably undertaken multiple times in order to obtain good correlation between the intended signal and the actual signal. To this end, provision can advantageously be made for the base functions to be predetermined as periodic functions and for the adaptation of steps h) to k) to be repeated periodically, wherein a time period of between 25% and 100% of the period duration of the base function with the longest period lies between two adaptations in each case.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for actuating an electric motor and a configuration for exerting oscillatory rotation of a driveshaft, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.
Referring now to the figures of the drawings in detail and first, particularly, to
It is either the sample torque M or the deflection w which is predetermined in advance in the form of an intended variable e(t) so that such a measurement can be undertaken overall. In this case, the intended time profile e(t) has a periodic, predetermined form and is predetermined for the regulator 3.
The configuration in
The assumption is made within the scope of the invention that the sample 2 exhibits nonlinear behavior. If the driveshaft of the motor 1 is only moved within a small deflection range about a work point, the sample 2 usually has a linear behavior around the relevant work point. However, if the deflection w is increased, this has as a consequence in the case of a nonlinear sample 2 that the measured variables y(t) and the manipulated variable u(t) behave nonlinearly in relation to one another, at least within a range between the maximum and minimum of the predetermined, periodic intended time profile e(t). Due to this nonlinear behavior, it is not possible either to already estimate or establish a manipulated variable u(t), which ultimately obtains the desired intended time profile e(t), in advance. Moreover, the problem of a sample 2 changing during the measurement, in particular having a behavior exhibiting hysteresis, may also arise, and so setting a manipulated variable u(t) in advance for the purposes of reaching a predetermined intended time profile e(t) is not possible. It is for this reason that the invention uses the iterative method described in more detail below, in which the predetermined intended time profile e(t) for the deflection w or the sample torque M is ultimately achieved in a simple manner.
Initially, that is to stay still before the iterative adjustment, an approximation function e′(t) is established for the intended time profile e(t), which approximation function is established as weighted sum of a number of predetermined, periodic base functions f1(t), f2(t), . . . which may be offset in time when necessary.
Advantageously, sine or cosine oscillations f1(t)=sin(a0t), f2(t)=sin(2a0t), . . . are used as base functions f1(t), f2(t), . . . , where a0 represents a base frequency of in particular 1 Hz, and the first base function f1(t) has a predetermined basic shape and the further base functions are in each case compressed in relation to the first base function by a predetermined integer value in such a way that fn(t)=f1(n*t). Preferably, use is only made of a few base functions in total. The present exemplary embodiment uses only three base functions in total.
By way of example, an advantageous example for base functions is depicted in more detail in
For the purposes of initially setting the manipulated variable u(t), a manipulated parameter vector U=[u1, u2, . . . ] is predetermined, the individual elements of which represent weights which—multiplied by the base functions—approximately reproduce the manipulated variable u(t) as a weighted sum.
u(t)˜u′(t)=u1f1(t)+u2f2(t)+ . . .
The intended parameter vector E, multiplied by a predetermined factor x, is predetermined as an initial value for the manipulated parameter vector U. The predetermined factor x is set in advance as follows: 0.5 if M is predetermined and 0.5*J*(2*pi*fn)2 if w is predetermined (J: inertia of the measurement drive).
An iterative method is now presented below, by using which the regulator 3 continuously adapts the manipulated variable u(t) in order to generate a deflection w or a sample torque M in accordance with the predetermined intended time profile e(t). As is depicted in
Subsequently, the sampled values of the measured variable y(t) within the time window W are also subjected to the same analysis as the intended time profile. An approximation function y′(t) is established as a weighted sum of the base functions; the individual weights, thus established, for the individual base functions are combined to form an actual parameter vector Y.
y(t)˜y′(t)=y1f1(t)+y2f2(t)+ . . . ; Y=[y1, y2, . . . ]
A difference D between the intended parameter vector E and the actual parameter vector Y is established in a further step. This difference D seen in
U
n+1
:=U
n
−D=U
n−(E−Y)*v
In a last step, the manipulated variable u(t) for the next iteration step is established as a weighted sum of the base functions on the basis of the newly established manipulated parameter vector Un+1.u(t)=u1f1(t)+u2f2(t). Subsequently, sampling is once again carried out within a subsequent time window W, an actual parameter vector Y is once again established, the difference D is established between the intended parameter vector E and the actual parameter vector Y and that difference is subtracted from the manipulated parameter vector U, and the manipulated parameter vector U is once again used for generating the manipulated variable u(t). This process is undertaken continuously by the regulator 3 in order to achieve appropriate adaptation to the measured variable, i.e. the deflection w or the sample torque M.
The adaptation can be repeated as often as desired. There is a time period between two respectively adaptations in each case of between 25 and 100% of the period duration of the base function f1(t) with the longest period.
| Number | Date | Country | Kind |
|---|---|---|---|
| A50864/2015 | Oct 2015 | AT | national |
This application claims the priority, under 35 U.S.C. §119, of Austrian Patent Application AT A50864/2015, filed Oct. 8, 2015; the prior application is herewith incorporated by reference in its entirety.
