Technical Field

The invention relates to an apparatus for measuring nuclear-magnetic resonances and for identifying substances having gyromagnetic proper- ties, said apparatus comprising means for generating a magnetic field, a transmitter coil placed in the magnetic field adjacent a sample of the substances, the nuclear-magnetic resonances of which are to be meas¬ ured, a receiver coil also placed adjacent the sample and optionally being identical with the transmitter coil, a frequency generator for feeding the transmitter coil and a circuit so as to change the frequency of the fre¬ quency generator in order to lock to a resonance frequency.

Background Art

In the known apparatuses for measuring nuclear-magnetic resonance in a magnetic field, said magnetic field is in most cases generated by means of a supraconducting coil, the field of which is typically of the magnitude 6 to 8 Tesla. The cooling of such a coil implies that the appa¬ ratus is relatively expensive.

As an alternative, the magnetic field can be generated by means of strong permanent magnets of for instance neodymium. Such permanent magnets render it possible to achieve a magnetic field of about 1 Tesla in a gap of about 14 mm. These permanent magnets are, however tem¬ perature-sensitive, as a temperature change of 1 /1 000 °C causes a change in the magnetic field of about 1 ppm. In practise it is impossible to maintain the temperature within a range of 1 /1000°C.

Brief Description of the invention

The object of the invention is to provide an apparatus for measuring nuclear-magnetic resonance in a magnetic field generated by means of a permanent magnet, where it has been accounted for that the tempera¬ ture cannot be kept completely constant.

An apparatus of the above type is according to the invention character¬ ised by the frequency generator generating a signal to the transmitter coil, and by the signal from the receiver coil being mixed with the signal from the frequency generator, whereafter the transmitted signal upon a fourier transformation and a signal processing can be used for controlling the frequency generator so as to compensate for possible temperature changes, said frequency generator being locked on a well-defined nuclear resonance frequency. As a result, a temperature change, if any, is automatically compensated for by a corresponding change in the frequency, an unambiguous relation between the resonance frequency and the field intensity being utilized.

From SE printed accepted specification No. 458,397 it is indeed known to lock the frequency relative to a proton resonance frequency. This is, however, conditioned by a magnetometer allowing a very accurate measuring of a magnetic field.

The apparatus according to the invention may furthermore be characterised by the signal from the frequency generator being fre¬ quency displaced by means of a signal of about 10 MHz in nega¬ tive/positive direction before it is transmitted to the transmitter, and by the signal received from the receiver and mixed with the signal from the frequency generator in negative/positive direction also being frequency- displaced by means of the above signal of about 1 0 Mhz. In this manner the phase of the signal from the frequency generator is of no import¬ ance. It is also possible to couple a narrow band-pass filter in the receiver circuit. When dimensioning this filter it is not necessary to

consider possible changes in the parameter values and accordingly the filter can be made correspondingly narrow. As a result, an improved signal-to-noise-ratio is obtained.

Moreover according to the invention the frequency generator may be changed in steps of for instance 1 kHz, while minor frequency differ¬ ences can be compensated for by changing the magnetic field by means of correction coils in the magnetic field means. As an alternative the frequency may be controlled by means of a digital signal processor capable of emitting the exactly desired frequency in response to a digital message.

Furthermore according to the invention the signal processing may be provided by means of a strong processor performing a reversion in time of the signal received in connection with feeding of a pulse-modulated HF-signal to the transmitter coil, whereafter a fourier transformation is performed of the sum of the received and the reversed signal. In this manner the signal processing is simplified. In addition an improved resol¬ ution is allowed.

A phase-displacement of the reversed signal may be performed by means of software until the fourier-transformed signal obtains a charac- teristic appearance.

Brief Description of the Drawings

The invention is explained in greater detail below with reference to the accompanying drawings, in which

Fig. 1 illustrates a magnetic yoke for an apparatus according to the invention for measuring nuclear-magnetic resonance so as to identify sugar solutions,

Fig. 2 illustrates a portion of the magnetic yoke with a gap shown in Fig. 1 , in which a sample of the substance is placed, the nuclear-magnetic resonances of which are to be measured,

Fig. 3 illustrates a circuit for analyzing output signals from the appar- atus,

Fig. 4 illustrates an output signal from the apparatus,

Fig. 5 illustrates the output signal of Fig. 4 after a signal processing including a fourier transformation,

Fig. 6 illustrates a corresponding fourier-transformed output signal, where the sample contains nothing but water,

Fig. 7 shows a program structure illustrating the signal processing, and

Fig. 8 shows a diagram of a circuit for controlling the entire apparatus.

Best Mode for Carrying Out the Invention

Fig. 2 illustrates a portion of an apparatus for measuring nuclear-mag- netic resonances and identifying substances having gyromagnetic prop¬ erties so as to perform an on-line determination of the ratio of the sugar content to the dry matter content (the quotient) of a sugar solution.

This quotient can be found at a laboratory where the sugar content can be determined by a polarisation measuring while the dry matter content can be determined by a refraction measuring.

The quotient can for instance be used for controlling a sugar crystallization process during a production of sugar.

According to the invention an on-line apparatus is provided for measur¬ ing nuclear-magnetic resonances and identifying substances having gyro¬ magnetic properties, said apparatus continuously indicating the above quotient. As a result it is possible to minimize the recycling amounts in the factory section handling the crystallization process and to control a so-called "molasse reduction".

A minimizing of the recirculating amounts in the factory section handling the recrystallization process results in a saving of energy during the production of sugar. Furthermore, it is now possible to control other factors, such as change of chute, covering amounts etc. to an optimum. Furthermore, an on-line measuring of the quotient implies that an alarm is quickly given in case of error situations or other undesired operational errors. Such control and alarm possibilities have not been possible pre¬ viously. The only way of determining the quotient involved time-consum- ing analyses at the laboratory.

An optimum reduction of the molasse implies furthermore that the enter¬ ing amount of sugar is utilized to an optimum.

The determination of the sugar content in a sugar juice by means of nuclear-magnetic resonances requires high-resolution apparatuses for measuring the changes of nuclear-magnetic resonances caused by chem¬ ical changes. In this manner it is possible to determine the chemical change of H ⁺ in a binding in the OH-molecule and the sugar molecule, respectively. A sugar molecule from for instance a sugar juice causes not only one, but several resonance frequencies (a total of 21 ) within an interval of approximately 0.6 ppm. In the apparatus according to the invention these resonance peaks are collected in one response signal from the sugar molecule at a distance of approximately 1 .2 ppm from the OH-resonance. The apparatus for measuring nuclear-magnetic reson¬ ances must be able to separate these two resonance peaks. A compari-

son of the intervals with a reference measuring can be used for deter¬ mining the sugar juice quotient.

Fig. 1 illustrates a portion of the apparatus according to the invention for measuring nuclear-magnetic resonance. The apparatus comprises a rec- tangular magnetic yoke 1 . Two pole shoes 3 are placed inside the mag¬ netic yoke 1 , said pole shoes 3 facing one another and defining an air gap therebetween. 4 refers to magnets of neodymium. The pole shoes 3 are made of an cobalt-iron alloy sold for instance by Vacuflux, where¬ as the magnetic yoke is made of XC06 (soft iron). The air gap 4 is of a width of approximately 1 6 mm and generates a magnetic field of ap¬ proximately 1 Tesla. Some electric heat sheets 6 are arranged on the outer sides of the rectangular magnetic yoke 1 . The entire magnetic structure is arranged in an isolated iron box not shown, said box shield¬ ing for outer electric and magnetic fields and keeping the temperature substantially constant. A problem applies, however, to the magnetic field changing in response to the temperature as domains in the perma¬ nent magnetic are influenced. The changes in the magnetic field involve corresponding changes in the nuclear-magnetic resonance frequencies. Even changes of 1 /1000°C are of importance. This problem has been solved by locking to a known resonance frequency thereby serving as a reference. In other words, a measuring is performed relative to a known reference whereby said measuring is independent of possible changes in the magnetic field caused by minor changes in the temperature. This is explained in greater detail in connection with Fig. 3.

Fig. 2 shows the air gap 4 between the pole shoes 3. A container or a pipe 8 is placed in the air gap 4, said container or pipe containing a sample of the material, the nuclear-magnetic resonances of which are to be measured. A receiver coil 10 is wound on the container or the pipe 8. A transmitter coil 1 1 is arranged perpendicular to said receiver coil. The two coils 10, 1 1 are arranged perpendicular to one another in order to

avoid a coupling therebetween. The transmitter coil can optionally be identical with the receiver coil. Furthermore, some correction coils 1 3, 1 4 are arranged at the end of the pole shoes 3, said correction coils ensuring a homogenous field which is very important for an identification of the resonance peaks.

Fig. 3 shows a diagram of a circuit for analyzing the output signals. A frequency generator 1 6 of 52 MHz transmits a signal of 42 MHz to the transmitter coil 1 1 after a frequency displacement of 10 MHz. This signal is influenced by the sample in the pipe 8 placed in the magnetic field in the air gap 4. The signal emitted by the sample is received and transferred to a mixing circuit 1 7 where it is mixed with the signal of 52 MHz from the frequency generator 16. The signal from the mixing circuit 1 7 presents then a frequency of approximately 1 0 MHz. Through a filter this signal is transferred to an additional mixing circuit 1 8 where it is mixed with the above signal of 1 0 MHz, whereby the mixing circuit 1 8 results in a signal of a few hundred Hz. This signal must be of 400 Hz when the sample contains nothing but water, and it is transferred to a processor 20 where it is mixed by means of software with 400 Hz to 0 Hz, whereafter the signals outside the intervals from -200 to + 200 Hz are removed. Then a fourier-transformation is performed. The processor 20 performs a software-tracking of a resonance peak indicating the presence of hydrogen atoms in the water. Based on the position of this signal peak relative to 0 Hz, an error signal is generated and transmitted to the frequency generator 1 6 so as to adjust the frequency thereof in such a manner that the error signal automatically moves towards 0.

In one embodiment, the transmitter 1 1 is pulse-modulated. The response signal from the sample 8 in the air gap 4 is then an oscillating attenuat¬ ed and extinguishing signal. When several resonance peaks are involved, the extinguishing signal is modulated as shown in Fig. 4. Then the signal processing by means of software implies that the processor 20 performs

a reversion in time of the signal received in connection with transmission of a pulse-modulated HF-signal to the transmitter coil 1 1 . It is assumed that the signal is in form of f(t), where f(t) = 0 when t < 0. The rever¬ sion and the addition of the reversed signal has then the form f(t) + f(-t). Subsequently, a fourier transformation is performed in the pro¬ cessor 20 of the sum of the received and the reversed signal f(t) + f(-t), said fourier transformation including a phase displacement by means of software of said signal until there is no phase-difference between the combined signals, which corresponds to the situation where the fourier- transformed signal presents a characteristic appearance in form of a relatively high peak with symmetrical signal portions with areas of sub¬ stantially equal size on both sides. A signal with such a characteristic can be identified by means of said processor 20. The processor 20 is for instance based on ADSP21020 Digital Signal Processor from Analog Device. This processor is well-suited for comprehensive calculations and increases the calculating speed approximately 1000 times compared to a personal computer.

The frequency generator 16 can be changed in steps of 1 kHz by means of a PLL-system. Minor frequency differences can be compensated for by changing the magnetic field by means of the correction coils at the pole shoes 3. An alternative solution for controlling the frequency is to use a microprocessor capable of changing the frequency in steps of a few Hz.

Furthermore, a number of field-correcting coils are provided on the pole shoes.