KR20040108497A - Operating Method for Multichannel Double Relaxation Oscillation SQUID Sensors - Google Patents

Abstract

PURPOSE: An FLL(Flux Locked Loop) driving method of a multi-channel squid sensor system and a method for obtaining an optimum bias current are provided to acquire rapidly a signal in comparison with a manual method by automating an operation of a squid sensor. CONSTITUTION: The external magnetic field is increased in a constant interval Φ0/4. A maximum value and a minimum value of output voltages are obtained by reading output voltages at four points. A voltage change width and a mean voltage are calculated thereby. A mean voltage value is inputted into an offset bias voltage value in order to set the mean voltage value as a reference point of an integrator. The external magnetic field is finely increased in a constant interval Φ0/16 in order to detect a flux point where voltage is increased from minimum. A flux bias generation voltage value is set by adding a predetermined difference to the rising flux point and an FFL is operated.

Description

Operating method for multichannel double relaxation oscillation squid sensors

The present invention relates to the automatic control of the optimum operating state of a high sensitivity multi-channel squid sensor for biomagnetic measurement. In particular, it is necessary to adjust a large number of double-relaxed squid type sensors by serial communication between the sensor drive circuit and the computer. The present invention relates to an FLL driving method and a method of obtaining an optimum bias current of a multi-channel double relaxed oscillation squid sensor, characterized by completing the adjustment of the bias current value of the sensor and the FLL operation adjustment in a short time by reducing the number of command strings.

The core technology in the operation of DC squid type high sensitivity squid sensor is search for optimum bias current value with the smallest output noise, search for bias magnetic flux value that maximizes triangular function-converting flux-to-voltage conversion ratio, and search It consists of a technique to linearize the magnetic field-voltage change by operating a flux-locked loop (FLL) that locks the magnetic flux at the set values, and this series of techniques measure the change in magnetic flux as a continuous voltage. Are automated for multichannel sensor systems.

On the other hand, the dual-relaxed squid sensor developed as the next generation squid sensor is more than 10 times the magnetic field-to-voltage conversion ratio compared to the DC squid sensor, which simplifies the sensor driving and shear amplifier circuits, which is suitable for a system having a large number of channels.

However, there is a problem in that the operation automation technology of the DC squid is impossible because its characteristics are different from the DC squid.

In other words, the dual-relaxed squid sensor, which has a much higher magnetic field-to-voltage conversion ratio than the DC squid whose voltage output changes to triangular wave form for the external magnetic flux input, has various characteristics such as the voltage output to the input changes to square wave form. As a result, the conventional DC squid control method cannot be adjusted or is inefficient. Therefore, the bias current search and the FLL operation of the sensor must be manually stored by a human.

On the other hand, there is a serial communication protocol developed for the purpose of writing values related to the FLL operation including the bias current value from the terminal to the squid sensor controller. However, since it is not automated, humans adjust and determine the value.

Therefore, the operation of the dual-relaxed type squid sensor is operated manually for each sensor while measuring the sensor output using an input flux generated by a person having expertise in the characteristics using a separate coil. There is the inconvenience of having to search for, determine and adjust the values.

As a result, as described above, the double-relaxed squid sensor should be manipulated by an expert person, and in the case of the medical multi-channel squid system used for measuring brain mass, the number of sensors ranges from tens to hundreds, so the manual adjustment method It is unreasonable to apply the system to a patient system of a hospital requiring quick and easy adjustment.

The present invention is to solve the above problems, and the operation adjustment of the dual-slack oscillation type squid sensor is automated without the need for an external device such as an external input flux applying coil and manual operation, the operation adjustment of the multi-channel squid sensor system Its purpose is to make it quick and easy.

In addition, the present invention is to control the automatic adjustment of the sensor using the serial communication between the sensor controller and the terminal, and to compensate for the slow speed that is a disadvantage of the serial communication by transmitting as few control signals as possible to adjust the operation required for the sensor It provides a set of control methods to complete the process.

As a means for achieving the above object,

First, the FLL driving method of the multi-channel double-relaxed squid sensor generates an external magnetic field. Reading the output voltage at four points at equal intervals to obtain the maximum (Vmax) and minimum (Vmin) of the output voltage change; Calculating a voltage change range (swing) and an intermediate voltage (field-to-voltage conversion slope maximum, V center) therefrom; Inputting an intermediate voltage value (V center) into the offset bias voltage value (9) to make the intermediate voltage value a reference point of the integrator; In this state, the external magnetic flux interval Finding a rising magnetic flux point (17, Vf) at which the voltage value begins to increase at a minimum while increasing (16) finely; {(Vcenter-Vf) / (Vf-Vmin)} × at rising flux point 17 It is characterized by the step of writing the difference (18) as much as the magnetic flux bias generation voltage value 12 and operating the FLL.

In addition, a method for obtaining an optimum bias current of a multi-channel double-relaxed squid sensor includes finding a point 19 where the voltage width exceeds the initial noise level and disappears after the linear increase; Designating a region of interest between the bias current value 19 (Ib, min) at which the initial voltage width occurs and the bias current value Ib, max being about 70% of the maximum voltage width; Determining a degree of white noise while reducing the bias current at minute intervals from Ib, max to Ib, min when the range is determined; Reducing the bias current to operate the FLL and measuring white noise if the output is overloaded 21 near Ib, max; Subsequently, when the output current is overloaded because the bias current is too small in the vicinity of Ib and min, the process is terminated at this point.

1 is an equivalent circuit diagram of a double-relaxed squid sensor and an FLL circuit applied to the present invention.

2 is an exemplary diagram illustrating a serial transmission protocol of various squid parameters.

3A and 3B are conceptual views for explaining a bias voltage value and a bias magnetic flux value determining method for an FLL operation for application to the present invention.

4A and 4B are conceptual views for explaining determination of an optimum bias current value of a sensor for application to the present invention.

Explanation of symbols on the main parts of the drawings

1: detection coil 2: input coil

3: signal squid 4: reference junction

5: Dua 6: DC applied current

7: DC shear amplifier 8: integrator

9: offset bias voltage value 10: output line

11: switch 12: magnetic flux bias generation voltage value

13: Return coil

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. First, in adding reference numerals to the components of each drawing, it should be noted that the same reference numerals are used as much as possible even if displayed on different drawings. In addition, in the following description of the present invention, if it is determined that a detailed description of a related known function or configuration may unnecessarily obscure the subject matter of the present invention, the detailed description thereof will be omitted.

First, the FLL operation circuit developed for the dual-relaxed squid sensor applied to the present invention is shown in FIG. 1, and the serial communication protocol between the terminal and the controller is shown in FIG.

As shown in FIG. 1, the double-relaxed squid sensor comprises a signal squid 3, a reference junction 4, a magnetic flux feedback coil 13, an input coil 2, and a detection coil 1. The circuit for signal processing is composed of a bias DC current applying unit 6, a DC shear amplifier 7, an integrator 8 and the like. The input coil 2 and the detection coil 1 are also referred to as a flux converter.

Referring to the operation of the double-relaxed squid sensor and the signal processing circuit unit configured as described above, first, the magnetic flux signal to be measured is applied to the detection coil 1 and transmitted to the squid sensor through the input coil 2.

In this case, the squid sensor applied to the present invention uses a double-relaxed squid having a magnetic flux-to-voltage conversion factor 10 times larger than a DC squid in order to simplify the driving circuit.

The double-relaxed squid has a signal squid (3) and a reference junction (4) connected in series. The signal squid (3) and the reference junction (4) are operated by superconducting at low temperature, and thus, 4K liquid helium It is contained in the dua (5).

A bias DC current applying unit 6 is used for driving the sensor, and the squid output is directly detected by using a DC shear amplifier 7 across the reference junction 4.

Since the detected voltage varies nonlinearly with respect to the magnetic field, in order to obtain a linear response, the current flowing through the magnetic flux converters 1 and 2 is made constant so that the magnetic flux applied to the signal squid 3 is made constant.

In order to make the current flowing through the flux converters 1 and 2 constant as described above, a current proportional to the voltage value accumulated in the integrator 8 is transmitted to the flux converters 1 and 2 through the feedback coil 13. The magnitude of the signal magnetic flux can be known by maintaining a constant magnetic flux and reading the accumulated voltage of the integrator 8 through the output line 10.

On the other hand, the serial transmission protocol used to control the squid driving circuit is composed of three transmission lines as shown in FIG. 2, and transmits one command data string consisting of a total of 32 bits at a time through the serial transmission line, and each data string performs a switching function. You can change the operation value of one of four.

In particular, the last bit LSB of the switch bits can operate and stop the FLL by turning on and off the switch 11 which starts and ends the feedback operation of the integrator output to the flux converters 1 and 2 inputs.

For automatic sensor adjustment, the sensor operating values changed by the serial communication are the bias current value 6, the offset bias voltage value 9, the bias magnetic flux generating voltage value 12, and the like.

The bias current value 6 is a DC current value applied for the operation of the squid, and the offset bias voltage value 9 is for adjusting the DC output voltage level of the DC shear amplifier 7, and the magnetic flux bias generation voltage value Reference numeral 12 adjusts the magnetic flux applied to the magnetic flux transducers 1 and 2.

As a result, the automatic operation adjustment for the squid sensor can be divided into the process of driving the FLL and the process of finding the optimum bias current value.

Hereinafter will be described the technology of the present invention for the method of driving the FLL.

In the double-relaxed type squid sensor, the output voltage of the magnetic flux quantum FLL operation in the external magnetic field corresponding to the rising edge of the square wave, which has a sharp increase in output voltage to the magnetic field, for the same signal polarity as the sensitive change in the output voltage to the external magnetic field. Should start.

The operation of the FLL is a feedback to the input of the integral output having the offset bias voltage value 9 as a reference point in the analog circuit as shown in Fig. 1, so that the capacitor inside the integrator is saturated when the starting point of integration is largely different from the reference point. Because the FLL operation can fail, you should start the FLL operation as close to the reference point as possible.

Therefore, the present invention finds the point where the output voltage first rises while increasing the external magnetic field in order to operate the FLL as close to the reference point as possible with as few signal strings as possible.

Since the output voltage of the sensor changes periodically, starting with an arbitrary magnetic field (14), as shown in FIG. By reading the output voltage at four points and increasing them at equal intervals, you can always find the maximum and minimum of the output voltage change, and calculate the voltage change range (swing) and the intermediate voltage (the maximum point of the magnetic field-to-voltage conversion slope). .

Subsequently, an intermediate voltage value V center is written to the offset bias voltage value 9 to set the intermediate voltage value as a reference point of the integrator (15).

In this state, as shown in FIG. While increasing finely (16), find the rising magnetic flux point (17) at which the voltage value begins to increase at the minimum.

If the voltage at this point is Vf, the integral starting magnetic flux point can be approximated by extrapolating from the intermediate voltage and the minimum voltage Vmin previously obtained.

That is, {(Vcenter-Vf) / (Vf-Vmin)} × at the rising magnetic flux point 17 × The difference (18) plus the value of the magnetic flux bias generation voltage (12) and write the FLL (11).

For reference, the difference between the external magnetic flux value and the reference at the time of FLL operation is integrated and appears as a DC offset of the output, which can be corrected by the magnetic flux bias generation voltage value 12 after the FLL operation.

On the other hand, squid sensors usually have different noise characteristics according to the operating bias current (6). This is a characteristic determined in the manufacturing process, and thus the level of the noise level according to the bias current for one sensor is usually unchanged and is characteristic. To measure the weak magnetic field, the sensor is operated at the bias current with the lowest noise.

That is, the bias current value is changed and the white noise of the sensor output (RMS value of the output over 100 Hz) is obtained and compared.

However, since the measurement of white noise at one bias current value involves the operation of the previous FLL (FIGS. 3A, 3B), it is time consuming to obtain this process by serial communication for all bias current values. In order to solve the problem, in the present invention, in order to obtain an optimum bias current value in a short time, the range of the bias current value is limited.

That is, in the case of a general double-relaxed squid, as the bias current increases, the maximum and minimum voltage widths (swings) linearly increase and then disappear suddenly (FIG. 4A). In the present invention, the measurement of the bias current with a finite swing is performed. To increase the bias current value at relatively large intervals, find the point 19 where the voltage width exceeds the initial noise level and disappears after the linear increase using the processes of FIGS. 3A and 3B.

At this time, when the swing is large (20), since the flux voltage conversion ratio is so large that the integrator malfunctions and the FLL operation fails, the bias current value (19; Ib, min) that generates the initial voltage width is about 70% of the maximum voltage width. Only between the bias current values Ib and max is defined as the region of interest.

Then, when the range is determined, the bias current is reduced at minute intervals from Ib, max to Ib, min, and the degree of white noise is measured (FIG. 4B).

At this time, since the FLL fails because the magnetic flux voltage conversion ratio is large in the vicinity of Ib and max for the first time, the output is overloaded (21) and the FLL operates as the bias current is reduced, so that the white noise can be measured.

Subsequently, if the bias current is too small in the vicinity of Ib and min, the FLL operation fails and the output is overloaded (21), the process ends at this point.

Since the bias current values Ib and optimun that give the lowest noise do not change, they are determined once and stored in a file, and from the next experiment, the FLL process can be completed quickly by using the stored optimal bias current value.

By using the bias current values of the sensors stored in the sensor file obtained by the optimum bias current determination method as described above, the FLL operation value determination process (clock frequency for serial signal transmission; 500 Hz) is performed in a magnetic shield room (magnetic shielding rate; 250). @ 0.01Hz, 1000 @ 1Hz,> 10000 @ 10Hz) The hemispherical 37-channel magnetometer sensor system was applied to complete the operation adjustment of all signal channels in 2 minutes and 8 seconds.

Under the same conditions, it took more than 30 minutes to operate FLL by manual adjustment after installing coil for external magnetic flux.

If it is necessary to open the magnetic shield door for simple experimental device change or patient posture change indication, the operation of FLL must be stopped due to the influx of large magnetic noise, so close the shield door and determine the FLL operation value again. It should work.

However, when the magnetic field change around the sensor is not serious, most of the sensors operate normally even if the FLL of the sensor controller is operated without re-determination of the operating values using the determined FLL operating values.

In this case, it took only 9 seconds to operate the 37-channel FLL.

On the other hand, since the method proposed in the present invention utilizes the characteristics of the normally operated double relaxation oscillation type squid sensor, the process of determining the optimum bias current value fails in case of a sensor which does not operate normally due to manufacturing problems or electrostatic damage.

Thus, the success or failure of this determination process enables normalization of normal, poor judgment and operational stability of the manufactured sensor.

That is, the above method can be applied to automating normal and bad operation check of the squid sensor mass produced by the thin film manufacturing process.

As an example, satisfactory screening results were obtained by applying this method to device screening of a double-relaxed squid current amplifier requiring stable operation.

Using the present invention as described above, by automating the operation adjustment of the sensor, there is an effect that it is possible to obtain a signal quickly and simply in the multi-channel biomagnetic measuring apparatus, compared to the conventional manual method.

In addition, since the adjustment method is based on the serial communication method, the device is simple even for a multi-channel system, and the optimized sensor parameter determination process enables quick completion of the squid sensor operation adjustment. It is simple because no additional device is required for adjustment.

In addition, this method is very useful when developing a medical biomagnetic measuring system using a squid sensor such as brain map or core map for patient care.

Although the present invention has been described in connection with the above-mentioned preferred description, various other modifications and variations may be made without departing from the spirit and scope of the invention. Accordingly, the appended claims may be determined to include such modifications and variations as fall within the true scope of the invention.

Claims (2)

External magnetic field Reading the output voltage at four points at equal intervals to obtain the maximum (Vmax) and minimum (Vmin) of the output voltage change; Calculating a voltage change range (swing) and an intermediate voltage (field-to-voltage conversion slope maximum, V center) therefrom; Inputting an intermediate voltage value (V center) into the offset bias voltage value (9) to make the intermediate voltage value a reference point of the integrator; In this state, the external magnetic flux interval Finding a rising magnetic flux point (17, Vf) at which the voltage value begins to increase at a minimum while increasing (16) finely; {(Vcenter-Vf) / (Vf-Vmin)} × at rising flux point 17 And writing the difference (18) as much as the magnetic flux bias generation voltage (12) and operating the FLL. Finding a point 19 where the voltage amplitude exceeds the initial noise level and disappearing after linear increase; Designating a region of interest between the bias current value 19 (Ib, min) at which the initial voltage width occurs and the bias current value Ib, max being about 70% of the maximum voltage width; Determining a degree of white noise while reducing the bias current at minute intervals from Ib, max to Ib, min when the range is determined; Reducing the bias current to operate the FLL and measuring white noise if the output is overloaded 21 near Ib, max; Thereafter, when the output is overloaded because the bias current is too small in the vicinity of Ib and min, the process is terminated at this point. The optimum bias current of the multi-channel double relaxation oscillation squid sensor is obtained. Way.