Stefan Mayer Instruments MK-1 Bedienungsanleitung

Stefan Mayer Instruments

MK-1

Single Axis Magnetic Field

Compensation System

User’s Manual

valid for instruments with serial number 292 and higher

with auto-reset function

– January 2021 –

Stefan Mayer Instruments GmbH & Co. KG

Wallstr. 7, D-46535 Dinslaken, Germany

Tel./Fax: +49 2064 479762/3

Contents

1 Safety precautions 2

2 Introduction 3

3 Main frame with power supply and current source 4

3.1 Description ............................... 4

3.2 Speciﬁcations .............................. 5

4 Summing ampliﬁer 5

5 Magnetometer 5

5.1 Description ............................... 5

5.2 Magnetometer speciﬁcations . . . . . . . . . . . . . . . . . . . . . . 6

6 Filter module 7

6.1 Description ............................... 7

6.2 Filter speciﬁcations . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

7 Operation and adjustment 8

7.1 Gainadjustment ............................ 8

8 Conversion of Measurement Units 9

1 Safety precautions

The following safety precautions should be observed before using this

instrument:

This product is intended for use by qualiﬁed personnel who are familiar

with the safety precautions required to avoid possible injury.

Read the operating information carefully before using the instrument.

The instrument must not be operated in explosive atmosphere.

The instrument should only be used in dry rooms.

The operation of the instrument is left at user’s risk.

The instrument must be placed in such a way that it can be easily

disconnected from the power line by unplugging the mains plug.

The ventilation slots and heat sinks must not be covered. Provide suf-

ﬁcient air ﬂow for cooling. In critical cases an air fan should be used.

Before operating the instrument, make sure the power cord is connected

to a properly grounded power receptacle.

Before performing any maintenance, disconnect the power cord.

The detachable power cord must not be replaced with lower rating

types. See section 3.2 for power cord ratings.

Replace blown fuses with same type and rating for continued protection

against ﬁre hazard.

Do not exceed the electrical and thermal operating limits, as deﬁned

in the speciﬁcations section 3.2. Disregard can cause destruction of the

instrument and consequential damages.

If the instrument is used in a manner not speciﬁed by the manufacturer,

the protection provided by the instrument can be impaired.

Stefan Mayer Instruments shall not be liable for any direct, indirect,

special, incidental or consequential damages arising out of the use of its

instruments.

The instrument and this manual may be subject to alterations without prior notice.

2 Introduction

The MK-1 is a single axis magnetic ﬁeld compensation system designed for image

improvement in magnetic resonance imaging (MRI) and other applications where

feedback magnetic ﬁeld control systems are not suitable. The system reduces slow

disturbances, e. g. caused by trams, elevators or vehicles, as well as magnetic noise

produced by power lines by establishing a counteracting magnetic ﬁeld with the

help of suitable compensation coils.

The magnetic ﬁeld disturbances are measured with sensitive ﬂuxgate magnetome-

ters in the frequency range from DC to 1 kHz. Sensitive DC magnetic ﬁeld sen-

sors cannot be placed close to an MRI scanner due to its large DC magnetic

ﬁeld. Therefore, the sensors are usually placed outside of the shielded MRI room,

whereas the compensation coils are installed inside the MRI room. The MK-1 is

a feed-forward controller with manually adjustable gain.

Figure 1: Operating principle of the compensation system MK-1.

The magnetometer signals are ﬁltered, ampliﬁed, and added. The resulting signal

controls a current source for direct connection of compensation coils which generate

the counteracting magnetic ﬁeld.

Maximum ﬁeld attenuation is achieved by manual adjustment of the system gain.

The gain for slow signals up to 10 Hz and power line frequencies 50 Hz and 16.7

Hz (or 60 Hz, depending on version) can be separately adjusted. The phase of the

power line frequency signals is adjustable too.

The instrument consists of a 19 inch main frame unit with power supply, a current

source for direct connection of compensation coils, ﬂuxgate magnetometers and

ﬁlter modules. The basic version contains one magnetometer and one ﬁlter module.

It is possible to install up to three magnetometers and ﬁlters. Compensation coils

are not part of the MK-1 and must be provided separately.

The mains power connector and switch as well as connectors for sensors and com-

pensation coils are located on the rear panel. The outer conductors of the BNC

connectors on the front panels are all connected with the case which is grounded.

This is important to know when connecting an external device like an oscilloscope.

In the following section the individual components of the instrument, their spec-

iﬁcations and operation is explained. After that the setup and adjustments are

explained.

3 Main frame with power supply and current

source

3.1 Description

The main frame contains on the right side a power supply unit of type Kniel low

emission CA 15.4 with automatic input range switching. The power line receptacle

with power switch and two fuses is located on the rear panel. Further fuses are

inside the power supply unit. The input power is 150 W.

Caution: The instrument must not be powered with any voltage other

than 216 to 240 V or 108 to 120 V, 50 to 60 Hz AC. The line cord must

be properly grounded.

The current source for connection of compensation coils is located at the backside

of the instrument. The compensation coils must be connected to the removable

clamp connector COMPENSATION COILS near the backside heat sink. The

minimum allowed DC resistance of the connected compensation coils is 3.0 Ohms.

Caution: Because of the current source’s diﬀerential output any load

connected to compensation coils connector must be ﬂoating and must

not be grounded.

The ventilation slots and heat sink must not be covered. Provide suf-

ﬁcient air ﬂow for cooling during operation. In critical cases an air fan

should be used.

3.2 Speciﬁcations

Current source gain 0.5 A/V

Output current max. 2.5 A

Minimum load resistance ≥3.0 Ω

Compliance at 2.5 A typ. ±7 V

Power supply built in, 115/230 V ±10%,

AC 50 to 60 Hz, max. 2.0 A

Power cord ratings 250 VAC, 10A

Operating temperature 0 to 40 ◦C

Dimensions of main frame enclosure 19 inch, 3 HU, 310 mm depth

Weight 8 kg

4 Summing ampliﬁer

The summing ampliﬁer (module AMPLIFIER) is located near the power supply

unit. The magnetic ﬁeld compensation system must not be operated without

ampliﬁer module installed because the ampliﬁer also provides the system’s analog

ground reference signal. The ampliﬁer adds and ampliﬁes the signals of up to three

magnetometer/ﬁlter modules. The ampliﬁer’s gain can be adjusted in steps from

0 to 11. The LED bar graph display shows the DC drive level of the ampliﬁer.

The display characteristic is non-linear. The ampliﬁer output signal is fed to the

BNC connector OUT.

The ampliﬁer module contains a window comparator which automatically resets

the magnetometers in case of an overrange condition, see section 5.1.

5 Magnetometer

5.1 Description

The measurement range of the ﬂuxgate magnetometer is ±199.9 µT. The mea-

surement direction is axial, pointing away from the sensor cable. The sensor is

labeled with the serial number and a letter A, B, or C. It must be connected to

the corresponding receptacle SENSOR A, SENSOR B, or SENSOR C on the rear

panel. If two or three magnetometer modules are installed, then they must not be

interchanged.

The digital display shows the DC magnetic ﬁeld in µT. The display range can

be changed between ±19.99 µT and ±199.9 µT with the switch RANGE. The

magnetometer is zeroed by pressing the RESET button. The actually measured

DC magnetic ﬁeld component is subtracted from the magnetometer signal. This

procedure is indicated by the RESET LED and takes four seconds. After zeroing

the display shows a small value. The maximum ﬁeld which can be zeroed is

±500 µT (5 Gauss). The magnetometer is automatically reset when the ampliﬁer

detects an overrange condition, see section 4. The auto-reset function can be

deactivated by replacing a jumper on the ampliﬁer board.

The analog output signal at the BNC connector has a bandwidth of 1 kHz (−3 dB).

The scale factor is 3 V/100 µT in range 199.9 µT (RANGE switch up) and

3 V/10 µT in range 19.99 µT (RANGE switch down). The scale factor of the

signal internally routed to the ﬁlter module does not depend on the range switch

position and is 3 V/100 µT.

5.2 Magnetometer speciﬁcations

Display 3.5 digits LED display with sign

Measurement range ±19.99 µT and ±199.9 µT

Resolution 10 nT

Noise <0.7 nT RMS

Accuracy 0.5% ±25 nT ±1 Digit

Operating temperature 0 to 40 ◦C

Max. ﬁeld at sensor ±500 µT

Analog output 0.03 V/µT or 0.3 V/µT depending

on range switch position, short cir-

cuit protected, BNC connector

Output voltage range −6 to +6 V

DC output impedance 100 Ω

Bandwidth 0 to 1 kHz (–3 dB)

6 Filter module

6.1 Description

The ﬁlter module provides analog ﬁltering of the magnetometer output signal. It

contains two parallel band-pass ﬁlter sections for 16.7 Hz (or 60 Hz, depending on

version) and 50 Hz, a DC channel, and a summing ampliﬁer. The output signals

of the individual ﬁlter sections can be switched separately and with selectable

polarity to the input of the summing ampliﬁer. The gain of the ﬁlter sections

can be adjusted between 1 and 11 with 10-turn potentiometers. The phase of the

band-pass ﬁlters is adjusted with the trimmers Φ.

The DC channel is ﬁltered with a three pole 10 Hz Bessel low-pass ﬁlter. Jumper

settings can be changed on the ﬁlter board in order to change further parameters.

Figure 2: Jumper settings on the ﬁlter board

6.2 Filter speciﬁcations

Band-pass ﬁlter type Biquad active ﬁlter, 2nd order

Bandwidth (−3 dB) ∼2.6 Hz

Center frequency 16.7 Hz or 60 Hz and 50 Hz

Phase adjustment range −120 ◦to 0 ◦and −30 ◦to +90 ◦

Channel separation >20 dB

Operating temperature 0 to 40 ◦C

7 Operation and adjustment

Before the instrument is switched on with the rear side power switch, the sen-

sor(s) and compensation coils must be properly installed and connected to the

corresponding receptacles. Turn the rotary gain switch on the ampliﬁer front to

position 0. When power is applied to the system, the digital displays and the

LEDs on the ampliﬁer and power supply front turn on.

The magnetometers can be zeroed by pressing the RESET buttons.

7.1 Gain adjustment

Gain adjustment can be performed with the help of an external magnetometer

as follows: First, the external magnetometer must be put at a place inside the

compensation coils, where maximum compensation is required. Special induction

coil magnetometers are available for MRI applications. The induction coil is placed

at the isocentre of the MRI magnet. If more than one ﬁlter channel (DC, 16.7 Hz,

or 50 Hz) have to be activated it is recommended to start with the DC channel.

Activate a ﬁlter channel by switching the corresponding toggle switch to + or −.

Leave every channel activated while adjusting the next one. The 10-turn poten-

tiometers on the ﬁlter modules should be turned to approximate center position

(position 4 to 6). Then, the ampliﬁer gain is increased by turning the rotary switch

on the ampliﬁer panel clockwise until maximum ﬁeld attenuation is achieved (when

the signal changes of the external magnetometer are minimum). If the signal does

not decrease with increasing gain, then reverse the polarity of the ﬁlter channel by

changing the corresponding toggle switch position and try again.

Finally, ﬁne tuning of the gain is performed by changing the position of the ﬁlter

10-turn potentiometers and phase trimmers.

8 Conversion of Measurement Units

Magnetic induction B

Unit 1 Tesla (1 T)

1 T = 104Gauss = 1 kg s−2A−1

1µT = 10 mG

Magnetic ﬁeld strength H

Unit 1 A/m

1 A/m = 4π×10−3Oersted

Connection between Band H

B=µ0Hfor non-magnetic environment

Vacuum permeability µ0= 4π×10−7m kg s−2A−2

Units:

1 A/m = 4π×0.1 µT / µ0'1.3 µT / µ0

1µT'0.8 A/m ×µ0

1µT = 10 mOe ×µ0.

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