HELMHOLTZ COIL INSTALLATION AND CALIBRATION
of secondary fields which may affect the signal within the coil. The closer the material is to the coil the
worse the interferences will become. The presence of conductive material should therefore be carefully
considered and avoided where possible. For example, the coils should not be set on the ground (where
metal beams or reinforcing bars may be located), or on or close to metal shelves and tables.
In both cases, the size of the object will dictate the distance of clearance to the coils. In order to
determine whether a site is suitable, a site survey using one or more magnetometer(s) is recommended.
This can be typically be carried out using Bartington’s Spectramag
-6 and two Mag-13 or Mag690 sensors.
The use of two sensors can allow to perform gradient measurements to determine the differences
between two points, typically the centre and edge of the homogeneous volume of the coil at the planned
site. A separate site survey document is also available.
To reduce the impact of local field distortion, the use of the CU2 closed-loop module can enable
external distortion to be cancelled out within the coil at the location of the reference magnetometer
typically mounted in the centre of the coils. However as only one reference sensor is used, the CU2 closed-
loop module does not cancel out the presence of gradients within the coils. Any gradient present will see
itself being shifted up or down so that the point where the reference magnetometer is located read 0 (or
whatever value field is applied to the coils).
Typically, this will mean having a clearance of a few meters (also coil dependent, the larger the coil,
the larger the clearance area) from structures, shelves, phones, laptop etc… and an increased clearance
from area accessible by vehicles, especially if these are larger vehicles.
Any equipment to be used in the vicinity of the coil (tables, c
hairs etc…) should also be checked for
hygiene as for example swivelling chairs tend to have a relatively large magnetic signature. All these
elements can be checked either during the site survey stage, or with equipment typically supplied with
the coils.
2.
System components
A complete Helmholtz coil system will include the coils, the PA1 Power Amplifier, CU1 Control Unit
(which is used to interface the PA1 with an ADC card/PC), the CU2 Closed-loop module (optional but
recommended if operating mainly at DC and low frequency below mains power frequencies), a National
Instruments ADC card and a PC (running the operating software enabling field generation).
Additionally, a Mag-13MS (range determined by the coil used), Mag-13 cable, Mag-13 Mounting Jig
will be recommended to be used for initial system calibration, and regular system checks thereafter.
The clearance recommendations apply not only to elements external to the system, but also apply to
parts of the system. The coils are supplied with a 3m long cable that connects to the PA1. The whole length
should be used, and the PA1 should be located as far as possible from the coils, together with the CU1,
ADC card and PC.