QTFM Vector Mode Operation

The vector coil attachment

An add-on three-axis coil attachment is needed for QTFM Gen-2 to output vector field measurements. The coil attachment slides over the sensor head and connects to the electronics controller with a flexible ribbon cable.

How vector mode works

QTFM is inherently a total field magnetometer and therefore it can only measure B₀. However, with the coil attachment, QTFM G2 is sequentially subjected to pre-determined B_x, B_y, and B_z offset fields. From a measurement of the shift in B₀ when the bias coil is on vs off, the vector components of the background field are mathematically calculated.

Interleaved timing and data rates

QTFM Gen-2 operates in a pulsed mode (link) in which one field measurement is made in a cycle that repeats every 1 ms. In scalar-only mode, a measurement of the total field [B₀ = √(B_x² + B_y² + B_z²)] is made in each cycle. When the hybrid vector-scalar mode is activated, the total-field and vector components are measured in interleaved cycles as shown in the figure below.

In cycles 1, 3, 5, … the sensor measures B₀. In cycles 2 and 4, B_x coil is activated from which one measurement of the B_x field component is derived. Similarly, the B_y component is derived from cycles 6 and 8, and the B_z component is derived from cycles 10 and 12. Because each vector axis consumes a pair of cycles, vector output is inherently slower than scalar output — the trade-off for getting four measurements from one sensing region.

Maximum data rates. In scalar-only mode the sensor outputs as fast as 1 ms/sample (1000 Hz). In hybrid vector mode the fastest rates are 4 ms/sample for the scalar field and 12 ms/sample per vector axis.

Benefits & limitations

Benefits

• Compared to fluxgate, QTFM vector output is extremely stable with no noticeable long-term drift or temperature coefficient
• High sensitivity. Because each component is derived from the same scalar measurement rather than read off a separate sensing element, vector sensitivity tracks the scalar performance instead of being capped by a second, less sensitive sensor
• B₀, B_x, B_y, B_z measurements from a single device and the same sensing region, so there is no baseline offset between the scalar and vector measurements

Limitations

• Designed for earth’s field operation, 10 µT to 200 µT
• Vector sensitivity degrades as the sensor head approaches a dead zone. See the dead zone guide for the affected orientations