As in the prototype fieldmill, the motor speed control is obtained by using a simple variable voltage regulator (LM317T). As the load on the motor is purely the rotating vane assembly, the load is essentially constant, hence also the motor speed for a given voltage. A more complex closed loop speed control system was considered, but was rejected on the basis that the additional complexity did not justify the small performance gains.

A centre-tapped battery supply was also adopted instead of the dc to dc converter used on the prototype. This was done mainly to reduce the possibility of switching noise, coupling into the front end, due to the close proximity and compact dimensions of the two PCBs.

The mechanical construction, although similar has several improvements, the most obvious of which is the use of a 4 bladed rotating vane as opposed to the two vanes of the original prototype. This results in a higher frequency signal for a given motor speed and makes the resulting synchronous detector, less susceptible to the effects of ac mains harmonics.

In addition, the screening can is now a more rugged aluminium housing, made from an empty gas canister. This enabled the electronics to be mounted in a sturdy plastic enclosure on the rear of the fieldmill assembly. This keeps the whole unit reasonably compact and affords the electronics a good degree of mechanical protection.

Caibration Details (Coming Soon)