Fieldmill Voltmeter MK 2

The fieldmill voltmeter detailed here is the second version I have built and is a logical follow on from my original prototype unit. The operating theory is identical to that previously described here.

The following sections show various pictures of my MK2 fieldmill voltmeter. In terms of general construction it is very similar to the original prototype and none of the component parts are particularly special. The main changes are the materials used for the sense electrode and the rotating vane, both now made from sheet brass. The outer screening can is now a more substantial aluminium housing, made from an empty compressed gas canister. Other differences are in the electronic signal processing which will now allow the polarity of the high voltage being measured, to be determined. Details of these changes will be found in the following sections.

Click on small images to see a larger version.

The photo on the left shows the Fieldmill head assembly mounted in its' aluminium screening can. The rotating, earthed brass vane can be seen with the brass measuring electrode (almost completely exposed) directly below. The number of vanes has been doubled from that of the original prototype. Thus for the same motor speed, the signal frequency will have been doubled. At the top left corner of the picture, can be seen the opto electronic sensor which senses the position of the rotating vane. An infra red emitter and an infra red sensor are positioned such, that when the rotating vane passes between them, the infra red beam is broken and the infra red detector senses this and outputs a signal to the electronic signal processing board. It is important to be able to determine the position of the rotating vane, in order for the polarity of the incident field to be determined. The overall diameter of both the vane and the measuring electrode is 67mm.

The motor is directly mounted to the acrylic base plate and the ground plane is also supported from the same base plate, supported on brass pillars. This form of construction, the motor and the motor drive pcb are very similar to that used on the protype version.There are three cables to be seen exiting from the head assembly. At the top of the picture is a 2 pin connector which is the supply connection to the motor control pcb. The gold plated coaxial connector is connected to the sense electrode with the screen connected to the ground plane immediately below the sense or measuring electrode. The cable takes the signal from the sense electrode to the signal processing board. The 5 way ribbon cable (lower rhs of picture) is connected to the opto electronic sensor at the top of the assembly. The opto sensor senses the position of the rotating vane, and is positioned such that a negative signal is generated when the rotary vane is clear of the sense electrode. Conversely, when the rotary vane is shielding the sense electrode, a positive signal is generated. Thus a bipolar squarewave is generated, phase locked to the rotary position of the vane.
The photo left shows the general assembly of the fieldmill in its aluminium screening can. On the RHS at the front can be seen the opto assembly used to synchronise the electronics to the position of the rotating vane.

Mounted on the rear is the plastic enclosure housing the fieldmill electronics. The single 9 pin "D" connector is used to connect the dc supply, the output signal and also the poolarity select switch wiring.

This view shows the electronics housing. The uppermost board is the signal processing pcb which carries all the circuitry necessary to amplify the low level signal from the sense electrode. It also uses the signal from the opto sensor to synchronise the electronics to the rotary position of the vane. Centre right is the coaxial connector which connects the amplifier input to the sense electrode. Located top right on the pcb is the DIL switch which selects the front end gain, along with the gain setting resistors. Bottom right is the dual op amp which comprises the front end amplifier and the second stage amplifier. Centre top is the quad analogue switch IC which is used to rectify the processed signal. Lower centre is another dual op amp, one half of which is used with the synchronous rectifier, the other half forms a low pass filter. Below this and out of sight is the power supply pcb.
 

See Next Page for circuit details.


This page last updated 23/11/2003