My Fieldmill Voltmeter Project

Click on the small images to see a larger version.

The electronics pcb measures approx. 58mm x 38mm and is made from prototype board. The dc to dc converter is located in the top left hand corner of the pcb along with a small brass screen. The dil switch on the rhs selects the front end gain in steps of 1, 2, 5, 10. Next to the dil swich (top right) can be seen the removeable link (LK). This connects a high value resistor (RA ) from the measuring terminal to ground. This protects the amplifier input from ESD damage when the unit is not in use. The front end op-amp is located on the lower rhs of the pcb.The small 2 pin connector (lower right) carries the ground and sense electrode connections
To the left is the schematic of the fieldmill electronics. Click on the schematic to see a larger version. See the separate parts list for component details. The functional blocks are transconductance amplifier IC1a, amplifier IC1b, active half wave rectifier IC2a and buffer/meter driver IC2b. The switches S1 to S7 allow the overall gain to be switched from 10 volt/mA to 1 volt/uA. This equates to FSDs of 1MV/m to 10kV/m. DCC1 is a dc to dc converter which converts the 5 volt input to +/- 12volts for the two op-amps. Potentiometer VR1 allows the ouput current to be trimmed to suit the meter used. Note: This simple circuit does not provide an indication of polarity. With the addition of a simple phase sensitive rectifier, this feature can be easily added.
   
Parts List:
IC1 - AD648JN, IC2 - TL072CP DCC1 - NMH0512D D1, D2 - 1N4148  
R1 - 10K R2 - 20K R3 - 50K R4 - 100K
R5 - 200K R6 - 500K R7, R12 - 1M R10, R11, - 10K
R13 - 15K R14, R15 - 100R VR1 - 20K RA - 10M
C1, C8 - 10uF C3, C4, C5 - 100nF C6, C7 - 4.7uF  
Method Of Calibration:

In order to calibrate the fieldmill voltmeter, we have to understand that what we are really measuring is in fact electric field intensity, which has units of Volts/metre. Thus in order to carry out calibration, we need a known voltage source (V) and a known separation or distance (d) to produce our voltage gradient. We also need to ensure that the fieldmill voltmeter is situated within an even, undistorted electric field. If we try to calibrate it as shown in the diagram right, the electric field or the lines of electric flux, become distorted towards the fieldmill voltmeter and false readings will result.One of the main problems encountered when trying to measure electric field intensity is the proximity effect of the field mill itself. The proximity of an object, such as the field mill voltmeter will distort the lines of electric flux. Thus when attempting to take a measurement, there will be an element of error due to the disturbing presence of the object present within the electric field. For example,when we operate the VDG generator, it is not surrounded by free space. It is by necessity either contained within or in proximity to buildings and other structures. This includes the earth upon which it stands. Hence the electric field is already distorted. When using the fieldmill voltmeter, we should endeavour to to keep as far away from the voltage source as possible, in order to minimise the effects discussed above
The basic arrangement for calibrating the fieldmill voltmeter is shown below. Two conductive metal plates form the parallel plates of a capacitor. The separation of the plates (d) is adjustable and each plate is connected to one side of a known stable d.c. voltage source (V). One plate has a hole cut centrally into it such that the fieldmill voltmeter is a close fit and the external earthed housing makes good electrical contact with the capacitor plate. When positioned correctly, the rotary vane of the fieldmill voltmeter will be positioned almost flush with the surface of the capacitor plate. The dimensions of the capacitor plates should be large in comparison to the diameter of the fieldmill voltmeter to prevent 'field fringing' effects at the edge of the plates from distorting the linear field impinging on the fieldmill voltmeter. The width and length of the plates should be something like 10 times the diameter of the fieldmill voltmeter.
 

See next page for more calibration details.

This page last updated 27/02/02