Adjusting the Hall sensors                                                     Latest change 2017-09-12

In brief:
Adjusting the Hall sensors and their electronics.
Measuring the accuracy of the setup.

This procedure can be done in the Lab or Workshop. When the TopMount is at its final destination no additional justage should be necessary.
Assumed here is that when the magnet is placed the voltage level on the Hall sensor outputs will be lower than the midscale level they have when the magnet is not present. As the amplifiers are inverting the readout is increasing when the magnet approaches a sensor.

1/ Lock the magnet to the center with the locking tool.
2/ Verify with a non magnetic ruler that the distances between the magnet and the Hall sensors are very much the same for all 4 sensors. If not, re-adjust the PCB or one or more of the sensors.
3/ Mount the TopUnit such that the wire can be manipulated from below and that it is under so much tension that it is not bended.
4/  Set the gain to maximal with the Gain potmeters.
5/ Adjust the Offset pots for a reading of 128.
6/ Lower the locking tool.
7/ Move the wire to the North, such that the 6 mm tube just touches the edge of the hole in the lower plate. Reduce the North Gain potmeter for a reading of 250. Repeat this for South, East and West.

I was able to measure the accuracy of the Hall system with an old A3 plotter from which I only had the mechanical part (plotting bed with stepper motors) and for which I realized some new electronics to control the steppers.

Fig 1. The A3 plotter with the wire attached.

The plotter was centered below the top mount and the wire was attached to the plotting head wit a small spring.
A specially written program moved the plotting head from -200 to + 200 mm in NS and EW direction in steps of 20 mm, and registered the readings from the Hall sensors. After that the readings were graphically presented.
To prevent the magnet to wobble the movements were done very slowly, after a movement during about 2 seconds no data were registered and after that the data of 2 seconds were averaged. One complete measurement lasted around 25 minutes.

Some results below:

Fig 2. Measurement without serious calibration.
The red circles are the measuring locations, the green circles are the measured results after some scaling. The green lines indicate which result belongs to which location.

Fig 3. After the calibration procedure above.
The errors in the corners result from the magnet stem touching the lower plate. A small rotaional error results from a misalignment between the plotter and the topmount.

The height of the clamp in the topmount was here 134 cm above the plotter head and the amplitude was 10 cm. When I map the amplitude/height ratio of the pendulum in my home I find the dotted circle to represent the actually used part of the range. The ratio of the pendulum at Vrijland will be much the same. For the time being I accept these residual errors.