Hemholtz coil positioning

[John PurdyParticipant @johnpurdy78347]

John Haine

I have finished the new suspension with the carbon fiber rod,(it’s totally non magnetic), and have 3D printed some trials for the coil forms and supports. After reading your paper in HSN,I have a couple of questions about the coil positioning.

I take it that the best position is for the coils to be as close to the top of the rod as possible.

The spacing between the coils is to be equal to the radius of the coils, I assume that is measured from the centre of the coils. How critical is that, as some initial trials indicate that the rod might hit the coils with the current swing amplitude I am using, just over 4 degrees.

John

[John HaineParticipant @johnhaine32865]

The reason for using the helmholtz configuration is that the field and therefore torque is predictable. In practice if you make the spacing a bit larger it won’t make much difference but you might need a bit more drive current. But 4 degrees amplitude seems high to me, that’s 8 degrees of arc? A regulator would normally run at 1 to 2 degrees to minimise circular deviation.

[John PurdyParticipant @johnpurdy78347]

John

That 4 degrees is total arc, so just over 2 degrees amplitude.

John

[John HaineParticipant @johnhaine32865]

👍

[John PurdyParticipant @johnpurdy78347]

Have finished the new suspension unit incorporating the Hemholtz coils, (still have to wind the coils), and have temporarily joined the threaded rod for the rating nut to the bottom of the carbon fiber rod. Now to mount it on the wall and set it swinging to check the period and confirm that the threaded rod is the correct length before fixing it permanently into the bottom of the carbon fiber rod.

John

[John HaineParticipant @johnhaine32865]

That looks very neat John. I made mine before I had a 3D printer so the plastic bits were machined from Corian, and also had to fit an existing suspension bracket.

[John PurdyParticipant @johnpurdy78347]

Got my Helmholz driven pendulum set up and it seems to be working although not quite as desired, will need some mods to the drive circuit. The drive to the coils, and the timing electronics are currently just on a bread board as can be seen in the pic and video. The green LEDs are the alternating pulses to the drive coils and the yellow ones are the one second pulses to the Lavet stepper driving the clock hands. I am currently driving the coils with only 5 volts supplied to the L297 drive unit which results in only 3.395 volts to the coils which with a total resistance of 52.62 ohms results in a drive current of 64.5 ma. I started with the pendulum displaced about 32mm from the BDC, giving a total swing of just over 3 degrees of arc. Over the next 3 hours the swing slowly decreased where it stabilized at an arc of 1.51 degrees, and has remained there for the last 12+ hours.
I have a 9 volt wall wart, will try that for the coil drive voltage and see if that will maintain the full swing with occasional triggering of the over swing circuit.

Haven’t quite figured out this YouTube business, as a result the video has come out at only 240p. So if you watch it, select “Settings” at the lower right an change it to 1080p, it will look much better.

John

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