My last project entry described a single-core fluxgate, and showed that it can measure primary currents. It also showed that the fluxgate injects a few hundred mV of ripple into the primary circuit. This entry describes a two-core fluxgate which offers reduced ripple injection into the primary.
First up, let's ask where this injected voltage (I'm calling it Ep, short for 'EMF injected into the primary' (EMF = Electro-Motive Force)) comes from. As we saw in the previous post, it is a funny-shaped waveform with a 5 kHz fundamental frequency. This is a dead giveaway; the only 5 kHz singal source around is the excitation supply. Therefore we deduce that Ep is caused by the transformer action of the fluxgate. We cannot avoid this action, because we have to excite the fluxgate in order to measure anything.
But we can compensate for it. We can do this by adding an opposing EMF to the circuit. A good way to do this is to add a second fluxgate with opposite 'polarity' to the first as shown:
We now have a vastly reduced Ep term - theoretically zero (we will have a look at component tolerances later). Also, we now have the opportunity to measure the difference between Vm1 and Vm2, which we see is (in principle) zero Volts at zero primary current:
And if we have a 1A primary current Ip, we get some nonzero quantites: the voltage between Vm1 and Vm2 is nonzero and Ep is nonzero.
If we sweep Ip over a -3A to +3A range (this is the X axis), and look at the average of Vx (i.e. output of a low pass filter) and the RMS value of Ep, things look pretty good:
Our current to voltage gain has doubled, and Ep is vastly reduced compared to the original 320 mV RMS. In fact, it should go to zero when Ip = 0 (note that component tolerances are not yet considered). So, we have gotten both more output signal and less voltage injection into the primary circuit, which is most welcome.
I must confess, I avoided one imporant issue in the first post: current measurement range. I mentioned in the project description that I was interested in a 25 A range. Let's simulate that...
Oh ****. Our sensor is nice and linear in the -3A to +3A range, but all kinds of nonlinear outside that (not sure what the glitch around +12A is, but I don't like it). This is not going to work as desired.
The solution: closed-loop current sensing.
See you next time