Doubling Distributor Hall Resolution

A stranger going by the handle of Red1600i was working on putting sequential fuel injection into his VW Beetle using a standard distributor with Hall sensor and an upgraded vane wheel to get accurate engine position and timing.

A+ for effort. (Both the photos are his.)

Turns out that his first attempt was by far too ambitious, with nearly 40 “teeth” machined into the vane wheel which is just 46  in diameter. Had he read a data sheet for a typical sensor and noted that the release distance between sensor and vane is just over 3 millimetres, he’d have ensured that the slots are at least 6 mm wide.

Vane wheel in distributor with Hall sensor

His second attempt was a wheel with just 12 teeth; one of them removed for providing a reference of engine position; nominally for the number 1 piston being near top-dead-centre. That worked much better. Here it is in the distributor body with Hall sensor visible in black.

The Hall sensors have a trick in that they pick up the vane more quickly than releasing it, so by altering the ratio of gap to metal, more teeth could have been fitted to a vane of the same size; as long as there’s a gap of about 6 mm between them.

There is an easier way to increase the resolution of the vane in the distributor: Add another Hall sensor offset from the first at some angle and XOR the signals from the two sensors to output to the injection/ignition controller.

Combining the two signals is nearly trivial; use a high-temperature CMOS chip; glue it, with its legs in the air, onto the carrier plate for the Hall sensors, connect to the same power supply. Hook up each Hall sensor output to an input of one of the XOR gate circuits on the chip. Connect the output of the gate to the distributor output socket. A small capacitor may be required to smooth the power supply for the chip but it can all be done without a circuit board. A blob of resin will protect the chip’s connections once it’s all connected up.

One offsets the second sensor by 180° plus half the angle subtended by the “mark” (vane or gap width if they’re the same). The red blotch in each or the following marks the missing tooth; as it’s picked up by Hall sensors A and B.

Offset Hall sensors XOR'd

The A⊕B signal has double the positional resolution of each individual sensor.

The signal train distinctly identifies when the missing tooth passed the A or B sensor. 100110 means it’s just passed B and 011001 that it’s just passed A. This allows the engine management unit to establish the phase of the engine within about one revolution when cranking to start, compared to more than 2 revolutions with just one sensor.

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