Electronic – What type of encoder is this (optical, magnetic, inductive… etc)? and how does it operate


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I have found this circuit mounted to a DC motor. It is probably an encoder but I don't know how it works. I have looked everywhere but I could not find a similar encoder assuming that this is in fact one.
The circuit is placed on a motor casing in a way that does not allow it to rotate or move. The PCB part with green solder mask is facing away from the motor. A metallic disk with a magnetic ring on top of it is attached to the motor shaft facing the green mask and is allowed to rotate freely with the shaft.
The electronic components on the PCB are caps, resistors, dual operational amplifier (IC BA728) and a regular NPN transistor (C3311).
Can someone identify the type of this encoder? or better yet explain how it operates?

(I Modified the schematic again. Thanks, curd.)

Best Answer

You'll notice that there are 8 magnets, but the "coil" (the serpentine traces) have 3-fold symmetry. This means that as the magnets rotate, the induced voltage in the pickup will vary with rotation, with a period 3 x 8 or 24 (EDIT - oops, 12, since the magnets need to alternate north and south) times the shaft rotation rate. This varying voltage is AC-coupled to the first op amp, where it is amplified and passed to the second. The second adds more gain and effectively half-wave rectifies the signal since the op amp is running single-ended. The output of the second op amp drives the base of the output transistor, so you get 24 (EDIT - 12) open-collector pulses per revolution.

Depending on the gains involved and the strength of the magnets, this will only provide reliable operation over a particular range of shaft velocities, so it is technically a shaft encoder, but unlike optical encoders it can only be used for velocity, not position, control.

It is probably used to provide precision locking of the motor to a reference speed, which cannot be done with "standard" tachometers due to voltage coefficient uncertainties and line loss, or to provide a precision rotation signal and is possibly intended for an application like use of a chopper wheel with a lock-in amplifier.