An optical interrupter or reflective sensor is used to determine RPM from the lathe spindle.  A slotted wheel or a disk with a tooth is used to break the beam or reflect a light to a photo sensor.  The reflective type of sensor has very poor gain and can be overwhelmed by electrical noise from stepper motors or DC servo switching.  The best way to combat the noise is to convert the optical sensor signal as soon as possible into a differential signal based on the EIA-485 specfication.

The ELS has the option of having an RS485 chip installed in addition to the signal comparator.  Or, alternatively,  the signal can be transmitted as RS485 and then converted back into TTL for input into an interface board before it is sent to the ELS.

The photos here show how this is done.


First we have the two little boards that interface to the slotted optical sensor and convert the signals to/from RS485.

Next is the pair connected to the stock ELS.

Finally only the sending unit is used and the ELS is populated with the RS485 receiver.

This photo is my encoder test setup.  A small surplust 12V electric model car motor with a disk and a single slot run with a small 555 Timer base PWM driver good to 150VDC and 10A.  I can test both the reflective and slotted sensors.  In this picture the surplus slotted sensor is connected to the RS485 driver which is connected directly to the optional RS485 receiver on the ELS.   Shouldn't be needed except in high noise environments.

These little boards are just prototypes and are two channel so that a high count slotted spindle sensor and a one pulse per rev sensor can be used together.  Or a two channel TTL quadrature sensor that is perhaps sensitive to noise.

Here's a link to a drawing of an simple interface to a quadrature encoder with index pulse that could connect to both EMC and the ELS.

Example Encoder Interface

And here's a set of links to the schematics and layout overlay of the above modules.

Transmitter Schematic  and Layout

Receiver Schematic  and Layout

I've had a bit of trouble trying to figure out how to add an spindle encoder to my South Bend 10L lathe.  As you can see from the picture,  the cast iron shroud around the gears is a really close fit.  There isn't a lot of room to do anything without modifying the cover.

I did some measuring and discovered there was enough clearance to create a sleeve that would slide over the spindle and still leave room for the collet closer.  A quick sketch with Alibre CAD and I think I can whip this up and create a disk that has both 100 holes and a slot for the index.  A small sheet metal shroud will have to be constructed to protect it but it looks like this may work.  Stay tuned.  The PDF has 3D capabilities with the correct Adobe Add On so you can look at it and rotate it to see if from all sides.  Just click on the image.


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