spindle speed DRO using tachometer output

[dhanger]

When I search the forum for 'tachometer' I get only 10 hits so I guess this is a little discussed topic, and maybe what I want isn't possible. It appears that UCCNC only accepts encoder signals to read actual spindle speed, but my spindle motor has a tachometer output rather than encoders. In fact the mill was equipped with a DC brushed servo motor with a Servo Dynamics servo drive for the spindle motor and control and I use PWM to the drive for controlling the spindle speed. The tachometer wire from the motor is fed into the drive and in fact requires it to function (I suppose this is common but I don't have much experience with servo drives). Anyway, the motor nameplate states output of 7V/1,000 RPM, I assume but haven't checked that is DC output signal. So is there a way to convert that for input into UCCNC, maybe some auxiliary piece of hardware like possibly an analog to digital converter?

Thanks,
Dan

[cncdrive]

You need to convert the analog signal to frequency signal, then you can configure the Index channel in the Spindle settings in the UCCNC to read that signal as the spindle speed reference signal.
I'm sure that there are premade electronics/circuits for that or even an Arduino could probably easily do the signals conversion.
Just one thing you should take care is that tachometers usually giving analog signals out of the TTL Voltage range, e.g. 0-10Volts or 0-30Volts etc.
You should check the max., Voltage and you should scale down that signal or check what the Arduino or the circuit you will be using can accept.
A simpe Voltage divider resistor pair could even be used to scale the voltage down with one resistor from the signal to the other resistor and connected to GND.
And the middle point is the scaled down signal. The ratio of the resistors set the ratio of the scaling, it is a simple Voltage divider circuit.

[dhanger]

Thanks, that's about what I was thinking only I couldn't pull it all together in my mind. I'm thinking that for all the trouble involved it might be easier to try and fit an encoder to the spindle. The spindle is belt driven by the motor so the motor tach output itself would have to be run through a calculation before sending on to UCCNC, actually 2 different calculations since it's a high and low belt drive (I suppose a program in the Arduino could do that). Plus I'm not sure just how accurate all that would end up being, not that I need so much accuracy but I don't like to bother doing something that I can't be pretty confident about the result. With the encoder directly on the spindle there would be no additional calculation and should be as accurate as it reasonably could be.

Thanks for helping me work it out mentally.

Dan