Q:

What is the difference between the servo track and the line track of a motorized potentiometer? 

I have this motorized potentiometer, which I would like to control with an STM32 microcontroller. 
As far as I understood there are several parts in this device:

a potentiometer  
a DC motor
a conductive knob for touch sense

I need all of these.  There are 2 connectors.  The 2 pins connector is for the DC motor.  Nothing special there.  But the 7 pins connector is described this way:

pins 1 2 3: line track (R1)
pin T: touch sense
pins 1' 2' 3': servo track (R2)

The touch sense is straightforward.  I guess 1 2 3 is the potentiometer.  But what is 1' 2' 3' then?  I measured with a ohmmeter, and it seems to be the inverse potentiometer, which doesn't make sense to me. 
The question is: what am I missing?  I found this related question, but with no answer. 

A:

I would suppose that pins 123 are what you use for the audio or other analog channel you're controlling, and 1'2'3' are what the motor/system uses to figure out where it is on the slide.  A similar device on the Sparkfun site explains:
"The slide contains two separate 10k linear taper potentiometers so that you can use one as servo-feedback in order to read the position of the slider and use the other to control whatever your target is"

A:

Servo track has a linear dependence of the resistance on the position and is used to monitor the position. 
Line track has a complex dependence of resistance on the position and is used to adjust the analog audio signals (main purpose). 
Depending on the last symbols marking three kinds of potentiometers characteristics are shown on the last page of the datasheet. 
And about the touch sense track link. 

A:

Unfortunately the spec sheet is not very explanatory.  It appear obvious to me that the 1'2'3' servo track is to be used by the uControler to know the absolute position of the pot.  You could not do that with the 123 pins because those are used for audio signal.  Hence, 1'2'3' is repeating the same action as 123 but on a totally different pot, the Servo Pot.  I would use them for sending a DC voltage to an ADC input on the uControler.  See proposed schematic. 
Furthermore, What puzzled me is the pin T. Is it only a one wire touching the slide metal ?  One could sense a 60hz buzzing signal at user touch and detect that someone is attempting to slide the pot?  A bizarre way of doing it but could work.  Any other idea on the T pin ?   Cheers :)

