"In my industrial world, traditional motor absolute speed control was obtained by either servo motors/ controllers or motors with speed encoder feedback.
However in past few years drive controllers have become so sophisticated that now best speed regulation is obtained by running " open loop" with zero encoder feedback and using current feedback at the controller itself.
Now is this a possibility for TT speed control or is this how some are already controlled?"
It depends on the type of table. For belt drive (the LFT1 is BD), the motors are relatively high speed (300/600 RPM) with low inertia rotors so open loop speed control is possible. The SOTA Eclipse package does this with a BLDC motor run as a 3 phase AC synch motor and the motor speed is very stable and accurate. The RR tach is used to counter long term speed drift as the table warms up.
Running a direct drive motor open loop is much more difficult because of the slow speed and high inertia of the platter. With no feedback, the platter speed will wobble considerably. Most of the DD tables that I’ve seen use a rotary encoder for speed feedback and a DC servo control to drive the motor. The VPI HW40 uses a magnetic ring encoder and drives the BLDC motor as a DC type using block (trapezoidal) communtation and Hall sensors. Because of the LPF in the feedback loop, the platter speed is still susceptible to oscillations, although with a heavy platter, it will move the oscillations lower in the audio band vs the light platter DD tables of the 70’s and 80’s. The HW40 does respond to variable drag on the platter, but it is quite sluggish, slowing down for ~250mS before compensation is applied and takes another 250mS to correct, so it does little to affect W&F.
Current feedback is still feedback, but if implemented correctly, it can eliminate both the encoder and the delay in the feedback loop. Field Oriented Control (FOC) monitors the current in the windings and can compute the rotor flux position on a de-rotated frame of reference so the control loop operates at DC. The current control loop regulates the torque of the motor and a speed loop is wrapped around the current control loop (changing torque changes acceleration and therefore speed). The speed feedback comes from an estimator circuit that derives the back EMF signal (rotor speed) without external sensors and works well at low speeds as well as in the presence of high noise.
I don’t know of any mfr that uses this method, but it would be interesting to try.