Stylus-Drag..Fact or Fiction?


Most audiophiles can't seem to believe that a tiny stylus tracking the record groove on a heavy platter could possibly 'slow-down' the rotating speed of a turntable.
I must admit that proving this 'visually' or scientifically has been somewhat difficult until Sutherland brought out the Timeline.
The Timeline sits over the spindle of the rotating disc and flashes a laser signal at precisely the correct timing for either 33.33rpm or 45rpm.
By projecting these 'flashes' onto a nearby wall (with a marker attached)....one can visualise in real-time, whether the platter is 'speed-perfect' (hitting the mark at every revolution), losing speed (moving to the left of the mark) or gaining speed (moving to the right of the mark).

RAVEN BELT-DRIVE TT vs TIMELINE 
Watch here how the laser hits the mark each revolution until the stylus hits the groove and it instantly starts losing speed (moving to the left).
You can track its movement once it leaves the wall by seeing it on the Copperhead Tonearm.
Watch how it then speeds up when the tonearms are removed one by one....and then again, loses speed as the arms are dropped.

RAVEN BELT-DRIVE TT vs TIMELINE
Watch here how the laser is 'spot-on' each revolution with a single stylus in the groove and then loses speed as each additional stylus is added.
Then observe how....with NO styli in the groove.....the speed increases with each revolution (laser moves to the right) until it 'hits' the mark and then continues moving to the right until it has passed the mark.

Here is the 35 year-old Direct Drive Victor TT-81 turntable (with Bi-Directional Servo Control) undergoing the same examination:-
VICTOR TT-81 DD TT vs TIMELINE 
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"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.

Thx Phoenix
Great post which actually makes sense to myself. I must admit some prior posts have shot right over my head but now I am trying to think in industrial terms which is something I am at least vaguely familiar with.
We rotate, amongst other things, 900 to 1200lb steel rollers with a required very high degree of speed accuracy precision.
Think in terms of  3 phase 5 to 25hp motors for this task.
Current feedback has proven very effective at precise control AND eliminating the pesky noise problem sometimes seen on 0-10vdc and 4-20ma control feedback circuits.
Hi phoenixengr. why do you think  VPI use trapezoidal commutation?
Surely this is not optimum if they want to minimize torque ripple. 


Cheers

"....why do you think VPI use trapezoidal commutation?
Surely this is not optimum if they want to minimize torque ripple."



The controller is made by Elmo Motion Control (Gold Twitter Solo), and while Elmo's PC control software is fairly comprehensive, I'm not sure if the controller is capable of sinewave drive;  it is not capable of sensorless operation (FOC) which would use sinewaves.  Sinewave drive is considerably more complicated and would result in smoother operation, but for whatever reason, VPI chose to operate the motor in the simplest mode available, as a DC motor which uses block commutation and Hall sensors. 


They do some other rather strange things:  The motor is designed for high power (500W) so the windings are 0.3R;  not the easiest load to drive especially from a class AB output amp.  The Elmo controller uses PWM output at 10kHz, which IMHO is a bad choice for use around a sensitive audio circuit like a cartridge pickup (especially MM).
I am getting a sneaky sensation that phoenixengr just might know a thing or two about TT motors.