Servo Controlled Arm

I have a Sony PS X800 TT that has a linear tracking as well as the "biotracing (vertical servo) arm. All the criticisms of servos that audiophiles cite do not apply to a well designed servo. No hunting, no offset. The arm pivots, so the servo does not need to deal with things like off center holes. The servo moves the pivot point, and the rate of movement follows any groove spacing variation. Tracking error is less than 0.05 degree.

There are five (count em) microprocessors in the darn thing.

Dan_ed...The main points are as follows...
1...The arm pivots, just like a conventional arm. This prevents any side forces due to the servo.
2...The pivot point is moved as the record plays.
3...The rate of movement is biased to match nominal groove spacing.
4...Actual groove spacing different from nominal will cause a slight arm angle to develop. This is measured by a Hall effect sensor in the arm, and the rate of pivot point movement is adjusted accordingly.

Dan_ed...If groove spacing changes abruptly there will be a momentary change in tracking angle WITHIN THE SPEC RANGE OF 0.5 DEGREES. (Remember, the arm pivots). A properly designed servo does not overshoot or hunt. Digital servos make it particularly easy to implement the necessary algorithms, which may be difficult or impossible to do with analog components.

Hiho...An important detail about the Sony linear tracking servo is that it does not move in and out, as you suggested. It only moves in. (Unless you pick up the arm). This is because the servo only modifies the RATE of movement, which is always in. Because of this feature, bearing friction (which is very low) does not result in jerky motion. (In technical circles this is called stiction). Finally, with a servo, friction, as long as it is constant and especially doesn't change direction, is not very important. The purpose of the servo is to create a virtual frictionless bearing.

Rotarius...I think you are talking about the vertical "Biotracing" servo, which is quite separate from the linear tracking arm. Sony made pivoting Biotracer arms also.

By the way, I made a typo in my last posting. The spec tracking angle error is 0.05 degrees, not 0.5.

The Biotracer arm tracks very warped LPs with not the slightest problem. (That's why). If you lift the arm manually is has a strange "limp noodle" feel to it. The downforce remains constant dispite movement of the arm up and down as with a warped record. The downforce, VTF, is applied electronicly, so it can be tweeked while a record is playing. The arm itself is mass balanced (no downforce) and this minimizes sensitivity to vibration.

Hiho...Most people, audiophiles included, don't understand servo design, or appreciate the performance potential. Perhaps they have experienced crude servos, like early cruise control in cars, and not been impressed. Most audio manufacturers don't have people with control system expertese, and would prefer to stay with tried and true pivoting arms. Design and development of a good servo arm requires a large investment, and the small market does not justify it.

Besides, pivoting arms work well (if you can ever get them set up right).