What does FG servo system mean?
- Thread starter tubeboob
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Hi tubeboob,
FG servo system means "Frequency Generated Servo system" - basically a circuit generates a frequency that the motor works with to derive its revolution speed and accuracy. A servo is a circuit that compares the actual sensed speed (frequency) with the reference frequency and then corrects it to the reference. Self correcting mechanism which should keep speed stability well. That is, till the frequency generator or servo system goes wonky, then you can have problems. Don't know if the JVC's ever had this issue but some tables do need some maintenance if the speed accuracy is off.
lini
just me...
That seems a bit misleading to me, 'cause the motor usually is a dc type and as such doesn't really derive anything from anything. Shall mean: The FG rather refers to the frequency generator (be it integrated into the motor or an external solution) that delivers the motor speed info to be compared with the reference oscillator by the motor control circuitry (which can also be integrated into the motor or an external solution).
Greetings from Munich!
Manfred / lini
Blue Shadow
Waiting for Vintage Gear from this century
grab the sm for your unit from vinylengine library and look at the diagram for the servo system if you want more info.
Pio1980
AK Member
Not quite so, the motor coils are driven by gated pulse-width modulated DC powered sources that may or may not alternate polarity, depending on the design. The FG is the precision tachometer for the speed locking circuit that compares it's waveform output to those of the drive circuits and provides constant correction. Earlier DD DC motors used a pulse generator with much coarser resolution than the FG system, relying on the flywheel effect of the platter's momentum to smooth between correction signal pulses. In a Quartz-locked PLL system a quartz clock circuit is the reference to which the motor drive circuit drive signals are synchronized, the quartz clock reference works much better with FG taching than with the pulse generator system.
lini
just me...
Pio: Yes, quite so - because you rather refer to the often pretty nifty solutions in many dd models. But fg servo regulation implementations can just as well much simpler - for example as you can find 'em in many belt-drive models. So the important characteristic of an FG servo regulation is that there is a continuous adaptation of the power delivered to the motor according to speed sensor information in form of a generated frequency. How exactly the sensor info is processed and how the result is transferred into the signal fed to the particular type of motor also are interesting topics, but I'd deem these only of secondary interest for the fg servo principle as such.
Greetings from Munich!
Manfred / lini
Greetings from Munich!
Manfred / lini
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Pio1980
AK Member
This is the section I was referring to;
"That seems a bit misleading to me, 'cause the motor usually is a dc type and as such doesn't really derive anything from anything."
With belt drive FG/quartz-locked TTs like the Harman Kardon T-60 the basic motor is a DC brush type motor with FG and that is the case. DD motors are brushless as a rule and the rotor coils must be commutated by rotor-timed switching drive circuitry, thus the locking pulses can be derived from the drive circuitry. I was reading your response with DD motor drive in mind, not considering the DC driven belt-drive version.
"That seems a bit misleading to me, 'cause the motor usually is a dc type and as such doesn't really derive anything from anything."
With belt drive FG/quartz-locked TTs like the Harman Kardon T-60 the basic motor is a DC brush type motor with FG and that is the case. DD motors are brushless as a rule and the rotor coils must be commutated by rotor-timed switching drive circuitry, thus the locking pulses can be derived from the drive circuitry. I was reading your response with DD motor drive in mind, not considering the DC driven belt-drive version.
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