i want to know the servo theory of dvd players,the key parameter of the servo system,especial the parameter of the motor driver.can anybody help me.if you have any message,you can send mail to:miaol@crhjcom.cn. thank you

 

sorry,the email is wrong,the right is miaol@crhj.com.cn

 

DVD players (like CD players) have multiple servo systems, focus servo, tracking servo, sled servo, spin servo. But the essential ingredient in any servo is FEEDBACK - the motor driver is usually just a driver IC, and an H-bridge if required.

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thank you ,Nigel Goodwin.i know the players have four servo systems ,i just want to know what is the most imporant parameter of the motor drivers,gain,or the other

 

It's ALL important, it's the balance of all parts that make it work - I wouldn't say any individual section is any more important than the others.

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The most important thing to know about any linear feedback control system is that the roots with positive real parts must disappear or the system will!

 

i know all parts is important.but our products is motor driver.so i want to know the key parameters required for dvd servo.

 

You should start with the motor itself. To a first approximation the torque of the motor is proportional to the current in the winding(s). This constant of proportionality is called KT (K-sub-T). The next thing you need to be concerned about it the load on the motor. Each particle of mass connected to the shaft, and the rotor, and the windings contributes to the moment of inertia. You can compute the moment of inertia for standard geometric shapes by doing the appropriate integrals or looking up the formulas in the Machinist's Handbook. The last mechanical thing to worry about is friction which is generally ignored in the initial analysis because well designed bearings generally reduce it to negligible proportions.

Using the rotational form of Newton's Second Law of Motion, the moment of inertia times theta double dot is equal to KT times the current. In simple terms "current in the coil will accelerate the shaft".

The next thing to worry about is position and velocity. These can measured with an optical encoder. An optical encoder will tell you if you are moving, and how far you have moved since you started keeping track. On large machine tools there are typically other sensors, called limit switches, that tell you where you are with respect to hard limits like the end of a leadscrew.

If the goal of the servo system is to run the motor in a direction at a constant velocity then you apply a bit of current to the motor and you measure the velocity. If it is too small then you increase the current a little bit and measure it again. If the velovity is too large then you decrease the current and measure again. This would be called proportional velocity control.

If the goal of the servo system is to move the motor to a position and you know how far that is away from where you are, then you apply maximum acceleration for half the distance and maximum deceleration for the remaining half of the distance. When you are stopped you will know if you arrived exactly where you want to be. If so fine. If not then a small current can be applied to correct any undershoot or overshoot. This is called Bang-bang control.

The final warning is to make sure that your system is mechanically balanced so that it does not matter what orientation the DVD drive is in. You absolutely do not want your servo motor working against gravity if you can help it.