According to this video from Khan Academy about electronics of a DVD drive the position of the sled (which moves the reading head) is controlled by a geared DC motor by using only timing and one end-stop switch.

I've also took apart some DVD drives and where there was a DC motor (instead of a stepper) I did not find any sensor as a feedback for closed loop control.

I could not find any reference for this online. If this is really the case can anybody point me to a page/site with the explanation? Why this is not used in hobby projects?


After the feedbacks I would focus on one specific question: is it possible that a DVD drive can operate with open-loop control and a DC motor?

  • $\begingroup$ i think that the dvd drive uses the read head for feedback $\endgroup$ – jsotola Feb 4 '18 at 21:38
  • $\begingroup$ Welcome to Robotics BalintPogatsa, but I'm afraid that it is not clear what you are asking. We prefer practical, answerable questions based on actual problems that you face, so it's a good idea to include details of what you want to achieve, what you tried, what you saw & what you expected to see. Please take a look at How to Ask & tour for more information on how stack exchange works and work through the Robotics question checklist to edit your question to make it clearer. $\endgroup$ – Chuck Feb 5 '18 at 15:46
  • $\begingroup$ I can't understand what your question is. You watched a video that says a position is controlled by a motor and an end-stop switch, and then you took apart a DVD drive and confirmed what the video said - there aren't any encoders. What is your question? How open-loop control works? $\endgroup$ – Chuck Feb 5 '18 at 15:48
  • $\begingroup$ Why this is not used in hobby projects? ... how do you know? $\endgroup$ – jsotola Feb 5 '18 at 17:49
  • $\begingroup$ @Chuck I thought that with the precision requirements of a DVD drive open-loop control cannot be used. $\endgroup$ – BalintPogatsa Feb 6 '18 at 17:18

Just because it is not directly measuring where the sled is does not mean that it is open loop control. What matters is where the laser is in relation to the tracks on the disc, where the sled is doesn't matter - unless it's run into the end of travel, and there's a switch for that - so that isn't the variable you close the control loop around.

The tracking needs to be done to less than 1μm so is not possible without feedback from the optical system. The optical signal is split into side beams and these are compared, this then can control both the micro system adjustments and the larger sled motor movements.

The side beams created by the diffraction grating are positioned forward and back of the main beam straddling the track of pits being followed (not directly on either side as shown in the diagram - but that was easier to draw!). Segments on either side of the photodiode array designated E and F monitor the side beams. Tracking is perfect when the E and F signals are equal.


So if you are oscillating a little bit off the track, then you can use the tracking coil to compensate, but if you trend continuously to one side you would move the sled. I don't know how this is done, but that looks like a job for a Kalman filter. The linked page also mentions open-loop heuristics for track seeks, but then you check the timecode where you landed and go back into closed-loop fine control.

The principle here of 'measuring the effect rather than the actuator position' is commonly used in both hobby and industrial settings; it arguably is what makes closed loop control closed.


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