I opened up a broken laser printer yesterday to find one of the important sections (this is an example photo from Google Images), trying to learn from the design of the laser+polygon mirror motor within:
I was able to find the pinout of the driver chip, and successfully got the motor running at a very high RPM, as well as the laser to reflect off the rotating mirror, forming a simple linear pattern on the end surface.
Now, here is the part that's mysterious to me:
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The mirror is just a standard BLDC (not a stepper nor an encoder-based servo).
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The hexagon of mirrors is rotating at unknown/inexact speed.
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There is such a high speed of rotation and such a short mirror length (I measured each side of the hexagon's mirrors to be about 2 cm long).
So how do they control the laser to reflect at the exact rotation-timing/angle of each mirror so as to (hit the photoreceptor drum at highly accurate positions and) produce printing quality in the thousands of DPI, i.e. better than 0.03 mm resolution?
In other words, how is the timing of the on/off laser pulsing coordinated with respect to the mirror angle in the below picture?
Best Answer
It's hard to know exactly how your specific unit works, but in general there is a timing sensor that is used to read back the mirror's position, as in the diagram below. It doesn't continuously read every position but only once per face change. The measured error is used to compensate the firing of the laser circuit.
There are more detailed patents on the kind of (digital) compensation circuit that makes it possible to use this non-continuous sensing method, e.g. US5754215A that enable the use of cheap motors.
The whole point of that being
The combination of patentese and Japanese authors is a killer :)
That particular patent actually goes on to talk about controlling a PWM motor with the resulting data.
But there are ICs for controlling a brushless motor that are specifically marketed for laser printer mirrors. ON Semi has whole bunch of them e.g. LB11872H, LB1876, LV8111VB. These use PLL speed control circuitry internally. The latter two chips boast "direct PWM drive" as well, which is not very clear to me what it means, but I assume they convert the control signal internally (from PWM). So as long as you have control data they probably work just as well. There's not much in the way of application notes for using these (in an actual laser printer). My guess is that those who need them know how use them. Rohm (which holds the aforementioned patent) also makes a bunch of these "direct PWM driver" ICs for brushless motors, also marketed for laser polygonal mirrors, e.g. BD67929EFV. There's even a paper talking about this [PWM] control technique for brushless motors: http://dx.doi.org/10.1109/ICEMS.2005.202797 (I have not yet read it.)
Re: "how exactly does this timing sensor receive the beam?" I think that was somewhat obvious from the diagram: through a mirror (labelled there "1st Reflection Mirror") which is struck only when the laser switches mirror faces. That's a different mirror than the main mirror used to illuminate the OPC drum. Presumably there could be other arrangements. For a color laser printer, there typically are (or rather were) multiple sensors, one per beam (color channel) as explained in a more recent Lexmark patent US9052513, which as you can see proposes a way to reduce the number of sensors. (That's probably among the reasons why you can buy a color laser printer for under $100 these says.)