How to convert pixel map to voltage/angles?

Question

My end goal is trying to get something rendered using lasers and a MEMS scanning mirror. There are not that many resources online for this, but it seems like the basic idea is pretty applicable to general embedded systems principles. I am building the driver from scratch and having trouble with part of it, and hopefully can get some direction on what to learn and make sure my thought process/approach is correct? The idea is straight forward:

1. Render a element and generate a ppm file to define each pixel value
2. Convert that to a set of voltages for the DAC
3. Based on the data sheet I can generate a look up table for voltage -> angles for the mirror

My main question is figuring out how to generate those sets of voltages. Is that even the correct approach? It seems like it will be a very similar approach to how LCD screens work with refresh rates.

HV_A (X+)
HV_B (X-)
HV_C (Y-)
HV_D (Y+)

HV_A = ((x_in / 8) + 1.25) * 56
HV_B = 140 - (((x_in / 8) + 1.25) * 56)
HV_C = ((y_in / 8) + 1.25) * 56
HV_D = 140 - (((y_in / 8) + 1.25) * 56)

Answer 1

So, the conversion from rectangular coordinates to angles is really just a bunch of trigonometric functions; that's not hard to do on paper (if your mirror is x meters from the wall, and you want to light up a point y meters right of the perpendicular, that angle is arcsin(y/x); do a drawing to clarify for yourself).

But:

Your approach isn't great. The mirror is an object with a mass and thus, hysteresis. Accelerating and breaking it is complicated, takes time and energy.

Your laser on the other hand can nearly arbitrarily fast be pulsed.

So, assuming you have one voice coil to deflect the mirror in vertical direction, and one in horizontal:

• Let the mirror do a full row scan as one continuous swipe. It's very possible that the easiest way of doing so is a simple current DAC, which you also find in cameras to control optical elements (stabilizers, autofocus), for example the AD5398A.
• Then, advance one row with the other coil, and do the swipe (hint: might be clever to do it in the opposite direction)
• all the while, you turn on your laser at the right time instants – not at inidividual "discrete" angles, but based on knowing that mirror travels continuously, at the right time.

That gives you a simple three-DAC system: One DAC for row slides, one DAC to define the vertical deflection, and one fast DAC for the laser. Your horizontal resolution is only defined by how quickly you can control the laser, your vertical by how many individual rows you can do.

Answer 2

question is how to go from pixel -> voltages for the DAC.

Short answer: You don't!

In a raster scanning system, the voltages are fixed ahead of time by the area of the raster scan. For example, if 1 volt input causes your mirror to deflect 1 degree on each axis, and you want to do a +/-1 degree image, you would start at [-1, -1] then [-1,-.999], then [-1,-.998] ... [-1,+1]. At that point you have finished one fast axis line and go to the next. Usually you do a so-called "triangle" scan, which just means you now turn around and scan the next line backwards: [-0.999,+1], [-0.999,+.0999], ... When you get to the end of the last line, you have finished your image, and you loop around back to the start and draw the next frame.

Usually this is implemented by loading a circular buffer into your DAC that just continuously feeds however many voltages are necessary to draw one frame. You can also significantly subsample your data since your mirror will have a lot of inertia and doesn't actually need to have the voltage updated for each pixel.