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Custom Drivers Underway!

A project log for 3D Printable 2-axis MEMS Mirror

A home buildable 2D mirror for controlling light deflection.

Zach BaldwinZach Baldwin 06/14/2021 at 02:092 Comments

[June 2ish]: Since the car stereo was designed for audio, there is a bunch of DC-offset removal from the output that is not desirable for controlling the mirror. As a result, it was time to build my own coil drivers.

Most of the time spent in the design for the drivers was determining if I could buy a COTS power supply that could provide the amount of power I needed in a dual rail configuration. From my estimates and data collected from the bench supply, I would need about 8A on a +/- 15V supply. This makes it infeasible to be constructed out of linear regulators due to the amount of heat that would be dissipated. The other challenge was that finding negative voltage switching regulators with such a high current capacity proved very difficult. In the end, a single-supply design was settled on due to its simplicity and my realization that I might be able to use two symmetrical drivers in a differential scheme for coil driving. Here, the drivers are a mirror of each other around the VCC/2 voltage.

A few hours in a circuit simulator later, I had a tentative design worked out that only required four opamps and worked off a single, variable supply.

In the above schematic, XFG1 is the output of the DAC and XSC1 is reading the output voltage of the output amplifiers. (The finalized KiCad schematic is on GitHub)

I transferred the design into KiCad, added the DAC, some extra regulators for the digital side, routed the board, and sent them off for production. A quick note about the DAC: I went for a parallel-interface unit (DAC7613E) instead of one that operates over a serial bus like SPI since I wanted the system to be easily controllable from old 8- and 16-bit computers (in case someone wanted to use this as a display for one). Also, the parallel interface allowed for me to use pullup resistor networks on the pins, making it easy to only use some bits of the DAC if all 12 bits are not needed.

Pretty soon, I'll have these boards built up and see how they perform!

Discussions

x3n0x wrote 06/15/2021 at 18:47 point

You might want to look at using something other than a 741 to drive your coils...The 741 can only source/sink up to 25mA.  There are several Power Op-Amps made by various manufacturers that may be able to do the job for you.  Coil driver design, specifically, WRT Galvanometers etc is a pretty specialized field!  Some parts to check out/try for your power output:  L272 (1A)  LM675T (3A) OPA548 (3A, 5Apk).  There are more options as well, specifically, some of the available Audio amplifier Chips, that are useful for these types of things.  Power supplies sourcing  +/- 15V are easy to get!  Good luck!

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Zach Baldwin wrote 06/17/2021 at 21:02 point

That's a good point! Your comment made me realize that I only put the simulation schematic in the project log and forgot to link to the finalized KiCad schematic (added in to project log now). The 741s were a stand in for the final op amps with ADA4522s for the first stage and voltage reference and LM1875s (3A) as the output drivers

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