Gas station without pumps

2017 November 22

Change to track-wire amplifier

Filed under: Uncategorized — gasstationwithoutpumps @ 20:32
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I decided not to use the track-wire amplifier in Track-wire detector sensitivity, without even building it.  I looked at the impedance of the tank circuit I was planning to use as a feedback element, and realized that the phase change was large enough that I would have less than 5° of phase margin when the gain of the amplifier dropped to 1 (possibly much less than 5°).  That is too close to being an oscillator for my taste, so I decided to not bother with filtering other than the resonant tank of the detector itself.

Here is the circuit that I breadboarded and tested today. It has no filtering other than the resonant tank of the detector itself.

Here is the response of the amplifier (with gain set near the middle) to a 3ms 25kHz burst.

The ripple is about 7% of the signal, so I’ll need at least 10% hysteresis in the software. I’ve still not decided whether finding the targets will just look for a big enough signal or will try to find the maximum signal as I roll past the target.

Another approach I’ve considered, though probably won’t implement, is to use a pair of inductors at right angles, and look for a large signal in one with a very small signal in the other—the null at the end of inductor is much sharper than then maximum out the side of the inductor.  Because this amplifier only uses 2 op amps, I could easily wire up another amplifier on the same MCP6004 op-amp chip.

I still have to solder up this amplifier—I’ve delayed a little bit, because I’m undecided about putting a 3.3V regulator on the board or take 3.3V from the Teensy board.  It may be better to use a regulator, so that the 3.3V power does not have long wire runs to pick up noise.  I’m also thinking that I should check out how much electrical noise I get from the current-limitation PWM of the MAX14870 H-bridges—I may need to add RC filtering to track-wire amplifier if the there is too much noise.

In the mail today I got more reflective sensors, so I need to solder up another board or two of them also.  The 8:1 mux also arrived, so I need to make the board that will communicate the tape sensor signals to the processor and write code to test it.

I’ve been trying to diagnose why my robot building is going so slowly.  I’ve come up with the following partial reasons:

  • I’m spending a lot of time creating the graphs and pictures and writing up these blog posts.  Almost half my mechatronics time is spent on documentation.
  • I’m still very slow with SolidWorks, so mechanical modeling takes a long time.
  • I’m not very comfortable with mechanical design in general—I can’t visualize stuff very well in my head, and I can’t sketch well enough to clarify designs, so I either need to use SolidWorks (which takes forever) or I need to build things (which also takes forever).

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