I’ve started working on one of the fundamental instruments on the plane: Attitude indicator.
The ones I’ll be using are from a 737-200 cockpit, and one can be seen on this close up from the cockpit:
Once opened, we can see the instrument is not difficult to interface with, it is basically a bunch of motors with some resolvers for position feedback. Also there are some coils for driving the localizer, glidescope, several flags and so on.
Once I’ve examined it in greater detail, though, a difficulty arised: The motors used in the instruments are not the usual DC motors, but some AC motor-tachometer arrangement. That accounts for the expected reliability and precision on the instrument. Essentially, the tachometer can be used to dampen motor movement.
Fortunately I found some datasheets from a different manufacturer, and I was lucky enough the cable colors matched the motors I have, which I confirmed just measuring DC resistance on them.
On the motor I can see: Control winding: 26v/400Hz. Reference: 26v/400Hz, Tacho: 26v/400Hz.
I have all the details now! I only need to figure out
easiest cheapiest way to generate a variable frequency/amplitude 26v/400Hz signal.
This is the easiest way I could find:
Generate a 400Hz sin using a PWM from the microcontroller. Varying PWM parameters is very easy modulate amplitude/frequency.
Run that signal thru a ULN2003, that’s a very nice mid-power darlington array. That will amplify the PWM on voltage and current. Then, filter the output signal with a simple LC network, and run it to the motor winding. This is the idea:
So after building it (this is a 3-channel version):
I connect the motor, I get a nice signal on the scope…
But the motor doesn’t turn 🙁
So, what’s going on here… After some head scratching I see what’s going on. I’m not sending real AC to the motor, it’s more a modulated pulse, but actually I never invert polarity on the motor. I need something more complex, something I try to avoid because I’m going to need a good number of these circuits to control all motors on the instrument.
I happen to have some TDA2030A IC’s on my lab. That’s a very nice power amplifier, I can use it as a proof-of-concept. I build a small demo board:
and this is the signal I get now (PWM filtering is much worse here, but that’s not a problem)
That’s very good, and the motor turns now!!! I really don’t want to use this as the final circuit, but I’m in the right track.
I’ve just ordered a bunch of N and P Mosfet, so I’ll be building a simple circuit to generate this.. More once the parts arrive!Share on Facebook