Success with driving vintage Frasca Altimeter with a polou jrk G2 21v3 and messageport

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brianbarr
Posts: 25
Joined: Wed Apr 01, 2020 4:44 pm

Success with driving vintage Frasca Altimeter with a polou jrk G2 21v3 and messageport

#1 Post by brianbarr »

After a great deal of trial and error I was able to get my vintage Frasca altimeter working with an Arduino UNO and a polou jrk G2 21v3 motor controller.

The jrk is a nice piece of hardware. You can connect it via i2c to an Arduino and via USB to a windows machine at the same time. There is a nice windows based GUI configurator provided and the same software monitors the jrk's operation in realtime -- so debugging is a snap.

As designed, the Frasca control chassis used an analog error feedback setup to sync the altimeter to the sim. It used -5..+5 volts across a 10k 10 turn pot for position (-1000 to 29,000 feet) and a planetary drive dc motor to make the needles move. The entire system is analog and controlled with some fancy circuitry on the sim's "servo" board. The jrk's feedback system is single ended 0-5v with 12 bits (4096 steps) of ADC resolution that you can scale any way you want. It also has its own regulated vref on the board if you want to use it. So I calibrated it for 0=0 feel msl and 4095 = 20,000 feet msl or about 5' per step. The major issue I ran into was stalling the motor at very slow speeds. I did several calibration runs to see what motor controller duty cycles corresponded to what "FPM" movement on the altimeter. The slowest I could reliably drive the needle was about 1100fpm or so.. so at slow climb and descent rates, the needle stops and starts a bit. For a high dynamic situation (aerobatics) I was able to get the needle movement speed to closely match an AM screen altimeter on an ipad by sending a vvi (first derivative) "hint" to the Arduino along with with the altitude. Based on a bunch of calibration runs I developed a formula to translate vvi values to a motor controller duty cycle to match same (it is actually very linear ! ) and it works pretty nicely. So in a high dynamic mode (> 700fpm or so) I switch on the vvi hinting and the needle gets moving much faster and closely matches the movement of a real altimeter. I received two altimeters with my lot (Pilots and Copilots) and the pot in the pilots was worn and noisy so I pulled it apart to replace the pot. It is a real altimeter with some modifications .. see attached pics. My next trick will be to get the AI working. It has two similar motors and two pots inside.
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Jeff_S
Posts: 2
Joined: Sun Mar 17, 2024 6:10 pm

Re: Success with driving vintage Frasca Altimeter with a polou jrk G2 21v3 and messageport

#2 Post by Jeff_S »

Would you share your code for the altimeter? I have a 142 simulator I'm trying to get setup with msfs.
Thanks,
Jeff

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