www.fransedano.net

Long time without updating the blog, hectic times here!

I managed to create a new controller board, got rid of the multidrop serial interface and now I'm using CANBus for communicating the master and slave boards. Doesn't make sense to reinvent the wheel, CANBus transceivers are very cheap and some uc already have the protocol implemented in HW. 

Regarding the altimeter, it's from a real Boeing 737, and I guess it's from the early 80's, so it's all analog. I'm still learning how it works, it has 4 differential resolvers, 1 DC motor and ... 1 stepper motor! I've still got no idea what the stepper is for. At first glance, I thought the 4 differential resolvers would be geared differently,  so 1st would be 1 turn per 1000 ft, 2nd 1 turn per 5000ft, 3rd 1 turn per 20.000ft, and so on, and then everything would be moved with the DC motor, using the resolvers as feedback elements. But, that's not the case: 2 resolvers are geared so 1 turn is 5.000ft (5 indicator turns), and the other 2 resolvers are 0.25 turns per 20.000ft (!). Given the full altimeter scale is 50.000ft, I don't understand the reason for this. Also, everything can be moved either using the DC motor or using the stepper...

Anyhow, the plan is first to get a good resolution from the resolvers, and see what can I get; 

The resolvers are special transformers, where primary is a rotating coil, and there are two secondaries, each 90º apart. So, knowing the phase of the signal applied to the primary, and reading phase and amplitude of the secondaries, you can get the shaft angle.

Of course, the limit here is the noise and resolution you can get.

To generate a known phase wave to excite the primary winding, I used a table driven PWM generator. You can see here the PWM train and the resulting wave. To drive the resolver primary winding I use a ULN2003. I'm not applying any filtering at all, as the coil itself is a very good filter. 

I experimented first reading the secondary in a single-ended way, that is, one end of the winding to ground, the other to an operational amplifier

Last Updated ( Monday, 08 June 2009 08:25 )

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I bought a real airliner altimeter some days ago in eBay and I just received it. The idea is, as I did with the HSI, convert it for use in the simulator with my master controller.

After disassembly I noticed this is not going to be an easy task. The altimeter is dated 1973, and it's an all-analog construction.

It has a DC motor and *a lot* of angular position sensors, called synchros. It's the same construction as the HSI has, and they are not easy to interface with.

The synchos are transformers, where the primary is stationary and the secondary rotates. Actually you have 3 secondaries, and, applying a sin wave to the primary, and measuring phase and amplitude on the secondaries, you can deduct the angular position.

 I wrote code for a PIC18 to use the synchro in the HSI, and I ran out of CPU power just monitoring one synchro. For this instrument,I will need to measure about 6 of them. The altimeter is built using a DC motor and a lot of synchros, with different demultiplications.

Looking at the value of the synchros, you can deduct the altitude the instrument is indicating.

I will post any progress here, but I guess this is a good candidate for using one of the dsPIC I have around.

These are the pictures of the dissasembled altimeter:

Last Updated ( Monday, 08 June 2009 08:26 )