Air-Core Movements in Simulated Instruments

[Mike.Powell]

Steam-gauge style instruments used in hobby flight simulators are generally based on air-core movements, RC servos, or stepping motors. This tutorial is an overview of air-core movements.

Air-core movements are the motivating force behind a great many automotive and marine instruments. The heart of the movement is a magnetized rotor. It’s frequently a cylindrical ceramic magnet which has been magnetized through its diameter so that the poles form on the curved surface. The rotor is housed in a plastic body which both provides the bushing surface for the rotor shaft, and acts as the bobbin for a pair of field windings. The two sets of windings are wound at right angles to each other. By controlling the currents through the two windings, the rotor can be turned to any desired position. The space around the rotor in the plastic body is often filled silicone oil to dampen the rotor motion. The air-core movement is surrounded by an annealed mild-steel cup to shield it from stray magnetic fields.



Air-core movements produce little torque. They are suitable for swinging a dial pointer, put nothing much larger. They’re great for making airspeed indicators and tachometers, but not for instruments with bulkier indicators like directional gyroscopes.
The windings typically take a peak current of 40 milliamps, though this will vary by manufacturer. The currents needed to position the rotor (and its pointer) depend on the sin/cos of the desired angular position:

Current(winding 1) = (40 ma) * (cos(pointer angle))
Current(winding 2) = (40 ma) * (sin(pointer angle))

Air-core movements are easy to interface to a PC. You can do it with just three chips. One chip is a data transceiver. This might be an RS-232C transceiver if you plan on connecting to the serial com port, or perhaps an RS-422 transceiver if connecting to a serial instrument bus (my current favorite). The second chip is a micro controller, perhaps a PIC16F648A (another of my favorites). The third chip is an H-bridge driver like an L293D. This is an inexpensive power driver used because 40 ma is larger than the micro controller can supply.



While air-core movements are useful devices, they are difficult to come by. Commercial movements, such as the one pictured below, are manufactured in large numbers by several companies. Unfortunately, the bare movements are rarely available on a retail basis to hobbyists.


As hobbyists we have several options:

We can buy simulated instruments or instrument kits that incorporate air-core movements. Companies such as Beta Innovations sell air-core movement based instruments. For example: http://www.betainnovations.com/hardware/downloads/AC360.pdf

We can convert after-market air-core movement based automotive gauges. For example, JC Whitney www.jcwhitney.com sells a variety of “low cost” gauges. Unfortunately, not all product descriptions clearly indicate if a gauge uses an air-core, and “low cost” is $30US and up.

If lucky enough to live near an automotive junk yard with reasonable prices, we can salvage instrument clusters for parts.

Finally, we can build them. Using diametrically-magnetized cylindrical magnets, hobby brass tubing, epoxy, and various scraps, we can make air-core movements every bit as functional as the commercial units. Here’s a cross-sectional illustration of a DIY movement.



And photos of prototypes (of a slightly different design).

[Trevor Hale]

Mike.

This is incredible Reading. Thanks so much. You contribute to this hobby more then anyone I Know.

Thanks for this post, and can't wait for Christmas 2020 for the new book to come out *wink* Just kidding.

Trev

[Mike.Powell]

Thank you for the kind words, Trev.

Christmas, 2020? Who moved the date up?

[Trevor Hale]

ROFL... I know your just dying to get it out, so I made is 2020. Better then 2036 that you had originally planned! ha ha ha

And your welcome. Its people like you that truly is the spirit of why this site exists.

Trev

[Joe Cygan]

Great stuff Mike!!!

[P1IC]

I really don't see why anyone would want to spend $30 and up on an air-core instrument when you can make perfectly good VOR, ILS glide, RPM, VSI, ASI, T&P, AI Bank, AI Pitch, Turn indicator, fuel level, etc. instruments using a PIC chip, a clear plastic Petri dish lid and a pot, for each instrument. And interfacing is simply a matter of sending a handshaked 8-bit word to the PIC, via USB.

At least I hope so!! I'm working on a simpit (I hate that word) which will use exactly that arrangement. I'm a light single instructor who wants the real deal when it comes to instruments: hands-on to the subscales on Alt, OBS on VOR, real dialing of frequencies, modes in the radios etc., and the instruments must look real too - I know that lots of folks are into glass, but my interest is in IFR procedures training in light singles - where it all starts - and so this must be just exactly like a real instrument platform. Cherokee Warrior etc.

So I've decided on using a PIC chip, a clear plastic Petri dish lid and a pot, for each instrument, as I have already said (OK, some, like ILS, will require more than one set). The Altimeter, DI, ADF and other instruments which rotate 360 plus will need to be steppers, but that's OK too.

I just don't think I will need to spend $30 for an instrument, then have to spend more to interface it! YMMV.

[Michael Carter]

Let me know when you get around to building the Collins HSI. I'd be very interested in that.

In fact I'd be interested in any of them. I need 15 engine monitoring gauges.

[P1IC]

Let me know when you get around to building the Collins HSI. I'd be very interested in that.

In fact I'd be interested in any of them. I need 15 engine monitoring gauges.

Thanks. Actually so far I haven't seen a light single with a Collins HSI, so I doubt if I'll get round to that!

The project is at the design stage (by the way, I omitted the simple detail of a small dc motor from my first post!) and so far I've written the PIC code and await the delivery of a breadboarding system so that I can check out speed of response, resolution, jitter, etc. I've also designed the flowcharts for the USB interface controller and that gets written and tested once this pot-controlled needle positioning system is debugged.

As you can see, I'm starting with the hard stuff. No point in getting to work on the actual mechanical framework, fun though that is, until the technology is proved!

I'm guessing that this forum is probably the best one to bore folks with this project, but not this thread! Once I've got a bit further down the road I'll look for a suitable place to post my stuff, in case anyone else is interested!

[Michael Carter]

Probably not a Collins, but I learned instrument flight in a 172 Cutlass that used the Bendix/King HSI. It was so much easier than a separate VOR and gyro compass.

Well, with all of the glass represented here from the 737 and up crowd, your audience is small, but we are paying attention.

Will the software and hardware be compatible with FS9 and WinXP Pro respectively?

Please keep posting on your progress.

[Padraig]

Thanks. Actually so far I haven't seen a light single with a Collins HSI, so I doubt if I'll get round to that!

The project is at the design stage (by the way, I omitted the simple detail of a small dc motor from my first post!) and so far I've written the PIC code and await the delivery of a breadboarding system so that I can check out speed of response, resolution, jitter, etc. I've also designed the flowcharts for the USB interface controller and that gets written and tested once this pot-controlled needle positioning system is debugged.

As you can see, I'm starting with the hard stuff. No point in getting to work on the actual mechanical framework, fun though that is, until the technology is proved!

I'm guessing that this forum is probably the best one to bore folks with this project, but not this thread! Once I've got a bit further down the road I'll look for a suitable place to post my stuff, in case anyone else is interested!

Please keep us informed, A glass cockpit I do have, but even at that, I still require alot of gauge's in the near future, and if I could build them with practically peanuts that would be great.