Aircraft instrument servo motor - Page 1
Forum section: electronics / projects

author: georges shed
author_rank: Contributor
timestamp: April 05, 2020, 03:39:40 pm
content: Hi all,I have a smiths radio altimeter that i wish to convert for use with a flight sim. It is driven using a 26v motor that has 3 coils. 2 are labelled control and one is labled reference. the 2 control coils are connected together so there are 3 pins. I have no idea how i would run this? any suggestions greatly appreciated.The control coils are 24each and the ref coil is 46

author: jmelson
author_rank: Super Contributor
timestamp: April 05, 2020, 04:29:53 pm
content: Yeesh! This can be messy. A very common device from WW-II era was the "selsyn". These had a wound rotor and a stator wound just like a 3-phase motor. You connected some excitation (typically 120 V 60 Hz for Navy ship applications, and 26 V 400 Hz for aircraft applications)to the rotor, and just connected the stator windings 1:1 together. If you mechanically drove one shaft to some position, the other unit would move to the same position. I think this could be a similar scheme, but with just two stator windings at 90 degree relationship.This is commonly called a "control transformer" and is actually identical to what is now called a resover. If the rotor is excited, the stator windings produce a singla which is either in phase or 180 out of phase with the excitation, depending on shaft angle. Their amplitude varies as the sine and cosine of the shaft angle.If you were to apply a sine wave that matches this relationship, it should work as a weak motor, enough to turn the instrument.So, you need to convert the shaft angle you desire to sine and cosine, and then produce sine waves of that amplitude and 0 degrees or 180 degrees to the rotor excitation. Not a real simple thing to do.Umm, one way to cheap out on this would be to rig a gear motor and pot or encoder to another of these motors, and use them like selsyns.Your computer drives the gearmotor to desired angle, the sending selsyn sends the 2-phase output to the instrument which displays the rightposition.Jon

author: Martinn
author_rank: Frequent Contributor
timestamp: April 05, 2020, 04:48:15 pm
content: Wouldn't it be possible to drive the coils with DC? Like connecting the reference coil to 5 V and the sin/cos coils to variable voltages (+- a few volts)? If you measure the inductance, you can calculate the DC level to get the same current as with 26 V 400 Hz (not burning the coils, but still delivering enough torque).

author: georges shed
author_rank: Contributor
timestamp: April 05, 2020, 06:12:08 pm
content: Thanks for the comment JMelson, As part of the mechanical arangement there is a potentiometer that is used to measure the position of the instrument. Ive got this working and can feed the diaplaed altitude within 10 ft. Is i can use the original motor that would be great but im now considering just replacing it with a normal dc motor. The next problem with that is making a suitable bracket and finding a correctly pitched gear. Either way im going to have to start thinking about a PID setup.

author: georges shed
author_rank: Contributor
timestamp: April 05, 2020, 06:13:02 pm
content: Thanks for the comment martinn, i will try to investigate that tommorow.

author: duak
author_rank: Super Contributor
timestamp: April 05, 2020, 07:55:00 pm
content: Just last week I was showing one of my sons a few old aviation instruments and trying to get him interested in building something with them, but no luck. Anyway, I looked into synchros a bit.Here are a few links to drivers on this site:https://www.eevblog.com/forum/projects/driving-a-synchro-from-a-uc/https://www.eevblog.com/forum/projects/driving-a-synchro-from-digital-logic/Analog Devices has a fairly detailed writeup on how it could be done using one of their products:https://www.analog.com/media/en/training-seminars/design-handbooks/synchro-resolver-conversion/Chapter4.pdfAn Arduino should be able to generate the low level signals for a 3 or 4 channel power driver. Note that it doesn't matter too much whether the sychro is three phase or two phase. There is a mathematical transform to convert from one to the other and it's a matter of the appropriate voltage and phasing of the windings.It would be interesting to try DC on the reference winding and try to turn the rotor with some AC signals. I expect it will turn but it will also require an encoder so the driver can supply the correct drive to build up the power angle. IOW, it will be easy to stall the motor otherwise.Cheers,

author: georges shed
author_rank: Contributor
timestamp: April 06, 2020, 07:52:24 pm
content: Thankyou for the comments, They sparked a long 4 hour deep dive into syncros and some old blog posts from 2006. It all seems very complex and outside of my current technological ability so this project is going to be shelved for a while but will always be taunting me in the back of my mind so will definatley be revisited. I think the jist of it is figuring out a way to generate 26v 400hz sine and cosine signals and the various control methods to vary their amplitudes.

author: georges shed
author_rank: Contributor
timestamp: April 06, 2020, 07:55:50 pm
content: some of the best sites i found during my reaserach, if youre intrestedhttps://www.electro-tech-online.com/threads/3-phase-syncrhos.142595/https://www.avrfreaks.net/forum/avionics-instrument-motorhttp://www.ddc-web.com/documents/synhdbk.pdfhttp://mikesflightdeck.com/oldnews/oldnews_2006.htmlhttp://www.pdp-11.nl/viperpit/altimeter/altimeter.htmlhttps://forums.eagle.ru/showthread.php?t=145264&page=4http://www.scottbouch.com/aircraft-cockpit-instrument-simulation.html

author: Martinn
author_rank: Frequent Contributor
timestamp: April 07, 2020, 06:37:20 pm
content: I don't think you should give up already. I am not sure how this instrument works, but my guess would be that you don't need the 400 Hz AC signal. I would think that if you connect the reference to some fixed DC value, you can use the two phase coils to drive it like a brushless or stepper motor.Why don't you simply connect reference and one of the drive coils together and apply a small DC voltage (lab supply, start slowly from 0 V) and see what happens.

author: duak
author_rank: Super Contributor
timestamp: April 07, 2020, 07:56:29 pm
content: Martinn asked a good question so I looked it up. A synchro or resolver has an additional set of windings called the Rotary Transformer than gets around needing slip rings, but can only pass an AC signal to the reference winding. seehttps://www.amci.com/industrial-automation-resources/plc-automation-tutorials/what-resolver/It occurs to me that if the indicator has slip rings instead of a current transformer then DC could be applied to the reference winding. It's still worth trying a DC signal to see what happens. Since it's an altimeter that's inside a closed loop, it might be a simplified design.