View Full Version : Synchro torque repeater

03-14-2011, 07:34 PM
I have several genuine gauges that use Torque repeaters to drive them. Aside from the 400 hz issue, how do I control them? I am familiar with the concept of operation but have no pracitcal idea of how to create a circuit to drive them. I have considered replacing them with servo motors but really want my attitude indicator to be able to rotate 360degrees plus. Does anyone have any suggestions or am I biting off too much?
Thanks in advance

03-15-2011, 05:00 AM
It could be very simple - you just need an identical torque sender connected to the receiver. Then you feed both with a 400Hz sinewave. Receiver's shaft will follow the transmitter's shaft becoming a virtual driveshaft.

03-15-2011, 10:39 AM
Thanks very much! Now I understand it completely. I thought I needed some sort of driver circuit to power it.Duh!

03-15-2011, 11:10 AM
Simple gauges likely contain synchro receivers. They can be electrically driven by synchro transmitters or by digital to synchro converters.

The synchro transmitter can be rotated by an RC servo connected to an interface card like the OpenCockpits variety. Synchro transmitters show up on ebay from time to time. Prices vary.

A digital to synchro converter is an electronic module which produces the synchro S1, S2, and S3 signals in response to a digital input. These are costly new though they also appear on ebay now and then. It's certainly possible to build a digital to synchro converter. It's covered in the "Using Real Instruments" chapter of my book Building Recreational Flight Simulators. The cost for building a single converter is on the order of $40~50 depending on how you build it and where you source your parts.

You mention an attitude indicator. These are more complex. Generally there is a set of motors that rotate the ball. The positioning of the ball is controlled by a feedback loop which uses a pair of synchro control transformers as position sensors. This also uses the S1, S2, and S3 signals, but now you have a chunk of circuitry and a big connector on the back of the instrument. If the circuitry works and if you know the connector pinning, then you can drive the instrument. Lack of proper documentation can be a big issue.

A downside of using surplus synchro instruments is that they tend to sing. The bearings are a bit loose and you get a 400Hz buzz. It can be really annoying. On the other hand, watching a real HSI swing around is really neat.

03-17-2011, 07:50 PM
Thanks to all that commented. I have determined it is way to complex to try use the gauges as they are. I have started to modify them with servos and dc motors.

07-29-2011, 02:43 AM
Wouldnt it be feasable to build some digital-to-synchro converters as a small series production, with SMD and PCB's machine fitted? I should think you could produce them for around 10$ each (if >100 units are made) and after all the handling etc. probably sell them for 20$ as a home builder product, or maybe even organize a group order on a non-profit basis. Of course some instruments will need 2 or even 4 channels but with careful planning (especially when deciding what kind of control bus to use) it should be doable? I havent looked at parts prices yet (just glanced at Mike's circuit and it's huge heat sinks, would I hope with some careful engineering its possible to reduce the heat loss significantly) so maybe I'm wrong...

07-29-2011, 10:29 AM
The heat sinks are for the linear output amplifiers. There is nothing particulalry special about them and they could be replaced with switching amps which would require minimal heatsinking.

07-29-2011, 01:46 PM
Sounds good :-) I hope I'll have some time to have a look at it closer soon.