Synchro Interface Revisited

[Mike Powell]

@Ian. The filter has four stages which is overkill. You get a fair waveform from two stages, but the cost of going from two to four is almost zero. Both filter channels together require only a single quad opamp. The passive components are surface mount devices costing pennies.

 

@Gadroc. There are $15~20 in semiconductor devices on the board along with a few dollars worth of passives. The bare board can cost anywhere from $2 to $20 depending on quantity, lead time, and choice of manufacturer. It's double-sided, 9.5 square inches. The board has a pair of power amplifiers which need heat sinking. I'm planning on mounting the board(s) on a chunk of sheet aluminum along with an Arduino which will act as PC interface as well as provide any needed local processing.

 

 

Re scopes: I used Tektronix scopes throughout my career, though toward the end I was spending a lot of time squinting at a logic analyzer. Tek gear started showing up frequently at swap meets and I picked up a pair of Tek454 scopes for about $35 each. (I must admit it almost broke my heart to see ten or so Tek 7000 series scope mainframes sitting in the dust at an outdoor swap meet. They had been the elite, high end scope of choice.) I've never gotten around to buying a DSO. I suspect the smaller size and weight will eventually be what gets me to do so. The '454 was the portable field scope of its day for a great many, but that was only when compared to 50 pound scopes on large four wheel lab carts.

[bnepethomas]

Nice work Mike - there may yet be life for those instruments I have in 'that' box that lives under the desk after all :)

[Mike Powell]

A progress update:

 

A design goal of the DTS2 is that it can be optionally used with a 400 Hz power supply, or that it can be used to drive a filter-amplifier combo that produces limited amounts of 400 Hz power. The second option is useful if you have a small synchro gauge and don't have a separate source of 400 Hz power. Since the cost of filter-amplifier combo looks to be perhaps $10 for semiconductors and $2~20 for the bare board (depends on quantity, source, etc.) this second option has a certain appeal.

 

The first option, however, looks useful when you do have a 400 Hz power source, or you have a need to interface an instrument with multiple synchro inputs. Both of these situations requires that the DTS2 boards be able to synchronize with an external 400 Hz timing reference.

 

Developing the synchronization functionality is what currently occupies my time. At present, it works but has an annoying amount of bouncing about as it responds to changes in the reference signal timing. Looks like a great application for a phase locked loop or for a PID controller. Unfortunately I'm working with an inexpensive 8 bit micro lacking multiply-divide capability and a somewhat vintage brain which is increasingly refusing to remember classroom discussion from the distant past.

 

Still, progress is happening, and it is fun.

[Gadroc]

Thanks for the update. I already have an inverter but would have loved the filter/amp combo. It's not easy to find a decent cost working w/ pin out static inverter!

[Mike Powell]

A brief update.

 

Synchronization has been implemented. The DTS2 board will synchronize itself to an external 400 Hz source, and will switch to its internal timing if the external reference is removed.

 

This simple sounding function took a surprising amount of time. The synchronizing function is basically a feedback circuit. Not too many eons ago I took a feedback and control class, and thought I did pretty well at it. Of course that was done with continuous variables and with the expectation that applications would be continuous voltages or mechanical positions. The synchronizing function is controlling a digital oscillator (a counter inside the PIC) using a rather low resolution 8-bit micro and binary arithmetic. Took me awhile to figure out how to begin to model the dynamics of the control loop and how to stop it from bouncing around.

 

If it worked the first time, I guess it wouldn't be any fun.

 

 

The next step is verifying the serial control input functionality. This is "proven" code lifted from earlier projects. What could go wrong? To find out, I've written a simple Arduino sketch...

[Hansolo]

Mike, just to an electronic noob. Your circuit doesn't require a 400Hz power supply, but if it is to be used to drive a multi syncro instrument a 400Hz source is needed in order to achieve syncronisation between the different boards? Is that totally incorrect understood?

 

cheers

Hans

[FSFIan]

Mike, just to an electronic noob. Your circuit doesn't require a 400Hz power supply, but if it is to be used to drive a multi syncro instrument a 400Hz source is needed in order to achieve syncronisation between the different boards? Is that totally incorrect understood?

 

cheers

Hans

 

The circuit will be able to work without any external power or clock source. But if you need more power than the board can provide on its own, you can power it from an external 400 Hz supply. To support that optional use case, it needs to be able to synchronize with the frequency of that external 400 Hz power supply.

[Mike Powell]

What Ian says is correct.

 

A bit more detail:

 

The DTS2 board provides a "modified sine wave" output that can be used with an external filter/amplifier board to produce 400 Hz power expected to be on the order of 30 watts. If you have an instrument with several synchros and the total power demand of that instrument is less than 30 watts then you should be able to use one DTS2 board to drive one synchro input and a filter/amp board, and additional DTS2 boards, synchronized to the filter/amp board, to drive the additional synchro inputs of the multi-synchro instrument.

 

If the multi-synchro instrument requires more power than about 30 watts then you need a separate source of 400 Hz power.

 

However, there's nothing to say you couldn't build a more powerful filter/amp board. The one I'm planning is based on a pair of LM1875T audio power amps. They cost about $3 each. The LM3886 costs $6~7 and handles about twice the power, but remember you still have to pay for the DC power supplies feeding the amps. This isn't the most efficient method of generating 400 Hz power, but uses readily available parts and does produce clean power. If you have a lot of synchro instruments, it's probably worth tracking down a 400 Hz power source.

[Hansolo]

Thanks for the replies Gentlemen. I guess then I better start checking whether it is easiest/cheapest for me to find a 400Hz power supply or a powerfull DC supply in EU :smilewink:

 

Cheers

Hans

[Looney]

HMA, I can make you one using this schematic:

 

http://www.pdp-11.nl/viperpit/altimeter/altimeter.html

 

It's relative low-cost, the mosfets are € 5,- per piece, the CD4047 something like 65 cents..

The transformer is the kicker at € 25,- or so.

 

This one can go to 25W using the small heatsinks but up to 100W using large heatsinks on the FETs.

[AAnnillo]

Hello, all.

 

I just discovered this posting and just enrolled to become a member of the forum.

 

I am wondering if there have been any other updates to this project.

I am very interested in learning interfacing techniques for synchro/resolvers.

 

Any updates to announce and/or any info on where I might purchase the control boards demonstrated here?

 

Thank you. 
 

Anthony