Demodulating a resolver signal.  - Page 1
Forum section: electronics / projects

author: vini_i
author_rank: Regular Contributor
timestamp: August 29, 2017, 03:08:28 pm
content: I need to demodulate an AM signal synchronously. To be more specific, I have a resolver with an excitation of about 7kHz. The out put of the resolver is what looks like an AM modulated signal at a carrier frequency equal to the excitation frequency. One output is a sin and the other is a cos. I need to demodulate one of those out put signals (it does not matter which one). See image below.What off the shelf product could I use where all I need to do is feed in the output of the resolver, the carrier and power and get out the demodulated signal? (My budget is $2k to $3k and the form factor is irrelevant. )Or what category of products can is search? I'm not having much luck, all i keep finding are diy type demodulators.

author: Ian.M
author_rank: Super Contributor
timestamp: August 29, 2017, 03:21:15 pm
content: Hint: add0-10V interfaceto your search terms to get the industrial stuff.

author: mjs
author_rank: Regular Contributor
timestamp: August 29, 2017, 03:50:30 pm
content: Google for 'synchro resolver'. Here's one example in box form, DDC has multiple other products as well. Don't know about the pricing, though.http://www.ddc-web.com/Products/226/Default.aspx

author: Benta
author_rank: Super Contributor
timestamp: August 29, 2017, 04:04:03 pm
content: I wonder a bit here, because normally the resolver exciter and demodulator is in the same unit. Also, it must be rotating relatively slowly, 7 kHz is not a lot.

author: jmelson
author_rank: Super Contributor
timestamp: August 29, 2017, 04:42:49 pm
content: Quote from: vini_i on August 29, 2017, 03:08:28 pmDo you need to read the resolver position on a resolver that is connected to some other electronic gear, ie. spy on the position that the other gear is already reading out? Or, do you just need to read resolver position? (Not sure why you want to read JUST the demodulated signal.)I make a device that excites the resolver and provides a simulated quadrature encoder output. It uses the Analog Devices 2S1200 resolver to digital converter chip. It is $150, so well within your budget. But, you don't tell us what you want to do with the demodulated signal, once you get them. (seehttp://pico-systems.com/osc2.5/catalog/product_info.php?cPath=2&products_id=32if a quadrature output fits your needs.)Your description of how it works in not clear, either. The "modulated AM" would be seen when the resolver shaft is rotated. When it is not being rotated, the Sin and Cos outputs would not vary in amplitude. Measuring their amplitude and phase (relative to the excitation) can be fitted to the Sin and Cos on a circle, and give you the shaft angle.Jon

author: vini_i
author_rank: Regular Contributor
timestamp: August 29, 2017, 05:12:48 pm
content: Quote from: jmelson on August 29, 2017, 04:42:49 pmI want to compare the demodulated signal to another sinusoid in the system. The source of the excitation for the resolver could actually be anything. The system will always be spinning and the case where it stops does not concern me.

author: Smokey
author_rank: Super Contributor
timestamp: August 29, 2017, 11:41:15 pm
content: http://www.analog.com/en/products/analog-to-digital-converters/integrated-special-purpose-converters/synchroresolver-to-digital-converters.htmlI'm sure there are dev boards. Don't bother rolling your own solution. This is what everyone uses.Keep track of the digital count and you can map that to any frequency sine wave you want. You can even use the digital count to drive a sine table into a DAC and get yourself a sine wave back out.

author: jmelson
author_rank: Super Contributor
timestamp: August 30, 2017, 07:07:49 pm
content: Quote from: Smokey on August 29, 2017, 11:41:15 pmYes, that links to the family of chips I use in our resolver to quadrature converter. They will deliver 10 KHz excitation to the resolver (with an amplifier chip) but they do NOT bring out the demodulated signal.Sin and Cos AC goes into the chip, and quadrature and parallel position data come out. Many years ago, they had a multi-chip solution for this that might have allowed the decoded Sin and Cos to be intercepted.Jon

author: Smokey
author_rank: Super Contributor
timestamp: August 31, 2017, 12:06:23 am
content: Ya, but my point is that the digital position (properly captured with a QEP port or something) you get out of those resolver to digital chips Directly Correlates to the phase of the equivalent sine/cosine waveforms coming from the resolver.Think of it this way. If you have a digital counter that has 8 bits (256 counts) for a full sine wave, and a 1Vp-p sine wave, and the initial offset is known (in this case 0), then:0 Counts = 0 Degrees = 0V64 Counts = 90 Degrees = 1V128 Counts = 180 Degrees = 0V192 Counts = 270 Degrees = -1VAnd you can determine any other point on the waveform the same way.

author: mikerj
author_rank: Super Contributor
timestamp: August 31, 2017, 10:34:49 am
content: Quote from: Smokey on August 31, 2017, 12:06:23 amThe point is that isn't very useful for the OP's requirements as he wants to compared ONE of the resolvers outputs to a separate signal in his system. Unless the other signal is from a resolver driven by the same excitation signal (and can therefore be fed into the other input of this IC) this probably isn't very useful.If the OP simply wants the amplitude without the carrier, then a precision rectifier and low pass filter should do the job.

author: vini_i
author_rank: Regular Contributor
timestamp: August 31, 2017, 12:27:49 pm
content: Quote from: mikerj on August 31, 2017, 10:34:49 amThe problem with that is the output signal of the resolver looks like suppressed carrier AM (forgive me if I'm messing up the jargon). This type of signal can not be demodulated with an envelope detector, a synchronous demodulator is required. A pre built solution to the synchronous demodulator is what I can't find.

author: Benta
author_rank: Super Contributor
timestamp: August 31, 2017, 05:36:32 pm
content: vini_i, I think what's throwing everyone off here is the graphic in your original post. It shows an excitation frequency that's half the frequency of the sin/cos outputs.Normally, excitation frequency and output frequency is the same, and you also state this in your post.In that case, multiplying the excitation signal with one of the output signals will give you the desired modulated signal which can be rectified and filtered.The output carrier will be distorted to a sin2carrier, but the envelope will still be the sine of the position.

author: vini_i
author_rank: Regular Contributor
timestamp: August 31, 2017, 06:17:44 pm
content: Quote from: Benta on August 31, 2017, 05:36:32 pmI see what you mean. I did not notice that before you said something. I inserted a better image that has a matching excitation frequency.Do you know any off the shelf products that will do what you describe?

author: Benta
author_rank: Super Contributor
timestamp: August 31, 2017, 06:51:46 pm
content: QuoteI'm sorry, no. I'm an electronics/semiconductor guy myself, not an industrial control guru. And I'm not sure that EEVBlog is the right forum for finding off-the-shelf industrial products.Perhapswww.eng-tips.commight be better, they have a forum for Electrical motors and generators that's quite active.Over there, I'd phrase the question differently, though. Like "I have a resolver, which supplier can supply the exciter/demodulator". Then you can go into specifics, if you like :-)(Am I allowed to point to other forums here? If not, I apologise in advance).

author: C
author_rank: Super Contributor
timestamp: August 31, 2017, 07:11:12 pm
content: First do you know what a resolver is?A simple rotary transformer.A transformer needs AC.The rotor has the reference frequency on it's windings.As the rotor turns different amounts of the reference is coupled to the two field windings that are offset by 90 degrees..If you make the max peaks of all three signals the same, you have some ratios.If the rotor is at 45 degrees then sin & cos are 0.707 of reference.When reference is1V sin is 1V * 0.7072V sin is 2V * 0.70710V sin is 10v * 0.7070V sin is 0V * 0.707so as the reference varies, The outputs varies by the rotation angle.The 7kHz is not a rotation speed limit, as much as the low voltage limits of converter which causes dead times.3 ADC's all sample at same time and then compute ratios.

author: capt bullshot
author_rank: Super Contributor
timestamp: August 31, 2017, 07:11:53 pm
content: A lock-in amplifier like this one might do the job:https://www.edmundoptics.com/testing-detection/amplifiers/dual-phase-lock-in-amplifier/

author: CM800
author_rank: Frequent Contributor
timestamp: August 31, 2017, 07:29:58 pm
content: Resolvers are pretty common place.Depending on your application, you could use these as a solution:Resolver to SSI:http://www.vegacnc.com/html/vega2790504.htmResolver to Quad:http://www.vegacnc.com/html/vega2795500.htmother:http://addi-data.com/products/resolver-to-digital-converter/

author: Benta
author_rank: Super Contributor
timestamp: August 31, 2017, 07:58:55 pm
content: Quote from: C on August 31, 2017, 07:11:12 pmYes, we do know what a resolver is.OP is looking for an off-the-shelf product with a budget of 2...3 k$. And what'syoursuggestion?