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Creating an oscillating waveform.

Creating an oscillating waveform.

Creating an oscillating waveform.

(OP)
I want to create a waveform that resembles a human heartbeat. (At this point I don't need the little "ticks" before and after the beat.) My idea was to start with a square wave and "process it" so that on the rising edge of the pulse, my voltage would rise to a positive voltage and then quickly taper back to zero V. Then with the falling edge of the pulse, the voltage would fall to a negative voltage, quickly tapering back to the baseline of zero V.

As far as parameter values: I'm thinking the Voltage spikes would be +/- 5V on the low side to maybe +/- 12V on the high side.

Is there a simple method of producing this  — with discrete components on a circuit board? …computer generated?

If it can be done with digital/analog components: Is there a simulation program that I can use to experiment with the circuit design so that I can see the output waveform result for various inputs and various circuit modifications?

RE: Creating an oscillating waveform.

I would use a micro for this... a look-up table would allow you to create whatever "heartbeat you want".  You said "at this point", which tells me parameters are going to change in the future, so make the product as future-proof as possible and stay away from the simple solutions... you'll spend less time up front, but it will bite you in the rear as time goes on.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Creating an oscillating waveform.

Several companies make arbitrary waveform generators.

Here's an example from Agilent, $1860.

http://www.home.agilent.com/agilent/product.jspx?nid=-536902257.536883183.00&cc=US&lc=eng

One of the built in functions is "cardiac".

If you're designing a product that needs to generate such a waveform, I agree with Dan, use a micro, they're very versatile.  But if you just need one box, to test or calibrate something, the development costs are likely to be much higher than just buying one.

Or you could download one or record one, like the first couple of seconds of http://media.healthhaven.com/find.aspx?play=40125606, before the talking begins, and there's a lovely heartbeat on Pink Floyd's "Dark Side of the Moon".  Dump it onto a cheap mp3 player and use an opamp to boost the level to what you need.  

(Legal disclaimer:  Don't steal music!  I'm only advocating whatever Fair Use you may have of legally purchased music.)

RE: Creating an oscillating waveform.

Hi Cincinnati10,

Yes, you can easily do what you want, with just a few  passive components.

Assuming that you already have the square-wave generator (if not, that is almost trivial, too), the basic circuit would be a resistor in series with an inductor, between the signal generator's output and its ground (inductor to sig gen's gnd), with your "heartbeat" waveform appearing across the inductor.  With a few more components, you can make it more realistic.

Go to http://www.linear.com and download the excellent, free LTspice circuit simulation software.  Or use the URL below, for a direct link to the download.

http://www.fullnet.com/~tomg/gooteesp.htm

I will come back, in a little while, with more details.

- Tom Gootee

RE: Creating an oscillating waveform.

(OP)
I'm hoping to go into production. So I will look at the micro approach.

This is a shoestring back yard project at the moment. Still R&D.

It's been 12 years since I've done automation/programming. Is there a code/language of choice for this sort of thing?

Do you know a good resource for learning how to accomplish this?

RE: Creating an oscillating waveform.

If you don't want to get involved with the μP world just yet, a trick I used a few times was to load a ROM with values corresponding to the values required and just step the memory addresses using a counter. The data out from the memory feeds a DAC. It was a simple way of generating three sinusoids with 120° shift which kept the phase relationship as clock frequency was swept. Old fashioned but effective for prototyping.
 

----------------------------------
  
If we learn from our mistakes I'm getting a great education!

RE: Creating an oscillating waveform.

to minimize the programming complexity, you could start with a BASIC Stamp processor from Parallax.  There are also a number of PIC processors that could easily do the job. Max pulse rate is around 200 bpm, so the time resolution is not that critical.  16-bit amplitude is about max that's needed, since that's the max dynamic range of most audio CDs.

TTFN

FAQ731-376: Eng-Tips.com Forum Policies

RE: Creating an oscillating waveform.

jim, IR... I believe the OP is looking for an ECG/EKG of the heart, not an audio waveform of a heart thumping.  The clue was his mention of "ticks" before and after the actual beats, so he's probably looking at things like ST segment elevation shapes, etc.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Creating an oscillating waveform.

But the ROM/counter/DAC is likely to have finer resolution in time and in magnitude.

These days, for a product, I'd use a RAM and load it from flash at boot time... i.e., the boot time of the uc that I'd use to provide a simple UI and the ability to modify the waveform in place, save a copy to flash, or accept an entire new waveform and store that in flash.

Mike Halloran
Pembroke Pines, FL, USA

RE: Creating an oscillating waveform.

In an ECG wave form, the P-Q-R-S-T (the "beat" segment) remains pretty constant in time no matter what the heart rate is.  What changes is the time from the last "T" to the next "P."  Here the Microprocessor can do this but the ROM/Counter/DAC woud compress the entire waveform with increasing frequency.

RE: Creating an oscillating waveform.

It would be a trival task to incorporate a variable dwell time between 'beats' by inhibiting the counter for a definable period.

Many ways to solve this one! It just depends where the OP has existing skills and how much he wants to, or has time to, learn new ones.
 

----------------------------------
  
If we learn from our mistakes I'm getting a great education!

RE: Creating an oscillating waveform.

Hi,

Good ideas posted. The cheapest one I'm thinking is using some midrange PIC MCU (like 16F873A) with reference voltage test output, a simple on-chip 32 step resistor ladder.  Then writing a static lookup table in ROM with your waveform values. As said before, You can control the table reading speed to get several frequencies and idle times.

The pin is a high impedance output, so you must add an opamp with some resistors and caps to build a low-pass filter, thus getting a buffered, cleaner signal.

This is another example with an external resistor ladder:
www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1824&appnote=en011071


Good luck with this!

Regards,
Gonzalo

RE: Creating an oscillating waveform.

the old EKG's used a simple circuit to generate typical heart beat. It was used to test the amplifiers and calibration.

RE: Creating an oscillating waveform.

(OP)
Clarification on Signal Requirement:

I need an electrical signal that is in the shape of a heartbeat pattern. I need it to carry 50-100W. I want to operate at 0V baseline. At t=0, I want to create a +5000V spike, and immediately drop it back to 0V, followed immediately by a -5000V spike, immediately returning to 0V. I want capability to repeat this cycle, varying the rest period between voltage spike pairs to generate a frequency between 40KHz and 1.5MHz.

I originally thought I would start with a +/- 12V signal and boost it to 5000V; I picked 12V because I planned on using an automotive coil to transform voltages from 12 to 5000V. However, since coils operate today in the magnitude of 25kV output, perhaps I could use a smaller input voltage to get the Vout in the 5000V range. I seem to recall the DACs I formerly used output 1-5V.

Although an ME, I have a little experience with Process Control. After reading the above responses, it seems doable for me to use a computer and a lookup table, or subroutine to create an output waveform.

It's been 20 years since I've done this sort of thing. Can anyone direct me to a resource or tutorial that might assist me?

RE: Creating an oscillating waveform.

Also try www.tek-tips.com , for specific help on programming.

Good luck!

Regards,
Gonzalo





RE: Creating an oscillating waveform.

Reminds me of the old movie (castle, lightning)...

"He's ALIVE!"

RE: Creating an oscillating waveform.

Everything eventually shows up on ebay.....

Just listed is an EXACT 202 complex waveform generator.  This seems to have 40 pots that you can adjust manually to create a  repeating waveform in steps.  Got to love early easy to understand technology!

RE: Creating an oscillating waveform.

That sounds good for DJ's :)

RE: Creating an oscillating waveform.

Don't miss his post of '4 Feb 08 22:09' where he mentioned +/- 5000 (!) volts and 50-100 watts (!). Also frequency between 40 kHz and 1.5 MHz (?).

Confusing.

RE: Creating an oscillating waveform.

(OP)
I'm not sure what is confusing about my design parameters.

I want to create a positive voltage spike immediately followed by a negative voltage spike—Have a period of "rest", then repeat the cycle. I want relatively little power dissipated.

From time t=0 until the two voltage spikes are complete, I anticipate that being about 10% of the Period of the waveform. But I want to be able to vary the "rest" time to end up with a waveform frequency that is adjustable between 40KHz and 1.5 MHz.

RE: Creating an oscillating waveform.

Considering the frequency and voltage requirements, I think we can safely assume this is NOT designed to simulate a human... or any other biological entity known to man.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Creating an oscillating waveform.

The confusing thing is that you want a power of about 50 - 100W in a pulse which lasts, according to your spec, zero time ('immediately'), something which requires a Dirac delta function or something close to it). What you're asking for up to now requires a very high instantaneous energy level during the pulse followed by a quiescent period. Normally a pulse is specified in Joules rather than Watts, and the rise time, pulse duration, and fall time of the pulse are defined. For a 1.5MHz repetition frequency your are looking at a very fast rise and fall time and to achieve 5kV peak voltage the dv/dt is going to be remarkably fast. Start putting values to the standard parameters for a pulse and you'll get more specific responses.
 

----------------------------------
  
If we learn from our mistakes I'm getting a great education!

RE: Creating an oscillating waveform.

It's one thing to create an arbitrary waveform at 'signal' amplitudes as per the original post. There's hundreds of methods to accomplish that. You can choose any method you like. Not quite trivial, but no big deal either.

It's another thing entirely to produce these incredibly high voltages and moderately high power level and at such high frequencies while maintaining an arbitrary waveform. It seems that you have no idea what you're asking for.

If my boss gave me those initial requirements, and then added those voltage and power and frequency specs a month later, I'd simply go home for the rest of the day for a nap. I probably wouldn't come back to work until the following week.


This thread reminds me of the old story about the bidders' conference for a comlicated military system to be installed on a submarine. Just as everyone was getting up to leave, draft specs and SOW in hand, the Admiral suddenly remember one more requirement that he'd forgotten to mention.

"Oh yeah, sorry folks, one more thing. The entire system has to enter the ship through a 3-inch diameter pipe, four feet long, with a 90-degree elbow about halfway along. Is that going to be a problem?"

RE: Creating an oscillating waveform.

(OP)
VE1BLL — From your UserName, I'm guessing that you served our nation at some point in your career. I am always thankful for people who sacrificed their time for the freedoms we all enjoy. I also appreciate your taking time to consider my design challenge and your time in responding.

Since I interpret your responses having a bit of bite to them, let me respond about the design parameters. In New Process Development, one often must make a best guess of requirements and go until we find a reason to vary the course. Sometimes it's just a small correction, other times it's a hard turn on the path. If you were leading men into battle and the conditions required a hard change to be successful, would you really tell your superiors you were going home for a nap and taking a week off? I somehow believe you possess much more character than to actually do this. However, I remember the frustrations of working in an engineering group when I too thought my clients didn't know spit from shineola — especially in how to accomplish what they were asking me to do.

It seems from other's responses that the best way to accomplish my waveform is with a lookup table and a micro, outputting a voltage signal through a DAC or video card, and then boosting the signal. Of course the biggest hurdle as you point out is the voltage spike and the frequency. My thought is that if I can product the shape of the waveform, I could amplify it or use a transformer (auto ignitions system coil or amplifier) to get the 5 kV.

I didn't intend on concealing details, just sharing what I discovered when it impacted the design spec. I also did not see value in writing two pages of requirements that I do know how to accomplish. Therefore I only asked about the portion on which I needed assistance. If anyone can direct me to a source that can lead me through the process to accomplish this, I would be grateful if you would share it with me.

RE: Creating an oscillating waveform.

No, I work for a company that supports the military. But I'm fulltime on military projects for many years. It was the closest fit on the pick list. And YCMV (your country may vary).

The 'bite' was only intended to ensure that the severity of your new requirement was made clear to everyone. It seemed that some were still answering the original question and hadn't noticed the 5kV etc.

The problem with creating a precisely-shaped spike, and then running it through a step-up coil or transformer, is that the precise shape is likely to be affected.

Personally, I don't see any (cheap and efficient) way to separate the two parts of the circuit. Of course, if you're willing to throw money at it, then you simply make the waveshape and run it through a monsterously-expensive wide-band amplifier. Even 50-watt wide band amplifiers can be a two-man lift and cost a small fortune.

If you're a bit flexible on the shape of the spike, then the voltage and power become much easier. But I don't think it will be quite as simple as an ignition coil.

As someone already suggested, start at the load and work backwards. Making the arbitrary waveform isn't the hardest part of this project. Making it 5kV is.

RE: Creating an oscillating waveform.

(OP)
AH! So you have had to endure our government bureaucrats. Your bite was rather mild for that scenario.

I do not have a lot of money to throw at this project. But it's a personal idea and I have to pursue it for a while.

At this stage, I need to keep energy consumption low to prevent process heat build-up. I believe using an alternating Voltage spike at low current and high voltage may accomplish this. I want to run as high a frequency as I can achieve but I don't know what the process may require - hence the 40KHz to 1.5MHz range. My thoughts are to vary the "rest time" between spikes to vary the frequency.

As far as the process, I won't have the hardware for several more weeks. At that point I will know the exact system load resistance.

I don't believe the spike shape is critical. Nor am I concerned about a perfectly clean signal. A bit of random noise riding on the waveform  should not be harmful in the process, and it may even help.

I got the idea about the auto ignition coil when I realized that the 12V battery system is used to create 25-30 kV pulses at the spark plug. So I thought why not run a +/- 2 or 3 V signal through the same coil, generating the approximate +/- 5kV pulse.

Am I overlooking something obvious?

Would anyone have a different way of accomplishing this?

RE: Creating an oscillating waveform.

If you run an ignition coil at a reduced voltage, then you're not likely to get 50-100 watts at the output terminal.

RE: Creating an oscillating waveform.

Is your 50 to 100 watts instantaneous peak or average  power?

RE: Creating an oscillating waveform.

The primary of an ignition coil is approximately 300V or 1:400 ratio.  This is why CD ignition systems operate at about 400V.  You might look at charging a cap and duping the charge with a FET instead of an SCR.  Google some CD ignition systems for some ideas.

RE: Creating an oscillating waveform.

"...primary of an ignition coil is approximately 300V..."

The primaries of traditional ignition coils are obviously a nominal 12 volts.

RE: Creating an oscillating waveform.

The collapsing magnetic field creates 300V on the primary, that is boosted up to spark voltage by 1:400 ratio. Therefore, most 12V waveforms fed into an ignition coil will not produce a voltage capable of sparking.  You know this and you time would be better spent explaining to the OP how to use an ignition coil for his purpose.  Personally, I don't believe the OP has any chance of success no matter how long this thread goes on.

RE: Creating an oscillating waveform.

I'd forgotten that an ignition coil (even a traditional one) only sparks on field collapse. I appreciate the correction.
 

RE: Creating an oscillating waveform.

25 years ago when I was last playing with this stuff, all ignition coils were really 6V with internal or external resistors to drop the voltage and kep inductance as low as possible.

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