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16

Topics / Science and Measurement / Re: problems in data measurement

on: May 26, 2014, 07:48:36 pm

It is important to mention the difference in circuit analysis in the time domain and the frequency domain. Your equations apparently are mixing them both and that's also part of the problems with your results. Although they are related through the Laplace Transform (fortunately something we don't have to worry about), the time domain analysis will give you instantaneous "behavior" of the variables, that is, at any given instant in time. The frequency domain on the other hand, is used in AC circuits analysis to obtain variables "behavior" with frequency and it is invariant in time. Therefore, if both are mixed it won't work, that's why you can't calculate the rms current (Iac) using instantaneous voltage (E(t)) and reactance. The E(t) is the voltage at any t=timeyouselect instant during the AC signal, while the rms voltage as it is a periodic sine wave signal does not change with time (in the analysis).



17

Topics / Science and Measurement / Re: problems in data measurement

on: May 26, 2014, 02:02:35 pm

E = L di/dt =>  224x10^6(0.036) => 8.064 micro volts. That's the instantaneous Voltage (E(t)) across an Inductor (L) through which a current I(t) is circulating. That's not the rms voltage which is what you are after. In order to get the rms voltage you would need to perform complicated calculations with that E(t) including integration in time ( http://en.wikipedia.org/wiki/Root_mean_square). If it is a periodic signal then the rms of the signal is equal to that of one period. Since all that is not very practical to do, then what you do, is to measure the rms voltage. To do that with the Oscilloscope, you measure the peak to peak Voltage (Vpp) and Vrms=Vpp/sqrt(2) if it is a sine wave signal, as I suspect. If its not a sine wave, then that's not the equation. Another way is to use a true rms multimeter capable of operating at that frequency (20KHz in your case) and it will read the rms voltage directly for you. That could be the reason for the differences you are observing, depending on which multimeter you are using. You can read the specs and find out if it can operate at that freq. I studied these things long ago and might not remember the details very well though. I = Vac/ XL = 10 / 2*Pi*20*10^3*224*10^6 => 0.36; That looks somehow true, except for the fact that the 10Vac is applied to the serial combination of the multiplexer and the coil. That creates a voltage drop across the multiplexer also depending on its ON resistance and the current passing through. Therefore the 10Vac is not applied in its totality to the coil, part of it is lost in overheating the multiplexer. If too much, multiplexer blows. So in reality it looks like: I=Vac/(ZL+Ron) and VL=Vac*ZL/(ZL+Ron) VL(Inductor Voltage) Ron(Multiplexer ON Resistance) ZL=R+XL (Inductor Impedance) The multiplexer datasheet should tell you its ON Resistance. In a normal case, the multiplexer Ron should be way less than the Impedance of the load you are driving through it, so the voltage drop across it is negligible and most of it gets applied to the load where you want it. Furthermore, you are not considering the Ohm Resistance of the coil and calculating its impedance (Z=R+XL). That could be acceptable, if it is very low compared to its XL at the operating freq; but I have no idea about the wire gauge they are made of and the amount of turns which directly affect its Ohm Resistance. Anyways, at f=20KHz, XL=2*PI*20*10^3*224*10^(6)~28 Ohms. That is low and very well in the same order of magnitude of the inductor Ohm Resistance, therefore its Z may differ substantially from its XL. Please measure the coil resistance with a regular Ohmmeter to check how much it is. Furthermore, that low XL value could be also in the same order of magnitude of the multiplexer ON Resistance (I don't know); please check that also. If that's the case then most of the Vac is lost in the multiplexer and not in your useful load (the coils) I have the impression the 224 uH value you are mentioning may not be right though. That looks like and odd value (particularly the 4 at the end). Like caps and resistors they are "mostly" manufactured in series of standard values (from what I have seeing). If that's what you are reading on the coil itself, then the 4 may mean the multiplier and the value could be 0.22uH or 22uH instead (I don't know). If you have not done it yet, I suggest you use an inductance or RLC meter and double check the inductors' real values. Can you post a picture of the actual coil you are using? EDIT:By the way, I made a mistake when explaining to you how to measure the rms voltage with the Oscilloscope . To do that with the Oscilloscope, you measure the peak to peak Voltage (Vpp) and Vrms=Vpp/sqrt(2) Wrong, it should be the "peak voltage" (Vp) and not the "peak to peak voltage" (Vpp). Peak voltage means from zero to peak. Therefore: Vrms=Vp/sqrt(2). Sorry about that, too many things going on at the same time, I guess...



18

Topics / Science and Measurement / Re: problems in data measurement

on: May 26, 2014, 05:37:33 am

HI: Glad you made it work somehow. Most of the times it gets burned That means destroyed? Now i am trying to improve the current being sent to the inductor, I am not sure how i can send more current to it. There is a multiplexer stage inbetween the sensor and the frequency generator and if i send more than 20 mA current to the multiplexer it will definitely get burned. So clearly I need to send it afterwards.
One way could be using relays to switch the signal instead of the multiplexers. You can still use the multiplexers to activate the relays. ...Voltage drop across the inductor = AC voltage / Inductor. => VL = Vac / L... Inductive Reactance= w*L=2*PI*freq*L The voltage drop across the inductor is the voltage you measure across the inductor, either with the multimeter or the oscilloscope. I don't think you need to calculate that; but measure it. I still think you need to lower the frequency to get better results with the devices you are using. It all depends of the frequency of the original signal you are sampling, that is, the one you are measuring.



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International / Proyectos / Re: Medicion de Volumen (Metodo Seco)

on: May 07, 2014, 05:59:15 pm

Bueno, retomando al tema que me ocupa aqui, como les decia, prepare un video con pruebas realizadas donde pueden observar como se obtuvieron los resultados mostrados en la foto anterior. Para las pruebas, utilice baterias como patrones de volumem, que aunque no lo son en realidad, sino solo una aproximacion aceptable, me ayudan a comprobar el metodo y sistema construidos. Las pruebas mostraddas se realizaron para una y 3 batterias dentro del envase. Estas son solo pruebas preliminaries y es necesario utilizar verdaderos patrones de volumen de los cuales no dispongo por el momento. Agradeceria sus sugerencias y comentarios. Aqui esta el video nuevamente, para los que tengan interes en el tema. https://www.youtube.com/watch?v=OwI0EfT3fg8



21

International / Proyectos / Re: Medicion de Volumen (Metodo Seco)

on: May 07, 2014, 02:23:46 pm

Solo que no logro comprender el por que de todas esas opiniones aqui y te agradeceria que en mis "posts" te enfoques en debatir las cuestiones tecnicas que es el tema que me trae a este foro. Lo demas como ya te dije, sinceramente no me interesa.



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International / Proyectos / Re: Medicion de Volumen (Metodo Seco)

on: May 07, 2014, 01:42:26 pm

No entiendo las motivaciones de tus opiniones, que en realidad no son mas que eso, opiniones que se basan en tus experiencias personales y que a mi en realidad no me interesan, ni creo que este foro dedicado a cuestiones tecnicas, sea el lugar apropiado para ellas. Tampoco necesito, ni me interesa tu coaching.



26

Topics / Science and Measurement / Re: Volume Meassurement (Dry Method)

on: May 06, 2014, 07:16:30 am

@dlloyd. Sorry, I missed your reply and only looked at the last one (Chagrin's one). Really sorry for that. I really appreciate your thoughts and suggestions and I know you were trying to sincerely help. I'lll carefully consider everything you've said and perhaps there is room for improvement with some of your thoughts about this. Discussion is always good and your points helped me explain better what I've have done here. Thank you so much!!



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Topics / Science and Measurement / Re: Volume Meassurement (Dry Method)

on: May 06, 2014, 06:29:52 am

Also, if the thing is scaled up, to reduce the times the Pr needs to be increased and not all objects can be submitted to high pressures without damage. That will seriously limit the application scope of this. In reality, as very often the case in practice, there is no universal instrument capable of measuring at all ranges with the same accuracy. This will have to be adapted to particular cases, as required by the nature of the objects to be measured.



29

Topics / Science and Measurement / Re: Volume Meassurement (Dry Method)

on: May 05, 2014, 10:28:09 pm

Continue... $$$ may not be required at this point yet ... there may be still room for lowcost or nocost improvements.
That is almost always true also. However, at this point I do certainly believe this has reached a plateau here with no further visible progress, unless real improvements are made, such as the leaks free container and temp compensation. Then I think, new findings will prompt new improvements. The reasoning for that is that I can't find ways of improving the repeatability in the volume calculations significantly more than what they are now. Perhaps its just me, not able to do it. One way or another, help is required though. Oops  missed this. If averaging (sensor resolution 3.1 hPa) among other things can get 0.05 hPa steps, then perhaps averaging readings with 0.098 hPa resolution could result in 0.01 hPa steps. The resolution of the 250mbar FSS sensor is not 3.1hPa as explained before and I was referring to the AD conversion process. Please take a look at this: https://www.youtube.com/watch?v=3tvSkV77MaYArduino and its ADC are resolving 10000 different values to drive the Gauges in that video. Yes  possible. All time values should also be floats (numbers like 20 could be expressed as 20.0 in calculations) I did not do it like that, I used a long type variable and micros() to improve precision in the detection; but perhaps it can be improved by using interrupts. That's possible. However, the Standard Deviation as % of Avg for the TC is really low already (as shown in the picture). There is margin for improvements there, sure; but I don't think I can use floats for time??. Valve Timing: The solenoids are DC type and I’m driving them with transistors (way faster than relays), so there is a negligible delay in the switching. If you look, I extended the deflation time to decrease the error added by the valves response time among other things. In other words, a few ms delay for the valve to open, are almost negligible in 4000050000 msec aprox range of the TC. The valves I’m using have a response time of 510ms@ 100 psi (probably lower at 250mbar) which represent 0.01250.025% of 40000ms (in the lower TC where the effect is greater). This is a very low added error and enough for now. Furthermore, since this error is almost constant it is probably cancelled by the way the volume is calculated. Volume determination: I think I tried that already long ago when I was solving the exponential equation; but there were not visible improvements, I think because of the fact that the valve commutation time is negligible and lower than the intrinsic error in finding the TC. I tried even more, I calculated the area under the curve and also played with the slope of the curve with no improvements observed; on the contrary worst results. Leaking problem: What if the pump just precharges a small tank to around 5 psi. Then the check valve (I think this is the leak source) is replaced with an on/off solenoid control valve (which is inherently leakfree). Another benefit would be air that is much more stabilized with temperature and humidity for performing the tests.
Very possible, I already thought about that as you can see in reply #2. Actually that's what I'm doing on the video on reply#5 where I use a tank and everything is smoother there and more stable. The inflation is more precise than with the one piston compressor I'm using now. I did not implemented that because I had to either build it myself or buy one (which I have not found so far for such low pressures) and that was not really necessary for this proof of concept stage. This is another area where $$$ is required. Also, I reduced the error in the Start Pr by placing another flow restrictor in the inflation circuit as you can see in the schematic, right after the check valve. That slows down inflation and Arduino can detect the 150hPa more precisely. It also helps reduce leaks back to the compressor through the check valve (which may appear) and helps reduce the pulsation effects from the one piston compressor. I'm also driving the compressor with PWM to reduce the voltage and speed and therefore increase the inflation time. When I was testing that part I needed the Inflation time in the table to be able to compare as I adjusted things. That's why its still there. Furthermore, the concept of the TC suggests it is independent of the Start Pr. There is more on this: The Functions Generator I built was a consequence of this Project. I'm planning to use that to control a temp chamber where I can test temp dependency for this Project precisely; but to do that I need to correct the leaks problem first. Although I don't have the exact equation or certainty on this, preliminary observations suggest there is an almost linear dependency with temp. I want to test the whole thing inside a temp chamber to be able to find the temp dependency of everything combined which will produce a much more effective cancelation and allow to use this device in a wide range of temps. That's is pending. Well really thanks for your interesting observations and comments.



30

Topics / Science and Measurement / Re: Volume Meassurement (Dry Method)

on: May 05, 2014, 10:26:55 pm

Most all the same considerations are needed. In both cases we have temerature, pressure, flow and volume.
I just did not know about this procedures you are mentioning and its really interesting to know about them. I'll make some time and check the links. Thanks. Re: The compressibility factor (Z) of air  the curve is similar to the deviations you have found. It would only take some simple mathematics or use values from a look up table to correct for this. I did not know about that either and that was the reason for my question. On the other hand, neither the graph nor the compensation is required to find the volume. That's why I did not put much effort on solving that. Thanks for that info also. (another subject to study) Approximating 3 decimal digits might be enough for the display, but its how the numbers are determined that can influence the effective precision in the results. Certain equations can have a precision reducing effect. That is absolutely true; but I'm not approximating until the results are send to the display. All calculations are done with the floats without any approximations. On the other hand, none of the results shown are used to calculate the volume with the only exception of the TC which is measured by Arduino and I did not placed any approximations in the Arduino code. In the Processing code, there aren't any approximations in calculating the volume either and its just a simple linear equation linking the TC and the volume as explained above. That's why such repeatability is obtained, which is way better than solving the exponential equation as there would be more measured variables involved, each of them contributing with errors to the results. If you try to solve the exponential equation with the values shown, there will be significant errors not corresponding to the volumes shown, because I did not do it that way. I realized that was a tough proposition. You can find the equation I used though, by simply plotting the Volumes vs. TCs values shown in Excel, (insert graph, add trend lineshow equation in the options). I left the other values there because I wanted to calculate the Standard Deviations for them and observe "behavior" of those variables and also to help explain the method. Same as with the Graph; but none of that is really necessary. The screen is probably saturated with information an operator does not need and will just create confusion; but I need it for now, while I get this right. From your link to the sensor, its pressure range is 1 psi to 150 psi (60 mbar to 10 bar) (69 hPa to 10,342 hPa). The datasheet notes High resolution (min. 0.03 %FSS), so this represents 3.1 hPa resolution. 1psi150psi is the possible Full Scale Span (FSS) of the sensors in that Series (SSC). In other words, in that particular Series you can find sensors with FSS inside that range; but there are many different ones in there with different FSS and not all of them are 150psi FSS. The FSS for the one I'm using is 250mbar (250MG in the part number) and I believe (could be wrong though) from the table in page 2, that its resolution is 0.03% of its FSS, in this case = 0.075 mbar and not 3.1 hPa. I'm not sure how you have preconditioned the signal levels into the 10bit ADC. Perhaps resolution could be improved as follows. If ADC code 0 represents the lowest pressure you'll ever measure, say 40 hPa, and ADC code 1023 represents the highest pressure you'll ever measure, say 140 hpa, then the resolution becomes only 0.09765625 hPa per step. You may only need an opamp or 2 to achieve this. Perhaps there is another way. The sensor does not provide that output. It is a linear analog sensor providing a voltage output already amplified from certain value above 0V to a little less than 5V (I don't remember exactly; but its in the datasheet somewhere) therefore the range is not 1023 different steps in the ADC as it looks; but less. That output corresponds to Pr values from 0 mbar (referenced to the Atmosphere as it is a gauge sensor and not absolute) to its FSS in this case 250mbar. That's why I chose it, as it requires no additional electronics, just power and ready to go. On the other hand it is a high quality device (check the recommended applications in the datasheet) Cont...



