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61  Topics / Science and Measurement / Re: Volume Meassurement (Dry Method) on: April 23, 2014, 08:05:41 am
Referring to the operation I can't find a convincing explanation for these 2 facts:

-The spike in the curve right when inflation finishes.
-The slight difference during the first half of deflation between the ideal exponential decay curve (pink) and the real pressure measured (top of the blue area). After that they are almost identical.

These are factors I've been weighing in to find answers:

-I'm using elastic (very flexible) silicone tubing which expands and contracts.
-Leaks in the system dependent on pressure, adding weight to the exponential decay at the beginning of deflation when the pressure is higher.
-The fact that air is a compressible fluid.
-Reduction in hose diameter to connect to the Pr. sensor.
-Delays in the transmission of the pressure waves inside the pneumatic system.
-Minute decreases in air temp as it enters the container and expands.
-The fact that I'm using a one piston compressor directly driving the pressure in the system and not from a stable pressure source such as a tank and inflating though a solenoid valve openning .
-The whole theory is wrong and I made too many approximations and assumed too many equivalences.

Even when these 2 facts appear to be constant and therefore systematic and the way I'm calculating the volume apparently effectively cancels them, it will be good to find answers to these questions.
Any ideas??
Thanks

62  International / Proyectos / Medicion de Volumen (Metodo Seco) on: April 22, 2014, 07:14:25 pm
He estado trabajando en este Proyecto por un buen tempo ya y ha sido mi "Proyecto Fenix". Habiendo sido derrotado en innumerables ocasiones por el mismo, he tenido que levantarme de mis cenizas con nuevas ideas y lecciones aprendidas. Aun cuando todavia hay que hacer ajustes en el mismo, al final pienso, la perseverancia triunfo. El objetivo era disponer de una alternativa al metodo de inmersion que ha estado es uso por milenios para medir el volumen de objetos irregulares, muchos de los cuales no pueden sumergirse en agua u otros liquidos. El obstaculo mayor fue encontrar/fabricar un envase que no tuviera fugas al ser presurizado. Trate mas de 10 diferentes envases y el mostrado es el que me dio los mejores resultados; pero es algo en lo que aun tengo que trabajar. esto es solo un prototipo para probar el concepto. El problema principal radica en que al abrir y cerrar el envase para introducir la muestra, las fugas por la tapa cambian y esto introduce un grado de incertidumbre inaceptable en los resultados.
El sistema puede ser extendido para medir las fugas de cualquier contenedor sellado.
Criticas, comentarios, sugerencias, ideas, opinions son todas bienvenidas.
Construido con Processing 1.5.1 y Arduino UNO
Gracias.
63  Topics / Science and Measurement / Volume Meassurement (Dry Method) on: April 22, 2014, 04:22:36 pm
Hi:
I have been working on this project on and off for a long time. It has been my "Phoenix Project". Being defeated several times by it, I had to come back from my ashes with new ideas and lessons learnt. Although still needs some fine tuning, I think perseverance prevailed. The goal was to have an alternative to the thousands of years old immersion method to measure the volume of irregular objects, some of which can't be immersed. By extension, the system can be used to measure leaks in a sealed a container. The greatest obstacle was finding/fabricating a leak free container under pressure. I have tried about 10 different ones and all of them leak. I still have to work on that.
I would like to hear your suggestions, comments, ideas, critics and opinions.
Thanks.

Project built with Processing 1.5.1 and Arduino UNO.
64  International / Proyectos / Generador de Funciones on: April 20, 2014, 12:29:19 am
Este es un generador de funciones para ser utilizado en el control de variables fisicas.

https://www.youtube.com/watch?v=0MGW8DekE10
65  Topics / Science and Measurement / Functions Generator on: April 19, 2014, 11:37:32 pm
This is a Functions Generator to be used to drive physical variables either directly by voltage variations using PWM or by means of a control loop implemented in the Arduino code. The latter will require using the corresponding sensors and actuators for the variable in question. At this time 12 different waveforms are available. Implementing the graphical representation and adjustments of the mathematical equations has been the hardest part in this project. Particularly difficult for me was implementing the triangular waveform even when it looks very simple  smiley-roll-blue. It took me a bit more than 2 weeks to build this Project despite the fact that I had most of the virtual instrumentation already built  At this time PWM is the only type of modulation available. The GUI includes buttons for Freq and Phase Modulation; but they are not active at this time. The Arduino code to handle these types of modulation is pending and honestly I have no plans to do it for now. The video shows a light bulb used as the load to observe the voltage variations from the PWM signal generated by Arduino. analogWrite(); is being used to generate the PWM driver signal through pin 3. A potentiometer connected as a voltage divider to analog input A1 is used to simulate the sample signal. "Full duplex" communication is established between the computer (Processing) and Arduino. It receives the values to drive the PWM signal and sends back to the computer the voltage sampled at analog input A1. This is the first time I implement "full duplex" communication involving data in both directions although high resolution is only one way. High resolution PWM is pending. All data collected while recording is logged as usual into an Excel file which can be viewed at the end of the video.
Built with Processing 1.5.1 and Arduino UNO.
Critics, comments suggestions, ideas, opinions, thoughts are all welcome.
Thanks.
http://youtu.be/0MGW8DekE10

66  Topics / Science and Measurement / Re: i need disolved oxygen sensor interface circuit on: April 16, 2014, 06:24:52 pm
Thanks for the link. I can't find a datasheet in the link provided or elsewhere for that particular sensor. In order to design an interface circuit its imperative to know what it gives at the output, bias supply, freq response, transfer function with respect to the O2 concentration, output impedance, etc. If you find the datasheet or any of that info please post here. I have no idea what are those parameters for that sensor neither if it is digital or analog. Request the datasheet from the supplier, they maybe able to provide it.
67  Topics / Science and Measurement / Re: i need disolved oxygen sensor interface circuit on: April 15, 2014, 08:02:02 pm
What sensor are you intending to use?. If you don't mention what sensor is it, we can't help you. At least provide a link to where we can find information about it.
68  Topics / Science and Measurement / Re: voltage divider for temp sensor on: April 07, 2014, 10:19:20 am
This is why you need a bias resistor and how to calculate it.
CAUTION: This method will give you a highly nonlinear voltage output with temperature, because of both, the thermistor is a nonlinear device for which its resistance changes nonlinearly with temp and because of the voltage divider used for bias. If you don't know the transfer function of the thermistor with temperature you will be in deep trouble to read the temp and making the linearization. I suggest you use a device giving an already linear output with temp like the LM35 for instance.
Good luck.
69  International / Proyectos / Re: Lectura pin digital defectuosa on: March 26, 2014, 09:27:27 pm
1-Pull up resistor ~ 1KOhm to 5V, switch to ground.
2-Rebote de los contactos (Bouncing). Puedes tartar de eliminarlo con:
-delay()
-Utilizando banderas como aqui: http://forum.arduino.cc/index.php?topic=222479.0
Quizas eso te ayude
Suerte.
70  Topics / Science and Measurement / Re: ranging cap meter not quite correct on: March 15, 2014, 09:56:53 am
I think it was a good starter project and the best part is that we have learnt from it.  If you have a chance just try with the BIFET OpAmp. Maybe it will let you go a little bit lower in the Cap value and improves accuracy.
Thanks for sharing and good luck.
71  Topics / Science and Measurement / Re: ranging cap meter not quite correct on: March 12, 2014, 07:49:53 am
Looks much better.
-Remember damper diodes for the relays coils to prevent negative spikes.
-The 100R I think is too low for the pF region.
-I would use more ranging resistors. You are changing the caps range in about 6 orders of magnitude I think.
I've been thinking the relays aren't perfect either as they will introduce other problems such as: bouncing of the contacts, probably oscillatory response and delays in the commutation; but testing will tell.
Good luck.
72  Topics / Science and Measurement / Re: ranging cap meter not quite correct on: March 11, 2014, 05:45:39 pm
Thanks for letting me know all that.
Quote
until I get my hands on some suitable relays
That will definitely help. I would also try to get a BIFET opamp just in case... smiley-wink
Good luck and enjoy your project. smiley
73  Topics / Science and Measurement / Re: ranging cap meter not quite correct on: March 11, 2014, 09:36:55 am
Quote
However I would expect any impedance mismatch to be constant during the  cap discharge as I can't  imagine the circuit  impedance changing as the  DC voltage falls or the capacitance is changed.

That makes a lot of sense; but still better nor to have it neither having to compensate. On the other hand, the impedance could behave non linear still at such low currents. Testing will tell. I don't know.

Although I have not clear idea of the connections you have made and I need to put more time analyzing the circuit. It looks, the resistors connected to pins 9 and 12 could be creating a voltage divider connected to the cap. When one of the pins let's say 9 goes to LOW, the other (12) I guess is HIGH to suppress the ground path. That in reality connects it to 5V and with the other to ground there is a voltage divider. If both are LOW or HIGH then they are simply in parallel to either ground or 5V. I think Arduino digital outputs can't take 3rd state of high impedance and they are either low or high??
74  Topics / Science and Measurement / Re: ranging cap meter not quite correct on: March 11, 2014, 03:31:12 am
One thing at a time.
The method in theory should work as it is already proven science. Implementing it in practice is a different story. Trying to achieve "auto ranging" at the same time is introducing additional problems, before implementing the basic solution to have a working circuit. So, I would try to go step by step and have a working thing first and then auto ranging.
-It looks this is a typical case where the measuring method is affecting the variable being measured.
The Arduino A0 and pin 13 are connected to the cap under test. Maybe  in the picofarad  region for the tested caps, the effect of the leakage currents involved are in the same orders of magnitude as the discharge currents through the discharge resistors. That could be contributing to the nonlinearities and errors you are getting in the results.
Possible solutions:
-Use a buffer OpAmp (as voltage follower) with BIFET high input impedance to tap the voltage on the cap and send it to A0. This will minimize loading the cap with the instrument of measurement.
-The discharge resistor should be high as to provide enough time for the readings to take place; but not too high as to be comparable with the leakage currents of the cap being measured. Also if the discharge takes too long then the voltage presented to the Arduino A/D converter may take too long to change one quantifiable step, reducing the resolution in the time measurement.
-Make sure when the measurement is done only the discharge resistor and the OpAmp input are connected to the cap. For that, you can use relays as the switching devices to avoid any leaks which will influence the measurement.
-Concentrate on having a working circuit first and then move into auto ranging.
75  International / Proyectos / Control de Voltaje Computarizado (Mejorado con alta resolucion PWM) on: March 10, 2014, 06:59:24 am
Esta es una version mejorada del Controlador de Voltage (iluminacion) construido anteriormente.
http://forum.arduino.cc/index.php?topic=223712.0
En el caso anterior, al utilizarse analogWrite() para manejar las salidas, la resolucion de la senal de PWM y los pasos de voltaje obtenidos estaban limitados a 256. Esto pudiera no ser suficiente en algunas aplicaciones donde se requiera mas precision. En la nueva version aqui mostrada, la senal de PWM y el voltaje pueden cambiar en 1000 pasos diferentes. Hay una pequenisima inestabilidad como se observa en el video; pero aun asi, en mi opinion es mejor que solo disponer de 256 pasos diferentes.. La inestabilidad puede ser causada por varios factores, tales como: el tiempo que demora Arduino en leer los valores recibidos desde la computadora, es decir, el tiempo de comunicacion o el ruido introducido por las lamparas entre otros. La frecuencia de la senal de PWM seleccionada es de 1/15msec~66Hz y el ciclo util varia de 0 a 100%, como se muestra en el video. Se emplearon para manejar las lamparas los pines de salida 8,9,10,11 correspondientes a Dim 1,2,3,4.
Este fue un projecto construido rapidamente, asi es que si encuentran algun problema, por favor dejenme saber.
Construido con Processing 1.5.1 y Arduino UNO.
Gracias.
http://www.youtube.com/watch?v=271qPIFaC2E&feature=youtu.be

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