I found a pressure sensor matrix that we could adapt to a pressure sensor patch on the stomach of the women in labor on this webpage (see below). It gives instructions how to build a pressure sensor by using two pieces of cloth and conductive thread! If I understand it right, you just have to sew everything together in the explained way and connect it with external pull-up resistors! They even provide the Arduino code. Click through the pictures on the top, all steps are explained in detail and a video is provided in which you can see how it works.
We should try this! Maybe we can make it as a sticky patch... something that looks like this guy!
What do you think?
It's crunch time now! MIT Museum 1-minute pitches this Saturday, and 15-minute presentations in class next week.
We've finally chosen a team name: Team Partayuda, from the Spanish words for "birth" and "help." So here's what's up, as far as I know...
Maysun is working on the breadboard and programming for the pressure sensor - a capacitor, with rubber in the middle, and [AB] is going to make it. We tried making the EMG last week, but it did not work out.
Karina found the cool conductive thread project that she just posted, so she and [LT] are going to try to make that happen; [LT] is also working on Arduino code for the button.
So in total we should have 3 different ways of input to test (which may or may not happen before class presentations): the capacitor, the conductive thread, and the button...I think right now the button is the closest to being finished.
I'm going to work on the processing code to take the input and provide an output of the results we want to show. However, we don't have the monitor yet, so this may end up just being displayed on a computer. [AB] and I are going to put together the poster, including the problem statement, our solution, Pugh chart, Design Compass, and design specs among other things, while [LT] is going to make a slide for the museum pitch.
Busy week, but we're almost there!
Our contacts in Nicaragua have kindly responded to our questions with very interesting information that has really defined our problem. See their responses below.
I think the Processing code this girl wrote could be helpful to us right now (although our pulse ox code might already do the trick for now); the entire Instructable might be useful in the long run for future work.
This bend sensor actually reacts (decreases in resistance) to pressure, not specifically to bend. But because it is sandwiched between two layers of neoprene (rather sturdy fabric), pressure is exerted while bending, thus allowing one to measure bend (angle) via pressure. Make sense? Watch below:"
By the way, our team name is now Babytrakr!
So I totally suck at blogging!!! I have a lot to say about the progress of our device that has happened in the last week. The last time I wrote, I had a plan for the EMG and I was about to build. Well I built it and realized that this was not the best way to do it... This is the schematics for it anyways.
This is the differential amplifier stage I described earlier. I built it, along with super cheap, super cool electrodes :) (See below)
Yes... they are pennies that I soldered to wires... but they work so whatever. So I build the circuit with LM741's (the most typical op amp you can get)...and... it didn't work. I know it was able to take the difference between two signal... but not muscle signals. So I did some research and found out that because it is such a tiny signal... and better op amp is necessary. So I used LT1637's for the buffer and a LM358 as my amplifier. And... it didn't work. After debugging though... I found my signal... But!!! It was tiny and very noisy!!! Also it was not the same shape as we thought it would be. It marked when the muscle started to contract, but not the whole contraction. This signal was so inconsistent that I felt it would not be the best for our prototype. That is why EMG was out... but what now... we have no input... Well this is where the capacitor idea that is mentioned in a few of the blog entries comes in. Sorry for the weird timeline... I am horrible at posting.
Thanks Mays for reminding me to post also! Sorry this is very late and happened 2 weeks ago but here's how the conversation with Dr. David Acker from Brigham and Women's Hospital went:
1) What do you use to measure contraction?
There are 3 subjective ways to measure:
2) How does the tocodynamometer work?
3) When do you know baby is coming?
4) What smarts are the best out of EMG, flexiforce or push button?
We took all these answers into consideration when brainstorming for our prototype two weeks ago. As you can see in the post below, our prototype changed numerous times because we wanted to accommodate all the ideas and comments from doctors in Nicaragua and the US. Ultimately, we went for a pressure sensor that is different from a toco, but still measures abdominal rise for contraction rate to correlate to a hand squeeze ball. Dr. Acker also agreed to let us see his operation room if we needed some more ideas, so if the project continues we have access to his resources. He was very nice about answering questions and very interested in helping out in the future. Here's his profile:
Ok, it's me again,
So, last time I told about the EMG circuit, which we rejected from our prototype because the signal was not what we expected and it was way too small and noise. I was reading up on other ways to do determine contractions (this was about 2-3 weeks ago), and discovered that EMG is not common method to detect contractions, even in the use. This is because there is also of other muscle interference, like fetal heart beat, maternal heartbeat, and maternal body movement artifacts. There is some cool research into additive filtering, and also how EMG's are better in detecting when the baby is going to be delivered, but this research is not even applied here in the US, and what doctors need now is something that works. That is why we switched focus to a tocodynamometer, toco for short. This is basically a pressure sensor on the stomach that detects the upward movement of the uterus during contraction:
We have been told that during a contraction, the uterus stiffens and moves about 1cm up. This is where the Toco comes in. It measures the pressure exerted by this upward force. After a long brainstorming section, we decided that a cool why to make a cheap and use to manufacture sensor was to make a capacitor. For a parallel plate capacitor, the equation is:
C= ϵ0 ϵr A/D
so as you squeeze the plates together, the capacitance changes, and all you need for the sensor are 2 conductive plates, which is locally available!!! I thought it was brilliant. To measure the difference in capacitance, I was going to make a simple LC oscillator, and have the sensor is parallel with the oscillator C. Thus, as the plates are squeezed together, the frequency of the oscillator changes, and we can plot the frequency changes over time. Here is the oscillator, the antenna in the picture is our sensor:
I figured that there was about 10 pF variable capacitance, so in order to get a good frequency range, I was going to have two oscillators and mixer them and filter out all the frequencies I didn't want:
I know, a little technical... but it needs to be documented.
The mixer is below:
Now!!! The really cool part of this is that these are all parts of a radio!!! A radio is cheap and there are already tons of mechanics who know how to work/fix them. The infrastructure exists and it would be easy to implement anywhere. I was so amazed with myself, I was telling a friend while debugging my circuit, and that's when Velostat came into the picture. My friend saw what I wanted to do, told a much simpler and cheaper way to do this. So this eliminates our second prototype. Next time, Velostat and how it works!!!
Karina and Nathan put together this logo last week for our poster - isn't it cute?! Sort of makes the whole project feel more legitimate to me (not that it didn't before, but having some sort of branding helps).
I feel I need to write this blog entry before the end of the term. So we decided at the end to use Velostat, a material I found through a friend, as our pressure sensor. So the technical description that 3M, the manufacturer of the material, provides is an "opaque, volume-conductive, carbon-impregnated polyolefin. Easily grounded, the electrical characteristics are not affected by age or humidity, and are suited for handling, shipping and storage." I found this material at a site called lessemf.com, a site that, hilariously enough, specializes in materials to prevent alien abduction. Velostat is used to make into helmets to prevent alien mind control. But the real use is basically, it is the material used sometimes as anti-static bags for electrics. Electronics are packed in this stuff and often trashed afterwards.
It costs about $2 for a 1'x3' piece. Now, what is Velostat? Basically, it is a material that changes resistance as you deform it, and it can be deformed by pressure, or bend. The resistance of the material untouched is about 28 K ohm, but as you bend or apply pressure to it drops. I hooked about a sq. inch of this up to a simple resister to make a voltage divider, high pass filtered it with a capacitor and resister, and voila, a very cheap pressure sensor.
Now working with it, I did find some problems. There is actually no stable baseline, but that's where the high-pass filter helps. The problem is there is a definite limit to the filter, since our actual signal is very low frequency. But, this is not a big concern after filtering since we can threshold the sensor in the Arduino. Our second problem was that Velostat seems to be sensitive to heat and to sweat. Now this is a huge problem in our application. Fortunately, a piece of scotch tape around the material seemed to help. Lastly, since it is a flexible thin material, there is no easy way to attach wires to the material so there is often lose material. A cool thing, that a lady during our presentation at the MIT museum mentioned was that we could use conductive tape, like copper tape, but this adds cost.
To display the waveform the pressure sensor picked up, I used this code to make a simple oscilloscope with an Arduino (http://accrochages.drone.ws/en/node/90). With this display, I tested out different methods of arranging the Velostat to make it work better. First, I tried bending the material like this:
Then I tried double layering it so to increase reaction. Both methods worked but doubling the layers was more sensitive.
So, after many iterations of the designing the input… ball, to EMG, to Capacitor, we finally reach Velostat. More stuff needs to be looked into, but it is definitely cool.
I would just like to give special thanks to [anonymous MIT student EZ]. I basically forced him to give me some Velostat to experiment with and graciously helped me to make the sensor work optimally.
A couple of action shots from the museum presentations last week!