Note: This text is a work in progress. If you find any inaccuracies or material that needs to be included, please contact us.
Prototypes
We have built a very simple data collection system for recording MMG signals (muscle sounds). It consists of a 3.5mm audio plug, a Panasonic microphone cartridge (part number WM-64PNT), some wire, and a laptop. Not including the laptop, the components cost less than $5. The microphone is an electret condenser type, and is powered by the laptop’s sound card. We used the free program Audacity to record and process the sound. We simply taped the microphone directly onto the skin with masking tape, plugged it into the laptop, and pressed record:
The results were much better than we expected, especially after we rolled Jon’s silicone suspension sock over the microphone for isolation, which made the sound much louder. We processed the data to make it more audible by filtering out all frequencies above 40Hz and raising the pitch by two octaves. Here’s an mp3 of the processed sound. In this recording, Jon is making the motions that would have opened and closed his hand. The microphone is over the muscle that opens his hand, so you can hear it loudest, but the muscle contracting on the other side is also audible. You can hear the muscle sounds getting louder over the course of the recording as he grips with progressively more force. You can download the raw data as a WAV file (1.5M) if you want to do your own processing (the popping noises are from the alligator clips connecting the microphone to the computer).
After some more tests with electret condenser microphones, we have found out some essential characteristics of an ECM based MMG sensor:
- Clean power supply – Just running our laptop on batteries made a huge difference in low-frequency noise. Obviously a prosthetic device would be battery powered, but we need to protect against power supply fluctuations from the motors, etc.
- No electronics-body contact – When the metal case of the microphone touches the body, it turns into a giant antenna, and the 60Hz radiation from household electrical power really interferes with our signal.
- Good sealing – There must be an airtight chamber between the skin and the microphone or a the signal gets dramatically quieter. Additionally, the size and shape of the air cavity plays a big role in the sensitivity of the sensor.
The first requirement is easily satisfied with electronic filtering and the second simply requires insulation and shielding. However, the requirement of a sealed air chamber imposes some significant manufacturing costs. Jorge Silva’s CMASP sensor units require two types of silicone cast around the microphone. Silicone casting in a small shop is very labor intensive and time consuming. The sealing requirement would be completely eliminated if we did not use air to conduct sound from the skin to the sensor. We are investigating alternative sensor types and have discovered the following:
- Piezoelectric film elements – These are thin sheets of material that produce a voltage when bent. An element costs from $5 to $15 in low volume depending on options from Measurement Specialties, Inc.. The chief advantage of these sensors is that they are low-profile and don’t contain easily broken connections. It seems like they could easily be integrated into a silicone suspension sleeve, especially the type that has integrated, flexible leads. Here is a technical discussion of piezoelectric films in general. An excerpt: “One major advantage of piezo film over piezo ceramic is its low acoustic impedance which is closer to that of water, human tissue and other organic materials . . . [which] permits more efficient transduction of acoustic signals in water and tissue.”
- Accelerometers – These are accelerometers that are mounted directly on a circuit board. From MSI they cost $20 for a one-axis chip. The disadvantage of this solution is that the measurement is taken from movement of the whole device, whether it is moved by muscle vibrations or by the whole arm being shaken. It also requires surface mount soldering and a circuit board built into the suspension sleeve.
- Vibration transducers and contact microphones – These are encapsulated devices that measure vibrations when mounted to a surface. One from Knowles Acoustics costs around $50 from DigiKey.
The piezoelectric film elements seem really promising, so we are ordering some to test out. The next step is to collect data from more than one microphone at once and begin testing some analysis methods.
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