Music from the Heart -- Music Suit
Introduction
This project attempted to create a natural form of musical expression by connecting sensors to the body. Six tilt switches were attached to the wrist, ankles, and shoulders, each controlling a single pitch from the pentatonic scale. The heart beat was obtained using photoplethysmography on the user's finger, and this signal was used to strike a drum in sync with the heart beat.
Team Members
- Thomas Peterson (Computer Engineering, 2010)
- James Rein (Biomedical Engineering and Music Cognition, 2010)
- Eric West (Mechanical Engineering, 2011)
INSERT TEAM PICTURE HERE
Subsystems
Although intended as a single, cohesive system that would allow the user to intuitively make music, the project was easily divided into two subsystems: drum heart rate and music tones. Below is more explanation about each subsystem.
Heart rate monitor
In brainstorming how to translate a heart beat into a drum beat, we decided that the least intrusive method would be the best. Attaching electrodes to the user would simplify the process of identifying a heart beat, but attaching the sensors directly to the skin would be a time-consuming and personally-invasive process. Instead, we decided a finger tip sensor would be much more comfortable and easy to use.
To make a finger tip sensor, we used the concepts of photoplethysmography. Photoplethysmography is typically used in pulse oximeters and finger-tip sensors in commercially-available devices. We originally tried to hack a heart rate monitor that we had purchased, but this proved difficult, so we decided to make our own.
The basic concept of photoplethysmography is that blood reflects a certain amount of IR light, and the blood density in the finger changes as the heart pumps, so the IR reflectivity of the finger changes as the heart beats. Using an IR emitter-detector pair found in the mechatronics lab followed by lots of amplification and filtering, we were able to obtain a decent signal with peaks when the user's heart beat.
The sensor itself was a INSERT COMPONENT NAME HERE that was encased in open-cell foam insulation. A round, finger-shaped groove was filed into the foam and the sensor sat flush with the bottom of this groove. The user's finger was held in the correct position with relatively constant pressure using a latex band around the finger and the foam enclosure.
INSERT PICTURE OF THE HEART SENSOR
Signal processing
The signal coming directly out of the IR detector was very weak and noisy. To obtain just the information we desired, we ran it through the following filters and amplifiers:
- Passive high pass filter - cutoff frequency = INSERT
- Inverting amplifier - gain = INSERT
- Active Low pass filter - cutoff frequency = INSERT
- Bandpass filter - Frequency range = INSERT
- Comparator - output to the PIC
The complete signal processing circuit is shown below.
INSERT CIRCUIT DIAGRAM
Drum beat
Once the signal from the heart rate sensor is sent to the PIC, the rising edge triggers an interrupt which starts the driving sequence for the motor that strikes the drum. The drive sequence was: drive down (into drum) 100ms, drive up 75ms, wait 150ms.
An additional interrupt was triggered by the falling edge of the signal; this falling edge interrupt added additional wait time that prevented the PIC from being "fooled" by a noisy falling edge. For instance, if the falling signal had some noise in it, the rising edge interrupt would be triggered again, thus causing the drum to strike on both the rising and falling edges of the heartbeat. We only wanted one strike per heartbeat, so the falling edge interrupt was trigged by the first falling edge, and any noise in the falling edge was ignored because the PIC was told to wait for 300ms. With a maximum delay of 150 + 300 = 450ms from the two interrupts, the fastest heartbeat our drum could play was 133 beats per minute, but we did not expect anyone to come play the instrument after running, so this was plenty high for our purposes.
The motor for driving the drumstick was driven by an L293D H-bridge. See Driving a high current DC Motor using an H-bridge for more information on how this works. Our particular circuit diagram is shown below:
INSERT CIRCUIT DIAGRAM OF H-BRIDGE AND PIC INTERFACE
Mechanical Design of Drum Actuator
We wanted to use a real drum for our drum beat, so our challenge was to attach a motor and drumstick to a drum. We chose a small rack tom for its resonant tone that is reminiscent of a heart beat.
To attach the drumstick to the motor shaft, we used a small block of acrylic (.5"x.5"x2"). The drumstick was cut down to 8" in length so that it would require less torque from the motor. In one end of the block, a hole was drilled and the drumstick was sanded to the correct diameter in order to snuggly fit into the hole. To ensure that the drumstick could not slide out of the block, a screw was placed in a hole drilled through both the stick and the block. The other end of the block was attached to the motor shaft using a split-clamp method: A hole the size of the motor shaft was drilled through the block, and then a slit was cut from the end of the block to the hole, allowing a screw to tightly clamp the block around the motor.
INSERT PICTURE OF DRUMSTICK-MOTORSHAFT ATTACHMENT
To attach the motor-stick assembly to the drum, we utilized the tuning screws on the drum. A sheet metal bracket with holes in the appropriate places attached the motor to two of these tuning screws. The bracket was easily made by cutting and bending a piece of GAUGE gauge steel sheet metal to the appropriate size. Ideally, this part would have been made out of one continuous piece of sheet metal, but we could not find a large enough sheet available in the shop, so we used rivets to connect two smaller pieces together.
After testing the drum striking, we decided it was too loud. To dampen the sound, we taped a piece of foam core and a shop rag to the head of the drum where the stick strikes it.
Parts list for drum actuator
- Bargain drumstick, at least .5" in diameter
- Small block of acrylic, .5"x.5"x2" (aluminum would work just as well, this is what was available)
- GAUGE steel sheet metal for bracket
- Screws and nuts
- Foam core, shop rag, tape for dampening
