Figure 1: The electrode antenna is mounted behind the bridge of a Jensen electric violin. ANT 100 kHz PICF16F877 Bandpass Peak filter Detector Antennamp Bandpass _P peak - filter Detector 50 kHz Figure 2: The amplified signal from the antenna board on the violin combines two signals sent from the bow. To determine the position of the bow, the combined signal first must be separated into two signals once again. This function is performed by directing the combined signal into two bandpass filters, one at 50KHz at 100KHz. The filter outputs are converted to DC by two peak detectors and sent to the PIC 16F877. to a circuit that ampli~es the combined signal and is sent via a cable to a remotely mounted board, whose task is to separate the two signals from each other and measure their varying strengths. Bow position is then determined from this data in software on a computer receiving the output of the board. For the Hyperbow, the implementation of the position sensor is completed with as little alteration to the rest of the existing sensing system as possible. So as to add a minimal number of hardware components to the small board mounted on the bow, the two square wave signals are generated by the PIC16LF877 manufactured by Microchip Technology, Inc. The board that receives the signal from the antenna consists of two bandpass filters that have been designed to separate the two different signals of different frequencies from each other. Peak detectors convert the signals to analog DC voltages equal to the signal amplitudes and a PIC 16F877 microprocessor with a built-in 10-bit A/D converter receives these voltages and sends them to a PC workstation as serial data (see Fig. 2). 2.2 Acceleration Sensing In order to sense the acceleration changes of the bow, ADXL202 accelerometers from Analog Devices, Inc. are employed. This MEMS (Micro-Electromechanical Systems) accelerometer has a measurement range of ~2g and is capable of measuring both static and dynamic acclerations. Because the ADXL202 is a 2-axis accelerometer, two of these devices, one mounted orthogonal to the plane of the electronics board containing the other, are used in the design in order to attain acceleration data along all three axes. This accelerometer has a digital output for each of its two axes of sensitivity that has a maximum resolution of around 14 bits. The acceleration measured by the accelerometers is encoded in the digital output signal by modulating the duty cycle linearly with the acceleration. The acceleration is thus retrieved by simply counting the duty cycle in a software loop. 2.3 Strain Sensing Because of its direct effect on the violin string and its close relationship to the experience of a violinist while playing, we developed a technique of measuring the downward and lateral forces on the bow stick. This measurement is similar to that used in the original Hypercello project, which employed a force sensitive resistor to indicate the pressure of the right hand index finger on the bow stick. However, in this method we measure the relative changes of the different strains on the carbon fiber material of the bow stick itself. This basic technique was also employed by Askenfelt (Askenfelt 1986; Askenfelt 1989). Strain Gauge Operation The sensors used in this project are commercial foil strain gauges from Vishay Measurements Group, Inc. The gauges are two-terminal devices that behave as variable resistors. Therefore, the proper operation of the gauges demands that they be securely and permanently affixed to the material that is under strain such that the stretching of the gauge is identical to the stretching of the material. The strain gauges used in this project possess a uniaxial pattern designed to measure strain along one axis (in the direction of the grid lines), as in a bending beam. Wheatstone Bridge Configuration The strain gauges were arranged in a Wheatstone Bridge configuration with the midpoints of each "leg" of the bridge connected to a differential operational amplifier. So as to allow for the best measurement possible, a full bridge con~guration, i.e., two strain gauges in each of the two legs of the bridge, is implemented. The sensor measures the strain at the point located approximately halfway between the two sets of gauges (see Fig. 4). At rest, the resistances of all of the gauges are approximately 490 0
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