Method and apparatus for optically sensing the position and velocity of piano keys

Abstract

An electronic keyboard musical instrument is provided having optical emitters and detectors both positioned beneath each of the instrument's keys, whereby optical energy is emitted upon the bottom reflective surface of the key and the reflected light is detected. The distance between the optoelectronic sensor and the bottom of the key is detected by the light detector which produces an analog output signal and, by measuring the time between a first and second threshold crossing, the key's velocity can be determined. The two thresholds are preferably chosen to measure a rising voltage as the key is depressed, and the threshold values are preferably chosen at 50% of key travel and 75% of key travel. By use of these thresholds, a relatively clean and useful waveform is available at both threshold crossings, particularly since the mechanical movement of the key has nearly reached its terminal velocity. Electronics in the keyboard instrument sequentially scan each of the output voltages produced by their corresponding light detectors so that a single microprocessor can measure all of the key position values. A single set of threshold voltages can be utilized for all of the keys of the keyboard instrument, if desired, or each of the keys can have its own individual threshold values depending upon the calibration values for its zero and span conditions. The time interval between threshold crossings can be determined, which in turn is used to determine the velocity of each key as it is being depressed. Once the velocity is known, the volume for that key's particular musical tone can be controlled by a sound engine, as well as its other characteristics, such as timbre, attack, and envelope.