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# Ultrasonic Position Indicator

IP.com Disclosure Number: IPCOM000043362D
Original Publication Date: 1984-Aug-01
Included in the Prior Art Database: 2005-Feb-04
Document File: 3 page(s) / 40K

IBM

## Related People

Koperda, FR: AUTHOR [+2]

## Abstract

Described herein is a means of locating the position of a printhead using ultrasonic sound. The time that a sound wave propagates through air is measured and that time is proportional to the distance from the transmitter to the receiver. There are several problems associated with a simple measurement of the transit time of the wave. First, the velocity of sound is 1100 feet/second at 25ŒC and one atmosphere of pressure. As the temperature increases, the velocity of sound changes by one foot/second per degree Fahrenheit. Second, the number of times per second the position can be determined is inversely proportional to the distance to be measured because, usually, only one pulse is sent and the next pulse is not transmitted until the first pulse is received. The distance to be measured is approximately 15 inches.

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Ultrasonic Position Indicator

Described herein is a means of locating the position of a printhead using ultrasonic sound. The time that a sound wave propagates through air is measured and that time is proportional to the distance from the transmitter to the receiver. There are several problems associated with a simple measurement of the transit time of the wave. First, the velocity of sound is 1100 feet/second at 25OEC and one atmosphere of pressure. As the temperature increases, the velocity of sound changes by one foot/second per degree Fahrenheit. Second, the number of times per second the position can be determined is inversely proportional to the distance to be measured because, usually, only one pulse is sent and the next pulse is not transmitted until the first pulse is received. The distance to be measured is approximately 15 inches. Within a printer enclosure there is considerable noise in the audio spectrum. Ultrasonic frequencies (150 KHz) were chosen to avoid the noise problem, and piezoelectric crystals are used because of their low cost. The position of the printhead must be determined every 200 microseconds. The transit time of the ultrasonic wave across 15 inches is approximately 1.15 milliseconds. To achieve the 200-microsecond sampling time, pulses are sent every 100 microseconds. This means that there are multiple pulses in transit at all times. To be able to identify which pulse is being received, one pulse is dropped periodically and the missing pulse may be used to identify the proper sequence of arriving pulses. One means of implementing this method is shown in the figure. The transmit portion is shown in the upper half of the figure and includes a counter which produces five pulses of the 150 KHz clock and delays a total of ten pulses for a cycle of 33 microseconds on and 66 microseconds off. Every 20 cycles, a pulse train is dropped. The receive portion amplifies the ultrasonic pulse and filters out only those frequencies in the 150 KHz range. The missing pulse detector senses when the missing pulse should have been received and provides a sync pulse. If the transmitter provides a start pulse to a counter and the receiver provides a stop pulse to a counter, then the transit time of the pulse can be measured. Since there are 20 pulse trains in-process, it might take 20 sets of counters to measure each pulse train's transit time. However, there is a way to measure the time using only two sets of 14-bit counters and a binar...