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Self Level Adjustment in Optical Detector System

IP.com Disclosure Number: IPCOM000053135D
Original Publication Date: 1981-Sep-01
Included in the Prior Art Database: 2005-Feb-12
Document File: 3 page(s) / 56K

Publishing Venue

IBM

Related People

Pettit, JW: AUTHOR

Abstract

In an optical system in which power to the light source is controlled to obtain an optimal level on a phototransistor (PTX), the control level is incremented up one step when each PTX signal passes the level of one-half the desired signal. When the PTX signal exceeds the desired signal, a rapid series of pulses is produced which are translated by a nonlinear digital-to-analog converter (DAC) to quickly bring down the level of the light source.

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Self Level Adjustment in Optical Detector System

In an optical system in which power to the light source is controlled to obtain an optimal level on a phototransistor (PTX), the control level is incremented up one step when each PTX signal passes the level of one-half the desired signal. When the PTX signal exceeds the desired signal, a rapid series of pulses is produced which are translated by a nonlinear digital-to-analog converter (DAC) to quickly bring down the level of the light source.

As shown in Fig. 1, a light-emitting diode (LED) 1 produces light of intensity controlled by the signal to the base of transistor 3. That light is directed by a suitable optical system to a symmetrical, rotary light chopper (not shown) and then to a PTX (not shown), the signal from which is applied to input terminal 5. The light chopper may be directly linked to a machine member, the movement of which is being optically tracked.

When the chopper is in movement, the signal from the PTX is generally sinusoidal, as shown in Fig. 2. Potential V1 is the value of the maximum signal desired. A potential of V1 is applied to the two resistors 7 and 9 in Fig. 1, which are of equal value and connected in series. Thus, the full reference value V1 is one input to comparator 11, while the reference value one-half V1 is the input to comparator 13. The second input to the comparators 11 and 13 is the input 5.

Each time the input signal reaches the value one-half V1, the output of comparator 13 changes state. This appears on line 15 as an output signal defining movement of the chopper. That signal also triggers pulse source 17 to produce one up-count pulse to 5-bit counter 19.

The status of counter 19 is translated by a nonlinear DAC made up of combinational logic which is conceptually comprised of chord selector 21, step generator 23 and summing circuit 25. The output of summing circuit 25 drives operational amplifier 27, the output of which changes the signal to the base of transistor 3.

Each time the input signal reaches the value V1, the output of comparator 11 changes state. While the input signal is greater than V1, pulse source 17 produces a continuous series of count-down pulses at near the maximum speed at which the affected circuit elements can respond. The count of counter 19 may decrease several increments, and the light magnitude from LED 1 is immediately reduced accordingly.

Fig. 3 illustrates the characteristics of the nonlinear DAC. The curve is roughly exponential so that when LED current is large, each single count change of counter 19 produces a large change in current. The characteristic curve is implemented by four linear segment denominated chords, which are separated by eight counts, denominated steps. Thus, chord selector 21 need only receive signals from two high-order output lines of counter 1q to defi...