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Low-Cost Symmetrical Photoamplifier

IP.com Disclosure Number: IPCOM000046853D
Original Publication Date: 1983-Aug-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 2 page(s) / 38K

Publishing Venue

IBM

Related People

Goldrian, G: AUTHOR [+2]

Abstract

The resolution of an optical encoder, such as a rotating transparent disk carrying concentric tracks of opaque marks, can be increased if the photoamplifier generates a symmetrical output signal. For reliably sensing the encoder signal, an automatic energy control is required, by means of which tolerances and a deterioration of the transducer performance are compensated for. The unsymmetrical performance of photoamplifying transistors is used to advantage by the amplifier circuit described below. The amplifier circuit utilizes the unsymmetrical characteristic of the phototransistor to control the duty cycle and the magnitude of the output signal by only one feedback signal. Fig. 1 shows amplified encoder signals of a typical shape. The encoder disk is assumed to have a symmetrical track.

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Low-Cost Symmetrical Photoamplifier

The resolution of an optical encoder, such as a rotating transparent disk carrying concentric tracks of opaque marks, can be increased if the photoamplifier generates a symmetrical output signal. For reliably sensing the encoder signal, an automatic energy control is required, by means of which tolerances and a deterioration of the transducer performance are compensated for. The unsymmetrical performance of photoamplifying transistors is used to advantage by the amplifier circuit described below. The amplifier circuit utilizes the unsymmetrical characteristic of the phototransistor to control the duty cycle and the magnitude of the output signal by only one feedback signal. Fig. 1 shows amplified encoder signals of a typical shape. The encoder disk is assumed to have a symmetrical track. This means that the width and spacing of the windows (marks) is equal. The dissymmetry can be increased by using smaller windows in the encoder disk. The signal is generated by amplifier 1 of the circuit shown in Fig. 2. This amplifier is a standard operation amplifier acting as a current amplifier. The feedback resistor R1 defines the amplification. Up to a certain signal frequency, the output signal u is the current I-PTX of phototransistor 5 multiplied by the value of R1. As current I-PTX is also a function of the light intensity, the current through the light-emitting diode (LED), feeding transistor 5, defines the output voltage u. The signal u2 can be made equal to u1 if the current through the light-emitting diode (not shown) is increased, while the same encoder characteristic is assumed for both signals. Signal u is fed to a Schmitt trigger 2, the...