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DUTY CYCLE FEEDBACK LOOP CONTROL

IP.com Disclosure Number: IPCOM000007248D
Original Publication Date: 1994-Jun-01
Included in the Prior Art Database: 2002-Mar-07
Document File: 3 page(s) / 126K

Publishing Venue

Motorola

Related People

Marc Osajda: AUTHOR

Abstract

The purpose of this publication is to describe a signal conditioning which improves the duty cycle and period stability from a signal coming from a slotted opto switch used as a speed or position sen- sor under harsh environments (High temperature, oil, dust,. .) old voltage. The principle ofthe duty cycle feedback loop is to monitor the output signal duty cycle and then to change the threshold voltage automatically to obtain a 50% duty cycle regardless of the input signal wave form coming from the opto switch (or other technologies sensing elements).

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MOTOROLA Technical Developments Volume 22 June 1994

DUTY CYCLE FEEDBACK LOOP CONTROL

by Marc Osajda

l-PURPOSE

The purpose of this publication is to describe a signal conditioning which improves the duty cycle

and period stability from a signal coming from a slotted opto switch used as a speed or position sen- sor under harsh environments (High temperature, oil, dust,. .)

old voltage. The principle ofthe duty cycle feedback loop is to monitor the output signal duty cycle and then to change the threshold voltage automatically to obtain a 50% duty cycle regardless of the input signal wave form coming from the opto switch (or other technologies sensing elements).

3.2-Measurement of the output duty cycle

  The mean voltage ofa logical signal is ratiometric to its duty cycle. For a square wave signal between OV and Vh with a duty cycle th/t the mean voltage is defined as: V mean = Vh x th/T. In this condition a 50% duty cycle is characterized by a mean voltage

V5O=Vh/2.

From a technical point ofview the mean voltage is measured with an integrator (Low pass filter R-C).

2-BACKGROUND

  In most cases it is very easy to use opto switches in industrial environments to sense speed or posi- tion with accurate signals. Those applications use a current source for the emitter and a couple of com- ponents for the detectors. This solution gives good results ifworking under "normal" conditions for elec- tronic and optoelectronic devices (i.e. working in the commercial temperature range -0 to 7o"C-, with regulated power supply, no dust, etc.,. .).

  The use of optoelectronic devices for speed or position sensors under harsh environments, like auto- motive, needs a stronger signal conditioning to respect the specification request in terms of accu- racy (duty cycle and period stability). The main deg- radation's that affect the signal under such environ- ments are due to the LED output power variation (temperature dependent), to the coupling transfer ratio (CTR) between the optoelectronics devices and to the mechanical tolerances necessary for low cost mass production processes.
3-DESCRIPTION

Ofv,,,,=

3.3-Comparison to a reference and feedback loop

  The mean voltage coming from integrator is now compared to a reference voltage (V50) which repre- sent a perfect 50% duty cycle.

  In the traditional servo loop system an inverting dif...