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Saw Tooth Feature on Code Wheel Hub Lateral Surface

IP.com Disclosure Number: IPCOM000241078D
Publication Date: 2015-Mar-25
Document File: 2 page(s) / 429K

Publishing Venue

The IP.com Prior Art Database

Abstract

This paper is to explain the concept of Twist Cap and its function to align radial position and gap between code wheel and detector IC in easier way. It shows how effective the slider reduces customer assembly steps and cycle times.

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1. Abstract

This paper is to explain the concept of Twist Cap and its function to align radial position and gap between code wheel and detector IC in easier way. It shows how effective the slider reduces customer assembly steps and cycle times.

2. Introduction

Encoder is an electro-mechanical device used to monitor and measure the position, rotation velocity and acceleration of the shaft in the application. Figures (a) to (c) below show various encoder working principles and their respective basic components. Basically, the device consists of an emitter as light source, lens, patterned code wheel/codestrip and an array of photodetectors and signal processor on detector IC. When the codewheel/codestrip moves between the emitter and detector, the light beam is interrupted by the pattern of bars and spaces on the codewheel/codestrip. Similarly, in reflective or imaging, the lens over the LED focuses light onto the codewheel/codestrip. Light is either reflected back to the lens over the photo-detector or absorbed by the codewheel/codestrip. As the codewheel/codestrip moves, an alternating pattern of light and dark corresponding to the pattern of the bars and spaces falls upon the photodiodes. The photodiodes detect these interruptions and the outputs are processed by the signal processor to produce digital signal. In order to obtain good signal for optimized encoder dynamic performance, the radial position of detector IC relative to code wheel pattern and the gap between the detector IC and code wheel must be aligned. Radial alignment is needed to ensure the code wheel pattern match with the PD pattern on detector IC. The gap between the detector and code wheel determines the optical contrast of the image formed by code wheel pattern. The nearer the gap, the better optical contrast is and the better dynamic performance of the encoder. However if the gap is too small, there is a risk of code wheel scratch during operation, especially when the code wheel wobble during rotation. As a result, the gap must be set accurately.

3. Current design #1

Figures 1a) through c) show the typical codewheel hub designs found in optical encoder modules.  The codewheel hubs are typically made of aluminum and are highly reflective.

Figure 1: a) Hub design #1, b) hub design #2 and c)hub design #3

3.1 Disadvantage of current design

When the light emitting diode (LED) is placed close to the hub, as shown in Figure 1, the light emitting from the LED that reflects off the hub and onto the photodiode (PD) area will have enough power to interfere with the modulated optical signals. Figures 2a) through c) show the light rays that reach the codewheel and cast an image onto the PD area after reflecting off the hub. Assuming that the hub surface is properly finished, Figures 3a) through c) show the impact of the light reflected off the hub on the...