Economical, high-performance optical encoders - technical

Economical, High-Performance Optical Encoders

A HIGH-RESOLUTION, HIGH-SPEED printer like the HP DeskJet printer needs an electronic ruler to know where it is one the page. For this function, the DeskJet printer uses a separately available HP product, the HEDS-9000 Shaft Encoder Module (Fig. 1). The version of this two-channel position sensor in the DeskJet printer provides 2000 marks (500 cycles) per revolution and gives direction information. The module facilitates a large reduction in the cost of servo motion control and is one reason the DeskJet printer is able to provide high-quality, speedy printing at a low price. Special geometries in the emitter/detector system and a "designed for manufacturability" architecture are key to the high performance and low cost of this transmissive optical encoder.

This article describes key elements of the module's design, manufacturing strategies, and performance. The box on page 100 presents the "Basics of Optical Incremental Encoders." The box on page 105 discusses how the module was made into a dust-resistant enclosed encoder with a self-contained code wheel, the HEDS-5500. A convenient gap-setting system allows quick assembly of the HEDS-5500 on a customer's motor with no special tools.

Background

An earlier HP shaft encoder, the HEDS-5000, was reported on in October 1981. he emitter/detector modules discussed in that article formed the basis for a family of encoding products that included a 1000-cycle-per-revolution encoder with index pulse and a panel-mounted digital potentiometer.

Building on the base of the earlier technology was a significant advantage in the HEDS-9000 design. Several veterans of the previous development led the product definition stage for the new module. The team also learned from a disciplined process of talking to users and potential users of encoders. Most engineers at the end of a design project have said to themselves, "I know what I would like to try next time." This team had their chance.

Market Research

Customers were generally satisfied with the performance and reliability of the HEDS-5000 encoder kits, but several producers of office automation products said they needed a revolutionary change in price and assembly methods. The HP ThinkJet printer, for example, uses an open-loop step motor for driving the print carriage. A closed-loop system would have offered increased speed and graphics quality, but the added cost of an encoder (about the cost of the drive motor) was too great for the target market.

The purchase price of an encoder is not the only concern. Encoder characteristics can determine motor selection criteria. An encoder that can operate on a hot motor with large shaft play allows the use of significantly lower-cost motors. Economics often favor small motors that can reach temperatures up to 100[deg.]C. As one customer put it, "If I'm not running a motor hot, I'm using too big a motor."

HEDS-5000 encoders require adhesives in the assembly process and an alignment that takes about 3 minutes. Gustomers told the design team that such assembly processes are not compatible with high-volume automated manufacturing. They need an encoder that can be located and fastened by ordinary mechanical methodw with no follow-up adjustments.

Logistics are also important in high-volume manufacturing. Some customers want high-volume capacity with quickly adjustable running rates to avoid costly inventories. Another issues is that count density and code wheel size vary with the application, so a variety of standard parts

COPYRIGHT 1988 Hewlett Packard Company
COPYRIGHT 2004 Gale Group