Electronic Design

Improved Encoders Create New Possibilities In The Motion Control Industry

In the wake of the recent economic crisis, the landscape has changed quite drastically for design engineers across multiple disciplines. This turbulent environment has produced new challenges that the engineer must now face on a daily basis. It has also magnified existing trends that had been present to a lesser extent before the economic crisis hit.

Most significantly, companies have slashed their research and product development budgets. Now design engineers have to put in longer hours with fewer resources at their disposal, all while wearing hats that are unfamiliar to them. In the meantime, corporate leadership continues to push innovation and time-to-market to stay ahead of the competition.

Many companies see the recent weakness in the economy as an opportunity to increase their market share. Design trends that were previously important such as energy efficiency and product reliability are now set up as key differentiators as companies try to dig themselves out from under the weight of slumping sales and stagnant product lines.

Like their peers in other component-market sectors, engineers in the motion-control industry are facing this new reality and are increasingly looking for help from their component suppliers to ease the burden.

START WITH KEY COMPONENTS

One of the key components in a motion control system is the encoder. The prevailing encoder design over the last few decades employs optical technology where an etched disk resides between an LED light source and a phototransistor light detector, providing speed, positional, and directional feedback to the motion control system.

Unfortunately, optical technology brings some challenges that can affect design and purchasing costs, time-to-market, energy consumption, and reliability. One method for addressing these issues comes by way of an encoder based on proprietary capacitive technology that uses a custom ASIC to sense capacitance changes in an RF circuit and to convert that information into position increments.

Digital calipers have successfully used this same technology for many years. As an example, the AMT series modular encoder offers benefits that can meet many of the new and existing challenges facing motion control engineers today (Fig. 1).

INVENTORY COST

Offering as much flexibility as possible, the AMT encoder provides 16 resolution choices: 48, 96, 100, 125,196, 200, 250, 256, 384, 400, 500, 512, 800, 1000, 1024, and 2,048 ppr (Fig. 2). Resolution is can be set via an on-board dual-inline package (DIP) switch located on the back side of the component’s metal cover. The encoder also comes with sleeve sizes of 2, 3, 3.175, 4, 4.76, 5, 6, 6.35, and 8 mm, allowing it to fit any round back shaft and numerous mounting options in each base.

An engineer could therefore implement the same encoder Stock Keeping Unit (SKU) number across multiple applications or utilize the same encoder as system requirements change during the design cycle, reducing inventory costs and increasing purchasing power. On the other hand, optical encoders are available with a single fixed resolution, a single bore diameter, and a smaller selection of mounting options. This greatly reduces their viability for use across multiple applications.

TIME-TO-MARKET

An important aspect of motion control is system optimization and loop time modification. When a system’s response requires modification due to irregular or undesired behavior, the correct change can be difficult to find. The engineer needs to either adjust the system PID times with timely code review and modification or adjust the line count (resolution).

The AMT series offers the ability to dynamically modify the resolution, thereby simplifying the system response modification process. The PID control engineer adjusts the line counts of the encoder, evaluating the overall response time until achieving the desired result. However, when using an optical encoder the process requires the purchase of several different encoder versions and testing each through trial and error, increasing costs and design time.

CONSUMPTION AND RELIABILITY

As designers look for ways to meet the market’s growing demands for green products, every milliamp saved becomes vitally important. A typical optical modular encoder meets a current consumption range of 20 mA to 50 mA while a component like the AMT provides a range of 6 mA to 10 mA, offering a much more efficient solution.

Like the majority of components, dust, dirt, and other contaminants influence an optical encoder’s performance. This causes repeatability issues because the LED cannot pass light through the disk to the optical sensor. Once an optical disk becomes contaminated, the encoder must be replaced. The AMT and similar ASIC driven encoders are impervious to dust and dirt build-up, resulting in a more rugged, reliable performance.

TEMPERATURE, VIBRATION, AND LED BURNOUT

Just as contaminants could influence the incremental output of an optical disk, temperature variations also impact the performance of an optical encoder. These variations change the mechanical structure of the glass disk, causing potential repeatability issues.

ASIC technology ensures the encoder is less sensitive to heat and cold, offering reliable operation between a wider temperature range. Additionally, ASIC-based construction is far less susceptible to vibration than the glass disk of an optical encoder.

All optical encoders use an LED for the light source that passes through the etched disk to the optical sensor and LEDs do burn out, leading to eventual failure of the encoder. The AMT, on the other hand, avoids this issue thanks to the use of a semiconductor ASIC instead of an LED.

MOTION FUTURES

Moving into 2010 and beyond, driving forces that will shape the motion control industry mirror the forces driving the electronics industry as a whole: cost reduction, time to market, energy efficiency, and reliability. In a product group that has been static for many years, the introduction of capacitive and ASIC technologies into the encoder market give the motion control design engineer powerful tools to succeed in this new landscape.

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