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27 April 2000
Defining the Optical Encoder
by Mike Grinter

Incremental Encoders
The incremental encoder (Figure 1) is a relative position feedback device in that its signal is always referenced to a home, or zero, position. The encoder produces a digital, square-wave pulse train that's fed into an up/down counter chip or clock to derive position or speed. A single output incremental is called a tachometer encoder. A dual-channel incremental, with one channel leading the other by 90°, is called a quadrature encoder.

The drawback with this type of sensor is that a loss or interruption of power causes it to lose its position and speed reference. The system has to be reinitialized or returned to a known zero position in order to sense motion again. Subsequently, incremental encoders are best suited for short-cycle and -rate (high-speed) applications.

Absolute Encoders
In contrast, the absolute encoder (Figure 2) provides nonvolatile position data as a function of the pattern on its code disk. The encoder provides a unique digital output for every resolvable shaft angle without regard to a zero position. Accurate position data is always directly accessible, even upon wakeup from a power loss situation—hence the term absolute.

Absolute encoders usually offer resolutions in powers of two. Nonbinary or binary-coded decimal resolutions are also available. Absolute encoders are used in lieu of incremental types when repositioning is impractical, unsafe, or undesired. They are also favored when precise motion control with high resolution and accuracy is required. Typical areas of use include cranes, packaging, printing equipment, elevators, computer numerical control turning centers, automatic storage/retrieval systems, and antennas.

Code Disks

The code disks of the incremental and absolute encoders are quite different, as a comparison of Figures 1 and 2 shows. The code disk for each type of encoder is the unit's measuring standard, in that it converts the mechanical angle or motion into a digital representation. As you can see, the absolute code disk consists of multiple, alternately clear and opaque, concentric tracks of precise patterns. Each track represents 1 bit of resolution. If the code track at a given position is transparent, the bit is on—in a digital sense, the square wave is high; as a binary number, it is a "1." If the track at a given position is opaque, the opposite occurs; the square wave is low, and the binary number is "0." The total number of tracks or bits determines the length of the binary word and the absolute encoder's resolution.

For example, five tracks result in a 5-bit-long word and 5 bits of resolution capable of 32 different words. Most absolute encoders generate a digital word by optically reading a radial line of light across the code disk. An individual detector is positioned under each track, and, from that, the digital representation is derived.

Gray Code
The most important element of any encoder is the accuracy of the code pattern on the disk, as this strongly influences the unit's performance. Alignment of the optics and the detectors is also very important. Table 1 shows the 5-bit words for natural and Gray code formats for words 0 through 9 and 31. While most interfaces prefer a natural binary output, most absolute code disks are printed in Gray code to minimize the ambiguities of straight binary code. The conversion from Gray code to natural binary is easily accomplished with a converter chip. In addition, newer interfaces available today will accept straight Gray code outputs.

Variations of the Absolute Encoder
The absolute encoder is available in two configurations: single-turn and multiturn. The single-turn unit provides output for a single turn of the encoder disk and is employed in applications that monitor a single turn of a machine mechanism or travel over a short linear distance. The multiturn absolute encoder is essentially two distinct encoders in one housing; the first, or fine, encoder provides the resolution per turn of the input shaft, while the second, or coarse, encoder counts the number of turns of the fine encoder. The outputs of both devices are combined to produce a single binary word. Multiturn absolute encoders are used in applications requiring multiple turns of machinery or having long, linear travel paths.

Traditionally, the most common way to connect the two encoders of a multiturn unit is with reducing gears; this type of device is called a geared multiturn encoder. More recently, the second encoder has been eliminated in designs that employ a battery and a simple electronic circuit for the turns-counting function. You'll see the name electronic multiturn encoder for this style. Both methods of achieving the turns-count function have their own distinct advantages and disadvantages. Consult your encoder supplier on which type is best suited to your application.

Application Considerations
There are many types of absolute encoders available in today's marketplace. Shaft-type, hollow-shaft, and kit-type devices are all available for easy system integration. Single-turn devices are available to 24 bits of resolution; however, most applications require 16 bits or less. Multiturns currently offer similar resolutions and packaging features. Once you've determined the resolution and type of absolute encoder most applicable to your system, there are other important factors to examine as well.

Environmental, operational, and interface requirements must be considered for a successful integration of the sensor to the system. An absolute encoder has many output interfaces available, and selection is very important, especially with the high number of buses available today. The two most popular outputs are parallel and serial. A number of new buses are also available that allow interfacing with a PC. These include CAN/DeviceNet, Interbus, Profibus, and SSI.

The absolute encoder market continues to grow. Advances in electronics and manufacturing techniques make absolute encoders more accurate and easier to use than ever before. These advancements have also substantially narrowed the price gap between incremental and absolute encoders. Historically, absolute encoders have been two to three times more expensive than incremental encoders, but now they're being used in many cost-sensitive applications.

Additional Information

Figures and Graphics

• Figure 1: Incremental encoder.
  
• Figure 2: Absolute encoder.
  
• Table 1: Binary codes.
  

Author Information

Mike Grinter is regional sales manager for BEI Sensors & Systems Company's Encoder Systems Division. Contact him at 13100 Telfair Avenue, Sylmar, CA 91342; tel: (818) 362-7151; fax: (818) 362-0458.