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02 October 2000

Motion Fundamentals: Encoder Innovations

by Jerry Gordon

As a user, you're probably familiar with both incremental and absolute encoders. However, you may be less aware of new technological developments in pseudorandom encoding. A pseudorandom encoder is neither incremental nor absolute. It's a new kind of device similar in construction to an incremental encoder, but behaves more like a conventional absolute encoder. Though like the two traditional encoder types in some ways, pseudorandom encoders have aspects of design and behavior which are unique. Thus, while incremental and conventional absolute encoders are hardly obsolete, they're also no longer your only choice.

How It Works

In its simplest form, the new encoder uses cyclic and index tracks, a single LED light source, and the same number of detectors as an incremental encoder. However, the index track looks something like a bar code, instead of just a single mark (Figure 1). Its pseudorandom code structure provides a unique nonvolatile optical code for each and every position of the disc or scale. Thus, absolute position is encoded serially along one track, rather than being dispersed over multiple parallel tracks.

A series of code bits must be accumulated to determine position. The incremental track signals provide direction sense and spatial timing only. You don't know position immediately upon start-up, as you do when using a conventional absolute device, but after a very short travel in either direction and starting from anywhere, you know exactly where you are. In a rotary encoder, the initialization angle is typically 1–2°, depending on the encoder's line count (arbitrary resolutions like 3,600 are easily accommodated); in a linear encoder, less than 1-mm motion is needed.

Pseudorandom output codes picked off the disc or scale need translation into a natural binary format. The necessary decoder can be implemented in an inexpensive programmable logic device that stands in place of the quadrature decoder and up/down counter used with an incremental unit. Since it doesn't count position increments from the cyclic track(s), it can't unwittingly gain or lose "counts" under duress like an incremental encoder system can. Thus, although this type of unit costs little more than an incremental system of comparable resolution, it's effectively absolute.

In addition to the binary position output, the decoder provides a status bit. This bit is logic-high whenever the supply voltage is interrupted, when the initializing motion is not yet complete, or when some other effect such as electrical noise, damage, or fouling of the disc interferes with the proper code sequence from the indexing track. When these self-tests are all satisfied and the encoder is initialized, the status bit is low, indicating the position output data is valid. Full-time position verification with real-time reporting of any problem is the most important feature of this type of encoder, in the opinion of some machine designers.

The decoder circuit is preferably located in the host system, just as an incremental up/down counter would be, to preserve frequency response. However, in low-speed applications, the decoder may be located inside the encoder housing and the absolute position data transmitted serially along with the status bit.

The principal advantages of pseudorandom encoding technology may be summarized thus:

• The initialization distance or angle is a fixed and very small motion, regardless of the starting position or direction of travel. Just "bump" it to find out where you are.
• The encoder generates the same information as a conventional absolute, so interfacing to digital signal processors, computers, PLCs, servo controls, etc., is straightforward.
• Self-test functions not found in any conventional device report various malfunctions, and may also help detect system problems such as excessive heat, noise, or speed.
• Simpler optics allow a rotary pseudorandom encoder to be smaller than a conventional absolute of equal resolution, or to have a larger through-hole.
• Compared to conventional absolute encoders, simpler readout electronics, reduced parts count, and forgiving internal alignments translate into higher intrinsic reliability.

Applications

Pseudorandom encoders don't belong in mice and trackballs, nor are they appropriate if you really must know position immediately on power-up, without even the slightest motion. However, Table 1 lists a few types of applications where this technology may prove ideal.

Most incremental encoders can be retrofitted to use pseudorandom technology. For high resolution/high speed applications, the decoder is a separate, compact printed circuit which also enhances the resolution of the encoder by absolute interpolation, and monitors analog signal quality to provide early warning of a range of optical and mechanical problems. MC

Additional Information

Figures and Graphics

• Encoder Innovations (PDF File)
• Figure 1. A comparison of (a) incremental, (b) absolute, and (c) pseudorandom encoder discs.
• Table 1. Applications suitable for pseudorandom encoder technology.
  

Author Information
Jerry Gordon is vice president of encoder sales at Gurley Precision Instruments. Contact him at 514 Fulton Street, Troy, NY 12181-0088; tel: 518-266-7742; fax: 518-274-0336; www.virtualabsolute.com.