Compact, High-Performance Modular Absolute Encoders from Gurley Precision Instruments

Gurley Precision Instruments of Troy, New York (USA) has extended its line of novel Virtual Absolute® encoders to meet the ever growing demands of design engineers worldwide.  “Smaller, higher resolution, and less expensive summarizes the current trend in design engineering”, claims Gurley VP of sales and marketing, Martin Gordinier.  In response to these challenges, Gurley has developed its 7700 Virtual Absolute modular rotary and linear encoders with resolution from 12-bits (4,096 words) to 19-bits (524,288 words) and with temperature options from –40ºC to 100ºC.  Standard outputs include SSI, multiplexed parallel, and Universal Serial Bus (USB), so interfacing it to digital signal processors, computers, PLCs, and servo controls is straightforward.

Each system includes a read head, disc/hub or linear scale, and patented, compact external decoding electronics.  Standard disc diameters are 33 mm and 44.5 mm, with bore sizes to 14 mm.  Disc and scale materials include glass, metal, and plastic.  Initialization angles of 1.92 or 3.52 degrees for rotary motion, or a few millimeters for linear motion, need to be read before achieving absolute position.  Optional three-phase motor commutation is available at 2, 4, 6 or 8 pole pairs.

Applications that have benefited from Gurley’s Virtual Absolute series of encoders include gimbals/gyroscopes, machine tool spindles, tracking pedestals (telescopes and antennas), robotics, laboratory instrumentation, and photonics (spinning mirrors and laser scanning). 

DOWNLOAD THE 7700 DATASHEET HERE

Virtual Absolute(R) Basics

Readers are probably familiar with both incremental and absolute encoders. However, they may be less aware of a new technological development called virtual absolute (VA).  It is an entirely new kind of device which has the same number of code tracks as an incremental encoder, but behaves more like a conventional absolute encoder.  Though like the two traditional encoder types in some ways, virtual absolute encoders have aspects of design and behavior which are unique.

A virtual absolute encoder uses just cyclic and index tracks, like an incremental encoder. However, you can see the index track is something like a bar code, instead of just a single line (Figure 1). Absolute position is encoded serially along this one track, rather than being dispersed over multiple parallel tracks. 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, this initialization angle is typically 1–3º, depending on the encoder’s line count (non-binary resolutions are easily accommodated); in a linear encoder, less than 1 mm motion is needed. From that point on, the encoder is truly 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 new encoder in the opinion of some machine designers.

The decoder circuit is preferably located in the host system, 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 this virtual absolute technology can be summarized as follows:

• 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 whole-word information as a conventional absolute, so interfacing it to digital signal processors, computers, PLCs, servo controls, etc., is straightforward.

• Built-in-test functions not found in any conventional device report various encoder malfunctions, and can also help detect system problems such as excessive heat and speed.

• Simpler optics allow a rotary VA to be smaller than a conventional absolute of equal resolution (or have a larger through-hole).

• Simpler electronics, a reduced parts count, and less critical internal alignments translate into a higher intrinsic reliability when compared to a conventional absolute device.

• Technology adaptable to any GPI-made encoders: rotary and linear; enclosed or modular.

Captions

Figure 1. Disc comparisons of (a) incremental, (b) absolute, and (c) virtual absolute encoders.

Table 1. Applications appropriate for virtual absolute encoder technology.

Figure 1.

Table 1.