Technical Articles

High resolution optical encoders in digital incremental motion systems.

Servo control of direct-drive brushless DC motors demands increasingly greater resolution and tighter speed control for smooth operation. This article describes the advantages of sine/cosine optical encoders as the feedback element of choice for applications such as the feed axis (C-axis) in machine tools and low-speed applications in general.

For good dynamic behavior in a digitally controlled feed servo, it is necessary to have a sample rate of between 50µs and 600µs and a signal processing time of 25% of the update rate (between 15µs and 150µs. To minimize dead zones in a closed loop, position and speed information will therefore have to be available within a few microseconds. If, for instance, a shaft needs to turn at 0.1 RPM at a sample rate of 500 µs and a desired minimum change of one measuring step per sample period, the feedback element needs to supply 1.2 million measuring steps per revolution (for a digital output encoder: 300,000 cycles per revolution).

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Possible feedback devices are:

Optical encoder with digital outputs.

An optical encoder with digital outputs at 300,000 cycles per revolution would clearly not be a practical feedback element for reasons of cost and an exceedingly high datarate at high speed (at 3,000 RPM the channel frequency of an incremental encoder would be 15 MHz). Lowering the counts per revolution makes an analog tachometer necessary for low-speed control. Additionally, some kind of absolute position feedback element would be necessary for commutation (see fig 1, analog speed control for a traditional configuration).

Resolvers.

The problem with a resolver is their poor low-speed characteristic due to low gain and latency between a change in position and the availability of the position signal. A very desirable feature, though, is the absolute nature of a resolver: commutation control and sinusoidal drives are easy to implement. Although a resolver is inexpensive, a drawback is the relatively expensive resolver-to-digital (R-to-D) electronics necessary for processing the signals generated by the resolver element.

Optical encoders with sine/cosine outputs.

With this type of encoder the position information is continuously available. Digitizing the signals yields resolutions only limited by the system noise. A way to visualize an encoder with sinusoidal signals is as a resolver that repeats n times per revolution instead of just one (n is the linecount: the number of full A and B cycles per revolution). Counting the cycles in the traditional way and digitizing between the A and B channel zero crossings yields very high numbers of measuring steps: a 2,000 c/r encoder with a simple and a fast 10 bit digitizing scheme yields more than two million measuring points (fig.2, which shows the vector sum of A and B). At high speed, when high position resolution is not necessary, the digitized portion of the position word can be disregarded. Transmitting sinusoidal signals from a 2,000 c/r encoder even at high speed is no problem for leads of up to 500 feet. The same encoder can supply absolute position information by adding a second channel pair which generates one sine/cosine signal per revolution, which can be digitized to supply commutation information in much the same way as a resolver.

A number of manufacturers specialize in this type encoders. Computer Optical Products in the U.S., Stegmann and Heidenhain in Germany market a series of hollow-shaft encoders with sine/cosine outputs of between 512 and 4096 c/r and an "absolute" one cycle per revolution output. This cuts down on the number of data wires per encoder: two for high resolution positioning, two for universal commutation and one for an index pulse. These encoders are not "amplified sine" encoders but are specifically built for low waveform distortion and amplitude stability over time and temperature. All optical encoders generate a sine/cosine signal (although digital types with less regard for amplitude stability and distortion) which is squared-off by a Schmitt-trigger where only the zero-crossing position information is retained.

Used in vast numbers for many years in printers, disk drives and instruments etc., sine/cosine encoders have been slow in making inroads in the motion control industry. This is changing and especially in Germany there have been several interesting developments in using sine/cosine encoders for the machine tool industry. The "SinCos" encoder, custom designed by Stegmann for Indramat, generates a 512 c/r signal for high-speed, high precision positioning, in addition to generating the commutation signals.

In order to simplify processing and integration into the system, a group of forty German companies in the motion control field have developed a two-chip set: an analog chip to take care of the front-end processing of the encoder signals, and a digital chip for the position and speed loop as well as the commutation signal generation. This "VeCon" chipset was developed by the "Institut f?r angewandte Mikroelektronik" (IAM) in Braunschweig, Germany. The analog chip is manufactured by Burr-Brown and available as p/n ADS 7833 N.

The key to digitizing the sine/cosine signals is the arctangent generation by means of taking the ratio of the A and B encoder signals. With the availability of low-cost DSP's, ROMS and processors, this is a simple matter. The most straightforward scheme is to use the outputs of the two A to D converters as the address of a ROM containing the appropriate values of the distance between the A and B zero crossings. Available ROM sizes limits this scheme to about 8 bits, for 10 and 12 bits it is better to use a DSP with a lookup table. Simple processors, e.g. the 68HC11 processors have been used with excellent results and minimum grief.

The cost of optical sine/cosine encoders are not fundamentally different from the digital variety. With the addition of standard, off-the-shelf electronics and some extra lines of code in the controller, it is possible to reduce the "last bit hunting" by orders of magnitude in positioning and obtain ripple-free velocity information very close to zero by numerically differentiating the position information. The second "resolver" sine/cosine track makes this type of encoder a very attractive and cost-effective package for the most demanding incremental motion applications.

Acknowledgements:

"Microprocessor based position control with high resolution and low torque ripple, using a polyphase brushless DC motor and a sinewave encoder"

By Joseph N. West, HP Lightwave Operation, "Motion" magazine Sep./Oct. '94 issue.

"Einbau-Drehgeber für Antriebe mit digitaler Drehzahlreglung"

By Dr. Rainer Hagl, Heidenhain Traunreut, "Elektronik Informationen" nr. 3-1994

"Der analoge VeCon-Chip"

by Dipl.-Ing. Edwin Kiel, Institut für angewandte Mikroelektronik, Braunschweig, "Elektronik Industrie" nr. 6-94

Biography

Kees van der Pool has been working in the encoder industry since joining Sensor Technology in 1974 and starting Computer Optical Products in 1983. A native of Holland, he received his EE degree from Eindhoven Technical College in 1970.