Design News, January 2013

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26 DATA MEASUREMENT & ANALYSIS Unbelievably useful info on data measurement, collection and analysis from the test expert Encoders Know All the Angles ENGINEERS USE rotary encoders to measure the angular position of an attached device, such as a motor shaft, or when used with a linear belt drive, for example, to indirectly measure distance. Rotary encoders come in two types, digital and analog. Digital rotary encoders comprise two Jon Titus categories — incremental and absolute. An incremental encoder produces n digital pulses for each 360 degree rotation of the encoder's shaft. An absolute encoder provides an x-bit binary code that indicates a specific angular position. Incremental encoders rely on an opaque disc with a concentric ring of small equally spaced holes placed near the disc's outer edge. Light from LEDs on one side of the disc shines through the holes onto two phototransistors on the opposite side. As the disc rotates, the "out of phase" transistors create two pulse signals. The number of holes in the disc range from hundreds to several thousand. Equipment can count these pulses to determine incremental angular changes. The two phototransistors each generate a square wave out of phase with each other, as shown in the figure. This phase relation shifts when the encoder changes direction. As an option, some incremental encoders produce a separate indexpulse signal once per revolution to indicate a specific position or the start of another revolution. A controller circuit can count pulses from one output to determine the shaft's angular position. The pulse-phase relationship indicates direction of encoder-shaft rotation. By measuring the time between pulses, or by counting the number of pulses over a specific time, software can calculate a shaft's rotational velocity and acceleration. When equipment power goes off , though, you lose track of an incremental encoder's position. An absolute encoder provides a unique binary output as the encoder's shaft rotates. If you require an absolute location with a 15 minute resolution, or 1440 unique codes, for example, a 12-bit absolute encoder with 4096 codes would work well. Jon Titus, a former designer and chief editor of EDN and Test & Measurement World magazines, remembers when "fast" signals operated at 10MHz and programs came on paper tape. LEDs Disc Phototransistors A B Clockwise rotation A B CounterClockwise rotation Out-of-phase detectors produce two pulse signals that indicate the amount of relative rotation as well as direction for an incremental encoder. An optional index pulse occurs once per revolution. You would have 360/(212) unique codes for a resolution of 5.3 minutes. Loss of power does not affect the encoder so you know the position of a mechanism attached to your encoder when power returns. Like incremental encoders, many absolute encoders rely on an encoder disk and a series of phototransistors — one per bit. Absolute encoders also might use a rotating magnet and a stationary Hall-effect sensor to produce binary angular-position codes. Encoder manufacturers offer straight-binary and Gray-code-binary outputs. Engineers use absolute encoders in robotic and other equipment that requires coordination of movements on many axes. By using the Gray code, which changes only one bit per increment, engineers can avoid any electrical glitches caused by multi-bit transitions such as 01111 to 10000. Do you have a measurement question you'd like answered here? Contact me at: DN For More Information: 1. "Encoder Application Handbook," Danaher Industrial Controls. Design News | JANUARY 2013 | www.d esign n –26–

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