When you must track motion, you’ll need a shaft encoder, or rotary encoder. Most of these sensors use a circular disk placed between an LED light source and a phototransistor light detector. Depending on the type of encoder you need, the disk can produce X pulses per revolution or a binary-coded output.An incremental encoder produces a quadrature output that indicates rotation direction and produces a series of pulses you can count. In some cases, this type of encoder includes an index output that produces a short pulse once per revolution. An absolute encoder supplies information in coded form that tells you the shaft’s actual position. You can specify the type of code–usually binary of gray code. Read more at www.ecnmag.com/rotary-encoders-take-the-shaft.aspx. In addition to determining position, you can use an encoder’s output to measure rotation speed.While looking for an incremental shaft encoder I discovered one that uses a capacitance sensor to detect position. This type of sensor can help reduce power in equipment–6 mA at 5V down to 3.2 mA at 3V. You can find brief descriptions of the AMT-102 and -103 encoders at the www.amtencoder.com web site. The parent company, CUI, uses a custom ASIC to sense capacitance changes in an RF circuit and to convert that information into position increments.
According to the AMT site: …electronics use a 5-MHz signal modulated with a 10-kHz signal for sensing the position of the rotor by measuring the phase between the receiver low-frequency modulation and the transmitter low-frequency modulation. The sensor output has a small lag time as shaft rotations accelerate or decelerate.The AMT sensors let users change DIP-switch settings to choose one of 16 resolution that can range from a high of 2048 pulses per revolution (PPR) at 7500 RPM down to as low as 48 PPR at 30,000 RPM. You don’t have that flexibility with an optical encoder. You can buy the AMT-102 and -103 sensors from Digi-Key for $US 29.95 each, (part numbers 102-1307-ND and 102-1308-ND respectively) so it doesn’t cost much to give them a try. Each encoder comes with a variety of shaft adapters that range from 2 to 8 mm in diameter and include 1/8-, 3/16-, and 1/4-inch sizes.
When you visit the AMT Web site, be sure to read the inventor’s biography. It’s nice to see that information and more companies should tell engineers about their creative colleagues. –Jon Titus
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