AUGUST, 20218 IN MY OPINIONPOSITION SENSORS FOR ROBOTICSBY CHRISTIAN FELL, CTO, POSITAL-FRABA INC.A key element of any successful robot is the collection of sensors and communications networks that enable the control system to accurately monitor the spatial orientation of its mechanical components­arms, grippers, actuators etc.The position sensors built into these systems must provide high levels of accuracy to enable the precise positioning of parts or tools. They must also have excellent dynamic response to provide the control system with real-time feedback of rapidly moving components. Reliability is a must, of course, even when the robot operates in a harsh, wet, dirty or hot environment. And, they should be reasonably inexpensive, especially for smaller robots where lower purchase and operating cost is a key competitive feature.ROTARY ENCODERS­POSITION CONTROL MAINSTAYSRotary encoders are transducers that are designed to measure the rotation of a mechanical component (shaft, axle, etc.) and send a digital signal to the control system. For robots that mimic a human arm, with `shoulder', `elbow' and `wrist' joints, rotary encoders located at each of these joints provide the control system the information it needs to accurately position each section of the arm. For mobile robots such as the autonomous transport vehicles that can goods through crowded warehouse or factory spaces, rotary encoders enable the control system to keep track of speed, distance traveled and steering.Collaborative Robot Working in a Shared Space (Photo Courtesy of Universal Robots A/S) Until recently, the `gold standard' for rotary encoders for robots was instruments based on optical measurement techniques. These devices are built around a `code disk': a glass or plastic disk mounted on the encoder's shaft with a concentric pattern of transparent and opaque areas printed on its surface. A light source is located on one side of the disk and an array of photocells on the other. As the disk rotates, a coded pattern of light will fall on the photocells, generating a digital `word' representing the absolute angular position. Optical encoders have excellent dynamic response and resolution. However, to achieve high levels of accuracy, the code disk needs to have a relatively large diameter. Optical encoders can also be sensitive to humidity (which can fog the code disk) and mechanical shock and vibration loads. This can limit their suitability for harsh environments. Optical encoders also require extremely tight tolerances with respect to the alignment between the code disk and the photocell array, which makes them relatively costly. Magnetic encoders were introduced a little over a decade ago as an alternative to optical encoders. They are more compact, less susceptible to dust and moisture and more tolerant of shock and vibration loads. Magnetic encoders have a small permanent magnet mounted on the rotating shaft, with Hall-effect sensors on the body of the encoder to measure the position of the magnet. The Hall-effect sensors were originally
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