Encoders are the sharp point of the machine safety arrow.
　　At its core, automation is defined by control loops, but automated machine safety is more aptly described as an arrow. And that arrow’s sharp point is often an encoder that makes it possible for the control system to know where it is and how fast it’s moving. With that knowledge, the system can not only avoid trouble, but act appropriately when circumstances bring trouble to its door.

　　Incremental encoders need only two tracks to

provide displacement, speed, and direction information. To get true position, howeverCONTROL ENGINEERING China版权所有, they maintain a running sum of displacements. Absolute encoders use much more space, but report true position at all times.
　　Trouble, of course, can take many forms. It is generally any event that can cause harm to the machine itself or to people or equipment around it. For moving-machinery applications, such events include intrusions into the machine’s work space, failures of machine components, or power upsets.
　　“The biggest machine safety issue is unannounced power outageswww.cechina.cn,” says Jim Marshall, consulting design engineer for Sick Stegmann. “On startupCONTROL ENGINEERING China版权所有, you need to know where axes are without physically moving them. For machine tools and tool changers that’s criticalCONTROL ENGINEERING China版权所有, as it is for automated warehouses.”
　　Encoders decoded
　　“What we’re concerned about is machine axis control from a position point of view as well as velocity,” says Tom Wyatt, national sales and product manager for the automation group at Heidenhain.
　　Of all safety sensors (e.g., light curtainswww.cechina.cn, proximity switches, and E-stops), only encoders can provide forewarning of trouble ahead. All types of encoders can report both position and velocity.
　　Encoder systems generally have three main components:
　　A source, such an LED or permanent magnet, which mounts on the static part of the machine and provides a steady excitation.
　　The encoder mounts on the machine’s moving part and modulates the steady excitation.
　　A detector receive