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In electrical engineering, a limit switch is a switch operated by the motion of a machine part or the presence of an object. A limit switch can be used for controlling machinery as part of a control system , as a safety interlock , or as a counter enumerating objects passing a point.
Switches can be designed to respond to any type of mechanical stimulus: for example, vibration (the trembler switch), tilt, air pressure, fluid level (a float switch), the turning of a key , linear or rotary movement (a limit switch or microswitch), or presence of a magnetic field (the reed switch). Many switches are operated automatically by ...
Many linear encoders include built-in limit switches; either optical or magnetic. Two limit switches are frequently included such that on power-up the controller can determine if the encoder is at an end-of-travel and in which direction to drive the axis.
A reed switch has very low resistance when closed, typically as low as 0.05 ohms, whereas the Hall effect sensors can be in the hundreds of ohms. A reed switch requires only two wires whereas most solid-state devices require three wires. A reed switch can be said to require zero power to operate it.
Similarly, a parallel set of instructions will perform a logical OR. In an electromechanical relay wiring diagram, a group of contacts controlling one coil is called a "rung" of a "ladder diagram", and this concept is also used to describe PLC logic. Some models of PLC limit the number of series and parallel instructions in one "rung" of logic.
The limit switches signal when an end position has been reached. The torque switching measures the torque present in the valve. When exceeding a set limit, this is signaled in the same way. Actuators are often equipped with a remote position transmitter which indicates the valve position as continuous 4-20mA current or voltage signal.
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The hysteresis thresholds for switching (specified as B OP and B RP) categorize digital Hall ICs as either unipolar switches, [9] omnipolar switches, [10] or bipolar switches, [11] which may sometimes be called latches. [12] Unipolar (e.g., A3144) [13] refers to having switching thresholds in only one polarity of the magnetic field. Omnipolar ...