It occurred to me recently that, while linear position sensors are used in a wide variety of industries and applications, all of these applications fall into three broad categories: controlling linear motion, monitoring linear motion, and measuring linear motion.
Controlling Linear Motion
When we talk about controlling motion, we are usually talking about some sort of closed-loop control where not only the position of the moving part of the machine is important, but also how the machine behaves on its way to that position. In a closed-loop control application, the prime mover, e.g., hydraulic cylinder, pneumatic cylinder, electric motor, etc., receives commands from the controller to accelerate, decelerate, stop, reverse, and so on. The linear position sensor is tasked with providing accurate, dynamic position feedback to the control system. A linear position sensor used to close a feedback loop must be accurate of course, but it must also be able to provide highly dynamic position updates to the controller. If you spot a hydraulic cylinder with a servo or proportional valve attached to it, chances are there is also a linear position sensor in the mix, providing critical position information to the controller.
Monitoring Linear Motion
In some cases, dynamic motion control isn’t required, but the machine still has to be accurately positioned. Unlike the dynamic closed-loop control scenario described above, sensors used in position monitoring applications do not “close the loop”. In other words, the sensor plays no part in controlling acceleration, deceleration, or complex motion profiles. Instead, the sensor is used to allow the machine to be positioned at a particular position, or positions, along its travel. A good example of a position monitoring application is a mechanical stamping press, where the linear position sensor is used to set up the shut height of the press.
Measuring Linear Motion
Finally, the third category of linear position feedback applications uses the sensor as a sort of electronic tape measure. In these applications, the sensor is used to provide accurate, absolute position measurement. In many cases, measuring applications involve producing parts of very specific dimensions. Linear position sensors that are capable of accurate, repeatable position feedback fill the bill quite well. An example of a measuring application might be a cut-off saw used in a cut-to-length role.