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Basics of
TRANSDUCERS IN One of the most common transducer types is the inductive proximity sensor, such as this one from Balluff, which uses a non-contact principle to detect metal objects.
general, a transducer is a device that converts one form of energy into another. Common types used in industrial applications include sensors to measure temperature, pressure, force, strain, liquid levels and flow rates. These physical quantities are converted to electrical signals in either analog or digital form and can be used to gain information about or control some process. In motion control applications, transducers can refer to any one of a number of sensors, such as rotary or linear encoders, or resolvers for position feedback, such as tachometers for speed sensing, and even proximity switches to initiate or halt some machine action. They also come in various types and the choices available depend on the quantity being measured or controlled. For motion control applications, the measured variable is typically position or speed. Depending on a number of factors, the right transducer may be an encoder or a resolver or a simple potentiometer. A common approach to measuring position is a magnetostrictive linear displacement transducer (MLDT), which is typically mounted inside the cylinder. MLDTs are best because they use moving magnets that don’t come in contact with the sensor tube, avoiding mechanical wear. They also provide an absolute position readout, requiring no homing step before beginning to work with the position information from the MLDT. Advances in MLDT technology have led to resolutions down to 1 μm, with fast signal processing of up to 1.5 MHz.
There are a few key factors to consider when selecting any transducer, including the desired variable to be measured, the accuracy or resolution needed and the type of output, as well as any size or space restrictions, environmental factors, and product lifetime and cost. For starters, what kind of motion is involved, rotary or linear? Specific transducer types exist for each kind of motion. Determine the accuracy needed for the application. This includes factors such as linearity, resolution and repeatability. What range must the transducer measure? For linear applications, is the range on the order of nanometers, a few millimeters or several feet? For rotary applications, if measured in degrees, is the angular distance more or less than 360°? Is the type of encoder needed a single-turn or multiturn device? Also, if selecting an encoder, does the output need to be absolute or incremental? Consider the type of output. Is it voltage or current? Digital or analog? Many transducers are programmable with a data connection, such as a PC-to-USB link. Other interface options can include encoder-specific communication links like SSI (synchronous serial interface), BiSS (bi-directional serial/synchronous) or PROFINET.
Linear position sensors, like this Micropulse linear position transducer from Balluff, use a non-contact magnetostrictive technology and produce absolute output position signals.
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