Magnetostrictive Position Sensors

Magnetostrictive position sensors are non-contact linear position sensors that use the momentary interaction of two magnetic fields to produce a strain pulse that moves along a waveguide. One field is from a magnet that moves along the outside of the waveguide. The other field is from the waveguide itself. Typically, the waveguide wire is enclosed within a protective cover and attached to the device that is being measured. Applying a current pulse generates a sonic wave that travels along the waveguide to a small piece of magnetostrictive material that passes through a coil and is magnetized by a small, permanent magnet. The stress induced by the sonic wave causes a wave of changed permeability in the magnetostrictive material, resulting in a change in its magnetic flux and the production of voltage output from the coil. Electronic circuitry detects the voltage pulse and conditions it into the desired output. Depending on the sensor’s characteristics, measurements can be either absolute or incremental.

Magnetostrictive position sensors that provide absolute measurements produce a unique signal for each point along the axis of travel. Typically, the position is read after power cycling. There are several types of outputs. Gray outputs require the use of a unit-distance coder in which only one bit changes with the transition from one measuring step to the next. Binary outputs are represented as a series of ones and zeroes. By contrast, binary coded decimal (BCD) outputs are represented by decimal numbers (integers) in which each digit is signified by four bits.

Magnetostrictive position sensors that provide incremental measurements do not produce unique position signals. They are zeroed after power-down and rely upon an external switch or sensor for initial reference. There are several types of channel types and outputs for incremental devices. Quadrature is a signal configuration in which two channels carry signals that are 90° out of phase. Combining these signals provides four or more counts per line. Digital output signal lines produce one or two square waves that can be resolved via quadrature to four counts per line. Analog outputs use two channels that are 90° out of phase to read and combine sine and cosine signals. The theoretically infinite resolution is limited only by the resolution of external digitizing device. Single-channel magnetostrictive position sensors provide one count per physical line. Pulse and direction devices combine a unidirectional channel with either single-channel counting or multiple-channel counting with quadrature. Reference or index signals produce a once-per-revolution mark or home position. 

Magnetostrictive position sensors vary in terms of performance specifications, dimensions, and features. Performance specifications include measurement range, maximum range, resolution, and operating temperature. Dimensions include cross-section height and cross-section width. In terms of features, some magnetostrictive position sensors are sealed to prevent the ingress of contaminants such as dust, dirt, or swarf. Others include a tip or plunger that can be placed directly against the surface of the measured device. Typically, these probe-style sensors are biased with a spring or light air pressure.

There are many electrical and digital outputs for magnetostrictive sensors. Examples include fieldbus, process fieldbus (PROFIBUS), Interbus, DeviceNet, Ethernet, SUCOnet, and serial real-time communication system (SERCOS). Serial synchronous interface (SSI) is a general standard for absolute measurement devices. Serial outputs such as RS232, RS422 and RS485 are commonly available. Parallel outputs for magnetostrictive sensors include the general-purpose interface bus (GPIB), a standard which is also known as Hewlett-Packard interface bus (HPIB) and IEEE 488. Analog current outputs include 4 – 20 mA and 0 – 2 mA. Analog voltage outputs include 0 – 10V. Sensors that can transmit position data along optical fibers are also available.

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Part Numbers for Magnetostrictive Position Sensors