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Magnetostrictive linear position transducer

Author:iTarget Sensors Date:2011-9-10 15:00:06
 From Design World

Linear position transducer measure absolute distance along a motion axis. They are available in several technologies, each having its own advantages and disadvantages. This article presents information on the application of magnetostrictive linear-position sensors, which are gaining popularity due to their accuracy and reliability. A comparison of magnetostrictive transducer to linear position transducer of other technologies is also included.

Many types of industrial processing equipment utilise sensors to provide the information needed to monitor and control the process. This enables the design parameters to be maintained in order to produce the end product at the desired level of quality and throughput.

Typical sensors may measure temperature, pressure, flow, force or position, for example. The type of measurement and the sensor technology will dictate the set of parameters which are important in specifying and sizing the appropriate sensor.

Theory of operation

A magnetostrictive position transducer measures the distance between a position magnet and the head end of the sensing rod . The position magnet does not touch the sensing rod, and therefore there are no parts to wear out.

The sensing rod is mounted along the motion axis to be measured and the position magnet is attached to the member that will be moving. The head includes an electronics module, which reports the position information to a controller (or other receiving device) in the appropriate analog or digital format.

Also incorporated in the electronics housing are the electrical connection interface - either an integral connector or cable - along with visual diagnostic LEDs to ensure proper wiring, power and magnet positioning.

A magnetostrictive position transducer comprises five basic components: the position magnet, waveguide, pickup, damp and electronics module. There is also usually a protective tube over the waveguide.

The position magnet is a permanent magnet, often made in the shape of a ring, which travels along the sensing rod. The waveguide is housed within the sensing rod, and is a small diameter tubing or wire made from a magnetostrictive material.

Magnetostriction is a property of certain materials, including iron, nickel, cobalt and some of their alloys, in which application of a magnetic field causes strain which results in a change in the size or shape of the material. This is due to the alignment of the magnetic domains, within the material, with the applied magnetic field. Magnetic domains can be envisioned as many tiny permanent magnets which are randomly arranged before application of the magnetic field. When the magnetic field is applied, the poles of the magnetic domains align themselves along the gradient of the flux lines of this field.

The waveguide is so-named because a sonic wave travels in it during operation of the sensor. The sonic wave is generated by interaction between the magnetic field from the position magnet and a second magnetic field generated in the waveguide by the application of a current pulse (called the interrogation pulse) through the waveguide from the electronics module. The vector sum of the magnetostrictive strain from the two magnetic fields results in the generation of a torsional strain wave in the waveguide at the location of the position magnet.

The strain wave travels in the waveguide, towards the head end at about 2850 m/s. At the head, a pickup device senses the arrival of the strain wave (called the return pulse). Another strain wave also travels from the position magnet in the direction away from the head. This unused wave is eliminated by the damp in order to prevent interference from waves that would otherwise be reflected from the waveguide tip.

The electronics module applies the interrogation pulse to the waveguide and starts an electronic timer. After a time delay, which is proportional to the distance between the position magnet and the pickup, the electronics module receives the return pulse from the pickup and stops the timer. The magnitude of the time delay indicates the location of the position magnet. For example, at a measured distance of one metre with a waveguide velocity of 2850 m/s, the time delay would be: 1 metre ÷ 2850 metres/second = 0.35 milliseconds.

The electronics module then uses the time measurement to produce the desired output. The output can be a logic level pulsewidth, an analog voltage or current, or a standard digital interface.

The interrogation rate can be controlled from an external controller, or can be internally generated at a rate anywhere from one time per second to over 4000 times per second. This is the update rate, and is the frequency at which new position information becomes available at the sensor output. The maximum update rate depends on the waveguide length, ie, a shorter waveguide allows a faster update rate to be used.

Installation considerations

An advantage of the magnetostrictive sensor over other types of linear position sensors is the ability to read the position magnet even when there is a barrier between the position magnet and the sensing rod.

For example: the barrier can be the cylinder wall when the position magnet is part of a piston, or a transmission case when measuring gear position, etc. This is possible whenever the material directly between the position magnet and the rod can be a non-magnetic material. Common materials for this duty include plastics, ceramics, aluminium and non-ferrous metals, and many stainless steels.

Another advantage unique to magnetostrictive position sensors is the ability to measure multiple magnets while using one sensing rod. This allows making more than one measurement by only incorporating additional position magnets. Some sensor models accept up to 15 position magnets. In an injection moulding machine, for example, the injector motion, mould closing and ejector can be measured using only one sensing rod. Or, a slitting machine can measure the positions of all of the knives using only one sensing rod and adding a position magnet for each knife. Some temposonics sensors are also capable of providing direct position and velocity outputs which is necessary for many high-performance servo control systems.

Comparison of technologies

There are many things to consider when 'designing in' a linear-position sensor. Proper attention must be paid to matching the sensor to the application requirements regarding power input, signal output, housing style, mounting configuration, sensing stroke, and ability of the sensing technology to make the measurement under the application conditions.

With all of these considerations and the number of options available, the task can seem a little daunting. However, there are some major product options to consider.

Selecting the appropriate type and size

Housing style

Linear magnetostrictive position sensors are available in several housing configurations to enable mounting in a wide range of applications.

Two hydraulic or pneumatic cylinder mount styles include the standard mounting and a two-piece version intended for installation into space-restricted clevis-type cylinders where the sensing element is separated from the electronics module by an interconnect cable.

Installation of the hydraulic-style sensor is accomplished by threading the unit into a cylinder that has been prepped with a hollow piston rod and an industry standard threaded port in the end cap.

Another popular way to mount a linear-position sensor is by bolting its base to the machine frame, using a profile style housing.

Examples of profile housings. Here, the sensing rod is enclosed within an aluminium extrusion. The extrusion provides the mounting base for the sensor as well as a means to locate mounting 'feet' (brackets) or screws to secure the sensor in place.

The position magnet can be a bar magnet ('floating magnet')passing along nearby the top of the extrusion , or it may be captured inside of a shuttle ('sliding magnet') that rides along a rail which is part of the extrusion.

These magnet variations allow customers to use standard 'off-the-shelf' mounting hardware such as ball-joints and extension rods or design their own to suit the application.

A clevis mounting system is also available. This 'rod and cylinder' style is similar to the profile housings, but the position magnet is moved via a metal rod, with a clevis on the rod end and also (optionally) on the opposite end of the housing. The sensor housing can be supported through the clevis mounts for use in articulated motion applications, or by mounting feet applied through the groves in the aluminium extrusion.

Length

When determining the proper size of a magnetostrictive position sensor to order for a particular application, it is important to consider the length and alignment criteria of the sensing rod and position magnet.

There is a minimum distance allowable between the head end of the sensor rod and the position magnet. This is to prevent interaction of the position magnet with the pickup, and is called the null. The specified length of the null depends on the mounting configuration of the sensor.

It is 12 mm; so, the motion system and sensor mounting alignment must be designed so that the front face of the position magnet will be no closer to the mounting flange of the sensor than 12 mm. The front face of the position magnet is the face closest to the sensor electronics housing.

At the sensor rod tip (the end opposite the head), there is an unusable area in which the damp is housed. This is called the dead zone.

Like the null, the system must be designed so that the front face of the position magnet will come no closer to the tip than the specified dead zone distance. The dead zone is 82 mm.

For example, when ordering an TEC model with the dimensions, if the motion axis has a travel of two metres, then a sensor with a stroke length of two metres should be ordered.

The total length of the rod, from the flange face (at the head) to the rod tip, will be:

2 metres + 12 mm + 82 mm = 2.094 metres.

Electronic interface

Electrical power

The standard power for industrial sensors is 24 VDC, but some older systems use 15 VDC. A special extended power option (9 to 28.8 VDC) is available for non-standard power supplies and replacement of older products.

Mobile applications usually utilise 12 or 24 VDC from the battery; but often require special consideration because of a wide battery load range and the interface to the charging system. Make sure that you know the range of voltage provided by your power source. Automotive applications often power the sensor from a regulated 5 VDC to avoid higher cost electronics in the sensor.

Output signal

The signal from the transducer, and measured by the electronics module, is a time delay. This is shaped into a digital pulse when the sensor is specified with a start-stop interface.

In operation, the user supplies a digital pulse to request a reading (starting a timer at the same time), and the sensor returns a stop pulse. The time between the two pulses indicates the location of the position magnet. Similarly, a pulse with modulated (PWM) output can be used to indicate the same time interval.

Analog current or analog voltage outputs are common interfaces. The signal can be 0 to 20 mA, 4 to 20 mA, or -10 to 10 V. iTarget analog sensors can be ordered with 100% or no field output adjustment.

Also available, and more frequently applied today, are absolute serial (SSI) and industrial network (CANbus, DeviceNet and Profibus) outputs directly from the same electronics housing.