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Sizing and Applying Magnetostrictive Linear Position Sensors

Author:Matt Hankinson Date:2011-9-10 15:14:57
 From Design World, By Matt Hankinson

Magnetostrictive displacement sensors have many technical and practical advantages over other measurement technologies, which include precision, reliability, cost, flexibility, and low maintenance.

Many types of industrial equipments use sensors to provide the feedback needed for monitoring and controlling a process. This maintains the design parameters and produces the manufactured product at the desired level of quality and throughput. Typical sensors may measure temperature, pressure, flow, force, or position, and the type of measurement and the sensor technology chosen dictates the parameters that drive the specification and the size of the appropriate sensor.

Magnetostrictive linear-position sensors are gaining widespread use because of their innate accuracy, reliability, and ability to provide continuous, absolute position feedback. Temposonics technology, based on the speed of sound through a waveguide, is a specific implementation of magnetostriction that increases the signal quality for improved performance and robustness. A magnetostrictive position sensor measures the distance between a position magnet and the head-end of a sensing rod. The magnet does not touch the sensing rod, so no parts can wear out. The sensing rod mounts along the motion axis to be measured and the position magnet attaches to the member that moves. The head includes an electronics module, which reports the position information to a controller or other receiving device in an analog or digital format. Also within the electronics housing is the electrical connection interface, either an integral connector or cable and visual diagnostic LEDs to ensure proper wiring, power, and magnet positioning.

Installation considerations

An advantage of the magnetostrictive sensor over other types of linear-position sensors is its ability to read the position magnet even when a barrier is placed 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. This is possible when the material directly between the position magnet and the rod is a nonmagnetic material. Common materials are plastic, ceramic, aluminum, other non-ferrous metals, and many stainless steels.

Another advantage unique to magnetostrictive position sensors is the ability to measure multiple magnets with one sensing rod. More than one measurement can be made by incorporating additional position magnets. Some sensor models accept up to 30 position magnets. In an injection-molding machine, for example, the injector motion, mold closing, and ejector can be measured with one sensing rod. A paper slitting machine can measure the positions of all the knives using only one sensing rod and a position magnet for each knife. Some Temposonics sensors also have direct position and velocity outputs, which is necessary for many “high-performance” servo-control systems.

Operating specifications

Various models of magnetostrictive position sensors have unique specifications. Depending on the intended use and output style, specifications for a typical model are listed for guidance. But how does a magnetostrictive sensor compare to other linear position technologies?

Magnetotstrictive sensors offer high resolution and low non-linearity, are innately rugged and durable and come in stroke lengths as short as 25mm to as long as 10m, which make them ideal for applications that require high accuracy and repeatability.

Compared to magnetostrictive sensors, LVDTs (linear variable differential transformers) have high resolution and medium non-linearity, and are rugged as well, but are usually only available in stroke lengths from 2mm to 200mm, which rule them out for applications that require longer stroke lengths. Also, inductive transducers have both medium resolution and non-linearity for harsh applications that require stroke lengths of 2mm to 500mm. Encoders, on the other hand, offer medium to high resolution and low non-linearity, but are more sensitive to environmental conditions and are not reliable in harsh environments such as mining, construction, or steel making.

Ultrasonic sensors may be an option for applications that do not require high accuracy and repeatability since they have low resolution and high non-linearity.

Finally, potentiometers are often ruled out in harsh environments as well as in precision applications because of lower resolution and higher non-linearity than those in magnetostrictive sensors.

After selecting magnetostrictive sensors, many other factors must be considered when “designing-in” a linear-position sensor. For example, 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 perform well under the application conditions are critical issues.

 

 

 

 

 

 

 

Figure 1–Several different housing configurations let the sensors mount in a variety of applications. This is a two-piece version for mounting on hydraulic or pneumatic cylinders. It contains rods and flanges that can withstand the high cylinder pressure.

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-mounting options include standard mounting, as well as the two-piece version seen in Figure 1. They both have rods and flanges, which are capable of sealing and withstanding the cylinders’ high pressures. The high-pressure mounting thread can be specified in English or metric units, and the hydraulic-style sensor can be threaded into a cylinder that has been prepared with a hollow piston rod and an industry-standard threaded port in the end cap. The appropriate torque is applied to the hex flats adjacent to the pressure flange threads as shown in Figure 2.

 

 

 

 

 

 

Figure 2–The high-pressure mounting threads come in either English or metric units. Hydraulic-style sensors attach to cylinders that contain a hollow piston rod and an industry-standard threaded port in the end cap. Torque is applied to the hex flats adjacent to the pressure flange threads.

Another widely accepted way to mount a linear-position sensor is bolting its base to the machine frame using profile-style housing. Examples of profile housings are shown in Figure 3. Here, the sensing rod is enclosed within an aluminum extrusion. The extrusion provides the mounting base for the sensor and a means to locate mounting feet, brackets, or screws to secure the sensor in place. The position magnet can be a bar magnet or floating magnet, which passes near the top of the extrusion, as shown in Figure 3. It may be captured inside a shuttle or sliding magnet that rides along a rail that is part of the extrusion, also shown in Figure 3. These magnet variations let customers use standard, off-the-shelf mounting hardware such as ball-joints and extension rods, or they can design their own hardware to suit the application.

 


 

 

 

Figure 3–These linear-position sensors fasten to the machine frame with profile-style housing. The sensing rods are enclosed within an aluminum extrusion.

A clevis mounting system is also available, as shown in Figure 4. 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 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 grooves in the aluminum extrusion.

 

 

 

 

 

 

 

Figure 4–This rod and cylinder is much like the profile housing, but a metal rod moves the position magnet with a clevis on the rod end, and for some models, on the opposite end of the housing. Clevis mounts or mounting feet on the aluminum extrusion can support the sensor housing when used in articulated motion applications.

Sensor Length and Stroke Length

When determining the proper size of a magnetostrictive position sensor for a particular application, consider the length and alignment criteria of the sensing rod and position magnet. A minimum distance is allowed between the head end of the sensor rod and the position magnet to prevent interaction of the position magnet with the pickup: This is called the “null.” The specified length of the null depends on the mounting configuration of the sensor.

Because the null is 12 mm, 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 (as shown in Figure 5). The front face of the position magnet is the face closest to the sensor electronics housing.

 

 

 

 

 

 

 

 

Figure 5–Because the null is 12 mm, the front face of the position magnet must be at least 12 mm away from the mounting flange. The front face of the position magnet faces the sensor electronics housing.

At the sensor rod tip, at the end opposite the head, there is an unusable area 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. In Figure 5, the dead zone is 82 mm. For example, if the motion axis has a travel length of two meters, then a sensor with a stroke length of two meters is needed. The total length, L, of the rod from the flange face at the head to the rod tip is:

L = S1 + S2 + S3

L = 0.012 m + 2 m + 0.082 m = 2.094 meter

Where:

S1 = null, m

S2 = stroke length, m

S3 = dead zone, m

Electrical Power

The standard power for industrial sensors is 24 Vdc, but some older systems use 15 Vdc. A special extended power option, 9 Vdc to 28.8 Vdc, is available for non-standard power supplies and replacement of older products. Mobile applications usually use 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. Verify the range of voltage provided by the power source. Automotive applications often power the sensor from a regulated 5-Vdc source to avoid more expensive electronics in the sensor.

Output Signal

The signal from the transducer, measured by the electronics module, includes a time delay. It is shaped into a digital pulse when the sensor is specified with a start-stop interface. In operation, the operator 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 (pulse-width modulation) output can be used to indicate the same time interval. Analog current or voltage outputs are common interfaces.

The signal can be 0 to 20 mA, 4 to 20 mA, or -10 to 10 volts. MTS Sensors’ Temposonics analog sensors can come with 100% or no field output adjustment. Also available and more frequently applied are absolute serial (SSI) and fieldbus protocol (CANbus, DeviceNet and Profibus), or industrial Ethernet (EtherCAT or PowerLink). For applications requiring high-speed motion control, SSI or EtherCAT are the most popular digital outputs because of their 10 kHz update rate.

 

 

 

 

 

 

 

Figure 6–This Temposonics sensor mounts in a fluid power cylinder with the position magnet attached to the piston. The sensing element contains the wave-guide, and the sensor head contains the electronics module. They can be removed to replace the sensor without affecting the cylinder pressure. 

Lastly, the application itself defines most of these parameters, so the design engineer need only find a magnetostrictive sensor that meets all performance requirements. For example, in Figure 6, a Temposonics sensor mounts in a fluid power cylinder. The position magnet attaches to the piston within the cylinder. The sensor flange threads into one end of the cylinder. The sensing element, which contains the wave-guide, and the sensor head, which contains the electronics module, can be withdrawn from the mounting flange in order to replace the sensor without venting the hydraulic pressure inside the cylinder.

 


 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 7–This is a control system for maintaining a specified roller gap. Position magnets attach to each end of the movable rollers. The controller processes the sensor signals and controls the servomotors.

In a different application, Figure 7 shows a sensing and control system for maintaining a specified roller gap. The magnetostrictive displacement sensors mount along the roller adjustment axis, with the position magnets attached at each end of the movable rollers. The controller accepts the sensor signals and sends the control signal to the servomotors.

A wide range of applications can benefit from magnetostrictive technology. For instance, the sensors are ideal for applications which require precision measurement and “non-wearing” parts. In electric actuators, the technology enables improved positioning accuracy by measuring linear position at the load. Many applications in steel, plastic, and wood machinery benefit from the shock and vibration resistance. With hydraulic/pneumatic cylinders, the sensor can be mounted within the rod and the magnet placed inside the cylinder. Magnetostrictive sensors also benefit food and beverage, liquid level, medical, metalworking, paper converting, plastics, wood and testing equipment applications.