Choosing the Best Position Sensor: Does Yours Measure Up?

Reliable electronic measurement is something that is always needed in industrial automation. Production interruptions and unexpected downtime will cripple even the best manufacturers if they do not have the appropriate measurement technology in place.

Whether it’s monitoring the position of a hydraulic jack or determining the proper position of a flood gate on a dam, be sure to choose the best option for precision, accuracy, and most importantly, reliability.

Strings Holding Down Production 

String potentiometers, also known as string pots, yo-yo sensors, cable-extension transducers, and a few other names, have been used for electronic measurement for the last 40 years.

These devices use braided steel wires (“strings”) wrapped around spools and require the release of the coiled string.

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In an industrial assembly application, a typical scenario might involve wire being integrated into a manufacturing platform that moves from one assembly station to the next. As the string pot’s spool extends or retracts, position is measured by a rotational sensor/potentiometer that rests outside the spool and will trigger based on the position of the metal wire.

While string pots are often used in many sectors (heavy industry, crude oil processing, waste water treatment, etc.), they come with potential issues that make then unsuitable for others:

  • The wire will eventually jam from rust, mechanical glitches, or other environmental factors
  • The springs in the reel often fail over time
  • The high contact nature of the devices causes friction among the components, which leads to excessive wear and failure after a few thousand rotations

Combined, these things lead to expensive downtime/loss of production and costly repairs. A measurement system should not be a consumable item or an item with an expected but unpredictable maintenance interval. A measurement system should be designed with longevity and reliability in mind.

Right Solution for Your Industry

The assembly industry is not the only one that benefits from highly accurate position measurement. Whether it’s metallurgy, plastics and rubber, energy, or woodwork —­  the advanced, versatile, and resilient technology is required to thrive in high speed and demanding applications.

Fortunately, magnetostrictive linear transducers were developed to provide the kind of reliable position measurement that industry demands.

Instead of a trouble-prone mechanism, magnetostrictive non-contact linear transducers work with a movable free-floating or captive magnet that rides the length of a sensing rod as it follows the target object.

During operation, a very short-duration pulse is generated along the sensing element. This is known as the waveguide. The resulting magnetic field interacts with the magnetic field of the position magnet and generates a mechanical force on the sensing element. This force ripples along the waveguide at a faster-than-sound velocity that is detected by the sensor electronics, and is converted into an electrical pulse.

Using a very accurate high-speed clock, the time interval between the initial current pulse and return pulse is measured and converted into an absolute position value.

The end result is constant, precise, accurate, and smooth position measurement.

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As an example: A high speed punch press requires position monitoring down to the millisecond. The punch press is designed to move very quickly back and forth in rows, punching holes in precise locations. When one row is finished, the unit moves forward and does the next. As the punch continues back and forth and up the rows, the sensor follows the position of the press, transmitting position feedback to the control system. This ensures the press stays on the appropriate track and punches where it should.

A contact device would not be suitable for this kind of operation, as the amount of friction caused from the speed and repetition would wear the sensor down too quickly and cause failure.

Fortunately, magnetostrictive linear sensors are widely available, come in a variety of form factors, and are truly non-contact, with some “floating” versions riding as much as 15mm off the surface of the transducer body. No contact means no wire binding issues and the lack of contact also means a lack of impact damage, which will help the sensor survive longer than a string potentiometer.

A measuring distance from 1 to 300 inches, offers short and long range capabilities.

Moreover, compared to string pots, magnetostrictive linear sensors, require fewer components. This means fewer parts to replace and maintain, which results in a reduction in overall equipment and maintenance costs.

Adaptable to nearly any industrial control system, these sensors are available in common analog (0….10V or 4….20 mA), as well as a variety of digital interfaces. This includes digital start/stop, synchronous serial interface, as well as network interfaces (IO-Link, Ethernet/IP, Profinet).

Tying it All Together

Though both string pots and magnetostrictive linear transducers are employed for electronic measurement, selecting the one that is best for your application will maximize manufacturing efficiency, increase machine uptime, and cut costs. All while ensuring your process keeps running smoothly and your customers get the parts and products they need on time.