Non-Contacting Linear Position Sensors for Mechanical Engineering Applications

Variohm EurosSensor presents the following application note on non-contacting linear position sensors that replaced potentiometer types for a tube processing machine manufacturer.

Depending on the application and the respective operating conditions, non-contacting measuring procedures are used for detecting linear movements.  The choice frequently falls on inductive linear transducers. Compact dimensions and integrated electronics, which render the measured value as an absolute, standardized current or voltage signal at the output, are important arguments for the usage of these linear position sensors. Their application in the mechanical engineering sector described below provides a good example. 

The company Uniflex Hydraulik GmbH (see text box 1), headquartered in Karben, Germany, is a world-renowned manufacturer of various tube production and processing machines.  Compact dimensions, high pressures, quiet and reliable operation as well as high flexibility are characteristics of the hose crimpers (Illustration 1), which are successfully deployed in industry and workshop settings around the world. Here the machines have to rely on dependable position sensors because the press stroke must be adjustable to the respective tubular form and size according to the values set by the control (Illustration 2).

Alternatives to the Potentiometer Wanted

"For a long time, linear conductive plastic potentiometers did a good job measuring the travel at the press lever," explains Hubert Poth (Illustration 3), Development, Software and Electrical Engineering at Uniflex. "With regards to accuracy, reliability and durability, we were very satisfied with this type of sensor from Novotechnik (see text box 2) and have no reason to be critical."  But in the long run, it was not always easy to deal with the functional principle of the potentiometer. Potentiometers work as voltage dividers.  Therefore, different input voltages always have an effect on the measuring signal. "This is a disadvantage, which plays a role especially in export machines, since we often do not know where in the world our hose crimpers end up being deployed and what type of supply voltage the sensor technology must tolerate," adds Poth.  A practical alternative was found when Novotechnik developed a new linear position sensor series with integrated electronics, which helped solve this problem.

The LS1 series of linear position sensors work based on an inductive principle, but are in size completely compatible with the T-series potentiometers previously used with hose crimpers. With dimensions of 18 mm x 18 mm and effective lengths from 25 to 200 mm, they are appropriate as alternatives to most standard potentiometers with a square cross-section.  The integrated signal processing of the inductive linear position sensor makes the measured value available as an absolute current or voltage signal at the output. "In our machines, we use the analog 0...10 V voltage signal; the sensors' wide input voltage range of 16 to 30 V particularly benefits our export machines," Poth is pleased to confirm.

Integrated Signal Processing and Wide Input Voltage Range

The integrated teach-in function with LED status display is particularly practical for many application types. By pressing a push button directly on the linear position sensor, one can choose zero and end points of the measurement, invert the measurement output slope, or set the desired signal amplitude. Unlike potentiometers, a separate signal conditioner is not necessary; as a result, control is simplified and the user does not have to get involved in the control program. Poth adds another advantage: "An auxiliary signal conditioner also requires its own installation slot and must be mounted and connected." Plus, any additional component in a machine or installation translates into a potential source of error.

Simple Installation, No Mechanical Wear and Tear

In addition, the linear position sensors are easily installed. "Mounting brackets are included in the delivery, and a ball coupling without play allows for a shear force-free application, even with parallel and angular displacement of linear sensor and measuring direction," explains Poth. With its compact design and actuating rods on both sides, this series is also suited for a variety of other industrial applications.

These linear position sensors are virtually maintenance and wear-free due to the contactless inductive measurement principle and offer high repeatability (up to 2 mV and 3 µA respectively) and linearity of better than +/- 0.15% of the measurement stroke. The update rate of the output signal is specified at approximately 1 kHz, which is more than sufficient for most applications. It is also advantageous for many applications that the inductive linear position sensor is completely unaffected by magnetic fields. The functionality of the wear-free alternative to potentiometers is easy to understand:

The Inductive Measurement Principle 

A sinusoidal and a cosinusoidal conductor loop are both located on  the printed circuit board along the measurement stroke and are being supplied with alternating currents with a phase-shift of 90° degrees. This generates alternating fields perpendicular to the printed circuit board, while their strength over the measurement area is also sinusoidal and cosinusoidal. Based on the trigonometric addition theorem, the following relationship applies to the site-dependent sum of both fields:

H * sin(x) * cos(ωt) + H * cos(x) * sin(ωt) = H * sin(ωt+x)

where “H” describes the magnetic field strength, “x” the path information, and “ωt”“ describes the periodic time dependence of the signal sum. The result is a signal with a phase shift directly proportional to the stroke. The position indicator, designed as an oscillating circuit, "floats" over the signal conducting plate (Illustration 3).  Its resonance frequency is tuned to the emitting frequency of the two signal feeds.  The oscillating circuit is stimulated by them and emits its alternating field back to the printed circuit board. The rectangular receiver coil that is integrated into the circuit board receives this signal and sends it to the evaluation electronics. There the received signal is compared to the transmitted signals. The resulting phase information is then transformed by the integrated evaluation electronics into analogue position information, which is linear over the measurement range. "The fact that the inductive linear position sensors work non-contacting and therefore without any mechanical wear is of course a nice bonus for us even though we were very happy with the reliability of the conductive plastic potentiometers," concludes Poth. Inductive linear position sensors could open up many other application areas.

For over 40 years, UNIFLEX Hydraulik GmbH, with its headquarters in Karben, a manufacturer and worldwide supplier, has been developing, producing and selling innovative systems for hose assembly production and processing.  With the latest in technology and robust design, U