Variohm EuroSensor, the exclusive UK distributor for Novotechnik, presents an application note on non-contact 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; inductive linear transducers are often specified. Compact dimensions and integrated electronics, which render the measured value as an absolute, standardised current or voltage signal at the output, are important arguments for the usage of these linear position sensors.
Hubert Poth, in the Development, Software and Electrical Engineering department at Uniflex, explains: "For a long time, linear conductive plastic potentiometers did a good job measuring the travel at the press lever. With regards to accuracy, reliability and durability, we were very satisfied with this type of sensor from Novotechnik 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. This type of sensor operates as a voltage divider, so different input voltages always have an effect on the measuring signal. Poth adds: "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."
A practical alternative was found when Novotechnik developed a new linear position sensor series with integrated electronics, which helped to solve this problem.
The integrated teach-in function with LED status display is particularly practical for many application types. By pressing a pushbutton directly on the linear position sensor, the user can choose zero and end points of the measurement, invert the measurement output slope, or set the desired signal amplitude. Unlike potentiometers, there is no need for a separate signal conditioner; 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." Furthermore, any additional component in a machine or installation translates into a potential source of error.
The linear position sensors are also easily installed, as Poth explains: "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." 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 2mV and 3uA, respectively) and linearity of better than +/-0.15 per cent of the measurement stroke. The update rate of the output signal is specified at approximately 1kHz, 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.
A sinusoidal and a cosinusoidal conductor loop are both located on the printed circuit board along the measurement stroke and are 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.
The result is a signal with a phase shift that is directly proportional to the stroke. The position indicator, designed as an oscillating circuit, 'floats' over the signal conducting plate. 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 with 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.
Poth concludes: "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."
Follow the link for more information about Novotechnik non-contacting linear position sensors from Variohm-EuroSensor.