Linear position sensors provide high accuracy steering

Jon Lawson

Precision Decisions Ltd specialise in hardware and software solutions for agriculture that maximise yield potential through the use of Global Navigation Satellite System (GNSS) automatic vehicle guidance.

The Yorkshire, UK-based company are particularly strong in an area of agronomics known as controlled traffic farming (CTF) where satellite guidance is employed as a key element to reduce vehicle travel over the field to the very minimum.

CTF essentially reduces overlaps and travelled ground in fields to provide farmers with significant benefits that include increased soil health, reduced tilling, seeding and fertiliser application costs, and overall increased efficiencies.

For the highest precision systems, with accuracy to better than ±2cm, the GNSS is often complemented with additional ground-based signalling equipment which can be from field portable transmitters and receivers or via mobile telephone masts. Such systems also benefit from on-vehicle wheel-angle position sensors (WAS) that provide steering position information to a guidance ECU as part of the hands-free steering servo loop.

As an integral part of a newly-developed high-accuracy CTF system that is easily adapted to a wide range of farming vehicles Precision Decisions has called upon Variohm EuroSensor to supply TX2 series potentiometric linear position sensors for wheel angle measurement. The highly durable IP67 rated linear sensor, from Variohm’s distribution partner Novotechnik, was selected as it offers many advantages over more traditionally specified rotary transducers that are externally mounted and require pushrods and associated linkages which contribute to increased backlash and linkage wear leading to inaccuracy and lost motion.

The compact TX2 is directly fixed to the steering arm using its backlash-free pivot head mountings without the need for additional linkages - and with a generous angle of free movement of up to ±12.5 degrees to compensate for misalignment, is easy to install and commission. With eight available standard measuring ranges from 25 mm to 300mm, input voltage up to 42VDC, independent linearity specification of 0.05% and positional repeatability to under 10 microns, the sensor provides the precision and the installation flexibility required to mechanically and electrically adapt to the steering systems of the widest possible range of vehicles. In this application the TX2 is typically powered with 5VDC to derive a 0–5 V output to a sophisticated PVED computer controlled hydraulic steering system which is capable of providing millimetre level accuracy.

The fully protected sensor is suited for adverse environments where its use is aided with a double sealed actuating rod and metal mounting flanges for maximum protection against dirt, dust, oil and humidity. The TX2 series has a rated life of more than 50 million movements and employs Novotechnik's conductive plastic resistance element technology and elastomer damped multi-fingered precious metal wipers to maintain complete contact under extreme shock conditions that are specified to 50g for 6ms, with vibration durability.