Self driving tractors using linear position sensors

| Manufacturing

Sensors provide high precision agricultural steering

Variohm Eurosensor is supplying high precision linear position sensors for the steering systems on GPS guided agricultural equipment.

Precision Decisions specialises in supplying hardware and software for agriculture that maximises yield potential through the use of Global Navigation Satellite System (GNSS) automatic vehicle guidance. The Yorkshire based company is 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 reduces overlaps and travelled ground in fields to provide farmers with significant benefits that include increased soil health, reduced tilling, seeding and fertilizer 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 300 mm, input voltage up to 42 VDC, 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 5 VDC 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 well 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 unique 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 6 ms, with equally impressive vibration durability.

Precision Decisions’ After Sales Support Manager, David Hart, is very pleased with the performance of the TX2. “The accuracy and repeatability of vehicle position within the field is paramount in order to obtain correct machine placement for any operation, not least controlled traffic farming. Precision Decisions has chosen the Variohm EuroSensor TX2 linear transducer on the grounds of accuracy, robustness and simplicity of construction. It is also cost effective.” Hart continues his evaluation. “The unit has proven simple to fit to a broad range of tractors and has first class positional accuracy. On-line steering acquisition and position interpretation is rapid, smooth and without oscillation.”

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