kubler articles

One Step Better: Powerful encoder use in continuously variable tractor transmissions

Kubler 4/1/2016

One Step BetterAgricultural machines apply their power and performances with highest precision and accurate dosage. The suitable drive technology ensures the stepless "sensitivity." Within a servo actuator, Kubler encoders help the used CVT gearboxes to make the defined high-torque motion sequences even smoother. This allows more accurate driving of the tractors. 

Continuously variable drives are not appreciated in an automotive stronghold like Germany. This may be due to the "rubber band effect." This phenomenon generally appears on small cars, translating in a yowling engine noise reminding a sliding clutch. It is generated by - very simple - continuously variable transmissions when the engine speed is ahead of the speed during acceleration. This noise of overstretched rubber may also be the reason why Mercedes, Opel and many other manufacturers totally abandoned the continuously variable transmission, in short CVT gearbox, for passenger cars.

But the picture is very different here when it comes to utility vehicles. For over 20 years, CVT gearboxes are an integral part of mobile machinery. Among all users of continuously variable transmissions in the area of working machinery, agricultural tractors have been the forerunners. They benefit from all of the many positive features, since the performance quality of the power-split gearbox is undisputed according to the mechanical and hydrostatic principles. In these branches, the major tractor manufacturers themselves brought this technology to maturity. 

Encoders as a part of the communication interface 

Two of the major tractor manufacturers are using Kubler encoder in their CVT systems. The encoders are used here in a special branch with highly specialized requirements. The opinion that a modern tractor includes more high-tech than any S-class car runs through this branch like a red thread.

While a car is simply "driving," the agricultural technology is accomplishing herculean deeds: tractors brave the most adverse weather conditions and plough loamy and stony grounds. They heave, tow or cut tons of crop. In brief: encoders for such applications cannot be found in the catalog. Both encoders that ensure today reliable positioning in modern tractors are the result of extensive development projects - based on strict customer specifications.

The used encoders contribute significantly to machine performance; they are integrated in the servo actuator, which is made of components perfectly matched to one another, electrical motor with gearbox and encoder. This is where the accuracy and the intelligence of the machine come together. Together, the three components of the actuator form the communication interface.

For mobile machinery, the technical goal is to combine efficiency and energy consumption improvement in relation with the maximizing of the so-called area performance, that is to say the performance at the work systems and longitudinal drive. In this project, the real challenge on the way to the suitable encoder consisted in a compromise. The balance between precision, that is to say high accuracy and repeatability, together with total insensitiveness to magnetic fields and resistance against shocks and continuous vibration.

A wide temperature range and reliable protection against moisture and humidity were also part of the basic project requirements. A servo actuator converts the signals of a controller in the required gearbox ratio in the vehicle. Here, the precision gearbox and the servomotor merge with the encoder to form one single unit. Used in tractors, it must master reliably the whole temperature and supply voltage range thanks to its torque and torsional rigidity. This task is made even more difficult by the fact that the values must be provided and represented under varying load situations. 
 
Encoder solution for ideal integration 
 
In addition, a servo actuator is part of a global solution. This complex task required a group of highly specialized companies that worked jointly to provide the impetus for this product innovation, up to series-production readiness. Besides the sensor and electronic suppliers, the project involved the developers of the electromechanical section, of the software and of the motors. The result of this joint work: the CVT gearbox ensures acceleration and deceleration processes almost without adjustment times. Tough environmental conditions with extreme temperature ranges and strong vibrations place very particular demands on the encoder solution.

The highly accurate optical singleturn encoders specially developed by Kubler for use in the servo actuators of agricultural machinery are based on the combination of two interfaces: an incremental one with 2000 pulses and the parallel 5-bit Grey code scanning. Thanks to their high optical resolution they provide accurate measured values and, at the same time, they are very robust. Singleturn resolutions reaching 17 bits are usual: for the desired actuator, a 13-bit resolution was ideal. The customer-specific mounting options for the flange and the shaft complete the very compact and slim encoder construction designed especially for the integration in the actuator.

Encoder solution for ideal integration 
 
The special features continue with the cable, which is flatter than usually. Within the actuator, the encoder converts with high accuracy a CAN bus instruction into the angular position of the axial piston pump. The instruction is triggered by a joystick in the driver's cabin. The angular position determines the hydraulic oil throughput. This oil quantity regulates steplessly the speed of the hydraulic motor, and thus gearbox ratio. In addition, a particularly robust housing protects the highly accurate electronics from vibrations, heat and humidity. 

Henry Ford said about teamwork, at a time when this notion was not fashionable: "If all single men move forward together, we will not have to be concerned about success". Large projects that start from customer requirement and eventually lead to an own production island, can only be realized in close cooperation among all departments and in perfect communication within the team. As a components manufacturer for the latest tractor generation of two world-wide renowned producers, Kubler passed through the whole production part approval process, the so-called PPAP, based on strict automotive standards. This is a structured sampling process for series parts in the automotive environment that defines comprehensive requirements for the advanced quality planning.

All project participants within the Company contributed to the various single steps: from sampling preparation, through accompaniment of the production operations, up to changes approval and coordination - always in direct consultation with the other suppliers involved. At Kubler, the Project manager is the main interface to the customer in this context. The inter-divisional team made of designers, electronic developers, QM management - always supported by their colleagues of the work preparation and purchasing departments, was also taking part at the milestone reviews, even on site.
 
Safe service 
 
Within the drive, the actuator, being a monitoring interface, has a great relevance also from the safety point of view. Accidental adjusting movements, which can lead to machine motion from standstill, or even adjusting movements not carried out, which in the worst case can result in braking processes not performed, belong to the sources of hazards that can lead to serious injuries and damages to the machine.

The communication between the gearbox controller and the adjusting unit is based on a safety-oriented CAN bus protocol. It integrates many diagnostic functions and plausibility checks, such as temperature, voltage and current monitoring. Within this process, safety comes from the correct understanding of the requirements. Relying on its many years of experience, Kubler can support its customers for all functional safety topics. In addition, this sensor specialist offers a comprehensive portfolio of services for all safety aspects.