Displacement sensor lifts tractor performance

Ostfildern, Germany-Ever since they replaced horses as the agricultural prime mover, tractors have continued to develop. One of the major advancements in the history of the tractor occurred in the 1920s with the three-point power lift, which enabled the application of a constant pulling force while providing the best maneuverability. This configuration has proven so successful that it is still used today, although it has been refined and improved. However, one drawback of this design is that the plow was taken as the principal attachment used, with pulling force the main control quantity. This meant that the lifting kinematics and guidance required for other agricultural attachments was far from satisfactory.

One solution to this problem is a hydrostatically controlled upper swing-arm with active length control, offered by GKN Walterscheid. Using one linear displacement sensor from Novotechnik Stiftung & Co., the swing-arm hydraulic cylinder supplies an accurate length signal. When this length signal is combined with the signal from an angle sensor on the lower swing-arm, it is possible to fully automate attachment control using the three-point linkage. The farmer simply sets the position of his attachment to suit the job, and saves angle and length as a pair of values in a task computer. This same procedure is used to define the attachment position when turning at the end of the field. The farmer then plows his field or sows his seed recalling the appropriate attachment position by pulling a lever in his cab.

In this application, Novotechnik's displacement sensor works on the potentiometric principle, but without making actual contact to the resistance track. Instead of the normal contact present in conventional linear potentiometric sensors, a moving probe samples the position along the linear resistance track. The probe is itself capacitively coupled to a stationary collector track to complete the circuit. The electronic unit, located in the base of the hydraulic cylinder, supplies the alternating voltage required by the system and evaluates the signal before passing it to the control unit of the automatic power lift.

The displacement sensor, which has a stroke of 10 inches and a specified resolution better than 0.0004 inch, must be rugged enough for farm work. Since the same oil circuit supplies tractor transmission and the hydraulic cylinder, contamination of the oil can be expected. Dust, sand, wide differences in temperature, and high dynamic loads when the tractor crosses hillocks and other obstacles, are all part of the agricultural environment. Such arduous ambient conditions restrict the choice of displacement sensor.

Commenting on this aspect of the power lift design, Uwe Kupka, product manager at Novotechnik, says, "In this application, inductive or ultrasonic displacement sensors for integrating into hydraulic cylinders are just too expensive. And contacting potentiometer technology doesn't perform so well under oil, particularly where fast movements are involved. But with the non-contact potentiometric sensor, we have a rugged, inexpensive, and reliable sensor that just fits the job."