Mechanical feelers are relative position sensors that provide a linkage between
the desired trajectory and the vehicle. Several commercial products are available
that use a mechanical feeler to track a ridge, furrow, or crop. Today these products
have evolved to two primary forms. One is a mechanical feeler system mounted on
the tractor or combine, which provides a control signal to an electro-hydraulic
valve added in parallel to the steering system for steering the vehicle. The second
type is based on providing precise lateral control of the implement, ignoring the
deviations of the tractor within some limits. In the vehicle-mounted systems, the
products are typically made to add onto most tractors and combines. The retrofit
includes a mechanical feeler to sense the crop, a wheel angle sensor (potentiometer),
an electro-hydraulic valve steering actuator, and a control module.