Scalable Control Architecture for Variable-Rate Turn Compensation

The objective of this study was to determine if a CAN bus could be used to implement variable-rate turn compensation in a manner that is scalable by encoding application rates for an entire implement into a single data message. A variable-rate turn compensation test fixture was developed that used a CAN bus to communicate application rates to 16 individual nodes using a 2-byte data message (80-bit extended identifier CAN messages). The system assumed that the physical structure of an implement was linear and that the control nodes were equally spaced. Application rates for the outer-most nodes were broadcasted and the remaining nodes calculated their application rate using a linear interpolation method. Node locations were determined using a 4-bit binary thumbwheel switch located at each control node, allowing all nodes to run an identical program. Servo-controlled gauges were used to visualize node application rate across the test fixture. A joystick interface was developed to simulate vehicle movements and desired application rates. The system transmitted Bluetooth serial messages at a rate of 20 Hz, which were received by the test fixture and converted to CAN messages before being broadcasted to the control nodes. Two USB to CAN interfaces were connected to the CAN bus to insert additional traffic and measure bandwidth utilization. Due to the minimal amount of bandwidth required (<1%) to transmit variable-rate control messages, the system functioned properly when the CAN bus was heavily loaded with traffic up to 99% of the available bandwidth of 250 kbps. The variable-rate turn compensation test fixture demonstrated that a CAN bus is a suitable protocol for communicating variable-rate data. The scalable encoding technique developed in this study resulted in a single message required to update all nodes, regardless of the number of nodes in the system. The system has broad applicability in future planting, fertilizing, and chemical application systems where deposition points are evenly spaced along an implement.