Covering large fields, potentially across varied terrain, and with sufficient accuracy to drive a tractor semi-autonomously (auto steered) down exactly the same tracks between furrows laid when the crops were sown requires extreme precision. The reasons behind this approach include increased density of crop sowing, and hence yield and, when used in conjunction with other satellite-based field moisture and crop monitoring applications, the ability to irrigate, spray insecticides and fertilizers in the most cost-effective manner possible.
Achieving such precision control however raises two technical challenges.
Firstly, the accuracy of the derived GPS data needs to be improved such that the repeatable accuracy is better than +/- 15 cm and, secondly, that changes in terrain and the tractor’s exact position over the ground can be determined.
Achieving the positional accuracy has been made possible using a technique called carrier-phase enhancement (CPGPS), also named real time kinematic (RTK). This approach measures the phase of the GPS signal carrier rather than the timing information content contained within the signal and many suppliers are quoting 3 cm repeatable accuracy using RTK.
Being able to position a tractor to this degree of accuracy is impressive but there is one fundamental problem. Measurement of the GPS data really means that you know precisely where the antenna is situated, and typically this is in the centre of the cab roof.
Achieving positional adjustments are an ideal application for MEMS-based accelerometers and gyroscopes. Recent advances in MEMS technology has resulted in significant improvements in stability, noise and immunity to mechanical shock, all of which are vital for such precision agricultural applications involving moving heavy machinery over undulating terrain. In this way the slope, direction of movement, and the change of the slope can be accurately measured. An accelerometer is used to measure the angle of inclination relative to the Earth’s gravity. Due to the accelerometer's measurement principle it is also disturbed by the machine’s natural movement and motion, making the inclination information imprecise. The gyroscope measures angular rate (output in degrees per sec), and therefore can be used to detect the change of inclination angle. In moving machine applications, such as in a large tractor, use of either an accelerometer or gyroscope alone are not enough to provide precise inclination information, but combining the two with an advanced algorithm gives accurate inclination information. The principle is that an accelerometer is used for the static inclination angle and the gyroscope output is used to compensate for tractor motion seen in the accelerometer signal and so enabling dynamic inclination measurement.
Bringing the GPS and RTK data together and processing the data acquired from the accelerometer and gyroscope will take place in an inertial measurement unit (IMU). Typically a combined accelerometer and gyroscope sensor such as Murata’s SCC2000 series gyro accelerometer combo MEMS sensor device will be used in the IMU. The output from the IMU will be used to drive the tractor’s steering mechanism, either via hydraulic controls or via a drive servo that uses a mechanical coupling to the tractor’s steering wheel. Kalman filter algorithms are