By Marcel van Hak, Product Manager, Xsens
Agricultural vehicles are getting smarter every day and that is largely because of motion tracking, achieved with Inertial Measurement Units (IMUs), Attitude and Heading Reference Systems (AHRSs) and Global Navigation Satellite System (GNSS) positioning systems. Where IMUs provide inertial data such as acceleration and rate of turn, AHRSs also incorporate a sensor fusion algorithm to provide orientation. A third category consists of combined IMU/GNSS or AHRS/GNSS systems that provide the highest level of integration. The features and accuracies have different consequences for the overall system and target performance. What are the best design choices for specific applications such as autonomous navigation, stabilization and drones?
While GNSS is widely used in autonomous navigation, it has its downsides when used as standalone technology. GNSS heading is optimal as long as the platform is in a (linear) motion. In most case, you will need to make a turn of exactly 180 degrees at the edges of the field. This cannot be determined by GNSS alone as GNSS heading information cannot be extracted from a rotational motion alone. The heading information can, however, be provided by an IMU or an AHRS. Many IMUs, AHRSs and GNSSs allow integration or can be sourced as an integrated, turnkey system. When the choice is made for a separate GNSS system and a separate IMU or AHRS, the goals must be to choose systems with solid time-synchronization and low latency in order to keep the system stable and accurate.
Using GNSS systems on slopes (as in the slope of the field) holds some challenges. With a height of several meters and antenna placement on top of the vehicle, the actual position of the agricultural tools on the ground differs from the antenna (measured) position when driving on a slope. This discrepancy is almost 10 cm per 1 meter vehicle height at a 5 degree inclination of the vehicle. In order to reap all the benefits of an expensive GNSS positioning system, you will also need an AHRS to compensate for the inclination. A low-cost AHRS with 0.5 degrees roll/pitch accuracy will already compensate sufficiently for the inclination, preserving the position accuracy.
Another use of IMUs and AHRS is steering compensation on a slope or incline. When the vehicle is rolled (i.e., parallel to the incline), the vehicle will steer down if power is applied to the wheels equally. The ‘lower’ wheel must be powered more in order to maintain a straight heading. For this, the incline must be known, and devices with costs of below 100 USD can supply accurate enough roll/pitch estimations for these kind of applications. Watch the video here.
There are two reasons why IMUs and AHRS (with our without GNSS) increase the efficiency of agricultural drones over a full-size airplane or ground vehicle, particularly when it comes to spraying crops. First, with laser scanners it is possible to choose the individual plants that need a little more care. Second, a drone can fly much lower than a regular airplane, wasting less fertilizer or pesticides due to wind or inaccurate placement. An IMU or AHRS, with or without GNSS, is an invaluable resource for navigation, stabilization of the drone and the correction of the laser scanner measurements. Special vibration-resistant gyroscopes are required for good performance.
IMUs, AHRS and combo IMU/GNSS systems are getting more common in all topics of agriculture. You can find the main players in the field by doing an internet search on precision agriculture and IMUs.