By Ryan Taffet
PRECISION MOTION SENSORS
As I find myself out and about in the field lately on a variety of boats, one thing I consistently see is a plethora of different IMUs (Inertial Measurement Units) onboard.
IMUs are used for all sorts of different real-time data to operate such devices as your chart plotters, radar/radar overlay, auto pilot, trolling motor, automatic trim tabs, stabilizers and engine joystick. They sense various vessel motions such as bow-to-stern pitch angle, port-to-starboard roll angle, acceleration, speed, GPS position, rate of turn, and heading. These luxuries all enhance our boating and fishing experience, but only when these units are properly installed, calibrated and maintained.
Let’s first talk about the basics of how these various sensors work. They typically come in the form factor of a “puck”, but they can also be buried inside various black box control modules onboard.
IMUs that receive GPS will communicate with satellites for that information. The rest of the information they collect will come from an array of internal sensors. Their heading capabilities are based off magnetic sensors inside and they can detect motion across multiple axes. They usually communicate multiple data points multiple times per second over a Controller Area Network (called a CAN) onboard your boat, such as N2K for your main electronics or your private engine network. You may find them on other private networks in some select cases.
Now that we have some foundational information about what these sensors are, it is important to know where and how they should be mounted depending on their usage.
IMUs have a certain tolerance for angles and orientation. They often have an arrow stamped on them that needs to point forward for proper heading data. It is best to orient them to be as level to the waterline as possible when the boat is floating. In select applications some manufacturers will prescribe placement in terms of how far forward or aft, and how close to centerline port to starboard. In general, IMUs tend to do better in lower, aft-ward and centered (port to starboard) locations.
You also want the IMU on a stable surface that is subject to as little vibration as possible, as vibration can disrupt the sensors beyond what they can filter. This means that tall standoffs or flimsy brackets are less than ideal. Also make sure your IMU is mounted tight and secure and cannot unscrew or wobble.
Interference also comes into play with IMU mounting. You want to make sure there is no significant magnetic field onboard that will distort the heading sensors in your IMU. There are electromagnetic field test devices and similar phone apps available to test for this.
You want to make sure the IMU is not near any stereo equipment, electronics equipment, pumps, motors, metal tools, or anything else that would create a magnetic field. Also, if your IMU receives GPS feed, you need to make sure your mounting location allows for connection to satellites.
After installation, the rest of the magic is in the configuration and calibration. One of the best practices I recommend on modern multifunction displays is to go into the “device list” (name may vary) and look at the N2K devices connected to your boat’s public network. You will also want to look for “sources” (name may also vary) to see which unit(s) onboard are providing IMU data to your electronics.
You may find with multiple sensors onboard that one performs particularly better than others for heading, and that is the sensor that you want your chart plotter, auto pilot and radar overlay feeding data from if possible.
Being that these are precision sensors, they do require calibration and occasional recalibration. Typically, the heading detection is the main feature that needs to be calibrated. The process often involves entering a “calibration mode” and then driving the boat in a series of one to three slow circles. During this process, the sensor is detecting magnetic heading and working to adjust for any local magnetic field that may impact performance. It is not uncommon to have to add a slight heading offset manually after this process depending on exactly how your sensor is oriented on your boat.
Be sure to test at slow speeds and high speeds to see if the heading looks right. Two easy ways to check are to have a handheld “known good” calibrated compass (like a Boy Scout compass) to compare and to look at the boat icon on your chart plotter relative to your track line on the screen while running the boat. If the boat icon is moving sideways relative to your track line while running the boat at cruise speed, your heading is still off.
Following these best practices will ensure your equipment is functioning properly onboard. Mounting location, orientation, sourcing, and calibration are key. Hopefully, if you are having IMU concerns, this clears up some details to get you squared away.
Ryan Taffet of Stuart, Florida, grew up fishing at the Jersey Shore. When he is not busy as product manager for NAVICO Group, a division of Brunswick Corporation, he’s fishing and boating on his Everglades 240CC Feelin Crabby. The views and opinions expressed in this editorial are only those of Ryan Taffet and do not reflect the views or positions of Brunswick Corporation or its affiliates.
