What is an electronic compass with gyro and why do you need one

The most important part of an autopilot is the heading reference or compass. How good an autopilot can steer the boat is of course very much dependent on the in-data it gets from the compass. We will try to explain today’s best practise of building compasses here and why the last generation of TMQ compasses are better than most other autopilot compasses.

Most of you maybe familiar with a fluxgate compass so we start with that. That’s what all autopilots used to have before, and still use in some cases. A fluxgate is a device that employs two or more small coils of wire around a core of highly magnetic material. By applying a voltage over some of the coils and measure the voltage over other coils the horizontal component of the earth magnetic field can be measured. However there is also a vertical component of the earth’s magnetic field and the more close to one of the poles you are the more relevant this component is. You can test that by tilting the coil of a fluxgate and note how much the course changes. When you hold it flat you get the correct magnetic course, when you tilt it you can see the course change by more than some 30 degrees. To avoid inaccuracies created by the vertical component, the fluxgate coils must be kept as flat as possible by mounting it on gimbals or using a suspension system. Is has to be maintained in a horizontal position relative to earth as much as possible. All the same, errors are inevitable when the vessel is turning sharply or being tossed about by rough seas. Only under very good conditions or near the equator where no vertical component is present these types of compasses works perfectly. Another problem is that due to continuous movements of the inner parts a fluxgate compass will eventually wear out. The first generation of TMQ compass were a gimballed fluxgate compass.

To avoid these problems many autopilot manufacturers have complemented the fluxgate with small 1-axis gyros that can measure the jaw of the boat. These gyros are small electronic devices. Gyros measure angular velocity, how fast something is spinning about an axis. So by integrating the values you get from the gyro you can get information of the heading of the boat. Problem is that the 1-axis gyro introduces a lot of errors which will accumulate over time so for that reason you still depend on the fluxgate to correct the gyro. Gyros are short time stable while the fluxgate is more long term stable. This is what most manufacturers of autopilots use today.

The latest developments in the aircraft and spacecraft industry has today made it possible to build fully electronic compasses. These are based on Intertial Measurment Units IMU which are built on the latest sensor devices. They consist of 3-axis accelerometers and 3-axis gyros. The IMU is complemented by 3-axis magnetometer or GPS to form what is called an inertial navigation system. All of these components are fully electronic chips with no moving parts. For those of you that wants to dive into all technical details can read more in Wikipedia here.

The new ELECOM compass from TMQ is based on this latest technology from the aircraft and spacecraft industry. Besides an inbuilt 3-axis magnetometer it has 3-axis accelerometers and 3-axis gyro sensors to give optimum performance. This means 9 sensors in total. Combined they give an outstanding course reliability under any weather situation and at any speed. It does take a lot of computing power to handle all these sensor readings and combine them into an accurate heading ten times per second so the ELECOM compass also has a powerful inbuilt micro processor. The more computing power you have the more advanced filtering and calculation you can make.

There is one important aspect of this technology. It’s the accelerometers and gyros that outputs the heading information to the autopilot. The magnetometer is only used to make it long term stable. Otherwise the compass would slowly drift away. Since magnetometers measures the earth magnetc field they are not suitable on steel ships which themself can have a magnetic field. For this reason you can use a GPS to make the compass long term stable and the ELECOM compass has such an inlet for GPS data. By connecting your GPS plotter to the compass you get the perfect compass for steel ships. When the boat speed is over two knots the compass will take the information from the GPS and under that speed it will take the information from the magnetometer. More information on the use in steel ships you can find here.

There is one more important aspect when choosing an autopilot compass and of course autopilot. The first generation compasses had an analogue output that was proprietary to each brand of autopilots. So when after some years it got broken there where no replacement for the unit. Today’s new all digital compasses have a standard NMEA output. They can be easily replaced and there wil be many different models to choice from. You don’t necessarily have to buy the same as before since they all use the same interface. With prices going down and performance going up every year for electronic sensors you might be able to update your autopilot in the future with modern technology without having to buy a complete new one.