The magnet can be attached to a flotation device so that the angle of the magnet changes with the height of the float arm. This setup, shown in Figure 2-8, works with a very small magnet compared to the other designs and can be used to measure any size of liquid tank simply by changing the dimensions of the float arm. This implementation only requires one sensor. The sensor can be placed on-axis with the magnet or off to the side.

Figure 2-8 Pivoting Magnet Float Arm Implementation

Common measurement topology include angular position measurements in on-axis or off-axis angular measurements shown in Figure 2-9. Select the on-axis measurement topology whenever possible as this offers the best optimization of magnetic field and the device measurement ranges. The TMAG5273 offers on-chip gain adjustment option to account for mechanical position misalignment.

Figure 2-9 On-Axis vs. Off-Axis Angle Measurements

Figure 2-10 shows example X and Y sensor data for on-axis measurement, where the strength of the two axes are equal. Figure 2-11 shows example sensor data for off-axis measurement, where the Y-axis is half the strength of the X-axis.

Figure 2-10 X and Y Sensor Data for Full 360 Degree Rotation for On-Axis Measurement

Figure 2-11 X and Y Sensor Data for Full 360 Degree Rotation for Off-Axis Measurement

Traditional vehicle fuel sensor systems use a rotational float sensor implementation. These conventionally use a foam piece on an arm attached to a resistor wiper. As the float drops or rises in height, the resistance of the wiper changes. Current is passed through the wiper resistance and into a heating coil around a bimetallic strip. As the bimetallic strip heats up it bends one direction, and moves the fuel gauge needle. This system can't compensate for the shape of the fuel tank, and is suscpetible to wear and tear of the resistor wiper and bimetallic strip. It also creates a non-linear behavior in the fuel gauge, where the tank looks full for a while before the fuel level starts to indicate it is decreasing and then the fuel level drops faster as the tank empties. Modern vehicles may use a microcontroller to read a voltage from the wiper resistance and display the fuel level digitally.

A float arm paired with a magnet and hall sensor can provide a low-cost, single-sensor design to liquid level sensing.