Sensor Solutions

Inductive Application Brief

The SSP1492 (Product Brief) can interface directly with miniature inductive magnetometer sensors (such as PNI Corporations shown in Figure 2) and perform the high level math functions necessary to create a tilt compensated Compass application. The internal 8051 microprocessor engine along with sensor acquisition and math function built into the SSP1492’s internal ROM does most of the work. The macro program stored in the serial EEPROM simply prepares the data for, and calls upon the internal ROM functions to do all the heavy lifting.

Alternatively, the steps can be directed by the users host controller thus eliminating the need for the EEPROM. All ROM functions are accessible as commands sent to the serial interface from the host or are callable from the user’s macro program. Figure 1 is a schematic of a tilt-compensated compass application. This one includes the EEPROM for macro storage, which means the microcontroller only needs to issue a start measurement command and then retrieve the resulting heading, pitch and roll values directly in radians or degrees when the conversion is complete.

Figure 1 Click to enlarge

Figure 2

COMPASS click to enlarge

With a single resistor (RG), the SSP1492 sensor interface produces the DC bias currents necessary to operate these sensors in their optimal linear region. In order to provide the highest accuracy sensor measurements, the SSP1492 supports the calibration and correction for individual sensor’s offset, gain and temperature correction coefficients within its EEPROM and ROM routines. This additional sensor calibration infrastructure ensures that compass module can achieve the highest heading accuracies possible given any particular set of magnetic and acceleration sensors.

When executing a read, the SSP1492 gathers the 3 magnetic (B) and 3 gravity (G) field measurements (for X, Y and Z) thus creating two 3-dimensional vectors—one of gravity and one of the Earth’s magnetic field. These two vectors and their resultant cross-product are used to form the basis of a three-space that is aligned with respect to the Earth’s reference frame. Next, the component of the Earth’s magnetic field projection onto the tilt-corrected X-Y plane (North and East plane) is calculated by a dot-product operation, so that a tilt independent heading can finally be calculated using the SSP1492’s built-in trigonometric arc-tangent function. All the above operations are internally supported by the SSP1492’s hardware math engine and pre-programmed floating-point ROM functions.

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