5.3.4 Fob Calibration Overview

In a PEPS system, the learned fobs receive the LF signal and measure the magnetic field strength as an RSSI value. This value is reported to the vehicle and determines the fob’s position with respect to the transmitting LF antenna. To ensure RSSI accuracy, the fobs must be calibrated, including normalization and compensation, along all the three LF antenna coil axes (X, Y and Z-axis).Typically, this calibration is performed during the final test using high precision equipment at the fob’s manufacturing facility. However, with the help of the CARS PC application, the end-of-line calibration sequence is approximated by clicking the Calibrate Fob button in the System Configuration window.

The normalization procedure is used to establish a relationship between a known magnetic flux density and a measured RSSI value. Ideally, an Helmholtz coil is used to perform this task but, considering the constraints of the kit, this is not feasible. Instead, the RSSI value for each fob’s LF antenna coil axis (X, Y and Z) is measured at a fixed distance (50 cm) from the transmitting LF antenna. Arbitrarily using the x-axis as the reference, the difference in measurements of the other two axes are stored in the EEPROM as the Normalization Constants. On the other hand, the compensation procedure accounts for any non-ideal influences; for example, magnetic flux disturbances due to adjacent ferrous bodies, temperature, aging effects and so on. The compensation procedure is based on the following:
  • “Ref. RSSI” – Denotes the internal RSSI values measured with reference conditions (end-of-line) [1]
  • “Int. RSSI” – Denotes the current internal RSSI measured with no external LF signal present [2]
  • “Ext. RSSI” – Denotes the current external RSSI measured with external LF signal present [3]

The actual RSSI amplitude is, then, calculated by adding the error term [1] – [2] to the values measured at [3]. The normalization and compensation procedures ensure that a constant RSSI is reported, regardless of the fob orientation, and ensures accuracy over a large set of influences.