Wheel speed algorithm improvement

Software Releases

July 30, 2010

200_pegasem_wheel_encoderOxTS has released a significant improvement to the wheel speed processing in the Inertial+. This improvement is designed to make the position output of the Inertial+ even better in poor conditions where GPS is still available but can have large errors on it. Often these conditions occur just before an obstruction, upsetting the Inertial+ just before GNSS position is removed. By improving this situation the drift in tunnels is reduced. The algorithms improvements will filter through to the RT products when the next RT software is released.

To make this improvement the wheel speed state has been added to the Kalman filter in the Inertial+. Previously the Inertial+ only used the wheel speed to update the Kalman filter when the external GNSS was not available. Now the wheel speed is used all the time and this lets the Kalman filter remove more of the poor GNSS measurements. The older processing platform in the original RT products was not fast enough to have the wheel speed state as part of the main Kalman filter but the new, faster processor used in the Inertial+ product does not have a problem with the extra processor load caused by this new state.

Wheel speed is an excellent addition to the Inertial+ since it makes a significant difference to the drift. Our tests show that the Inertial+ drifts by about 2.5m per kilometre (or minute) in “ideal” conditions when using wheel speed. When there are significant errors in the GNSS position and velocity before an obstruction, like a tunnel, then the drift can be higher. Using the new wheel speed this is reduced so that we should be closer to our specification in non-ideal conditions. Quantifying this exactly is very difficult.

Wheel speed is an effective way of reducing the drift. By contrast, the drift after 1 minute without wheel speed can be more than 10 times larger. The wheel speed input is integrated in the Inertial+ electronics so that it is easy to incorporate a wheel speed in the inertial navigation system and obtain great results.

Image: Pegasem wheel speed encoder, perfect for the Inertial+, see www.pegasem.de.