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1. NovAtel APN 036 Rev 1 January 31 2007 Application Note on Using a Wheel Sensor with SPAN NovAtel s SPAN system supports wheel sensor updates The wheel sensor also known as the Distance Measurement Instrument DMI is external to the SPAN system SPAN with the iMAR iIMU FSAS IMU has an optional wheel sensor the iMWS that is fully integrated Other SPAN IMU variants for example HG1700 and LN 200 and users requiring a higher resolution wheel sensor require extra hardware not supplied by NovAtel to integrate a wheel sensor SPAN accepts properly formatted WHEELVELOCITY commands at 1 Hz derived from a user supplied wheel sensor system It is the user s responsibility to issue the WHEELVELOCITY commands to the receiver Using the WHEELVELOCITY commands SPAN creates an update measurement for its filter and can output on request TIMEDWHEELDATA and WHEELSIZE logs The SPAN Technology for OEMV User Manual describes the commands and logs in this application note User manuals are available from our website at http www novatel com support docupdates htm WHEELVELOCITY Command Format The WHEELVELOCITY command in either ASCII or binary format inputs wheel sensor information to the OEMV or OEM4 receiver Send this command through one of the OEMV COM ports with receive INTERFACEMODE set to NovAtel Timing and Frequency Typical wheel sensor hardware accumulates wheel ticks constantly as the wheel rotates The SPAN interface2. configured to accept cumulative tick counts at a rate of 1 Hz aligns with the GPS one second boundaries The GPS second boundary is available from the OEMV 1PPS pulse Use this pulse to trigger DMI hardware to send the accumulated tick count back to the OEMV through the WHEELVELOCITY message SPAN does not accumulate raw measurement ticks from a wheel sensor device Additional hardware is required to accumulate the tick counts and pass the accumulated count to the SPAN system at 1 Hz triggered by the 1PPS If you are using the iMAR FSAS with the iMWS option the wheel sensor integration is done for you If the iMWS is connected and powered SPAN automatically uses the wheel sensor information and provides TIMEDWHEELDATA and WHEELSIZE on request DMI Update Logic The SPAN system receives the WHEELVELOCITY command through the COM port and applies a time to the message based on the time of the last 1PPS pulse and the latency reported in the log This timed data passes to the SPAN filter to perform the update The timed data is available to the user through the TIMEDWHEELDATA log Use the TIMEDWHEELDATA log to apply wheel sensor updates in post processing 1 of 2 APN 036 Rev 1 January 31 2007 When wheel sensor information is available the SPAN filter includes a wheel scale factor state The wheel circumference is estimated accurately throughout the run because of the wheel scale factor state Wheel circumference can change due to tire pressu
3. escriptions of these logs Final Points If you require further information regarding the topics covered by this document please contact NovAtel Customer Service 1120 68 Ave N E Calgary Alberta Canada T2E 8S5 Phone 1 800 NOVATEL in Canada or the U S or 1 403 295 4500 Fax 1 403 295 4901 E mail support novatel com Website www novatel com 2 of 2
4. re changes changes in dynamics or because the setup has changed Also if you do not have an accurate measure of the ticks per revolution and or wheel circumference the wheel scale factor state adjusts for that and allows you to use the wheel sensor information effectively The SETWHEELPARAMETERS command lets you set the number of ticks per revolution the nominal wheel circumference and resolution of your wheel sensor The resolution of the wheel sensor can be approximate as it uses it to weight the wheel sensor information The conversion of tick counts to a distance in metres uses the ticks per revolution and nominal wheel circumference If you do not use the SETWHEELPARAMETERS command the default ticks per revolution is 58 and the default wheel circumference is 1 96 m If these are close to your actual setup the estimated wheel scale factor available in the WHEELSIZE log is close to unity If the scale factor is much different from unity it means the wheel parameters you entered or defaulted to do not represent how your setup actually is The wheel sensor information is not useless in this case but it is not optimal It means the scale factor state has much more to model rather than just modeling a residual amount The modeled wheel circumference is available in the WHEELSIZE log Information on how the wheel sensor updates apply in real time is available in the INSUPDATE log Refer to the SPAN Technology for OEMV User Manual for the d
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