This screen is used to set up various general parameters in the SPEEDBOX, including low speed clamping of the speed signal, direction detection and the local time offset from GMT.
Select speed source
This setting selects whether the speed source used for the distance calculation and the pulse output is taken from the 200Hz combined speed calculated from GPS and accelerometer data, or from the 20Hz GPS only speed. It is normally recommeded to use the default combined speed option. However, there are some circumstances in which GPS only speed may be preferred:
- Where there are pitching movements that may potentially confound the combined speed algorithm, such as on a small boat in rough seas, the GPS only option may give better results.
- Where distance measurements are being made that include violent swerving manouveres, and where it is not possible to mount the SPEEDBOX base unit directly underneath the GPS antenna, the GPS only option may give more accurate speed and distance measurement through the swerving sections.
Note that firmware v3.5.2 or later is required for this option to apply to the distance measurement; for earlier versions of the firmware it applies to the pulse output only. Also note that contrary to the caption in the software this option does not apply to the analogue output, since it is now possible to select either combined or GPS speed for each analogue output independently in the analogue setup section.
Combined Speed Parameters
A low speed clamp can be applied to the speed signal, this will remove the tendency to get very low speed readings when the vehicle is actually stationary. This is very useful for applications where the vehicle will not be moving for prolonged periods of time but the distance measurement is important. Without clamping the speed there will be a very gradual accumulation of error in the integrated distance during stationary periods.
Entering a value of zero will disable this option.
Entering a non-zero value will cause the speed clamp to be applied when the speed drops below the entered clamp value. The speed clamp will have the following effect:
- The speed will be output as zero below the clamp threshold.
- The integrated distance will use the clamped speed, and hence not increment when the speed is below the clamp threshold
- The heading will be held constant at the last non-clamped heading value when the speed is below the clamp threshold
- The gradient will be held constant at the last non-clamped gradient value when the speed is below the clamp threshold
Enable direction detection
With direction detection turned off all speed readings will be positive. With direction detection enabled the SPEEDBOX will output a positive speed value for forwards movement and a negative value for backwards movement. Only the combined speed is affected, any GPS only based measurements will always show a positive value.
NOTE: We recommend that direction detection is turned off if using the pulse output whilst testing hard speed transients close to zero, such as brake testing to a halt.
Time offset from GMT
This value can be set to the local time offset from GMT. It is re-transmitted in the Race Technology GMT time message and the NMEA GPZDA message. This enables the user local time to be correctly interpreted from these messages, and may be useful, for example, if the time is used for file timestamping. Currently only integer hours are supported.