Optimising the output of the RT

1. Introduction:

The way that out units calculate and output slip angle has the following major dependencies:

• Good heading accuracy
  • Antenna separation (Dual antenna systems)
  • Speed of vehicle (Both dual and single antenna systems)
• Correct initialisation
  • Dynamic via driving in a straight line
  • Static via the dual antenna heading solution
• Warmed up settings (e.g. RT-to-Vehicle orientation)
• Velocity accuracy

 This guide will go over how you can ensure the above factors are optimised

2. Pre-test procedure

Go through the following procedure step by step in order insure that all the elements required for the slip angle to be output as accurately as possible are in place.

2.1 Configure the RT as normal

The RT should be installed and configured according to the instructions in the RT User Manual.  Take care with the following aspects:

• When inputting the measurements for the location of the antenna, they need only to be accurate to about 10 cm
• You can make more accurate measurements to reduce the warmup time, but make sure to not input accuracy that are higher than what the actual measurement technique will allow (i.e. don't overestimate the accuracy)
• Ensure that Advanced Slip settings have been configured in step (see page 60 for further details) the "Get Improved Settings" feature (see page 55 for further details) will not be able to extract the RT orientation settings:
  
• Videos also available here and here to help the procedure

2.2 Initialise the system

Ensure that the RT has a GNSS position and velocity solution at the maximum precision available before continuing to the next phase.

2.2.1 Single antenna set up

• Accelerate straight forward until your speed exceeds the configured initialisation speed
• To prevent a heading offset and thus a slip offset from being observed, make sure you are driving in as straight a line as possible

2.2.2 Dual antenna set up

If you have a dual antenna system you can opt to initialise statically using the option on the Secondary

Antenna page. Take care of the following factors: 

• Make sure the antennas have a good separation, a minimum of 1 m is recommended (you can see the full effect of this in section 3.1)
• Measure the separation between the antennas as accurately as possible—preferably 3 mm accuracy
• Make sure both antennas have excellent visibility of sky (obstructions to the antennas could cause delays initializing statically or even initializing statically with an incorrect heading)
• Make sure each antenna in mounted on a minimum of 10 cm radius of metal to act as a ground plane (i.e. not next to a car sun roof). This is to counter GNSS multipath signals:
  
• Make sure the antenna cable exits the antenna body in the same direction:
  

2.3 Warm up

• Perform a warm-up as display as below accelerating and braking at different points (see pages 95 to 98 of the RT User Manual for further details) until the unit is performing at its specified accuracies.
  
• Going in both directions on these paths is important to make sure the IMU is excited in each direction
  
  

In NAVdisplay, the warm up template can help check that the unit is warmed up and ready to use. There are widgets that show the accuracy of the: Heading, Pitch, Roll and Position and others. If all the accuracy numbers are all green and remain constant the unit is warmed up. 

In the picture bellow the velocity accuracy north, the 0.01 on the right, is green but the heading accuracy, the 2.329 in the middle, is in orange. The unit is not yet warmed up .

You can select these from File>Open>Templates>Browse, and then select the template from:

C:\Users\userconfig\Documents\OxTS\NAVdisplay\Templates 

Once this is complete make sure to not turn off the system until the next phase

2.4 Get Improved Settings

This is an overall adjustment that will improve the RT orientation (if Advanced Slip was used) and GPS antenna locations.

This step will save the warm up settings that you have reached at the end of section 2.3. You can do this by:

• Opening NAVConfig and navigating to the orientation page. Here you can click "Select Get improved settings" on the orientation page:
  
• Select "Read configuration from Ethernet" and select the relevant improvements, and then complete the configuration as normal and commit the settings to the RT
• The selected improvements will have then been applied to unit, and the warmup will have been saved
• This only needs to be done once after a warmup, and will allow you to turn off the system without losing the improvements from the warmup session

2.5 Zero the offsets

2.5.1 Slip offsets

This setting adjusts the RT orientation to line up better with the vehicle and give the desired slip angle. Note:- Advanced slip needs to be configured in order for this to work

The "Get improved settings" would have corrected any slip offsets that RT is seeing but you can double check this by going to in NAVdisplay/Enginuity>Quick Config>Slip Offset. You can then zero the offset if there is one.

2.5.2 Orientation offsets

The RT will measure orientation with respect to gravity so in order to correct this for the test area that you are working in to correctly display these with respect ground you can.

• Drive to a location known to be level, with no slope and no camber, and then zero the offsets on the orientations by going to to NAVdisplay/Enginuity>Quick Config>Pitch/Roll Offset
• When doing this, you may choose to apply them to measurements only until the unit is reset, or to commit them to the configuration, so that they will be saved even if it is reset

3. Influencing factors and solutions

3.1 Influencing factors for slip angle:

3.1.1 Antenna separation, velocity, heading offset and accuracy

Heading accuracy and velocity affect the accuracy of the slip angle as it is calculated based on those vales. You can see how these affect the overall slip angle accuracies (coloured in degrees) in the table below:

Single antenna set ups can be subject to heading drift during stationary periods. The effect of this can be minimised by utilisation of the heading lock feature. This limits the amount that the heading can drift and is illustrated in the diagram below

3.2. Influencing factors for Pitch and Roll 

3.2.1 Tyre wear

Tyre wear is not likely to have adverse effects on measurements unless it is uneven.  This could occur after a sustained period of high-speed laps around an oval, but in most cases, the tyres would be changed before the effect was significant enough to affect the measured roll angle

1.1.1. Tyre pressure

In most testing, tyre pressures will be regularly checked and are unlikely to present a problem during testing.

1.1.2. Driver change

This is a factor that could cause variation, if the weight of the driver differs, or the position of the seat.

1.1.3. Weight change or redistribution

The issue is similar to driver change and could affect pitch and roll angles.

4. Conclusion 

In general, any of the above factors could have a small effect on roll or pitch angle, but are less likely to affect the slip angle, which is calculated in the level plane