VBOX Slip Angle sensor The VBOX Twin Antenna Slip Angle sensor is a significant step forward in technology for the measurement of slip angle. Two VBOX 20Hz GPS engines are harnessed together to give an accurate measurement of the speed, position, acceleration, slip angle and pitch angle of a vehicle. The slip angle is measured using GPS alone, no inertial sensors are required. The VBOX Slip Angle sensor enjoys a number of significant advantages over conventional methods to measure slip in that it is very quick and easy to fit, the signal has a fast response.
How does it work?
The most important parameter to measure in the quest for slip angle is the direction that the vehicle is pointing. This is measured by measuring the relative position of the primary antenna and the secondary antenna, and using this information to measure the angle between the antennas relative to true North. In the picture on the right, the angle between the primary and secondary antennas is shown. This is calculated to within an accuracy of 0.1 degrees depending on the separation of the antennas.
The second measurement required to measure slip angle is the angle the vehicle is travelling. This is measured by the primary antenna, and is the normal GPS "heading" measurement or Course over Ground. This is calculated from the direction of motion of the primary antenna, and is derived from the primary components of the doppler velocity (north velocity and east velocity). By subtracting the direction the vehicle is pointing from the direction it is travelling will give you a direct measurement of slip angle.
	Slip angle = (Direction vehicle is pointing) - (Direction of travel)
Where is the measurement made?	All measurements are made at the primary antenna. The primary antenna can be placed almost anywhere on the vehicle, and as long as the distance between the two antennas is fixed, they can be at any angle. If the antennas are placed away from the roof, then an antenna with a built in ground plane must be used. This is usually the case when you are comparing the system with an optical sensor which cannot be mounted within the confines of the vehicle.

What tests can it be used for?
The Slip Angle Sensor can be used to provide measurements for lane change manoeuvres, steady state cornering tests, trailer angles relative to the towing vehicle and any test involving speed, distance, acceleration, slip, roll or pitch angles. The speed and distance are as accurate as a normal VBOXII 20Hz, so you can still measure stopping distances down to 10cm, meaning you have one sensor which does many different kinds of test, and fits in seconds.
What calibration is needed?
To eliminate any offset in angle, there is an auto-calibrate function whereby the vehicle is driven in a straight line for 5 seconds, and any offset in the antenna alignment is measured and then removed from any further measurements.
How does it measure pitch/roll?
The relative positions of the primary and secondary antenna are calculated very accurately by using RTK techniques (RTK = Real Time Kinematic). This is possible by fixing the distance between the antennas, and telling the system what that distance is. This constrains the RTK positioning routines within the GPS engines and gives a very accurate relative displacement between the primary and secondary antenna, including the relative height between them.

The graph below shows the output from the pitch channel during an acceleration in the first four gears. You can see the small changes in pitch between gear-changes is captured in fine detail by the pitch measurement. This was carried out using a separation of 1m between the antennas, and you can see the system is sensitive enough to measure a 0.6 degree change in pitch angle over a very short time period.

If you need to measure pitch angles, roll angles and yaw angles, then just by fitting two Slip Angle Sensors, and mounting three antennas on the vehicle you can do just that.

Measured parameters

The following parameters are output simultaneously at 20Hz on the CAN bus, or via the analogue/digital outputs:

Time
Velocity over ground (kmh/mph)
Lateral acceleration (g)
Longitudinal acceleration (g)
Heading (degrees true north)
Distance (metres)
True heading
Slip Angle
Pitch/Roll Angle

Latitude (minutes)
Longitude (minutes)
Height (meters/feet)
Vertical velocity (kmh/mph)
Satellites
Lateral velocity
Yaw rate

OUTPUTS

2 x Digital
Output frequency proportional to Velocity, Slip angle, Pitch, Yaw Rate etc.
2 x Analogue
Velocity -5v to +5v, scale adjustable, Velocity, Slip angle, Pitch, Yaw Rate etc.
CAN
All parameters - 1Mbit default, configurable baud rates.
RS232
All parameters - 115200 baud.

What else can the Slip Sensor be used for?

The VBOX Slip Angle Sensor is two VBOXII 20Hz GPS engines in one box. Therefore all the normal VBOX tests can be carried out with the same degree of accuracy. You do not have to fit both antennas if you are not interested in measuring slip angle. This means braking distance can be measured to within 10cm, velocity to within 0.1kmh and distance to within 0.5%. The Slip Sensor can also be connected to the normal Multifunction Display and braking distance, acceleration etc can be displayed.

In competitive sailing, the slip angle or leeway is normally a very difficult parameter to measure, as it requires a precise measurement of the direction a boat is pointing relative to the direction of travel. In the past this has required expensive gyrocompasses or magnetometers, but by using the VBOX Slip angle sensor, the leeway or boat side slip is measured to a high degree of accuracy very easily.

A data sheet for the VBox SLip Angle Sensor is available here

VBOX Slip Angle sensor

How good is the pitch/roll measurement?

What else can the Slip Sensor be used for?