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VBOX 20 with Slip |
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VBOX 20 with Slip
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,
To see a video presentation on this sensor, use
this link.
Click
here for
a datasheet
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.
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 AnglePitch/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.
Further Documentation
You can find data sheets, user guides, application
notes and more
here.
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VBOX 20 with SLIP |
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 left below, 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)
What tests can it be used for?
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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.
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How does it measure Pitch/Roll?
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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.
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How good is the pitch/roll measurement?
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.
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.
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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 mangetometers, but
by using the VBOX Slip angle sensor, the leeway or boat side slip is
measured to a high degree of accuracy very easily. |
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