Before you begin connecting up and using your Vicon system, to ensure its precision and accuracy:
- Choose an optimal measurement volume for a given experiment
- Place cameras to achieve uniform precision in all directions
- Consider the mechanical stability of the cameras and their mountings.
As the resolution of Vicon cameras has increased, mechanical stability has become increasingly important, because a very small shift in position can have an impact on system measurements, as shown in the following example.
Example of the effect of camera position on system precision
A Vicon T160 camera with a standard 18mm lens has a horizontal field-of-view of 54°. Each pixel subtends an angle of 0.0115° or 200 micro-radians.
In other words, a change of 200 micro-radians in the angular position of the camera and its sensor represents a one pixel shift in the system's measurements. This shift is equivalent to about a quarter of the diameter of a 12mm marker at a range of 16m.
This is a 2D shift. All 3D measurements are estimated from the intersection of several 2D rays, so the resulting 3D shift may be smaller.
Maximizing data quality
The most common causes of imprecision of 3D data are:
Scenario: Cameras are often clamped onto a framework that allows their position and orientation to be easily adjusted. The framework is commonly cylindrical tube and the clamps depend on friction.
Problem: If a camera is cantilevered so that its weight may rotate the clamp, the amount of slippage or creep at the clamp/frame junction needed to introduce 200 micro-radians of angular change is tiny: about 5 microns or about 1/50th of the diameter of a human hair. This slippage is far too small to be seen.
Solution: To minimize the risk of movement, mount cameras so that their weight does not rotate their mounting point either by bending the mounting frame or by causing a clamp to slip or creep.
Scenario: Many buildings are of steel-frame construction. A steel framework can transmit vibrations caused passing footsteps, elevators, and passing vehicles. Most building vibrations are locally translational and, while undesirable, have little direct effect on camera rotation.
Problem: If a camera is mounted on a bracket or cantilever, building vibration combined with the cantilevered mass of the camera can cause a rotational oscillation of the camera mount.
Solution: Ensure that camera mounting brackets, and the structure to which they are attached, are extremely stiff and cannot wobble if there is any vibration in the building frame. This applies whether the camera mounting is vertical or horizontal.
Scenario: Thermal expansion and contraction in large structures such as a building can be very large but the temperature changes that drive them tend to be relatively slow compared with the duration of a Vicon calibration/trial cycle.
Problem: One part of the system that changes temperature much more quickly is the camera itself. The inside of a Vicon camera reaches a steady temperature of around 50° Celsius. While the camera is warming up from the ambient temperature of its surroundings, its internal components inevitably change dimension. However, when the components reach operating temperature, their dimensions remain stable.
Vicon measures the effects of warm-up and ambient temperature changes on all its cameras. All current camera models reach their steady operating temperature in approximately 30 minutes. This time is relatively independent of ambient temperature over the normal operating range of 0°–30°C. During warm-up, the equivalent positional change varies between 0.25 pixel for lower resolution cameras to approximately 1 pixel for the T160.
Solution: Allow Vicon cameras to warm up for at least 30 minutes before calibration and measurement.