The ifm O3D series of cameras and sensors provide parameters to change the frequency mode. What is a frequency mode and which settings to choose?
What means frequency?
The word frequency refers to the modulation frequency of the illumination. The measurement principle of O3D, O3M and O3X cameras uses a continuous modulation of the illumination. Principally it’s a square waveform.
The O3D provides three different “modulation frequencies” or “modulation channels” (both terms have same meaning): 28, 29 and 30 MHz. When using the single frequency mode, one of these frequencies or channels can be chosen. When using multiple devices close by, it is advisable to set each of the devices to different modulation frequencies. This way interferences can be minimized.
The principle is analogous to receiving different radio channels at different frequencies – in case of ToF cameras in the form of light.
What are frequency modes?
The standard frequency mode is “single frequency”. In this mode, the O3D uses only one modulation frequency for measurement. However, that way, the possible measurement range is only up to 5m. Every object at a distance of 6m is measured at a distance of 1m again. Why is that?
As mentioned, the measurement principle uses a continuous wave. A wave is periodical, with repeating values after each 360°. Therefor the measurement value is repeating. See below sketch.
For measurement of object distances, the distances shown in above figure need to be divided by 2, because of the way forth and back.
If you are interested in a more detailed explanation, see subheading The Ambiguity Problem for more details.
To be able to measure beyond 5m (10m light path), the O3D supports multiple-frequency modes. Internally the camera takes multiple images with two or even three different modulation frequencies and combines the data to one frame without ambiguities.
Here is a list of measurement ranges with no ambiguity:
| Unambiguity range | Parameter string | |
|---|---|---|
| single frequency | Up to 5m | under5m_[exposure_mode] |
| double frequency | Up to 30m | upto30m_[exposure_mode] |
| triple frequency | Beyond 30m | morethan30m_[exposure_mode] |
Whenever the O3D uses multiple frequencies for each frame, be aware of potential interferences with other devices.
The Ambiguity Problem
Example: Let’s assume a modulation frequency of 30MHz. Given the speed of light with c_0=299 792 458 m/s, we get a wave length \lambda_{mod}=\frac{c_0}{f_mod} = \frac{299 792 458 m/s}{30 000 000 Hz} \approx 10m
That means, that our light wave has a wave length of 10m. Beyond 10m it repeats. When measuring, the light wave needs to travel forth and back, thus the distance to objects without ambiguity is 5m.
See also the below figure.
