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What is the technical principle of TOF depth camera? many novices are not very clear about it. In order to help you solve this problem, the following editor will explain it in detail. People with this need can come and learn. I hope you can get something.

Introduction to the Technical principle of TOF depth camera 1.TOF

TOF, or time of flight, literally translates to flight time. Therefore, the name implies, that is, to measure the time of light flying in space, by converting into distance, you can measure the distance between the depth camera and the object. Usually, the TOF depth camera consists of a transmitting module and a receiving module. The transmitting module can be a LED, a laser and other transmitting elements, which will emit modulated infrared light such as 850nm. After the object is reflected, the reflected infrared light is received by the receiving module. Because the transmitted and received waves are modulated waves, the TOF depth camera can calculate the phase difference between transmitting and receiving, and get the depth value by conversion, that is, the depth distance between the depth camera and the object.

2.TOF depth camera-single point ranging principle

The composition of a simple single point TOF system is as follows:

It consists of a transmitting diode, a receiving diode, a modulation module, a demodulation module and a processor. The modulation module is responsible for modulating the transmitted infrared modulation wave and transmitting the signal through the transmitting diode; the demodulation module is responsible for demodulating the reflected infrared wave received by the receiving diode; the processor includes ADC and data processing, ADC is to convert the analog signal into digital signal, what is the analog signal will be described in detail below; the data processing is to convert the measured phase difference into depth information.

3.TOF depth camera-principle of multi-point ranging

The composition of a complete TOF depth camera system is as follows:

Its structure is similar to that of the traditional RGB camera.

In addition, it is different from the order TOF system in that the measuring range is not a point, it is a surface, so the receiving module becomes a lattice photosensitive sensor, usually using a CMOS sensor. Optical lenses are added in front of the sensor and in front of the emitting diode, one is for the radiation range of infrared waves, and the other is to filter out light other than 850nm, that is, to ensure that the light entering the sensor is only 850nm, so as to ensure the accuracy of the measurement.

Because the phase difference represents the distance between the object and the camera, that is, the depth, because the phase difference of each point is different, it can form the whole depth image of the object.

Working principle of 4.TOF sensor (photosensitive chip)

There are two main working principles of TOF sensor: the technical principle based on pulse wave (paulse base) and the technical principle based on continuous modulated wave (CW).

Before that, you need to know how the demodulation module works and how to read the charge on the sensor. Take the reading measurement of single-point pixels as an example:

The whole process of reading and measuring is mainly divided into four steps:

Respectively, Reset;Integration;Readout;Deadtime.

Reset: as you can see in the image above, the Rst switch needs to be closed before measuring the charge, so that the capacitor CA and CB are charged, and then disconnected after a good charge point.

Integration:Integration period, DMIX0 and DMIX1 are switches controlled by the demodulation module. When to switch is related to the frequency of modulation and demodulation. When the photon is received by the photon sensor (blue circle arrow), and when the DMIX0 is closed, the charged capacitance CA will neutralize with the electrons released by the photosensitive element, and the capacitance CA will discharge until the neutralization is completed, and the resulting capacitance level represents the level of the number and time of receiving photons.

Readout: after neutralization, the Address Decode switch is turned on to read the level on the capacitor.

Deadtime: dead time

From the above four links, Integration Time is the most important.

Why are there still DMIX0 and DMIX1?

Technical principle based on pulse wave (paulse base)

The transmitting module emits a rectangular pulse wave, and the object has a pulse wave with phase difference after it is emitted. The method of measuring the phase difference is as follows:

The transmitting pulse can set a frequency f and open two reverse windows, DMIX0 and DMIX1 are used to receive the number of photons, one window frequency and phase is consistent with the transmitted pulse, the other is opposite to DMIX1, and the phase difference is 180.

When the two windows are open, that is, when the level is high, and there is a pulse reflected by the object, the two different windows collect the charge at their own time. It has been colored in red on the picture. Suppose the amount of charge collected by DMIX0 is Q0. The amount of charge collected by DMIX1 is Q1. This part of the addition of Q0 and Q1 is the amount of charge collected in the time of the reflected pulse.

Td represents the time of existence of phase difference. In order to calculate td, we can calculate the time of Q1 in the reflected pulse. In order to calculate the time of Q1, we can read the level on the window of DMIX0 and DMIX1, that is, Q0 plus the ratio of Q1 to Q1, and multiply it by the time when the emission level is high. Therefore, the phase difference d can be calculated by the formula:

Note: the whole process above is called IntegrationTime.

Technical principle based on continuous Modulation Wave (CW)

The continuous modulated wave is based on the pulse wave, and the continuous modulated wave is usually modulated by continuous sinusoidal wave. Different from the pulse, the continuous modulated wave opens four windows: C1-C4. As shown in the following figure:

Since continuously modulated waves are usually modulated by continuous sinusoidal waves, modulated waves can be regarded as sinusoidal waves, as shown in the following figure:

The measurement method of continuous sine wave modulation is deduced as follows. The serial number 1-8 corresponds to the formula 1-8 of the following figure.

1. Suppose the amplitude of the transmitted sinusoidal signal s (t) is an and the modulation frequency is f

two。 After the delay △ t, the received signal is received r (t), the amplitude after attenuation is A, and the intensity offset (caused by ambient light) is B.

3. The time intervals of the four samples are equal, all of which are Tamp 4.

4. Four equations can be listed according to the above sampling time.

5. Thus the phase offset △ φ of the transmitted and received sinusoidal signals can be calculated.

6. Accordingly, the distance d between the object and the depth camera can be calculated according to the formula in (6).

7. Calculation result of amplitude An after attenuation of received signal

8. The calculation result of the received signal intensity offset B reflects the ambient light.

A more detailed derivation process is as follows:

In fact, the two methods of calculating phase based on different principles are similar, and the continuously modulated waves are relatively complex. As for how to deduce it, it is more about mathematics. In the final calculation of d formula, the meaning is actually the same, but the symbols are different.

5. Analysis of advantages and disadvantages of two different measurement methods based on the technical principle of pulse wave (paulse base)

Advantages:

1. Because there is no need to calculate the amplitude and ambient light, the measurement method is simple, the response is fast, and the frame rate of the camera can be improved.

two。 Because the pulse duty cycle is much narrower than the CW method in the whole cycle, it is theoretically designed to detect longer distances.

Disadvantages:

1. Because it does not counteract the influence of ambient light on the measurement, it will be affected by ambient light. But it can be made up by setting up a window to receive ambient light components.

two。 Compared with CW's scheme, the measurement accuracy is lower, but it is not so low as to be outrageous.

Technical principle based on continuous Modulation Wave (CW)

Advantages:

1. Phase offset (r2-r0) and (r1-r3) in Formula 5 eliminate fixed deviations caused by measuring devices or ambient light relative to pulse modulation. Continuous modulated waves use multiple measurement windows. (Q 3-Q 4) and (Q 1-Q 2) can subtract the common components (reset voltage, ambient light, capacitance gain, etc.) and deviations in the measurement.

two。 The accuracy (variance) of the depth measurement results can be estimated indirectly according to the amplitude An and intensity offset B of the received signal.

3. The light source does not have to be a short-term high-intensity pulse. Different types of light sources and different modulation methods can be used.

Disadvantages:

1. Multiple sampling is required, and the measurement time is long, which limits the frame rate of the camera.

two。 Because of the large amount of acquisition and calculation, the depth resolution is reduced in products with the same cost.

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