Johannes Seiter

FPGA based time-of-flight 3D camera characterization system

In this paper we present an FPGA based characterization system for our 3D TOF distance sensors supporting up to 128 × 128 pixels. The system is capable of flexibly generating all control signals required for a typical TOF measurement. Their properties can be changed within a very broad range. The cycle-to-cycle jitter of those signals was reduced to 1 ps by ECL circuitry. This is equivalent to a standard deviation of the measured distance of 0.15 mm. Furthermore, the system is able to preprocess the distance information before transferring the data to a terminal PC, which reduces the data load on the USB interface. The system includes an averaging function with a maximum of 256 elements to reduce the standard deviation of precision distance measurement sensors. A novel fiber based setup is introduced to systemize the characterization process. By means of averaging a standard deviation of 2 mm could be achieved with one of our 3D TOF distance sensors.

Correction of the temperature induced error of the illumination source in a time-of-flight distance measurement setup

A systematic investigation of the combination of a reference path and a reference pixel as correction method for a systematic error induced by the light source of a time-of-flight (TOF) distance measurement sensor is presented. A change of the bandwidth of the light source, e.g. caused by drifting temperature of the used LEDs results in a bandwidth dependent distance error. The presented method allows reducing this error over a large operating range by ~97 %, i.e. to 3 %.

Investigation of the distance error induced by cycle-to-cycle jitter in a correlating time-of-flight distance measurement system

Abstract. Time-of-flight (TOF) range sensors acquire distances by means of an optical signal delay measurement. As the signal travels at the speed of light, distance resolutions in the subcentimeters range require a time measurement resolution that is in the picoseconds range. However, typical clock synthesizers and digital buffers possess cycle-to-cycle jitter values of up to hundreds of picoseconds, which can potentially have a noticeable impact on the TOF system performances. In this publication, we investigate the influence of two common types of cycle-to-cycle jitter distributions on the measured distance. This includes a random Gaussian distribution, which is caused by, e.g., stochastic noise sources, and a discrete jitter distribution, which is found when timing constraints fail in synchronous digital designs. It was demonstrated that a Gaussian cycle-to-cycle jitter has only a negligible impact on the performance of the TOF distance sensors up to a standard deviation of 1 ns of the Gaussian jitter distribution. However, even the discrete cycle-to-cycle jitter investigated in its simplest form lowers the distance precision of the TOF sensor by a factor of 2.86, i.e., the standard deviation increases from 2.9 to 8.3 mm.

Correction of a phase dependent error in a time-of-flight range sensor

Time-of-Flight (TOF) 3D cameras determine the distance information by means of a propagation delay measurement. The delay value is acquired by correlating the sent and received continuous wave signals in discrete phase delay steps. To reduce the measurement time as well as the resources required for signal processing, the number of phase steps can be decreased. However, such a change results in the arising of a crucial systematic distance dependent distance error. In the present publication we investigate this phase dependent error systematically by means of a fiber based measurement setup. Furthermore, the phase shift is varied with an electrical delay line device rather than by moving an object in front of the camera. This procedure allows investigating the above mentioned phase dependent error isolated from other error sources, as, e.g., the amplitude dependent error. In other publications this error is corrected by means of a look-up table stored in a memory device. In our paper we demonstrate an analytical correction method that dramatically minimizes the demanded memory size. For four phase steps, this approach reduces the error dramatically by 89.4 % to 13.5 mm at a modulation frequency of 12.5 MHz. For 20.0 MHz, a reduction of 86.8 % to 11.5 mm could be achieved.

A Background Light Resistant TOF Range Finder with Integrated PIN Photodiode in 0.35µm CMOS

Within this work a single pixel Time-of-Flight (TOF) based range finder is presented. The sensor is fabricated in a 0.35 μm 1P4M CMOS process occupying an area of 45 × 60 μm2 at ~50% fill factor. It takes advantage of the integrated PIN photodiode, representing, to the best knowledge of the author, the first reported TOF device done in this technology with a PIN detector. The measurement results show a standard deviation of 1 cm for a total integration time of 2.2 ms and a received optical power of 10 nW. Furthermore, the maximal measured integration time per single phase step is slightly below 1 ms, being an improvement by the factor of 40 over the previous work using a similar approach. As proven with the measurements, the background light influence on the measured distance can be neglected even if the dc light is by the factor of 600 larger than the modulation signal.

Correction of the temperature dependent error in a correlation based time-of-flight system by measuring the distortion of the correlation signal

Correlation based time-of-flight systems suffer from a temperature dependent distance measurement error induced by the illumination source of the system. A change of the temperature of the illumination source, results in the change of the bandwidth of the used light emitters, which are light emitting diodes (LEDs) most of the time. For typical illumination sources this can result in a drift of the measured distance in the range of ~20 cm, especially during the heat up phase. Due to the change of the bandwidth of the LEDs the shape of the output signal changes as well. In this paper we propose a method to correct this temperature dependent error by investigating this change of the shape of the output signal. Our measurements show, that the presented approach is capable of correcting the temperature dependent error in a large range of operation without the need for additional hardware.

Development of a frequency-shifted feedback fiber laser at 777.5 nm for range sensing applications

A frequency-shifted feedback (FSF) laser in combination with an interferometer is a very accurate range sensing tool. In this paper, an FSF fiber laser with an output spectrum in the 777.5 nm range is presented. The cavity of the laser works in the 1555 nm range, enabling the use of cheap standard telecom products. Since a wavelength of 1555 nm is not detectable with silicon semiconductor devices, the output of the laser is frequency-doubled by a periodically poled lithium niobate (PPLN) crystal, which shifts the output spectrum from 1555 nm to 777.5 nm. It could be shown that frequency doubling is a feasible way to shift the output spectrum of the laser to a range which is detectable by silicon, without destroying the special properties of the FSF laser.

A processing approach for a correlating time-of-flight range sensor based on a least squares method

A novel processing approach for the output data of a correlating time-of-flight range sensor based on a least squares method is presented. Until now, the fast Fourier transform and a trigonometric approach have been widely used to derive the distance information from the output signal of the sensor. Compared to these methods, the presented approach does not suffer from a systematic phase-dependent error for ideal signals. Moreover, this method allows the detection of multipath propagation, i.e., it is possible to detect if light from different distances is received at the same time. Under certain circumstances, it is even possible to extract the distances of the different paths. Simulation results are presented, comparing the performance of this novel approach to the existing ones. Moreover, first measurement results prove the feasibility of this method and show a reduction of the phase-dependent error by 90% compared to the alternative approaches.