A 2×32 range-finding sensor array with pixel-inherent suppression of ambient light up to 120klx

Abstract

Real-time 3D imaging has reached a state of development where several fields of application are possible, such as factory automation, consumer electronics, security, and robotics. The first commercial products have already been launched, or will be available soon. Accuracies in the centimeter range or even below, low cost, and robustness to disturbances in realistic indoor and outdoor environments are the key performance demands for these sensors, which are mostly based on the time-of-flight (TOF) principle either in pulsed or continuouswave (CW) mode.Results with single-photon avalanche diodes (SPADs) and an array size of 128×128 pixels [1] permit a standard deviation of 1.4mm between two range maps after a measurement time of 1s with 0.25W solid-state laser pulses. Ambient light and/or temperature increase the dark-count rate of SPADs significantly, which dramatically reduces accuracy. The CW phase-correlation method is used in [2] and in PMD sensors [3], both based on opto-electronic mixer devices and realized in CCD-CMOS and CMOS technology, respectively. IR bandpass filters are used to attenuate undesired background light in order to guarantee functionality up to 50% of bright sunlight [2]. A filter attenuation factor of ~10 and burst-mode operation, which enhances the optical signal-to-noise ratio (SNR) by an additional factor of 5, make distance measurements feasible at conditions up to 726W/m² sunlight, which corresponds to about 90klx [3]. Similar techniques are applied to the 3D sensors in [4]. A triangulation-based VGA range imager [5] achieves up to 65fps with an error of 0.87mm at a distance of 1.2m utilizing a 0.3W laser scanning system, neglecting any ambient light considerations.We present a TOF-pixel circuit managing up to 120klx of ambient light without using any optical filter or burst-mode operation for optical SNR enhancement. The challenging task is to suppress the huge DC photocurrent I BGL due to ambient light that is decades larger in the worst case than the contribution of modulated light I MOD . Subtraction within one integration period might be the most obvious approach to solve this problem, by introducing the bridge-correlator concept [6]. Unfortunately, this topology suffers from insensitivity due to the parasitic capacitance of the photodiode. Figure 20.7.1 shows a pixel circuit based on a correlation principle. The received current I MOD due to modulated light, which is transmitted by an external light source and backscattered at a range object, is correlated with the on-chip clock at switches S 1 and S 2 . Capacitors C 1 and C 2 (C 1 = C 2 ) accumulate the received photocurrent I PH = I MOD + I BGL generated by photodiode D PIN . The modulation clock of the external light source is locked to the pixel modulation clock (at S 1 and S 2 ), and shifted by 2π/16 with every phase integration step, as outlined in the timing diagram of Fig. 20.7.1 for I MOD . In total, 16 phase steps are used to obtain the discrete triangle correlation function, out of which the distance information d is...

A 33 × 25 µm² low-power range finder

“…A more detailed description of the cross-correlation based distance acquisition can be found in [8][9][10].…”

“…However, a total pixel area of 125 × 125 µm² prevents the high-count integration. Another approach that uses additional capacitors for storing dc light information, and afterwards subtracts the charges is reported in [9]. Similarly as in [8], the pixel area was the limiting factor for lateral resolution enhancement.…”

Time-of-flight distance sensor with MOSFET lowpass filter as key element for suppression of ambient light

“…The operating range is limited to V LP-IN within V PD + 1 V. Fig. 3b shows the influence of ambient light generated by a cold-light source with a colour temperature of 3200 K. A maximum influence of Dd ¼ 26 cm is observed at 150 klx, which exceeds the efficiency of the BGLX mechanism reported in [6] by 30 klx at comparable performance.…”

“…Circuit description: In a previous design [6], a correlator circuit based on a differential integrator was investigated, which exhibits a different BGLX mechanism requiring two additional memory capacitors. The proposed circuit in Fig.…”

Range finding sensor in 90nm CMOS with bridge correlator based background light suppression

“…Techniques, based on a PIN photodiode detector, are presented in [6,7]. These detectors are able to completely electrically suppress the BGL.…”