A CMOS chip set for accurate pulsed time-of-flight laser range finding

TL;DR: A laser rangefinder device based on pulsed time-of-flight (TOF) distance measurement techniques was constructed and tested and is capable of measuring the time position, rise time and pulse width of incoming optical pulses with ps precision in the amplitude range of more than 1: 50 000.

Abstract: A laser rangefinder device based on pulsed time-of-flight (TOF) distance measurement techniques was constructed and tested. Key blocks of the system are the integrated receiver channel and the integrated time-to-digital converter (TDC) fabricated in a 0.35-um CMOS technology. The receiver-TDC chip set is capable of measuring the time position, rise time and pulse width of incoming optical pulses with ps precision in the amplitude range of more than 1: 50 000. The timing detection is based on leading edge detection in the receiver channel, and the amplitude-dependent timing error is compensated for by utilizing the multichannel TDC. A measurement distance of 100 m is achieved to a target with a reflectance of about 10% at the signal level of SNR = 6, with an optical output power and receiver aperture of 12 W and 18 mm, respectively.

Summary

I. INTRODUCTION

• The time-of-flight (TOF) radars are typically based on either continuous wave (CW) phase comparison method or on the measurement of the transit time (ΔT) of a short laser pulse to an optically visible target and back to the receiver [1] .
• Optical distance measurement principles have important advantages over microwave radars, for example.
• Laser radars have found use in several industrial distance measurement applications, for example in proximity sensors and in positioning of tools, and in automotive applications such as velocity control [2] - [4] .
• This measurement principle enables cm-level precision even with a single transmitted pulse to distances up to tens of meters to noncooperative targets.
• The CMOS integrated receiver channel detects a weak optical echo and generates logic level timing signals for the multi-channel TDC, which measures the time intervals between the transmitted and received pulses as well as the pulse widths and rise times used for the timing walk error compensation [5] .

A. Receiver channel

• An integrated receiver channel was implemented in a standard 0.35-µm CMOS technology [5] .
• The timing comparator is triggered only when the received pulse exceeds a predetermined threshold voltage (Vth_L) which is related to the noise level of the receiver [7] as shown in Fig. 3 .
• Unfortunately, the leading edge timing discrimination principle will produce a relatively large timing walk error (nanosecond range) in its basic configuration for the received optical echo whose amplitude varies a lot [8] .
• It is only important to use the same threshold voltages for the calibration measurement (measurement of compensation tables) and for the actual distance measurement in which the compensation table is used.
• The measured characteristics of the receiver channel are: a signal bandwidth of ~250 MHz, a transimpedance of ~100 kΩ, the input referred rms noise current of the receiver ~100 nA (Cin,total ~3 pF) and the power consumption of about 180 mW.

B. Time-to-digital converter

• The multi-channel TDC circuit (10 parallel channels), composed of the blocks depicted in Fig. 5 , was fabricated in standard 0.35-µm CMOS process.
• TDC solves time intervals accurately between the start and 3 timing signals from the receiver IC.
• Δt1 defines an actual distance information (including walk error), Δt2-Δt1 corresponds to the pulse width and Δt3-Δt1 to the rise time information, used for the timing walk error compensation.
• The multi-channel TDC (10 parallel channels) is based on a counter and delay line interpolation in two nested, different resolution levels.
• One delay is divided into 32 LSB phases with parallel load capacitor-scaled delay elements [9] .

III. CONSTRUCTION AND MEASUREMENTS

• The receiver channel, multi-channel TDC and laser transmitter were controlled using an Opal Kelly XEM6001 FPGA board.
• In addition, the transmitter PCB consists of an ECL comparator (ADCMP553) which is used to generate a start signal for the TDC from the driving current of the laser diode.
• The focal lengths of the transmitter and receiver optics are 30 mm and 20 mm, respectively.
• The wide dynamic range for the reflected echo was achieved by using different kind of materials as the target.
• This includes the jitter of the TDC (~10 ps), jitter induced by the compensation and the jitter of the receiver channel (due to noise).

IV. CONCLUSIONS

• In conclusion, a CMOS chip-set was developed, fabricated and tested for a pulsed TOF laser radar.
• In addition, the power consumption could be scaled down by using the shut-down principle while the laser transmitter is not operating.
• The resulting dynamic range (1:1600) in [11] is narrower than in the proposed receiver.
• It should be noted that the designed and measured receiver-TDC chip-set includes also the high performance time interval measurement unit which is in any case needed and is not shown in [11] , [12] .
• The integrated receiver channel, multi-channel TDC and the time-domain walk compensation presented here, paves the way to miniaturized laser radar sensor systems.

Figures

Fig. 5. The block diagram of the TDC.

Fig. 1. Pulsed TOF laser rangefinder.

Fig. 4. Simplified block diagram of the receiver IC.

Fig. 3. Principle of timing walk compensation based on the measurements of pulse width and rise time.

Fig. 2. Optical pulse shape (905D1S2J03Y with MOS pulse driver) of the transmitter recorded with a 25 GHz optical measuremnt head.

Fig. 6. TDC measurement core based on counter and 2-level interpolation.

Fig. 8. Transmitter and receiver optics.

Fig. 7. Photograph of the measurement PCB including the receiver and TDC chips.

Fig. 10. Single-shot distribution of 5,000 hits of walk-compensated results at 34 m measured for a) a black piece of cardboard (ρ~0.12) and b) a directional diamond grade reflective sheet (ρ >>1).

Fig. 9. Pulsed TOF laser radar measurement track.

Frequently asked questions

Q1. What have the authors contributed in "A cmos chip set for accurate pulsed time-of- flight laser range finding" ?

In this paper, a laser rangefinder device based on pulsed time-of-flight ( TOF ) distance measurement techniques was constructed and tested.