Fast optical distance sensing through System-on-Chip integration

Optical sensors are successfully used for measurement in many areas, such as the position sensing of moving objects or the measurement of distance. The types of sensor elements used in these applications range from simple photodiode PSDs (Position Sensitive Detectors)…

Optical sensors are successfully used for measurement in many areas, such as the position sensing of moving objects or the measurement of distance. The types of sensor elements used in these applications range from simple photodiode PSDs (Position Sensitive Detectors) to photodiode lines and CCD/CMOS arrays. Modern submicron CMOS technologies permit higher integration of photo elements or magnetic sensors [1] together with the overall mixed-signal evaluation circuitry. This allows for a complete optical or magnetic system-on-chip (SOC) device to be created.

This higher level of integration reaps considerable benefits with regards to performance, speed, space, and system costs as described in the following examples of a single–chip optical encoder and optical triangulation sensor device. The Single-Chip Optical Sensor System Trend Integrating optical components, signal conditioning, processing, and communication to form a single-chip optical system not only reduces the amount of board space required, it also boosts performance. This follows the same trend line as seen in the area of magnetic sensor based single-chip encoders [2]. However, a reliable optical sensor package must also be optically permeable and allows for automatic SMD mounting. These challenges have been met with specially developed optoBGA and optoQFN packages which have proven successful in these respects. Chip dimensions of the single-chip optical sensor may vary and are largely determined by the number and type of photo-elements to be integrated.  For optical encoders, these are used to detect an encoder code disc or a reflected light source for example. Figure 1 shows a typical single-chip optical encoder system with 26 phototransistors of various sizes. These phototransistors have been placed in an optimum array for optical encoder code disc scanning. As shown in Figure 1, the block diagram of the iC-LNB 18 bit absolute rotary encoder chip contains the signal conditioning unit, the sine to digital converter with FlexCountth, the LED power control, and I/O interfaces on one substrate. The optical optoQFN package, as well as the die photo, is also shown in perspective with their dimensions and pin count. Given that the photo-element array and position of the functional units roughly match the actual distribution on the chip, this illustrates how optical scanning features dominate the chip’s geometry. This is even more pronounced with photodiode arrays which are used for other optical sensors, such as distance measurement systems that exploit the principle of triangulation. By applying reliable, submicron, mixed-signal CMOS technology, this makes possible the integration of such fast optical systems onto a single-chip. Figure 1: Single-Chip optical system: an absolute encoder with 18-bit resolution and 80 ns position update

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Dipl.-Ing. Bernd Schrörs and Dipl.-Ing. Marko Hepp, iC-Haus GmbH, Bodenheim, Germany

– September 29, 2012

Fast Optical Triangulation for Distance Control
Measuring distances using triangulation is based on a very simple geometrical principle. A beam from a LASER or LED light source is directed at the object to be measured. The reflected light is then captured by an optical system and transmitted to a photo-receiver. This can be a photodiode array or a position-sensitive photo-element (position-sensitive detector or PSD). The intensity of the reflected light depends on the object surface and the position of this light to the distance of the object.  Figure 2 depicts this relation to the object (positions 1 and 2) and gives the positions of the reflected light on the receiver element (P1 and P2).

Figure 2: Measuring distance using the principle of triangulation

Typical applications for sensors based on the principle of triangulation are:
–          Measuring distances in general
–          Gauging thickness and/or minimum/maximum monitoring
–          Level sensing and limit value monitoring
–          Surface scanning
–          Monitoring of deflection
–          Non-contact remote sensing of registration marks
–          Position tracking for robots and automatic machinery
–          Measuring presence with background suppression.
Depending on the area of application, various demands are made of the light source, optical system, and geometry of the triangulation sensor.  The distances to be measured from the object, the resolution required, and the dynamics of the measurement range in particular specify which light source is to be used and which type of photo-receiver is suitable. The required scan rate is also important when it comes to sensor selection.
LASER diodes are often used as light sources for distance sensors as they have a high concentrated light output with high efficiency [3]. Rapid progress in the luminous efficiency of high-brightness LEDs also permits these to be used as light sources for distance sensing in a variety of ranges. LEDs are particularly interesting as they are an inexpensive source of light.
Integrating LED drivers, photodiodes, signal conditioning, and evaluation circuitry with programmable parameters on a single-chip is another means of reducing costs while still providing maximum flexibility and configuration.
Classification, Measurement, and Readout with the Single-Chip Optical Sensor
In many applications, the distance sensor has the function of monitoring limit values and end positions that activate a digital switching output. Alternatively, this output can be used to directly control a relay or pass on information to a robot controller or drive through a field bus.
Integrating system elements to form a single-chip optical system has a number of clear advantages, especially where cost-sensitive retro-reflective photoelectric sensors are concerned. To this end, a programmable PSD has been developed using a special geometric arrangement with multiple photodiodes. The iC-LO contains two end photodiodes for classifying both the near and far fields and a configurable measurement range consisting of 127 middle photodiodes. Figure 3 shows the structure of the 7 mm photodiode array with its 129 elements and connecting pads as they are geometrically arranged on the 15-pin optoBGA package.

Figure 3: Photodiode structure for sensing the measurement range

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Dipl.-Ing. Bernd Schrörs and Dipl.-Ing. Marko Hepp, iC-Haus GmbH, Bodenheim, Germany

– September 29, 2012

Photodiodes
The active photodiode area of the fixed near-field diode is 0.927 mm2, the configurable measurement range diodes 2.23 mm2, and the area of the fixed far-field diode is 0.16 mm2. By classifying the measurement area, a wide scan range can be set. In addition, any programmable number of the 127 photodiodes in the measurement range can be allocated to the near field, with the remainder automatically assigned to the far field.
The block diagram of the single-chip triangulation device can be seen in Figure 4, as well as the 15-pin optical optoBGA-package, and the photodiode arrangements on the chip itself.

Figure 4: Block diagram of the single-chip triangulation sensor, optical BGA-package, and the device layout
Click on image to enlarge
Signal Amplification
The combined currents of the configured near and far fields are converted via the internal AC transimpedance amplifiers. The gain characteristic is also dynamically adjusted based upon the strength of the received light.  This becomes a logarithmical characteristic for strong input signals, such as those emitted by highly reflective objects.
The internal AC transimpedance amplifiers also offer high ambient light suppression which can be further increased up to 100 kLux by coating the device’s glass with a filter. Together, these feature result in a dynamic range of 100 dB for the single-chip triangulation sensor.
Signal Processing
The amplified far and near-field signals are sent to comparators as a combined signal to indicate light levels that may be too low. This warning output is available as an additional output signal. A differential amplifier also generates a differential signal which uses a comparator to classify the object. As this differential amplifier can be programmed, the possible resolution of the sensing distance is substantially higher than the geometry of the measurement range photodiodes alone.
Configuration
All configuration and parameter settings are made through the integrated SPI interface from the Microcontroller or PC-User interface. Figure 5 shows the adjustable parameters that may be used to teach-in the sensor parameters during development at the factory or in the field.

Figure 5: Typical User Interface for Teach-In and Development

These programmable parameters can be stored on the PC following calibration and reloaded as required. During the production phase, the system can also be calibrated and configured through an IO-Link or other field bus interface. After teach-in, the sensor parameters can then be stored with write protection and checksum in the microcontroller flash or EEPROM. The system can also be reset or altered at any time through the master controller.

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Dipl.-Ing. Bernd Schrörs and Dipl.-Ing. Marko Hepp, iC-Haus GmbH, Bodenheim, Germany

– September 29, 2012

Light Source
A programmable, 16-stage, low-side constant current source of 112 mA to 1.15 A has also been included so that an LED light source can be directly driven by the device. As an alternative, a LASER light source can be triggered instead using a digital CMOS output signal.  The pulse duration and type of measurement cycle can be adapted to suit various light sources and reflective properties of the objects to be scanned.
Scan rate and Outputs
Given the internal 2-MHz clock generator, this allows for high scan rates of up to 13.9 kHz for the device.  The internal programmable digital filter also controls the two complementary switching outputs for the triangulation sensor.
Flexible Sensor Design through Integration and Programmability
Full integration of the single-chip optical system for triangulation only requires a few external components; these being an LED, a low-cost microcontroller for parameterization, and an interface component for the switching outputs (NPN/PNP/PP) and/or the digital sensor bus (such as IO-Link [4]). Figure 6 depicts the entire circuitry for the sensor comprising of just three ICs.
Given that the 15-pin optoBGA SMD package requires just approximately 4 mm x 9 mm of board space, a full distance sensor can thus be implemented in an area of approximately 10 mm x 30 mm on a PCB. A DC/DC-converter with two output voltages internal to the IO-Link transceiver powers the complete triangulation sensor by way of an external +24 Volt industrial voltage.

Figure 6: Block diagram of a triangulation sensor with an IO-Link Interface
Click on image to enlarge
The diversity of digital sensor outputs demanded by the worldwide market creates a considerable amount of extra effort for development, stocking, and sales. This demand can be met by flexible parameterization of the iC-GF transceiver by separating the two switching outputs into an IO-Link and NPN/PNP/PP interface for example. This in turn saves development time by cutting down on the range of products needed during the development phase.
Product variety is also ensured during the product’s entire life cycle without any changes being made to the hardware.  Costs are further reduced as fewer parts have to be warehoused, less capital is tied up, logistics are simplified, and follow-up costs for customer service are lowered.
Conclusion
An integrated optical sensor system based upon modern VLSI-CMOS-Technologies increases system performance in terms of speed, space, and functionality.  As the chip-area is mainly dominated by the optical sensor requirements, and design of the external optical scanning arrangements, a smaller and more highly integrated single-chip optical system can be achieved.  This has been shown and proven with the iC-LNB single-chip absolute rotary encoder and the iC-LO single-chip triangulation sensor.  Reliable optical QFN- and BGA-packages are the enabler for this integration trend.

[1] Dr. David Lin,Speed acquisition made simple, EDN, September 2008
[2] White Paper, iC-Haus, Boost Performance in Motion Control Systems with Single-Chip Encoder
[3]Uwe Malzahn, Driving Diode Lasers Is Straightforward, EUROPHOTONICS, August/September 2004
[4] David Lin, Uwe Malzahn, and Álvaro Pineda Garcia, PNP/NPN/PP or IO-Link?, Electronic Engineering Times Europe, February 2012

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