Based on the optical-geometric technology, PixArt has taken a step forward to deliver a distance-sensing solution through its recently launched Single-Axis Distance Sensor (SAS) products under the new Distance Sensor product line.
Figure 1. Working Principle of the SAS Technology
We will now use Figure 1 as an example to explain the working principle in a one-dimensional detection scenario. As shown at the left-hand side of the diagram, a CMOS image sensor can emit a spotlight source to project to objects within its detection range. The reflection of the objects that bounce back will travel through the sensor’s lens and form an image spot somewhere on the sensor. When the distance between the sensor and the object changes, the reflection path changes accordingly, which causes the image spot on the CMOS image sensor to shift its position. If we collect and compute these reflected data wisely, we will get to know a lot about the sensor’s surrounding environment, including the distance and depth information.
In PixArt’s SAS technology, we further expand the detection dimension from a single light spot to a light plane, as shown in Figure 2. This plane-wise mapping concept allows the SAS sensor to achieve more sensing capabilities and dramatically increases the number of applications it can serve.
Figure 2. Plane-Wise Mapping Concept of PixArt’s SAS Products
The SAS is a highly integrated and compact module embedded with an image sensor, a low-latency DSP processor, a flash memory, and a wide-angled light-plane source to provide a relatively comprehensive distance sensing capability. With the help of the microprocessor and flash, the module can output readily usable distance information and further offer the flexibility to increase additional detection functionalities according to customers’ application requirements.
A SAS module diagram (with the I2C interface version to communicate with the external processor) is shown in Figure 3. Note that PixArt also provides SAS modules that come with both the I2C and SPI interfaces.
Figure 3. SAS Module Diagram (with I2C interface version)
Figure 4 shows the top view of the SAS module, where the CMOS image sensor and the laser diode are mounted. Both the CMOS image sensor and the laser diode need a matching lens to achieve their desired performance.
Figure 4. Top view of an SAS Module
POTENTIAL APPLICATIONS OF THE SAS MODULE
An electronic device that adopts PixArt’s wide-angled distance-sensing SAS module can be used in various applications. Here we list out a few to trigger the brainstorming process and let your ideas flow.
A system developer may install a SAS module either vertically or horizontally to a device to provide light planes at different angles.
Integrating a SAS module into a vacuum robot can provide the vacuum robot with obstacle avoidance capability. For instance, a SAS module that projects a horizontal light-plane can sense the presence of obstacles within its detection range, including walls, table-foot, and small toys, and report their positions and widths. Such capabilities allow the vacuum robot to turn to another direction before collision occurs.
A robot that projects a vertical light plane can sense taller obstacles or objects with depth, allowing it to report information about overhang furniture, staircases, cliffs, etc.
When applying a SAS module to an automatic drink dispenser, such as a soft drink dispenser or a coffee machine, it is able to:
a) detect whether a cup exists
b) identify the height and diameter of the cup
c) calculate the estimated cup volume
The two above diagrams are examples of the depth information been detected by a SAS sensor. By calculating using the formula below, we can easily obtain the percentage of the liquid volume been filled in a cup.
PixArt provides two standard SAS modules that come in different sizes – the PAC7088J1 and PAC7088J2. The PAC7088J1 module is designed to provide mid-range distance sensing capability, whereas the PAC7088J2 module (with a tiny form factor of 29.8mm*19mm*10.95m) is designed for short-range distance sensing. System designers can make their selection according to their target applications.
Drawings of the two SAS modules are shown in Figure 5 and Figure 6 respectively.
Figure 5. Image of the PAC7088J1 SAS Module
Figure 6. Image of the PAC7088J2 SAS Module
The default firmware running in the PAC7088J1 sensor module and the PAC7088J2 sensor module are built for vacuum robot applications that can output profile information about the target obstacles (e.g., staircases, cliffs, overhang, etc.) and their distance raw data. Note that PixArt can assist with the customization based on customers’ target applications.
I2C: 4 slave ID configurable
1.8V or 3.3V for interface
Note 1&2: A SAS module can be used to detect staircases or cliffs ahead if it is installed in a machine to project a vertical light plane. Taking the PAC7088J1 as an example, it can sense a staircase/cliff 3~15cm away from the machine and report its height and distance information. Note that the staircase/cliff height has to be at least 1cm to be detected.
Note 3: A SAS module can be used to detect overhanging objects ahead if it is installed in a machine to project a vertical light plane. Taking the PAC7088J1 as an example, when used for overhanging object detection, it can sense objects 6 to 20 cm away from the machine and report their height and distance information. In such an application, only objects taller than 5cm will be identified as overhanging objects, whereas objects between 1cm to 5cm of height will be identified as staircases.
Note 4: A SAS module can be used to detect objects on the ground if it is installed in a machine to project a horizontal light plane. According to PAC7088J1’s default setting for ground detection, the SAS module should be installed 4.5cm above the ground to project up to 24cm of distance. At its furthest 24cm mark, the minimum height for an object to be detected is 2cm. If an object is as close to the module as 3cm, it has to be at least 5cm tall to be detected. Once detected, the SAS module will report the object’s distance and boundary information.
The distance-sensing capability can also be achieved by a few other technologies, each with pros and cons. The TOF (Time of Fly) technology is the one that has frequently been compared to the SAS technology in similar application fields.
A single-point ToF sensor can provide highly accurate depth information but has a pretty narrow view angle. Its relatively low resolution can make it suffer when it comes to detecting small objects on the ground. Although the array-type ToF sensor can improve the view angle issue, the limitation to its sensing resolution still awaits to be overcome. Most of the ToF sensors only provide raw depth data and rely on the algorithm inside the host processor for calculation. This external calculation process may increase the CPU loading and is likely to delay the response time.
The SAS products from the newly launched Distance Sensor product line of PixArt is developed to provide dimensional sensing within their 100-degree view angle. They can report both the distance raw data and readily usable information about the detected objects, with the least CPU load. With its built-in DSP processor and distance calculation accelerator, the SAS modules can reach a high response time of 60Hz.
The highly integrated microprocessor and flash memory of the SAS module also help to preserve the module’s total flexibility and feasibility in further application-based customization, allowing the sensor to serve in a wide variety of applications.
Please go to PixArt’s Website (https://www.pixart.com) to learn more about the Distance Sensor product line or its SAS series, or browse through our comprehensive application-based product portfolio.
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