The current bottleneck in machine vision is the need for greater bandwidth based on increasing sensor resolution and frame rate along with market demand for HD imaging. Current systems are forced to compromise between image quality and compression. Higher levels of compression limit the accuracy and performance of machine vision systems.
With 5.0 Gbps of bandwidth, USB 3.0 improves upon the current interfaces in use, including USB 2.0, IEEE 1394b, Gigabit Ethernet, and Camera Link. Greater bandwidth enables designers to increase image resolution and frame rate while preserving image quality. Without the need for compression, designers can use smaller FPGAs and reduce the amount of memory required in systems. This translates to a lower BOM, smaller PCB, and reduced power consumption.
Recently, VISION, the leading world trade fair for machine vision, celebrated its 25th
anniversary with a new attendance record – 372 exhibitors from 32 countries. The machine vision industry has enjoyed the benefits of continued advancements in image sensor technologies and is flourishing with growing adoption in industrial, medical, surveillance, scientific, manufacturing applications.
Among existing interface standards used in machine vision cameras – Gigabit Ethernet, IEEE 1394b, and Camera Link – all offer unique capabilities but none are one-size-fit-all due to trade-offs in bandwidth, footprint, cost, and power. While USB 2.0, the most ubiquitous and versatile interface standard in the computing and consumer markets, also plays in the machine vision market, it falls short to other standards when it comes to bandwidth. This all changes with USB 3.0.
This article explores the benefits of using USB 3.0 in machine vision applications and discusses key design considerations when building a USB 3.0-based camera.
Benefits of USB 3.0
- Higher bandwidth than USB 2.0, IEEE 1394b and GigE
- Provides power and data over a single cable
- Lower cost implementation than Camera Link
- Plug-and-play and easier to set up than Gigabit Ethernet
- Adopted as “USB3 Vision” standard by AIA
One of the main challenges designers face in machine vision today is to keep pace with ever-increasing data rates required by higher resolution and higher frame rate image sensors. Designers struggle to fit the required frame size and frame rate within the bandwidth offered by interface standards. Considering a machine vision camera design using Gigabit Ethernet, resolution is limited to VGA at 120 fps due to available bandwidth. If a 5 Mega-pixel image sensor is used, frame rate drops to a meager 5-10 frames per second.
Some systems overcome the lack of bandwidth through the use of compression, which allows higher resolution and greater frame rate data to transfer over a slow interface. Compression, however, is not a preferred approach, especially in machine vision applications, due to two main drawbacks: image quality and design footprint. Modern compression algorithms are designed to progressively remove detail from an image to reduce the necessary bit rate.
Unlike consumer applications where most missing details go unnoticed by human eyes, images captured by machine vision systems are meant to be processed by precise computation of image analytics. Thus, it is crucial to capture raw data that preserves every bit of image detail. Moreover, miniaturization has been witnessed as a main driver in machine vision products - most machine vision cameras sport an ultra-compact “ice cube” form-factor that is no more than 1 cubic inch.
However, the need to support compression would require additional hardware, such as an FPGA for encoding and additional memory for frame buffering, resulting in a larger PCB footprint and end product size. Furthermore, introducing more compression components increases the system bill of material (BOM), complicates the design, and requires significantly more engineering effort and design time.
With USB 3.0, designers now have additional bandwidth at their disposal. USB 3.0 is fast with a 5 Gbps data rate, a 10x increase over USB 2.0 (480 Mbps). After 8b/10b encoding, USB 3.0 has 4 Gbps available bandwidth for data. USB 3.0 continues to support both the bulk and isochronous transfer mechanisms offered by USB 2.0 which guarantee data delivery and bandwidth respectively. When it comes to isochronous transfers, USB 3.0 has been upgraded significantly: USB 3.0 offers a 16x speed improvement over USB 2.0, increased from 24 MB/s to 384 MB/s. Applications replying on real-time data will gain the most benefit from this improvement.