When designing a machine vision system, considerable attention is given to selecting the right machine vision camera, lens and lighting. The camera cable is often not selected until the rest of the system has already been defined. In practice, that is the wrong order. The cabling forms part of the signal chain and directly affects whether the camera can maintain its specified frame rate reliably over time.
A machine vision camera continuously transfers image data to the industrial PC. At the same time, the I/O connection is often used for triggers, encoder pulses and lighting control signals. An unsuitable cable can therefore disrupt both image transfer and inspection timing. The camera may stop working altogether, but the fault can also be far less obvious, appearing as a reduced maximum frame rate, dropped frames or intermittent connection errors. The correct camera cable should therefore not be selected solely on the basis of the camera interface. Cable length, connector type, electromagnetic environment and the mechanical loads imposed throughout the life of the machine must all be considered.
Camera cable quality determines the available signal margin
At high data rates, the electrical characteristics of the complete connection must remain within the tolerances of the interface. Cable construction, conductor quality, shielding and connector quality all determine how much margin remains for stable data transfer.
A lower-quality cable may appear to work perfectly during an initial test. Problems often become visible only when the camera runs continuously at high resolution and maximum frame rate. The connection may remain active while frames are being lost or the configured frame rate is not being achieved. As a result, the cause is often sought in the camera, driver or image-processing software, even though the actual limitation lies in the physical connection. Successful camera detection is therefore not proof that the cable is suitable for the application. The connection must be tested at the maximum expected data load, with the final cable length and in the actual industrial environment.
USB3 Vision requires a short, high-quality connection
USB3 Vision provides high bandwidth, but it places demanding requirements on signal integrity. When designing a system with a passive USB3 connection, a cable length of approximately three meters should be used as the initial assumption. A high-quality industrial USB3 cable can operate reliably at five metres in many configurations, but this should never be taken for granted. The practical maximum length also depends on the camera, USB controller, power consumption and cable quality.
Low-cost USB3 cables of five metres or more may allow the camera to be detected while failing to operate reliably at full load. In practice, this can result in dropped frames, a lower maximum frame rate or a connection that fails only under certain operating conditions. Cameras that use a large proportion of the available USB3 bandwidth leave particularly little margin for signal attenuation. Where a longer distance must be covered, daisy-chaining several passive cables is not a reliable solution. An active USB3 extension cable or a suitable USB3 extender is required to regenerate the signal along the connection. Even with an active link, the complete chain must be validated for bandwidth, power delivery and compatibility with the camera.
USB3 is not the right choice for every industrial installation
USB3 Vision is well suited to applications where the industrial PC is located close to the inspection point, the cable remains short and the installation is static. As distance, electromagnetic interference or mechanical movement increases, the installation becomes more demanding.
Servo motors, variable-frequency drives and power cables can generate electromagnetic interference. A well-shielded USB3 cable and correctly planned cable routing reduce this risk, but they cannot fully compensate for a poor system layout. A data cable running parallel to motor cables over a long distance remains more susceptible to interference than a short, physically separated connection. We do not advise against USB3 for industrial use in general, but we mainly apply it where the environment and system layout provide suitable conditions. USB3 can be an excellent solution for a compact inspection system. For long cable routes, continuously moving cables or a camera mounted on a robot arm, GigE Vision is generally the more robust choice.
GigE Vision provides greater freedom when positioning the computer
GigE Vision is based on Ethernet and can therefore cover a considerably greater distance between the camera and the industrial PC. The computer can be installed outside the machine or robot cell, with only the camera and essential cabling located at the inspection point. This simplifies maintenance and reduces the computer's exposure to vibration, heat and contamination.
The greater permitted distance does not mean that every network cable is suitable. Cable category, shielding, connectors and mechanical construction must match both the data rate and the industrial environment. A poor-quality cable or connector can cause packet loss and reduce the usable bandwidth. With Power over Ethernet, camera data and power are also carried through the same cable. This reduces the number of cables routed to the camera, but makes the conductor size, connector integrity and PoE supply more critical. Voltage drop or an unreliable connector can then affect not only communication, but also the camera's power supply.
Use a locking connector wherever possible
A standard USB or RJ45 connector is not designed to withstand vibration, tensile loads and repeated movement. For an industrial camera, a cable with a screw-locking or otherwise secured connector on the camera side is therefore preferable. The locking mechanism prevents vibration from gradually working the connector loose and makes the connection more repeatable during installation and maintenance.
A locking connector does not replace proper strain relief. The weight of the cable and forces transferred from a cable carrier must not be carried continuously by the camera connector. Securing the cable mechanically shortly after it leaves the camera keeps movement and tensile load away from the connection. This reduces both the risk of contact problems and the mechanical load on the camera interface.
A cable carrier requires a cable with a specified flex life
A standard industrial cable may be suitable for fixed installation, but that does not automatically make it suitable for a cable carrier. Inside a cable carrier, the cable is bent again during every machine cycle. Repeated flexing can break conductors, damage the shielding and alter the electrical characteristics of the cable.
This application requires a cable specifically rated for use in a cable carrier. These cables typically use fine-stranded conductors, adapted shielding constructions and abrasion-resistant jacket materials. Important specifications include the permitted number of flex cycles, minimum bend radius, travel speed and acceleration for which the cable is designed. The minimum bend radius applies to the complete cable route, not only to the straight section inside the carrier. An excessively tight radius at the camera connection or at the transition to the fixed installation can still cause premature failure. The cable must also have sufficient freedom of movement inside the carrier. If cables are compressed against one another or installed under tension, the mechanical load during each cycle increases.
A robot cable must withstand torsion as well as bending
The loads imposed on a cable mounted on a robot arm differ from those in a cable carrier. A cable carrier primarily causes repeated bending in a predictable plane. On a multi-axis robot, the cable is also subjected to torsion. A cable specified only as high-flex or cable-carrier rated is therefore not automatically suitable for robotic use.
For a robot application, the manufacturer should specify the permitted torsion per metre, torsion angle, number of motion cycles and minimum bend radius. Without these figures, it is impossible to determine whether the cable can withstand the expected robot movement over the required service life. Early-stage cable damage is not always immediately apparent in image transmission. The camera may operate normally through most of the robot cycle and lose frames or disconnect only in one specific robot position. This makes the fault difficult to reproduce. Selecting the correct mechanical cable specification therefore prevents not only downtime, but also lengthy fault-finding.
Why we usually choose GigE Vision for robotic applications
For cameras mounted on robot arms, we choose GigE Vision rather than USB3 Vision in most cases. This decision is not based solely on the nominal bandwidth of the interface. The combination of cable length, torsional loading and the availability of suitable industrial cable constructions is usually decisive.
Ethernet is supported by a wide range of cables designed for robotic motion and torsional loads. Its greater permitted transmission distance also allows the industrial PC to be installed outside the moving assembly or outside the robot cell. This removes the need for active USB3 extensions on or close to the robot and keeps the transmission chain straightforward. USB3 Vision can technically be used on a robot, but only where a suitable torsion-resistant USB3 cable is available and the total cable length remains within a reliably validated configuration. Because of the high data rate and shorter passive transmission distance, the available design and test margin is usually smaller. GigE Vision therefore often provides a more robust system architecture for robotic applications, even where USB3 would offer sufficient bandwidth on paper.
The I/O cable determines the timing of the inspection
The data cable carries the images, but the I/O cable often determines when those images are captured. The I/O connection is used for signals including triggers, encoder pulses, strobe signals and digital status outputs. These signals require little bandwidth, but they are time-critical.
A disturbed trigger signal can cause an image to be captured too early, too late or not at all. With a moving product, this changes the position of the object in the image. The image data may be transferred without any technical error while the inspection remains unreliable because acquisition did not take place at the correct moment. Use a shielded I/O cable that matches the camera connector and pin assignment. Wherever possible, route trigger and encoder signals separately from motor and power cables. For longer connections, it is also necessary to verify that the selected signal type, switching thresholds and cable length provide sufficient noise margin together. More information about the relationship between triggering, camera exposure and system latency can be found in our article on triggering cameras in a machine vision system.
Select the cable for the actual application
A suitable camera cable must match the system both electrically and mechanically. A statically installed USB3 camera over a short distance has different requirements from a GigE camera in a cable carrier or a camera mounted on the wrist of a six-axis robot. An I/O cable must likewise be selected according to its signal function, shielding, connector and cable route.
Anyone who integrates a machine vision system themselves should define the required cable length, data and power requirements, cable movement, and required bending or torsional lifetime before selecting the cable. Then verify that the connector can be locked securely and that adequate strain relief can be installed close to the camera. The correct cable does more than prevent a complete camera failure. Above all, it ensures that the camera continues to deliver the same performance under maximum load as it did during the initial test. When selecting suitable USB3, GigE and I/O camera cables, consider not only the connector and length, but also the shielding and mechanical specifications required by the application.
