Introduction
Cables play a major role in electrical systems. A properly engineered sensor cable and connector system helps attach the sensors and actuators to a control system without worrying about accidental disconnections. In this informative article, we’ll be talking about sensor cables and connectors and their industry standard specifications.
What is a Sensor Cable?
In the world of machines, sensors are everywhere. Wired, wireless, detachable, permanent and everything in between. Every wired sensor or device requires some form of physical interconnection to interface with the controllers. This is when sensor cables become very useful. Sensor cables are used to transmit signals and data between sensors and transmitters, transducers and control units.
Unlike regular cables/wires, cables designed for interfacing sensors with controllers are specially designed to be immune to electrical noise and mechanical wear and tear.
Along with their robust material and construction, interconnects play a major role in providing a highly robust, solid and lasting connection with the fixed terminals of the sensor and the controller’s interfaces.
Sensor Cable Construction
Let’s have a look at how sensor cables are built. Typically, sensor cables typically consist of a two main parts we can clearly identify:
- Cable assy/harness
- Cable termination/connector
Sensor Cable Assembly
Sensor cable assembly is also known as ‘cable harness’. This is the part that is responsible for carrying the electrical signals back and forth between the sensor and any other device. Cable assemblies can have varying degrees of complexity. Some of the cable options are as follows.
- Twisted pair cables
- Twisted pair sensor cables are some of the most common types of sensor cables. They usually have one or more pairs of twisted wires forming the cable. These sometimes have an additional shielding mesh that can also act as a grounding connection to
- Coaxial cables
- Coaxial cables are used to transmit signals that are very susceptible to noise. For example, signals from piezoelectric sensors such as vibration sensors and radio signals are transmitted through coaxial cables to reduce electrical interference.
- Coaxial cables have multiple layers of dielectric insulations and a grounding screen to provide ground coupling for external noise.
- Fiber optic cables
- Fiber optic cables are still somewhat rare in the sensor cable industry due to extremely special use cases. Nevertheless, they are used in optical sensing systems such as reflectometers.
- Fiber optic cables are extremely resistant to environmental conditions such as high heat, vibration and moisture.
Sensor Cable Termination
Cable termination goes hand-in-hand with cable connectors. In many cases such as automotive applications, sensor cables have replaceable connectors attached to them which we can remove.
The wires are connected to these connectors using special metallic contacts called ‘crimps’. These require special tools to properly install them onto wires.
There are also some sensor cables that do not come with full pluggable connectors, rather with ‘lugs’ that can be screwed into a wire-to-board connector such as a screw terminal or a specialized binding post.
How Do Sensor Connectors Work?
Sensor connectors form the ends of a sensor cable assembly. They are used to join the cables and components with sensors and equipment to build the sensor control system.
As mentioned before, there are many types of sensor connectors that are available in the market. Some are universal, industry standard and some are proprietary. Some connectors are built to last several thousand connections/disconnections and some are designed to form a solid connection that creates a watertight seal to handle extreme environment conditions.
Connectors are usually available in two types: wire-to-wire connectors or wire-to-board connectors. Wire-to-wire connectors are for connecting two sensor cables together to extend the length of a cable when needed. When the cable needs to be connected to a circuit board or an industrial controller, the cable with a connector can be connected to the board mount or panel mount receptacle that matches the connector on the cable.
Usually, connectors are identified as ‘male’ and ‘female’ or ‘plug’ and ‘receptacle’ to denote the mating types of one another.
Sensor connectors can also be classified according to their mating type. There are many types of matings available in sensor connectors such as,
- Screw type connectors (M8, M12 etc.)
- Latch-lock type connectors
- Friction-lock type connectors
- Screw terminal connectors
- Aviation connectors (screw+friction)
Screw type connectors usually have friction contacts that slide into the receptacle. To make a secure connection, there is a screw that couples the two parts of the connector together. M12, M8 are some examples of this type of connector.
Latch lock type has a mechanical latch that locks into place when pushed into the receptacle. The connector can be disconnected by holding the latch releasing lever and pulling the connector away from the receptacle. Ethernet and ProfiBUS are two examples for these.
Friction type connectors require least force to connect and disconnect. They are mostly used where low robustness and smaller footprint is needed.
Screw terminal connectors are commonly found in PLC I/O cards and other industrial automation equipment where frequent removal is not expected. The cables are simple and have terminations with metallic lugs to screw into the terminals.
Aviation connectors are an extension of screw type connectors that offer extremely robust connections with high immunity to dust, corrosion and moisture.
Sensor Cable Applications
Sensor cables are not only found in industrial and automotive machinery. They are also found in marine, military and even oil and gas industries. Here are some of the use cases for sensor cables:
- Automotive Industry
- Modern automotive industry heavily relies on electronic systems to provide extra functionalities such as drive assist systems and advanced engine management systems. Sensor cables are found in vehicles to connect sensors such as speed sensors, engine oil/coolant temperature sensors and crankshaft position sensors to the ECU and other submodules.
- Temperature, Pressure and Level Sensing
- Sensor cables are used to connect digital/analog temperature and pressure sensors to industrial controllers to measure the physical quantities.
- Marine and Aviation Industry
- Sensor cables are used to connect level sensors to allow the electronic systems to measure water, oil level, oil pressure and gas to determine the operational state of the aircraft/ship.
- Submersible applications
- Specialized sensor cables are used in wells, tanks and other submersible applications to carry signals from water level, water quality and flow sensors.
Sensor Cable Color Code
When it comes to color coding, sensor cables have a multitude of options depending on where they are being used and for what application. There are
For example, industrial sensors manufactured nowadays typically have blue, brown and black wires. Brown denotes positive voltage supply to the sensor while black is the negative (zero volts/ground) connection. For a three-wire sensor, the blue wire is the signal output. This can change from manufacturer to manufacturer therefore always read the user manual before installation.
The diagram below shows another set of common sensor cable colors along with their wiring schemes. These color codes are typically valid for proximity sensors.
If you’d like to read more information on automotive color code standards, you can refer to this link by Auto Electric Supplies.
Sensor Cable Types
Sensor cable types vary by few factors.
- Number of wires
- Depending on the sensor type, sensor cables that interface them with other devices can have 2, 3, 4, 5 wire configurations. On some cables, there are multiple pairs of wires twisted together to allow both sensor signal and power transmission.
- These wires can range from 30AWG to 10AWG.
- External insulation and protection
- Depending on the application environment, sensor cables are designed to be robust and resistant to mechanical and chemical stress. Below are some of the commonly used materials used to make sensor cable sheathing and their properties.
- Internal construction and shielding
- Some sensor cables have additional grounding/shielding mesh around the wires inside the cable to reduce electromagnetic interference for sensitive signals. The shielding is available as braid, foil, serve or a combination of either.
- Some high performance cables have internal ‘vent tubes’ to allow the cable to deform without disrupting the conductivity of the cable for maximum transmission accuracy.
Sensor Cable Specification
When selecting a sensor cable, the specifications must be carefully selected according to the applications. Make sure to select a sensor cable according to,
- Voltage rating
- Temperature resistance
- Waterproofing
- Chemical corrosion resistance
- Strength class
- Length
Some sensor cables can withstand up to line voltages while some can only carry signals at very low voltages. Selecting the correct voltage rating helps reduce cost and maintain system integrity.
Temperature, moisture and corrosion retardant outer sheathing helps protect the cable from elements at exposed areas. Factory terminated cables with connectors complements this abilities.
Some cables require over-head cabling and need to support itself physically. Special cables that are reinforced with kevlar and fiber are available for this kind of specific application.
When selecting a sensor cable, always select the sufficiently long ones to prevent slicing and introducing unnecessary structural and electrical weak points.
What is the difference between M8 and M12 connectors?
One of the most common types of screw type sensor connectors are M8 and M12. The number following letter M denotes the connector diameter in millimeters; M8 is 8mm and M12 is 12mm.
M series cables are available in a few different configurations according to the number of pins they offer and the keying slot in the connector housing to prevent unintended connections. They typically come in 3, 4, 5, 8 or 12-pin configurations.
Here are some of the applications of popular M8/M12 connector types:
| Connector Subtype | Application |
| M8A/M12A | Signal transfer (low voltage) |
| M8B/M12B | 3, 4, 5-pin Fieldbus connections |
| M8D/M12D | Ethernet, <=100MBps |
| M8X/M12X | Gigabit Ethernet, highspeed (>10Gbps)/ |
The primary difference between M8 and M12 connectors are their size and the wire gauge. Naturally, M8 connectors and cable assemblies have a more compact footprint than M12 ones. But M12 have a higher current carrying capacity due to higher contact area and larger wire diameter they can afford.
Conclusion
Sensor cable and sensor connectors are a broad area of discussion. Different industries have different standards implemented by different manufacturers. We, at OMCH have are happy to introduce our very own line of sensor cables that you can use in your sensor interfacing applications to help you select the perfect.
