Cables and their impact on test and measurement equipment

Test and measurement know-how and high-quality components are indispensable to reliably get accurate results when using modern test and measurement equipment. Many users take for granted that transducers, amplifiers and measurement software need to meet high quality requirements. However, high demands are also made on the measuring cables connecting transducer and amplifier. Since the measuring chain can only be as strong as its weakest link, we recommend that you opt for high-quality components.

Measuring physical quantities is pivotal to many applications. Reliably and precisely measuring the relevant quantity is important in both development, where test and measurement equipment is used, for example, in test benches, and in production lines, where test and measurement equipment is used as a key component in quality assurance. The right transducer matching the measured quantity has to be used to convert the physical quantity into a measurement signal. A suitable cable transmits the signal to an amplifier which makes the measured value available for downstream analysis software or higher-level control architecture. Small signals and electromagnetic interference in the environment are the major challenges such measuring cables have to cope with.

High requirements

Cabling projects in industrial applications, in the test shop or in automation environments pose substantial challenges. System availability is of particular importance in production. Correspondingly, requirements are significantly higher than, for example, in office environments. The cables as well as the matching connectors for such applications not only need to be particularly robust, they also must have excellent transmission properties. It goes without saying that the measuring cables, in addition to meeting these mechanical and test and measurement requirements, also need to comply with the relevant directives and standards. Besides the ROHS directive (2011/65/EC) and the WEEE directive (2012/19/EC), there are other directives for specific fields of applications. The ATEX directive (94/9/EC), for example, plays an important role in applications in potentially explosive atmospheres.

Robust mechanical design

Mechanical aspects are key to the quality of cables and connectors. Mechanically robust design is the prerequisite for good transmission properties. High contact force, excellent conductivity and the right raw material are the most important factors impacting connectors. When test and measurement equipment is used in varying applications, the number of connection/disconnection cycles also needs to be taken into account. The insulation resistance or dielectric strength as well as potential static charging of non-conductive components must be considered as well. Depending on the ambient conditions of the intended application, cables have to meet additional requirements. Examples include resistance to oil and water as well as weathering, if cables are used outdoors. Applications in which the transducer is mounted onto a moving part, furthermore require that the cable is suitable for drag chains. This means, the cable's properties must not be negatively affected by frequent movements and bending with a predefined radius.

EMC and shielding

Besides its mechanical characteristics, the measuring cable's electrical properties are, of course, of central importance. It is essential that the cable transmits the signal actually generated by the transducer to the amplifier. Interference-free transmission is crucial, since such signals are often very small - for example, amounting to few µV. Electromagnetic interference generated by power electronics components such as electric drives, frequency converters or power supplies can be considerable, especially in industrial environments. This also applies if all devices comply with the EMC directive (2004/108/EC), since with small signals even small interferences can have an impact. A measuring cable that is to transmit signals without interference in such an environment needs to have adequate shielding. In general, cables are shielded to ensure sufficient insensitivity to interference. In most cases, non-magnetic materials such as copper or aluminum are used as cable shield. A cable shield usually consists of two sets of braided wires running in opposite directions. The advantage of this design is that it shields from electromagnetic fields while still allowing for high flexibility of the cable. The density and strength of the braided wire determines the shielding quality. It is essential that the shielding covers as wide a surface of the conductor or pair of conductors to be protected as possible and thus minimizes punch-through. To ensure good shielding about 95 % should be covered. Reliable interference protection is not guaranteed when less than 90 % are covered; usage for very small signals is thus not possible.

Measuring cables from the experts

Contrary to network cabling in office environments, there are no relevant standards for measuring cables uniformly defining major characteristics such as the shielding design. Test and measurement expert HBM offers measuring cables for a wide range of different applications to meet the special requirements of test and measurement technology. The series7.5/00-2/2/2 six-wire cable, for example, has been specifically developed for applications using the carrier frequency method. The cable features a very low capacitance between wires and shielding as well as among the wires themselves. Both the outside sheathing and the intermediate sheathings are made of polyurethane, which ensures high flexibility of the entire cable. So-called drag chain tests were carried out during development in which the cable was able to demonstrate its capability even under repeated bending with a radius of 75 mm. The wires are twisted and shielded in pairs. Additional shielding covers all wires and ensures extremely high interference resistance. Thanks to its robust, abrasion-proof and weather-resistant outside sheathing the cable can also be used outdoors.

Conclusion

Reliable and precise test results require optimum conditions. The complete measuring chain, from the transducer through the measuring cables, amplifier and downstream software needs to meet high quality standards. HBM, with 60 years experience in test and measurement, provides you with the solution that best fits your application, also in terms of measuring cables.