Choosing Connectors for Test and Measurement Equipment
Solid body combo D-Sub connectors with rugged die-cast housings withstand field abuse.
Engineers who are tasked with choosing connector sets for test and measurement (T&M) equipment face a long list of choices. There are more than 100 connector manufacturers, and each has its slick marketing campaign or web page touting its products’ features and benefits for different target markets or applications.
T&M applications may include a broad range of environments, so it can be difficult to craft a one-size-fits-all set of specifications or requirements for connectors. On the “clean” end, equipment is often used in research and development labs, quality assurance product labs, and medical testing rooms. On the “dirty and abusive” end of the spectrum is all the equipment used on factory floors or even outdoors, by service technicians. This test equipment will be tossed into service trucks, dropped from tailgates, sprayed with water and fluids, or even dropped in puddles during everyday use.
Virtually every supplier will claim its products are reliable. However, the design engineer must assess the real-world definition of that word by looking at the design attributes and materials of the connector in relation to the expected product use, life span, and usage environment.
How does a designer filter through the marketing jargon, and instead zero in on what is really important for the connector’s application? Here are some parameters to consider when reviewing product drawings and specifications.
It’s a given that the lowest-cost product will not have high-temperature plastics or rugged contact alloys that provide years of stable operation, and this is typically a key requirement for T&M applications. Since test equipment tends to be expensive, it is expected to last a long time. Such equipment is seldom considered a one- or two-year disposable item.
For the connector’s insulators, you should specify glass-reinforced resins with a flammability rating defined by UL standards. Cheap plastic insulation might last a few years, but it may incur stress cracking, causing degradation of the insulator or even total connector failure when the contacts fall out.
For applications that will see hard use or accidental abuse (such as service techs that may be in a hurry), it is inevitable the product will undergo drop shock and vibration when being transported. Thus, a robust connector design — including high-quality, glass-reinforced plastics — is a requirement. Only these types of high-grade plastics will withstand years of 50g drop shocks. In addition, connector housings made of metal, including die-cast zinc, are typically required in this market.
Stamped and Formed Versus Machined
The contacts, specifically the female contacts, must retain their spring characteristics, over many mating cycles or long periods of time with few cycles, especially if exposed to higher temperatures. Less-expensive brass contacts are not recommended; instead, tempered phosphor bronze alloys or better materials are required. Connector contacts are often produced using two basic manufacturing methods, stamped and formed or machined (a.k.a. screw-machined). The general industry consensus is that machined contacts are superior to the stamped versions. While more costly, this style of contact lends itself to use of better spring alloys, post-machining heat-treating, smoother finishes and lead-ins, and cylindrical shapes (which are especially important if a shrouded female contact is required). Shrouded contacts can withstand harsh mating conditions, and offer better protection against improper pin probing. This style of contact is popular on high-end I/O connectors.
Most signal contacts are gold-plated. Since gold has become so expensive in recent years, designers must look carefully at the thickness of the gold plating. Gold flash (an extremely thin layer) offers the lowest cost, but may not provide the lifespan needed. Thicker gold, such as 30 microinches, is popular for professional applications that are typical of the T&M market. Most engineers understand that more gold means a higher number of mating cycles — typically 500 or more for 30-microinch gold. The added gold offers the benefit of stability. Some T&M devices are used in factory processes, and thus might sit untouched for years. Thicker gold provides a more stable contact resistance and less drift in the instrument’s readings over time.
Connectors that must be mated/unmated thousands of times, such as those used in a manufacturing process, require special contacts that have helical, or more precisely, hyperboloid contacts (configured like a wire mesh). These are not the typical beam-type sockets, and are specifically designed for the most extreme number of mate cycles possible, with cycle counts to 100,000. They are available for a variety of connector families from specialty contact suppliers.
Electromagnetic interference (EMI) can corrupt measurements, so for applications that might be sensitive to such reading errors, two features can be incorporated in connector selection — shielding and filtering. While many connectors are available with plastic housings, a connector family with metal housings grounded to the device chassis, such as a D-sub or circular connector, is the preferred choice for EMI-sensitive areas.
As it happens, the D-sub family is the largest family of connectors that is also available with built-in filters. These filter connectors contain capacitors, and sometimes ferrite materials, to limit and control any undesirable high-frequency noise. Of course, if the desired signals are also high frequency, then shielding is the only method available. For the RF range, there are connectors (combination D-subs and some circular connectors) that include coax contacts, along with signal contacts, all in the same body.
Environmentally Sealed Options
Environmentally sealed connectors are now in high demand for T&M applications. With the proliferation of electronics in automation, and the need for numerous interconnections on machines that are subject to fluid-based environments (whether water, food products, or other chemicals), connectors with an IP rating are required. It might not have been possible a few years ago, but now Ethernet and USB connectors are available in IP67 or better ratings.
Many sensors that measure processes or product attributes are necessarily close to liquids and dust exposure that would ruin a normal interconnect device. The most common rating is IP67, which indicates a connector capable of withstanding liquid spray and even directed low-pressure water jets. As a result of this requirement, IP67-rated connectors, such as the circular families of M12, M8, and again D-subs, are now available from several manufacturers.
Cable Sets and Strain Relief
While all the previously mentioned considerations apply to both the male and female connectors in a set, one half of that set usually has a cable attached, which brings additional factors into the selection. Although most instruction manuals say NOT to pull on the cable but to grab the connector or hood instead, design engineers know that this doesn’t always happen. When selecting the cable end hardware, consider the application, the likely users, and the robustness of the cable strain relief requirements. Many cable sets use overmolded hoods, which usually provide the best in terms of strain relief, tolerance to flexing, shielding, and even liquid sealing. For T&M applications that are custom-fitted on site or in low volumes that preclude overmolding, discrete hoods are used. Discrete hoods (a.k.a. backshells) are available with careful specification and comparison shopping. Strain relief systems can include crimped sleeves, compressible rubber boots, or discrete clamps inside the hood entrance. IP67-rated hoods for connectors such as D-subs are now also available.
Size and Latching
A broad range of latching mechanisms is available. Many small circular connectors use either threaded connections, bayonet coupling, or proprietary push-pull latching systems, while some are simply friction-fit. Others employ separate threaded hardware, external bail latches, or spring-loaded slide locks, which are commonly found on rectangular connectors. The design engineer’s selection should be guided by whether this is a portable test device with removable test cables or a fixed machine making measurements on an assembly line.
As is always the case with connector selection, a seemingly simple set of parts involves complex decisions, largely because there are so many choices, and multiple solutions to your problem.